Sodium-independent active potassium reabsorption in proximal tubule of the dog

The role of active transport in potassium reabsorption in the proximal convoluted tubule of the anaesthetized rat

1. We have previously assessed the contributions of diffusion and convection to net potassium reabsorption in the rat proximal convoluted tubule (PCT). The present study was conducted to evaluate the role of active transport in convective potassium reabsorption by measuring the transepithelial potassium reflection coefficient (sigmaK) in the presence and absence of cyanide in anaesthetized rats previously prepared for in vivo microperfusion. 2. Osmotic water permeability (Pf) was measured in double-perfused tubules (lumen and peritubular capillaries) by manipulating the applied transepithelial osmotic gradient between -30 and +40 mosmol (kg H2O)(-1) using raffinose added to or subtracted from luminal perfusates. Pf was unaffected by the presence of cyanide when the estimated dissipation of osmotic gradients along each tubule were taken into account. 3. The proportion of K+ (and Na+) convectively transported with water fluxes (i.e. sigmaK) was not affected by cyanide. In the absence of active transport and following correction for any diffusive component, sigmaK was 0.56 +/- 0.13, indicating substantial solvent drag which probably occurs via the paracellular pathway. 4. However, cyanide caused a reduction in net potassium flux over the entire range of fluid fluxes used in double-perfusion experiments. Subsequent single-perfusion experiments (tubule lumen only) using the specific K+-H+-ATPase inhibitor, SCH28080, failed to reveal any direct evidence for a primary active K+ transporting mechanism involved in K+ reabsorption in the PCT.

Estimated potassium reflection coefficient in perfused proximal convoluted tubules of the anaesthetized rat in vivo

1. As yet there is no definitive description of the mechanism and route by which K+ reabsorption is achieved in the proximal convoluted tubule (PCT). We have assessed the contribution of convective K+ transport to net potassium ion flux (JK) by estimating the reflection coefficient of K+ (sigma K) in the proximal tubule of anaesthetized rats previously prepared for in vivo microperfusion. 2. Alterations in the luminal concentration of the impermeant solute raffinose in single-perfused (lumen only) and double-perfused (lumen and capillaries) PCTs were found to change fluid reabsorption in a predictable fashion. 3. Net potassium ion flux (JK) in single- and double-perfused tubules was significantly correlated with net fluid flux (Jv), suggesting that convective K+ transport may be a significant factor in overall K+ transport by the PCT. 4. Estimates of sigma K in single- and double-perfused tubules were very similar (0.14 +/- 0.06 and 0.13 +/- 0.05, respectively), even though K+ diffusion was not strictly controlled in the former group. The maximum effect of 'pseudo-solvent' drag in double-perfused tubules was estimated to give a sigma K of 0.40. This low value for sigma K suggests that true convection/solvent drag may be an important driving force for the reabsorption of K+ from the PCT of the rat.

Effects of dobutamine on renal function in normal man

The effects of graded doses of dobutamine on renal function were studied in eight male volunteers. The infusion rates were 2.5, 5 and 10 micrograms/kg/min. Systolic blood pressure increased by 19% (P less than 0.01), 31% (P less than 0.01), and 44% (P less than 0.01), respectively, while diastolic blood pressure decreased by 17% (P less than 0.02), 17% (P less than 0.02) and 25% (P less than 0.01), respectively. Heart rate increased at the highest dosage by 32% (P less than 0.01). Glomerular filtration rate (GFR) diminished at all three infusion speeds by 10% (P less than 0.02), 9% (P less than 0.05) and 14% (P less than 0.02), respectively, while renal blood flow (RBF) was unchanged. Urine flow rate (UF) decreased by 36% (P less than 0.05) and fractional free water clearance (CH2O/CIn) diminished by 37% (P less than 0.05) at the rate of 10 micrograms/kg/min. Fractional potassium excretion (CK/CIn) decreased by 34% (P less than 0.01) and 44% (P less than 0.01) at the two highest rates. Fractional sodium excretion (CNa/CIn) and fractional chloride excretion (CCl/CIn) were unchanged. Catecholamine levels were unaltered. Plasma renin activity (PRA) rose significantly (P less than 0.05) at the highest infusion rate of dobutamine. It is concluded that dobutamine influences GFR, the clinical significance of which, however, is difficult to evaluate.

