Altered Venous Function in Hypertensive Rats

412

Altered Venous Function in Hypertensive Rats

GEZA SIMON, M.D., PH.D.

SUMMARY The vascular beds of the upper or lower body of ratswere perfused through the aorta with oxygenated Krebs-Ringersolution containing dextran (7 g/100 ml), at 37°C. Perfusion wasstopped every 10 minutes, and the pressure rise in the jugular or thefemoral vein was recorded during rapid infusion (15.3-90.0 ml/min)of Krebs-Ringer solution into the inferior vena cava. The followinggroups of rats were studied: (1) six male genetically hypertensive rats(GHR), 9-11 months old, New Zealand strain; (2) seven femaleGHR, 5-6 months old, New Zealand strain; (3) eight male spon-taneously hypertensive rats (SHR), 4 months old, Okamotostrain; (4) five male rats with two-kidney Goldblatt hypertension(2-KGH), 30 days postclipping; (5) seven male 2-KGH rats, 65 dayspostclipping; (6) eight male rats with one-kidney Goldblatt hyperten-

sion (1-KGH), 40 days postclipping; (7) weight- and sex-matchednormotensive control rats of the appropriate strain; and (8)weight- and sex-matched two-kidney and one-kidney, sham-clippednormotensive rats. Preliminary studies showed that rapid infusioninto the venous circulation fills the veins, but there is no entry of fluidinto the arterial side of the circulation. Compared to controls, thevenous pressure-volume curves of GHR (male and female), SHR,1-KGH rats and 2-KGH rats, 65 (but not 30) days postclipping, wereshifted toward the pressure axis {P < 0.05). The shift of the venouspressure-volume curves persisted following the reduction of vasocon-strictor tone by killing the rats or by the administration of sodiumnitroprusside (0.1 mg/ml perfusate), or by both. The findings suggestdecreased venous capacity in hypertensive rats.

THERE IS evidence that venous compliance is decreased indogs with one kidney and renovascular hypertension.1'3 Theevidence for a reduction of venous compliance in rats anddogs with two kidneys and renovascular hypertension isinconclusive.2"* Greenberg and Bohr* observed a decreasedpassive extensibility of portal vein strips in spontaneouslyhypertensive rats. Since the veins are not exposed to the highpressure associated with arterial hypertension, decreasedvenous compliance in hypertension is probably not second-ary to increased intraluminal pressure but must be the resultof neurogenic or humoral vascular influences. In the case ofthe spontaneously hypertensive rats, the possibility of ge-netic differences in vascular wall anatomy also exists.

The purpose of this paper is to investigate further thevenous function in rats with spontaneous and Goldblatthypertension to see whether venous changes are a constantfinding in the various forms of experimental hypertension.

Methods

Studies were performed in genetically hypertensive(GHR), spontaneously hypertensive (SHR), Goldblatt hy-pertensive, and normotensive control (NCR) rats (groupsI-VI in Tables 1 and 2). The GHR and their controls wereobtained by brother-to-sister inbreeding from the originalOtago stock of albino rats." The National Institutes ofHealth was the source of the SHR and the Wistar-KyotoNCR.7 Goldblatt hypertension was produced in maleSprague-Dawley rats (5-6 weeks old) by applying a sil-ver clip with an inner diameter (i.d.) of 0.2 mm to theleft renal artery, with or without removal of the right kidney

From the Hypertension Section, Department of Internal Medicine,University of Michigan Medical Center, Ann Arbor, Michigan.

During a part of these studies Dr. Simon was the recipient of U.S. PublicHealth Service Research Fellowship Award 1 F 22 H10 4038.

Portions of this work were published in abstract form in Fed. Proc. 34:398, 1975.

Address for reprints: Geza Simon, M.D., R6669 Kresge MedicalResearch Building, University of Michigan Medical Center, Ann Arbor,Michigan 48104.

Received July 18, 1975; accepted for publication January 29, 1976.

1 week later. Male Sprague:Dawley rats, age- and weight-matched, and sham-clipped (i.d. = 1.0 mm), with or withoutcontralateral nephrectomy, served as controls for the Gold-blatt hypertensive rats. The hypertensive and the normoten-sive rats were kept under the same conditions, fed regular ratchow and given water ad libitum. The rats were weighed,and their blood pressures were measured by tail plethysmog-raphy on the day of the hemodynamic studies.

