Effect of chronic potassium loading on potassium secretion by the pars recta or descending limb of the juxtamedullary nephron in the rat.

Abstract

A B S T R A C T Recently we demonstrated potassium secretion by the pars recta or by the descending limb of the juxtamedullary nephron. The purpose of this present investigation is to study the effect of a chronic high-potassium intake on this phenomenon. Fractional reabsorption of water and sodium by the juxtamedullary proximal nephron was decreased when compared to that in normal hydropenic rats. There was a striking increase in the fraction of filtered potassium at the end of the juxtamedullary descending limb from 94+11% to 180+18%, which was principally a result of enhanced potassium secretion. When the concentration of potassium in the collecting tubule fluid of potassium-loaded rats was reduced after the administration of amiloride, a sharp fall was observed in the amount of potassium which reached the end of the descending limb (64+8%). A direct correlation was observed between the fraction of filtered potassium at the end of the descending limb and the potassium concentration in the final urine (P < 0.001). The findings suggest that potassium, like urea, normally undergoes medullary recycling, which is enhanced by chronic potassium loading.Portions of this work were presented to

Consequences of potassium recycling in the renal medulla. Effects of ion transport by the medullary thick ascending limb of Henle's loop.

“…The results of the present experiments demonstrate that an acute increase in ambient K concentration to ranges found in vivo (1) can produce a substantial reduction in VT and NaCl absorption across the MTALH of both the mouse and the rabbit. The effect is reversible and occurs under hypertonic and near isotonic conditions (Figs.…”

“…Recent micropuncture experiments have demonstrated that K is recycled to the renal medulla (1)(2)(3).…”

“…The recycling process is dramatically illustrated when, under conditions of acute or chronic K loading, K delivery to the bend of the loop of Henle exceeds 100% of the filtered load (1,2,4). The currently accepted pathways for the recycling process involve K secretion by the distal nephron (1,5), passive absorption across the outer medullary collecting tubule (OMCT)' (6) and secretion into the pars recta or descending limb (1,4).…”

“…The currently accepted pathways for the recycling process involve K secretion by the distal nephron (1,5), passive absorption across the outer medullary collecting tubule (OMCT)' (6) and secretion into the pars recta or descending limb (1,4). K concentration in the medullary interstitium can thus reach 35-50 mM (1).…”

Electroneutral K+/HCO3- cotransport in cells of medullary thick ascending limb of rat kidney.

“…With respect to transepithelial bicarbonate transport, Good (25) has observed in the isolated rat MTAL perfused in vitro that increasing external potassium concentration from 4 to 24 mM had no effect on steady-state bicarbonate absorption; the latter result is, however, difficult to analyze with respect to the K+/HCO3 cotransporter described in the present study because potassium concentration was raised in both the perfusate and bath, which could have resulted in various direct, indirect, and possible opposite effects on basolateral bicarbonate transport and luminal proton secretion in the steady state. Selective large variations of the potassium concentration in the medullary interstitium, which may occur in vivo due to potassium recycling depending on the state of potassium balance (26)(27)(28), may modulate the K+/HCO3 cotransporter and thus bicarbonate absorption by the rat MTAL. The latter possibility could contribute to the acid-base disorders that are observed in a variety of pathologic conditions that include potassium imbalance (29).…”