Transport Of Potassium in Kidney Presented 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 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+ Secretion Increase 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 excretion Chronic acidosis- loss of potassium excretion.