UPPER BODY VENOUS PRESSURE-VOLUMEMEASUREMENTS

In these experiments, an in situ preparation was used inGHR and NCR (groups I and II in Table 1). Rats wereanesthetized with ether, the right jugular vein was can-nulated (PE 90), and heparin, 500 units, was given intrave-nously for anticoagulation. A midabdominal incision wasmade and the inferior vena cava (IVC) was cannulated (PE240) above the left renal vein with the cannula pointingtoward the heart. A 16-gauge needle was inserted into theabdominal aorta and the systemic vascular beds proximal tothe renal arteries were perfused retrograde (Sigmamotorpump) with oxygenated (95% O2, 5% CO2) Krebs-Ringersolution containing dextran, 7 g/100 ml, at 37°C and at aflow rate of 3.8-5.4 ml/min. The perfusate drained from thecannulated IVC into a reservoir and was recirculated.Perfusion pressure, monitored through a T-tube arrange-ment between the pump and the aorta, was detected with aStatham P23Db pressure transducer and recorded on anoscillographic recorder. Mean perfusion pressure was keptat 40-50 mm Hg to minimize formation of edema. Sponta-neous respiration of rats ceased 1 -3 minutes after the startof perfusion.

For venous pressure-volume measurements, perfusionthrough the aorta was stopped and the perfusate was allowedto drain from the IVC catheter for 50 seconds. During thistime, jugular venous pressure, measured with a StathamP23Db pressure transducer, fell to 0-2 mm Hg in every rat.Krebs-Ringer solution containing dextran at 37°C then wasinfused (Harvard infusion pump) through the IVC catheter

by guest on April 9, 2018

http://circres.ahajournals.org/D

ownloaded from

http://circres.ahajournals.org/

VENOUS FUNCTION IN HYPERTENSION/5/mo/j 413

TABLE 1 General Observations in Genetically Hypertensive (GHR), SpontaneouslyHypertensive (SHR), and Normotensive Control (NCR) Rats

Group andtype

Group IGHRNCR

Group 11GHRNCR

Group IIISHRNCR

Strain

New ZealandNew Zealand

New ZelandNew Zealand

OkamotoWistar-Kyoto

Sex

MaleMale

FemaleFemale

MaleMale

No. ofrats

66

77

89

Age(mo)

9-118-10

5-64-5

44

Blood pressure(mm Hg)*

173.7 ±4.7t124.3 ±3.4

158.3 ± 1.2t123.1 ± 1.2

160.0 ± 3.9t121.7 ±3.4

Body wt (g)*

387.0 ± 4.6402.2 ±10.5

233.6 ±6 .1219.6 ±3 .5

292.2 ± 8.2307.9 ±12.4

• Results are means ± SE.t P < 0.001, for comparison of hypertensive group with normotensive control group.

at 90.0 ml/min. A jugular venous pressure of 30-35 mm Hgwas attained in 5-15 seconds. The infusion then wasstopped, the IVC catheter was opened to the atmosphere,and perfusion through the aorta was restarted. Venouspressure-volume measurements were repeated at 10-minuteintervals, until the pressure-volume curves were reproduci-bile to within ±2 mm Hg at any given volume.

The average of two or three series of measurements wasused to construct a venous pressure-volume curve for eachrat. Using Student's /-test, volumes producing intravenouspressures of 5, 10, 15, 20, and 25 mm Hg and pressuresreached after the infusion of 1.5, 4.5, 7.5, and 13.5 ml offluid in hypertensive rats were compared to correspondingvalues in normotensive control rats. Entire pressure-volumecurves, in the pressure range of 0-30 mm Hg, in hyperten-sives and normotensives also were compared by profileanalysis.8 This analysis takes into consideration the fact thatdata points along the pressure-volume curves are notindependent and that correlations of errors may be induced.The null hypotheses of parallelism and of lack of stratadifferences were tested. Null hypotheses were rejected at P< 0.05.

LOWER BODY VENOUS PRESSURE-VOLUMEMEASUREMENTS

In these experiments an in vivo preparation was used inSHR, Goldblatt hypertensive, and normotensive controlrats (groups 111—VI in Tables 1 and 2). Pentobarbital-anes-

thetized (30 mg/kg, ip) rats were allowed to breathespontaneously through a tracheal cannula. The left femoralvein was connulated (PE 50), and the femoral venouspressure was measured with a Statham P23Db pressuretransducer. Heparin (500 units, iv) was given for anticoagu-lation. A midabominal incision was made and the IVC andthe aorta were cannulated below the left renal vein, thecannulas pointing toward the hindquarters. Mass ligatureswere placed below the level of the kidneys to include thelumbar and abdominal musculature. The lower body vascu-lar beds of rats were perfused at 1.4-2.6 ml/min through theaorta with Krebs-Ringer solution (see above), again keepingperfusion pressure at 40-50 mm Hg. Lower body venouspressure-volume curves were obtained similarly to the upperbody pressure-volume curves, except that the rate of infusionwas kept at 15.3 ml/min, reaching 25-30 mm Hg femoralvenous pressure in 2-10 seconds.

