The mechanism of the urinary concentrating defect in potassium deficient rats

Metabolic alkalosis in the rat. Evidence that reduced glomerular filtration rather than enhanced tubular bicarbonate reabsorption is responsible for maintaining the alkalotic state

Maintenance of chronic metabolic alkalosis might occur by a reduction in glomerular filtration rate (GFR) without increased bicarbonate reabsorption or, alternatively, by augmentation of bicarbonate reabsorption with a normal GFR. To differentiate these possibilities, free-flow micropuncture was performed in alkalotic Munich-Wistar rats with a glomerular ultrafiltrate total CO2 concentration of 46.5 +/- 0.9 mM (vs. 27.7 +/- 0.9 mM in controls). Alkalotic animals had a markedly reduced single nephron GFR compared with controls (27.4 +/- 1.5 vs. 51.6 +/- 1.6 nl/min) and consequently unchanged filtered load of bicarbonate. Absolute proximal bicarbonate reabsorption in alkalotic animals was similar to controls (981 +/- 49 vs. 1,081 +/- 57 pmol/min), despite a higher luminal bicarbonate concentration, contracted extracellular volume, and potassium depletion. When single nephron GFR during alkalosis was increased toward normal by isohydric volume expansion or in another group by isotonic bicarbonate loading, absolute proximal bicarbonate reabsorption was not substantially augmented and bicarbonaturia developed. To confirm that a fall in GFR occurs during metabolic alkalosis, additional clearance studies were performed. Awake rats were studied before and after induction of metabolic alkalosis associated with varying amounts of potassium and chloride depletion. In all cases, the rise in blood bicarbonate concentration was inversely proportional to a reduction in GFR; filtered bicarbonate load remained normal. In conclusion, a reduction in GFR is proposed as being critical for maintaining chronic metabolic alkalosis in the rat. Constancy of the filtered bicarbonate load allows normal rates of renal bicarbonate reabsorption to maintain the alkalotic state.

Mechanism of hyperreninemia in the potassium-depleted rat

Although dietary potassium deficiency (KD) results in an increase in plasma renin activity (PRA), the mechanism of this effect has not been elucidated. In the present study, isolated kidneys from normal rats or from rats made KD by diet were perfused at constant pressure (120 mm Hg) with a Krebs-Ringer-Bicarbonate medium containing albumin. KD led to an increase in PRA (3.6 vs. 1.1 ng angiotensin I ml per h, P less than 0.01), which was associated with a decrease in macula densa (MD) fluid delivery as estimated by urine flow (70 vs. 166 microliters/min per g, P less than 0.005), and an increase in renal vascular resistance (RVR) as perfusion flow rate was decreased from 34 to 24 ml/min per g, P less than 0.005. The increase in PRA was independent of the MD because PRA could not be suppressed when macula densa delivery was increased by perfusing KD kidneys with hypooncotic albumin. Moreover, when kidneys were made nonfiltering by perfusing with hyperconcotic albumin, PRA remained increased in KD kidneys (8.1 vs. 3.5 ng angiotensin I ml per h, P less than 0.01) despite the absence of MD delivery. Because the increase in PRA in both filtering and nonfiltering KD kidneys was associated with an increase in RVR, filtering and nonfiltering kidneys were perfused with the vasodilator papaverine. Despite lower tissue K levels in KD kidneys (278 vs. 357 mu eq/g, P less than 0.01), RVR and PRA were normalized in both filtering and nonfiltering KD kidneys perfused with papaverine. In conclusion, PRA is increased in the KD isolated perfused kidney. This increase occurs independently of both the MD and of tissue K levels and is mediated by the renal vascular receptor.

Urine concentration and dilution in hypokalemic and hypercalcemic dogs

The urine-concentrating mechanism was studied in chronic hypokalemia (seven dogs given a low K(+), high NaCl diet plus injections of deoxycorticosterone acetate [DOCA]) and chronic hypercalcemia (seven dogs given vitamin D). In the potassium-depleted dogs, muscle, serum, and urine K(+) fell markedly, but glomerular filtration rate (GFR) and body weight varied little. Maximum urine osmolality fell in all dogs (mean decrease = 45%); however, solute-free water reabsorption (T(CH2O)) at high rates of solute excretion remained normal in three of four dogs. Free water excretion (C(H2O)) increased normally or supranormally as a function of increasing Na(+) delivery to Henle's loop in six dogs so tested. Hypercalcemia of several weeks duration caused a decrease in both GFR (mean 36%) as well as in maximum urine osmolality (mean 57%). Maximum T(CH2O) was not invariably depressed; in fact, when the values were adjusted for the reduced number of functioning nephrons (T(CH2O)/C(In)), four of seven studies were normal. C(H20)/C(In) increased normally (or supranormally) with increasing fractional Na delivery to Henle's loop in four of five dogs.I conclude that the lowered maximum urine osmolality in these hypokalemic and hypercalcemic dogs was not related to abnormal water reabsorption from the collecting ducts. Although not specifically measured in this study, it is very likely that solute accumulation in the renal medulla was reduced. This probably was not caused by abnormal delivery of sodium to, nor reabsorption of sodium from Henle's loop. It is likely that a more subtle defect exists in the countercurrent mechanisms for establishing a steep concentration gradient in the renal medulla. In the few hypercalcemic dogs in whom GFR was very low, I believe that injury to, and blockage of medullary tubules could account for most of the reduction in maximum U(Osm). Although not specifically ruled out, there is no evidence here to suggest that high serum Ca(+) or low serum K(+) per se causes a defect in sodium and water reabsorption in the mammalian nephron.