POTASSIUM AND SODIUM EXCRETION AND POTASSIUM HOMEOSTASIS DURING ACUTE HYPOKALEMIA

Decreased potassium and increased sodium excretion were observed in dogs acutely potassium-depleted by hemodialysis. Potassium excretion at constant blood pH varied directly with plasma [K+]. When filtered Na load and urine flow were constant during K+ depletion, increase in Na excretion was equivalent to decrease in K+ excretion, suggesting close coupling between transtubular movements of the two ions. Large changes in plasma [K+] (30%) and K+ excretion (50%) were produced with removal of a relatively small amount (estimated 3%) of total body K+. Plasma [K+] decrease during depletion was rapidly decelerated by movement of intracellular K+ into the extracellular fluid (ECF). When ECF [K+] stabilized, further decrease in K+ excretion ceased despite continued reduction of total body K+. It appears: 1) that tubular secretion of K+ is directly and rapidly responsive to reduction in ECF [K+]; 2) that low ECF [K+] may simultaneously impair contraluminal K+ uptake and Na extrusion, reducing K+ excretion and Na reabsorption during acute hypokalemia. Stoichiometry of changes suggests a 1:1 coupling between K+ secretion and moiety of Na reabsorption in the distal nephron.

DUAL EFFECTS OF SALINE LOADING ON RENAL TUBULAR SODIUM REABSORPTION IN THE DOG

Saline (0.9–1.5%) was administered at increasing rates to hydropenic dogs. Constriction of one renal artery performed early in the course of the infusion and maintained thereafter produced unilateral reductions in glomerular filtration rate (GFR) up to 50%. Since GFR increased bilaterally, the filtered load obtained late in the infusion on the constricted side was often comparable to that noted earlier on the control side. At comparable filtered loads sodium reabsorption was approximately 15% lower, later in the infusion. It was proposed that a saline infusion, apart from its effects on filtered load, progressively inhibits sodium reabsorption by decreasing the percentage of the sodium load reabsorbed. These data virutally excluded a fixed transport maximum for sodium. On the basis of filtered sodium, Tch2o, Uosm, and solute excretion data it was suggested that during saline loading sodium reabsorption in the proximal tubule and ascending limb generally remains proportional to the filtered load while simultaneously there develops an inhibition in sodium reabsorption which may exert a major effect at a site distal to the ascending limb.

EFFECT OF PROLONGED HYPOXIA UPON GRANULARITY OF RENAL JUXTAGLOMERULAR CELLS

The various experimental models used in previous studies of the juxtaglomerular apparatus do not permit the effect of changes in oxygen tension of the renal parenchyma to be separated from that of changes in distension of the renal arterial bed. To study the isolated effect of prolonged hypoxia, three groups of rats, matched for weight, were kept in low oxygen, room air, and high oxygen environment for two weeks. The animals were pair-fed with the hypoxic rats serving as the determinant group. Supplemental injections of sodium chloride were given daily to avoid the effect of sodium deprivation on the juxtaglomerular cells. It was found that the hematocrits and also the granularity of the juxtaglomerular cells were significantly increased in the hypoxic rats as compared to the other two groups. Blood pressures, serum sodium levels, and urinary excretion of sodium were comparable among the three groups. These findings demonstrate that increased granularity of the renal juxtaglomerular cells can be induced by prolonged hypoxia.