After the third set of measurements the rats were killed byasphyxiation. Perfusion through the aorta was continued,but the perfusate no longer was recirculated. Additionalpressure-volume curves were obtained at 10-minute intervalswith or without perfusion of the hindquarters with sodiumnitroprusside (0.1 mg/ml of perfusate) for 10 minutes. Thelower body venous pressure-volume curves were analyzedsimilarly to the upper body pressure-volume curves. Thefirst, the average of the second and the third, and thepostmortem pressure-volume curves were analyzed andcompared separately. For purposes of intergroup compari-

TABLE 2 General Observations in Two-Kidney (2-KGH) and One-Kidney (1-KGH) GoldblattHypertensive Rats and Normotensive Control Sprague-Dawlev Rats

Group andtype

Group IV2-KGHControl

Group V2-KGHControl

Group VII-KGHControl

No. ofrats

55

78

87

Daysafter

clipping*

3030

6565

4032

Blood pressure(mm Hg)t

184.0 ± 9.8t122.0 ± 3.4

205.0 ± 7.It128.1 ± 5.0

210.6 ± 7.6t125.0 ± 2.9

Body wt (g)t

341.6 ± 8.0333.2 ±16.0

384.6 ±14.4401.2 ±11.1

301.1 ± 5.1317.8 ± 6.9

• Clipping was accomplished at 5-6 weeks of age.t Results are means ± SE.t P < 0.001, for comparison of hypertensive group with normotensive control group.



ownloaded from


414 CIRCULATION RESEARCH VOL. 38, No. 5, MAY 1976

sons, the average of the second and third curves is reported.For purposes of intragroup comparisons, venous pressure-volume curves obtained before and after killing the rats werecompared by the paired Student's /-test. Null hypotheseswere rejected at P < 0.05.

VALIDATION OF METHOD

An additional 30 pentobarbital-anesthetized (30 mg/kg,ip) male and female Sprague-Dawley rats, 4-10 months ofage, were studied. Venous pressure-volume curves wereobtained (1) with the aortic perfusion needle open and closedto the atmosphere, and (2) before and after perfusion of thelower and the upper body vascular beds with norepinephrine(10 Mg/ml of perfusate) or epinephrine (10 nglm\ ofperfusate) for 10 minutes. The effect of reduction ofvasoconstrictor tone on the lower body pressure-volumemeasurements was investigated by killing the rat or byperfusion of the lower body vascular beds with sodiumnitroprusside (0.1 mg/ml) for 10 minutes, or by bothprocedures. Lower body venous pressure-volume measure-ments also were performed during asphyxia for 3 minutesand before and after ligating the major tributaries (femoraland lumbar veins) of the IVC. In six rats, Evans blue dye inKrebs-Ringer solution was infused rapidly (15.3 ml/min)through the IVC catheter into the lower body venous beds(see above), and the distribution of the dye was viewed undera dissecting microscope. Finally, in one male Goldblatt

hypertensive rat and in one male normotensiveSprague-Dawley rat, rapid sequence (exposures at 1-secondintervals) venography was performed during rapid infusionof equal volumes of metrizoate sodium (Isopaque 440,Winthrop Laboratories) and Krebs-Ringer solution into thelower body venous beds.

Results

VALIDATION OF METHOD

The upper and the lower body venous pressure-volumecurves in the same rat were reproducible to within ±2 mmHg at any given infused volume, with the exception of thefirst curve which was displaced toward the pressure axis.Opening or closing the arterial side of the circulation to theatmosphere had no effect on the shape or the position of thepressure-volume curves (n = 10). Infusion of fluid into thelower body venous beds did not increase pressure on the ar-terial side of the circulation; this was a constant finding inevery rat of which the lower body venous pressure-volumecurves are reported in this paper. A shift of the venous pres-sure-volume curves toward the pressure axis, suggestingvenoconstriction, was noted after perfusion of the vascularbeds with norepinephrine and epinephrine (n = 6). In thestudies of the lower body vascular beds, aortic perfusionpressure at constant flow rose sharply, while femoral veinpressure increased slightly or in some rats remained un-

F«mof al vainprassur*ImmH,]

P»rh»lon

pv«Mur«

{mm M,] SO

UJ

1/1 10

UJ

8: .