Evidence for feedback mediated reduction of glomerular filtration rate during infusion of acetazolamide

Systemic administration of acetazolamide (ACZ) causes glomerular filtration rate (GFR) to fall. Clearance and micropuncture experiments were done to define the mechanism of this drug effect. When rats were infused with ACZ intravenously, kidney GFR fell by 30% and single nephron (SN) GFR (measured by collecting distal tubule fluid) fell by 23%. Changes in arterial blood pressure, arterial pH, extracellular fluid volume, and proximal tubule pressure were not sufficient to account for the decrease in GFR. When SNGFR was measured by collecting proximal tubule fluid, with the loop of Henle having been blocked for 2-5 min, SNGFR was higher than the distally measured value and was not different than control. The results are consistent with the fall in GFR being caused by activation of the tubulo-glomerular feedback mechanism.

Effects of acetazolamide on proximal tubule C1, Na, and HCO3 transport in normal and acidotic dogs during distal blockade

It has been suggested that the establishment of a tubular fluid to plasma chloride gradient in the late proximal tubule by the reabsorption of bicarbonate (and other anions) in the early proximal tubule is responsible for a significant part of sodium chloride and water reabsorption in the proximal tubule. In the present study the effects of acetazolamide on proximal tubule water and electrolyte excretion were examined in 6 normal dogs and 10 chronic ammonium chloride-loaded dogs during distal blockade produced by ethacrynic acid and chlorothiazide administration. During distal blockade control urine/plasma osmolality and urine/plasma sodium were close to unity in all experiments. Urine/plasma chloride and urine/plasma bicarbonate were 1.21+/-0.02 and 0.75+/-0.07 in normal and 1.24+/-0.01 and 0.04+/-0.01 in acidotic dogs, respectively. After the administration of acetazolamide (20 mg/kg i.v.), there was a significant increase in urine flow, absolute and fractional excretion of sodium, bicarbonate, and chloride in all animals. Associated with these effects, urine/plasma osmolality and urine/plasma sodium remained unchanged but urine/plasma chloride decreased significantly to 1.15+/-0.01 in normal and to 1.19+/-0.01 in acidotic dogs. In acidotic dogs there was a significant correlation between the increase in bicarbonate, sodium, or chloride excretion after acetazolamide and the plasma bicarbonate level (range 6.8-12.5 meq/liter). These data demonstrate a significant effect of acetazolamide on bicarbonate, sodium, and chloride reabsorption in the proximal tubule even in the face of severe acidosis. Moreover, the data suggest that the decrease in chloride reabsorption (and accompanying sodium) after acetazolamide is related to the decrease in bicarbonate reabsorption and the associated decrease in the transtubular chloride gradient.

Effects of nephrectomy on renal salt and water transport in the remaining kidney

Fluid, sodium, and potassium transport was studied in proximal and distal tubules in rats in which one kidney had been removed two weeks after a suprarenal aortic clamp had been placed to prevent adaptive changes in glomerular filtration rate (GFR) in the experimental kidney. Free-flow micropuncture techniques were used and tubular fluid (TF) samples analyzed for inulin, sodium and potassium. In addition, peritubular total protein concentrations and luminal and peritubular hydrostatic pressures were measured. The following changes were observed 15 hr after unilateral nephrectomy: (1) a significant increase in single nephron GFR; (2) unchanged absolute proximal tubular reabsorption rates of fluid and sodium; (3) increased delivery of fluid into distal tubules; (4) increased distal tubular reabsorption of sodium, but of insufficient magnitude to prevent natriuresis; and (5) an augmentation of distal tubular potassium secretion. Reduction of single nephron GRF to control levels by aortic clamping abolished the natriuresis following nephrectomy.