Transport Of Potassium in Kidney Presented By HUMA INAYAT.

Transport Of Potassium in KidneyPresented By

HUMA INAYAT

Introduction… K+ extracellular 4.2 mEq/L Increase in conc to 3-4 mEq/L causes

cardiac arrhythmias causing cardiac arrest and fibrillation.

98% of K+ in cells. 2% in extracellular fluid.

Daily intake ranges between 50-200 mEq/L.

Hyperkalemia and hypokalemia.

Cont… Excreted through urine. 5-10% in urine. Redistribution of fluids provides first line

of defance.

Internal Potassium distribution Ingested potassium moves into the cells untill

kidney can eliminate the excess. Insulin stimulates potassium uptake. Aldosterone helps potassium uptake into the

cells. Excess aldosterone conn’s disease-hypokalemia. Decreased aldosterone-addison’s disease-

hyperkalemia. Beta adrenergic stimulation increases

potassium cellular uptake.

Cont….. Acid base abnormalities changes

potassium distribution. Cell lysis causes increased extracellular

K+ conc. Strenuous exercise cause hyperkalemia. Increased ECF osmolality increases K+

conc.

Renal Potassium Excretion Rate of potassium filtration. 756 mEq/L Potassium reabsorption by tubules. Potassium secretion by tubules. 65% reabsorption in proximal tubules. 20-30% reabsorption in loop of henle.

Daily Variation K+ secretion in Distal And Collecting Tubules Principal cells of the late distal tubules and cortical

collecting tubules. K+ absorbed or secreted depending n body need. 100 mEq/day intake…92mEq secreted in

urine….8mEq secreted in feaces. High potassium diets, the rate of potassium

excretion exceeds potassium in the glomerular filtrate,

Potassium intake reduced below normal, secretion rate of potassium in the distal and collecting tubules decreases,

Decreases urinary potassium excretion. Extreme reductions in potassium intake, net

reabsorption of potassium in the distal segments of the nephron,

Potassium excretion can fall to 1 per cent of the potassium in the glomerular filtrate.

Low potassium intake, hypokalemia develops. Day-to-day regulation of potassium excretion

occurs in the late distal and cortical collecting tubules depending on body needs

Principal Cells Cells of late distal and cortical collecting tubules. Make up 90% of the epithelium. Potassium excretion occurs in two steps: Uptake from the interstitium into the cell by the

Na/K ATPase pump. Passive diffusion of K+ from the cell into the

tubular fluid. Luminal membrane of principal cells highly

permeable to K+ due to highly permeable channels.

Control Of K+ Secretion By Principal Cells (1) the activity of the sodium-potassium

ATPase pump,

(2) the electrochemical gradient for potassium secretion from the blood to the tubular lumen, and

(3) the permeability of the luminal membrane for potassium.

K+ reabsorption By Intercalated cells Severe potassium depletion, cessation of K+

secretion and a net reabsorption of K+ in the late distal and collecting tubules.

Reabsorption occurs through the intercalated cells;

This occurs due to hydrogen-potassium ATPase transport mechanism in the luminal membrane.

This transporter reabsorbs potassium in exchange for hydrogen ions secreted into the tubular lumen

Cont… Important in potassium reabsorption

during extracellular fluid potassium depletion,

Controls K+ excretion in normal conditions.

Summary of K+ Excretion Increased extracellular fluid K+

concentration stimulates potassium secretion.

By stimulating Na/K+ ATPase pump. K+ gradient between renal fluid and the

cell. Increased potassium concentration

stimulates aldosterone secretion by the adrenal cortex.

Cont…..

Aldosterone stimulates potassium secretion.

ATPase pump Increases permeability for potassium. Increased extracellular K+

concentration stimulates aldosterone secretion.

Negative feedback.

Blockade Of Aldosterone Feedback Impairs Control of K+ Concentration

Decreased aldosterone-addisson’s disease- hyperkalemia.

Increased aldosterone-primary adlsosteronism- hypokalemia.

Increase Distal Tubular Flow Rate Stimulates K+ secretion Rise in distal tubular flow rate with volume

expansion, high sodium intake, or diuretic drugs stimulates potassium secretion.

Decrease in distal tubular flow rate by sodium depletion, reduces potassium secretion.

high sodium intake decreases aldosterone secretion that decrease the rate of potassium secretion and reduce urinary excretion of potassium.

Cont….

However, the high distal tubular flow rate that occurs with a high sodium intake tends to increase potassium secretion.

These two effects of high sodium intake, decreased aldosterone secretion and the high tubular flow rate, counterbalance each other.

Acute Acidosis Decrease K+ SecretionIncrease in H+ conc decreases K+ loss.Decrease H+ conc increases K+ loss.Occurs by reducing the activity of Na/K ATPase pump.Prolong acidosis increases K+ urinary excretion.Acidosis leads to a loss of potassium, whereas acute acidosis leads to decreased potassium excretionChronic acidosis- loss of potassium excretion.