-

-

• yV 1

/ ,

^^

A

• •O - - O

asphyxiapre -asphyxiapost-asphyxia

O 60 10O

VOLUME

1 40

(ml)

FIGURE 1 Top: Continuous recording of femoral veinpressure and aortic perfusion pressure at constant flow inthe pump-perfused hindquarters of the rat before andafter clamping of the tracheal cannula (arrows). Bot-tom: Lower body venous pressure-volume curves (means)in Sprague-Dawley rats (n = 3). Horizontal bars repre-sent standard errors of mean volumes: * *P < 0.01: ***P< 0.001 by paired Student's t-test, for comparison ofvenous pressure-volume curves before and during as-phyxia.



ownloaded from


VENOUS FUNCTION IN HYPERTENSION/S/mon 415

changed during the first 2-5 minutes of asphyxia (Fig. I).Thereafter, as the rat died of asphyxiation, aortic perfusionpressure returned to a level lower than that prior to theclamping of the tracheal cannula. Similar changes weredemonstrated in every rat subjected to the lower body ve-nous pressure-volume measurements, indicating that thevasomotor innervation to the lower body vascular beds wasintact and that killing the rats reduced the vascular resist-ance. Lower body venous pressure-volume curves obtainedduring the peak rise of aortic perfusion pressure were shiftedtoward the pressure axis (P < 0.001 by paired Student's/-test) (Fig. 1).

Reduction of vasoconstrictor tone by killing the rat orby the administration of sodium nitroprusside, or both,shifted the lower body pressure-volume curves towardsthe volume axis. This finding is illustrated in Figure 2B,showing these responses in hypertensive and normo-tensive control rats. Tying off the major tributaries (femoraland lumbar veins) of the IVC in eight rats did not changethe shape or position of the lower body venous pressure-volume curves in the pressure range of 0-10 mm Hg. Inthe higher pressure ranges (10-25 mm Hg), there was a

ILLJ

cc

LLJ

*•-» SHR 0* (N = 8)•—• NCR (S <N = 9)

E

wcrc/)

LU

ccQ.

SHR d* (N--8)

NCR d1 (N=8>

O 0 20 0 40 0 60 0 80 10O 120 1-40

VOLUME ( m l )

FIGURE 2 A: Lower body venous pressure-volume curves (means)in male spontaneously hypertensive (SHR) and normotensivecontrol (NCR) rats (group III, Table 1). Horizontal bars representstandard errors of mean volumes; ***P < 0.001. B: Lower bodyvenous pressure-volume curves (means) in SHR and NCR (groupIII, Table I) before (open symbols) and after (solid symbols)reduction of vasoconstrictor tone (see text); *P < 0.05; **P < 0.01by paired Student's t-test, for comparison of pressure-volumecurves in hypertensives and in normotensives before and afterreduction of vasoconstrictor tone.

FIGURE 3 Lower body rapid sequence venography in the rat at 12(top), 22 (middle), and 40 (bottom) mm Hg intravenous pressure.Left femoral vein is cannulated at the level of the hip.

small (1-3 mm Hg at a given volume) shift of the pres-sure-volume curves toward the pressure axis.

Rapid infusion of Evans blue dye in Krebs-Ringer solu-tion, producing pressures up to 40 mm Hg, resulted in thefilling with dye of the small veins (0.01 -0.05 mm) surround-ing the intestines, bladder, and testes in the six rats tested.There was, however, no staining of the wall of the intestines,the muscles of the thigh and back, or the parenchyma of thetestes as viewed under a dissecting microscope. Rapidsequence venography in two rats confirmed the findings ofthe Evans blue dye infusion (Fig. 3). At 10-15 mm Hgintravenous pressure the large veins and their majorbranches were visualized. At 20-25 mm Hg pressure therewas filling of the smaller veins and of the venous plexussurrounding the epididymis and the vas deferens withoutevidence for capillary filling ("capillary blush"). Evidencefor capillary filling was still lacking when intravenouspressure was raised to 40-45 mm Hg.



ownloaded from



VENOUS PRESSURE-VOLUME MEASUREMENTS

Six groups of hypertensive and normotensive control rats,were studied (Tables 1 and 2). The hypertensive andnormotensive control rats were matched with respect tobody weight. The blood pressure in each group of hyperten-sive rats was significantly higher than that of controls. Thefemoral venous pressures of SHR and of two- and one-kid-ney Goldblatt hypertensive rats was 1.6 ± 0.28 ( ± S E ) mm

3O

h15

UJtzCO 10oOUJccQ- 5

* - * GHR 9 (N = 7)

•—• NCR 9 (N= 7)

1 1 1

C 20 40 60 SO 100 120 14 0 16 0 180

VOLUME (ml)

FIGURE 4 Upper body venous pressure-volume curves in male (A)and female (B) genetically hypertensive rats (GHR) and theirnormotensive controls (NCR) (groups I and II, Table I). Horizon-tal and vertical bars represent standard errors of mean volumes andpressures, respectively; *P < 0.05; **P < 0.01; ***P < 0.001.

Hg (n = 5), 2.0 ± 0.15 (n = 6), and 1.9 ± 0.25 (n = 8),respectively. These pressures were similar (P > 0.5) to thoseof controls, 1.8 ± 0.28 (n = 5), 1.8 ± 0.29 (n = 8), and 1.9 ±0.16 (n = 8).

Figures 2 and 4 illustrate venous pressure-volume curvesin GHR, SHR, and their controls. Compared to values innormotensive control rats, there is a statistically significant(P < 0.05 by Student's r-test) shift of the upper body venouspressure-volume curve in male GHR (group I, Table 1) inthe direction of the pressure axis. Profile analysis corrobo-rated this finding (P < 0.01 for parallelism and P < 0.001for lack of strata differences). The shift of the pressure-volume curve of the hypertensive group is statisticallysignificant when either pressure or volume is taken as theindependent variable (Fig. 4A). The same observationapplies to the analysis of the other venous pressure-volumecurves. In the female GHR (group II, Table 1) the shift ofthe upper body venous pressure-volume curve is statisticallysignificant (P < 0.05 by Student's Mest) in the low andintermediate pressure ranges but not at 25 mm Hg pressure;this is due to the greater experimental variation in the higherpressure range (20-30 mm Hg) (Fig. 4B). However, profileanalysis in the pressure range of 0-30 mm Hg showed both ashift (P < 0.05) and a lack of parallelism (P < 0.05 forparallelism) between the pressure-volume curves of GHRand NCR.

In SHR (group III, Table 1), there is a statisticallysignificant shift (P < 0.001 by Student's r-test; P < 0.01 forparallelism and P < 0.01 for lack of strata differences) ofthe lower body pressure-volume curve toward the pressureaxis at all pressures examined (Fig. 2A). In both SHR andtheir controls, the lower body venous pressure-volume curveis shifted away from the pressure axis following reduction ofvasoconstrictor tone by killing the rat or by the administra-tion of sodium nitroprusside, or both (Fig. 2B). However,the shift of the venous pressure-volume curve of hypertensiverats toward the pressure axis was not abolished.

At 30 days after clipping and sham-clipping, the lowerbody venous pressure-volume curves of two-kidney Gold-blatt hypertensive rats and their controls (group IV, Table 2)overlapped the entire pressure range (0-25 mm Hg) (datanot shown). At 65 days postclipping there was a statisticallysignificant shift (P < 0.05 by Student's /-test; P < 0.01 for

I 20

UJ

COLU

<r

- * 2-KGH (Ni7)CONTROL (N=3

O O 2O O 4O

VOLUME (ml)

* - * 1-KGH (N;8)CONTROL (N = 7)

O 20 O 40 O 60

VOLUME (ml)

FIGURE 5 Lower body venous pressure-volume curves (means) in two-kidney Goldblatt hypertensive (2-KGH) (left), one-kidney Goldblatthypertensive (I-KGH) (right) and their normotensive controls (groups V and VI, Table 2). Horizontal bars represent standard errors of meanvolume; *P < 0.05; **P < 0.01; ***P < 0.001.



ownloaded from


VENOUS FUNCTION IN HYPERTENSION/SVmon 417

parallelism and lack of strata differences) of the lower bodyvenous pressure-volume curve of two-kidney Goldblatthypertensives rats (group V, Table 2), toward the pressureaxis (Fig. 5). In one-kidney Goldblatt hypertensive rats(group VI, Table 2) the shift of the venous pressure-volumecurve toward the pressure axis (P < 0.05 by Student's /-test;P < 0.01 for parallelism; and P < 0.001 for lack of stratadifferences) was apparent by 40 days postclipping (Fig. 5).The shift of the venous pressure-volume curves of two- andone-kidney Goldblatt hypertensive rats persisted followingthe reduction of vasoconstrictor tone by killing the rat or bythe administration of sodium nitroprusside, or both (datanot shown).

The apparent differences in the lower body venouspressure-volume curves of control rats (Figs. 2 and 5) reflectdifferences in strain (Wistar-Kyoto vs. Sprague-Dawley),age, and body weight (Tables 1 and 2).

Discussion

Disturbance of venous function in experimental hyperten-sion has been suggested by several investigators.1- * Directevidence that venous function is altered in experimentalhypertension comes from the work of Overbeck2 and ofSimon et al.,3 who measured pressure-volume relationshipsin collateral-free segments of femoral and mesenteric veinsfrom one- and two-kidney perinephritic hypertensive dogs.In one-kidney perinephritic hypertensive dogs, the pressure-volume curves of the femoral and mesenteric veins wereshifted in the direction of the pressure axis, indicating adecreased venous compliance. In dogs with early (less than 2weeks) two-kidney perinephritic hypertension, femoral andmesenteric vein compliances were unchanged. More re-cently, Greenberg and Bohrs demonstrated a decreasedpassive extensibility of portal vein strips in vitro fromspontaneously hypertensive rats (Okamoto strain).

The experimental method used in the present study was anadaptation of Alexander's technique for measuring mesen-teric venous distensibility in dogs.9 Alexander's techniqueconsisted of rapid retrograde infusion of fluid into thevenous side of a collateral-free mesenteric vascular arcade,after arterial inflow had been temporarily occluded, and thecontinuous measurement of the resulting venous pressure.The preliminary experiments of the present study indicatethat the venous beds of rats respond to pharmacological andneural stimuli in a manner similar to the response of themesentric veins of dogs. Venoconstrictor drugs (norepineph-rine and epinephrine) and sympathetic nervous systemdischarge (asphyxia) shift the venous pressure-volumecurves toward the pressure axis. The venodilator agent,sodium nitroprusside J° and the reduction of neural vasocon-strictor tone by killing the rat shift the curves toward thevolume axis.

Our preliminary experiments also indicate that in thevenous pressure range of 0-10 mm Hg, rapid infusion offluid results in the filling of the large veins primarily. In thehigher pressure range (10-25 mm Hg), there is also filling ofthe smaller veins. The infused fluid does not enter thearterial side of the circulation; therefore, arterial compliancedoes not contribute to the shape or the position of the venouspressure-volume curves. The limitations of the presentmethod are similar to those of Alexander's technique." The

initial unstressed volume, although probably small becauseof the collapse of veins during draining, is unknown. Venouscompliance, AV/AP, cannot be calculated. The contributionof branching of veins to the pattern of pressure-volumecurves is uncertain.

The findings of this study indicate that the venouspressure-volume curves in genetically hypertensive, sponta-neously hypertensive, and two-kidney and one-kidney Gold-blatt hypertensive rats are shifted toward the pressure axis.The shift of the pressure-volume curves persists followingthe reduction of vasoconstrictor tone, suggesting thatfactors other than active venoconstriction account, at leastin part, for the shift.

In view of decreased intravascular volumes in geneticallyhypertensive rats" and normal intravascular volumes inone-kidney Goldblatt hypertensive rats at 4 weeks postclip-ping,12 increased initial unstressed volume of veins inhypertension is an unlikely explanation for the findings ofthis study. The shift of the venous pressure-volume curves ina low pressure range (5-10 mm Hg) is evidence against thepossibility that the differences between hypertensive andnormotensive rats were due to a more rapid capillary fillingin normal rats. Furthermore, capillary filling could not bedemonstrated following addition of a dye or a radiographiccontrast material to the infusate, even if venous pressure wasraised to 40-45 mm Hg. Capillary filling during the rapidinfusion must be small and due mainly to the displacementof residual perfusate from the veins into the capillaries. Agenetically determined abnormality of venous anatomy,such as a reduction in the number of veins, in rats withspontaneous hypertension is also unlikely to account for thedifferences between hypertensive and normotensive rats,because similar shifts of the venous pressure-volume curveswere produced in normal rats by constricting one renalartery.

Assuming that the initial unstressed volume of veins inhypertensive and control normotensive rats was similar, theshift of the venous pressure-volume curves toward thepressure axis in hypertensive rats suggests decreased venouscapacity. Venous capacity is defined in this paper as theamount of fluid on the postcapillary or venous side of thecirculation at a given pressure.

In this study, the mechanisms responsible for the venouschanges in hypertensive rats were not investigated. In thisregard, Lucas and Floyer13 found increased interstitial tissuepressure and unaltered interstitial fluid volume in one-kid-ney Goldblatt hypertensive rats at 60 days postclipping,suggesting a decreased interstitial space compliance. De-creased interstitial space compliance may in turn reduce thevenous capacity and shift the venous pressure-volume curvestoward the pressure axis. Pamnani and Overbeck14- l s

demonstrated increased water, sodium, and potassium con-tents of veins in rats with coarctation and two- andone-kidney Goldblatt hypertension. The same authors foundincreased water content in veins of genetically hypertensiverats.15 Venous "water logging"16 or increases in ion-bindingcellular and paracellular matrix,1' or both, may account forthe abnormal venous pressure-volume relationships in hy-pertensive rats.

In view of unchanged venous pressure in hypertensiverats, the venous changes do not seem to be adaptive or



ownloaded from



secondary to increased intraluminal pressure. The postmor-tem venous pressure-volume measurements in rats and theuse of an artificial solution to perfuse the vascular beds ofrats eliminate the possibility that neural or humoral factorsare immediately responsible for the venous changes. Thepossibility that the venous changes were the residual effect ofneural or humoral vascular stimuli was not investigated.

References

1. Ferrario CM, Page IH, McCubbin JW: Increased cardiac output as acontributory factor in experimental renal hypertension in dogs. Circ Res27: 799-80, 1970

2. Overbeck HW: Hemodynamics of early experimental renal hypertensionin dogs. Circ Res 31: 653-663, 1972

3. Simon G, Pamnani MB, Dunkel JF, Overbeck HW: Mesentric hemody-namics in early experimental renal hypertension in dogs. Circ Res 36:791-798, 1975

4. Floyer MA, Richardson PC: Mechanisms of arterial hypertension: Roleof capacity and resistance vessels. Lancet 1: 253-255, 1961

5. Greenberg S, Bohr DF: Venous smooth muscle in hypertension: enhancedcontractility of portal veins from spontaneously hypertensive rats. CircRes 36 (suppl I): 208-215, 1975

6. Smirk FH, Hall WH: Inherited hypertension in rats. Nature 182:

727-728, 19587. Okamoto K, Aoki K: Development of a strain of spontaneously

hypertensive rats. Jap Circ J 27: 282-293, 19638. Morrison DF: Multivariate Statistical Analysis. New York, McGraw-

Hill, 1967, pp 186-1899. Alexander RS: The peripheral venous system. In Handbook of Physiol-

ogy: Circulation, vol 2, edited by WF Hamilton. 1963, pp 1075-109810. Johnson CC: The actions and toxicity of sodium nitroprusside. Arch

Int Pharmacodyn 35: 480-496, 192911. Greeson CR, Bird DL, Simpson FO: Plasma volume and extracellular

fluid volume and exchangeable sodium concentrations in the NewZealand strain of genetically hypertensive rat. Clin Sci 44: 349-358,1973

12. Ledingham JM, Cohen RD: Changes in extracellular fluid volume andcardiac output during the development of experimental renal hyperten-sion. Can Med Assoc J 90: 290-292, 1964

13. Lucas J, Floyer MA: Changes in body fluid distribution and interstitialtissue compliance during the development and reversal of experimentalrenal hypertension in the rat. Clin Sci 47: 1-11, 1974

14. Pamnani MB, Overbeck HW: Vascular wall composition in coarctationhypertension (abstr). Fed Proc 33: 358, 1974

15. Pamnani MB, Overbeck HW: Vascular wall composition in renalhypertension (abstr). Fed Proe 34: 383, 1975

16. Tobian L: interrelationships of electrolytes, juxtaglomular cells andhypertension. Physiol Rev 40: 280-312, 1960

17. Friedman SM, Friedman CL: The ionic matrix of vasoconstriction. CircRes 20/21 (suppl II): 147-155, 1967

Effects of Acetylsalicylic Acid on the DuctusArteriosus and Circulation in Fetal Lambs in Utero

MICHAEL A. HEYMANN, M.D., AND ABRAHAM M. RUDOLPH, M.D.

SUMMARY Intra-arterial and intravenous catheters were insertedin six fetal lambs at 125-130 days of gestation.On the following day,fetal arterial pressures and blood gases were monitored and fetalcardiac output and its distribution were measured by injection ofradionuclide-labeled microspheres 15 ̂ m in diameter. Acetylsalicylicacid, 55-90 mg/kg of estimated fetal weight, then was admin-istered into the fetal stomach. Fetal pulmonary arterial pressurerose significantly after an average of 58 minutes, increasing thepressure difference between the pulmonary artery and the aortafrom 2 ± 0.3 (SEM) mm Hg during control to 11.2 ± 1.6 mm Hg.Resistance across the ductus arteriosus rose from 4.2 ± 0.5 (SEM)

to 27.4 ± 4.01 units, and flow fell from 495 ± 44 (SEM) to 409 ± 20ml/minute. The proportion of combined ventricular output distrib-uted to the placenta, adrenals, heart, and lungs increased, whereasthe proportion of combined ventricular output distributed to thebrain, liver, intestine, kidneys, and upper and lower body fell. In twofetuses infusion of prostaglandin E, reversed the pulmonary hyperten-sion. Inhibition of prostaglandin synthesis in fetal lambs producedconstriction of the ductus arteriosus and redistribution of cardiacoutput. It is probable that prostaglandins, particularly E,, areinvolved in regulation of blood flow through the ductus arteriosus andvarious vascular beds in the normal resting fetus.

IT GENERALLY has been thought that the ductus arteri-osus is dilated passively during fetal life and activelyconstricted after birth.1 However, Coceani and Olley2

showed that in the presence of low Po», prostaglandins E,and E2 produce active, dose-dependent relaxation of isolatedstrips of ductus arteriosus obtained from fetal lambs. Theysuggested that these prostaglandins may play a role inmaintaining normal patency of the ductus arteriosus duringfetal life.

To define the possible active physiological role of prosta-

From the Cardiovascular Research Institute and the Departments ofPediatrics and Obstetrics and Gynecology, University of California, SanFrancisco, California.

Supported by Program Project Grant HL 06285 from the National Heartand Lung Institute.

Address for reprints: Michael A. Heymann, M.D., 1403 HSE. Univer-sity of California, San Francisco, California 94143.

Received October 13, 1975: accepted for publication January 26, 1976.

glandins in controlling the fetal circulation we studied theeffects of inhibition of prostaglandin synthesis in fetal lambsin utero. This inhibition was produced by the administrationof acetylsalicylic acid, which blocks prostaglandin synthe-tase, an enzyme system essential for production of prosta-glandins.3'6

Methods

We studied six pregnant ewes with time-dated gestationalperiods of 125-130 days.7'8 Under epidural anesthesia withtetracaine HC1, 20 mg, polyvinyl catheters were insertedinto the maternal femoral artery and vein. The uterus wasexposed through a midline incision and a small hysterotomywas performed. Catheters were inserted into a fetal hindlimbvein and artery and passed centrally into the inferior venacava and abdominal aorta, respectively. An incision wasmade in the fetal neck through a second hysterotomy, and a



ownloaded from


G SimonAltered venous function in hypertensive rats.

Print ISSN: 0009-7330. Online ISSN: 1524-4571 Copyright © 1976 American Heart Association, Inc. All rights reserved.is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231Circulation Research

doi: 10.1161/01.RES.38.5.4121976;38:412-418Circ Res.

http://circres.ahajournals.org/content/38/5/412World Wide Web at:

The online version of this article, along with updated information and services, is located on the

http://circres.ahajournals.org//subscriptions/

is online at: Circulation Research Information about subscribing to Subscriptions:

http://www.lww.com/reprints Information about reprints can be found online at: Reprints:

document. Permissions and Rights Question and Answer about this process is available in the

located, click Request Permissions in the middle column of the Web page under Services. Further informationEditorial Office. Once the online version of the published article for which permission is being requested is

can be obtained via RightsLink, a service of the Copyright Clearance Center, not theCirculation Research Requests for permissions to reproduce figures, tables, or portions of articles originally published inPermissions:



ownloaded from

http://circres.ahajournals.org/content/38/5/412

http://www.ahajournals.org/site/rights/

http://www.lww.com/reprints

http://circres.ahajournals.org//subscriptions/


Altered Venous Function in Hypertensive Rats

Documents