Control mechanisms of bicarbonate transport across the rat proximal convoluted tubule

Abstract

Bicarbonate transport (JHCO3) was studied in rat proximal convoluted tubules by luminal and peritubular microperfusion, and the effects on tubular bicarbonate transport of selective changes in luminal and peritubular bicarbonate concentrations and of changes in luminal flow rate were evaluated. A pH glass electrode was used to measure [HCO3(-)] and gave results similar to those of a microcalorimetric method. Increasing the tubular and peritubular [HCO3(-)] at constant luminal perfusion rate (10 nl.min-1) augmented JHCO3, but JHCO3 increased more when pH changes were prevented by PCO2 adjustments (constant peritubular pH) than when pH was allowed to rise with the increase in [HCO3(-)] (constant PCO2). Elevation of the tubular HCO3(-) load by raising [HCO3(-)] stimulated JHCO3 more than when the HCO3(-) load was raised by enhancing luminal perfusion rate at constant [HCO3(-)] An increase in PCO2 at constant peritubular pH increased JHCO3. Diamox and benzolamide inhibited JHCO3 at luminal concentrations of 2-4 X 10(-4) M, yet a small but significant fraction of JHCO3 remained intact. Capillary perfusion with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (5 X 10(-4) M) depressed JHCO3 by 70%. Acute changes in luminal and peritubular potassium concentrations (range, 2-6 meq/liter) had no effect on JHCO3, but JHCO3 increased moderately but significantly in severe dietary hypokalemia.

Mechanosensory function of microvilli of the kidney proximal tubule

“…When Na ϩ reabsorption (with the control perfusate) is plotted as a function of the estimated microvillous torque, the relation is strikingly linear, with no suggestion of a transport maximum. Such linearity of reabsorption, over a broad range of delivery rates, has been a consistent finding in studies of HCO 3 Ϫ transport in the proximal tubules of rat kidneys (6,26,27). Note, however, that in the dextran perfusions, the effect of microvillous torque on reabsorption is not linear (3.6-fold torque increase in viscosity resulting in a 1.2-fold increase in reabsorption).…”

“…Because water reabsorption is driven by Na ϩ transport, a prerequisite for precise glomerulotubular balance is that luminal fluid flow modulates proximal tubule epithelial cell Na ϩ reabsorption. This flow effect has been termed ''perfusion-absorption balance'' (2,3) and has been demonstrated in microperfusion studies in rats (4)(5)(6)(7)(8). In mammalian kidneys, the primary pathway for proximal Na ϩ transport is into the cell through the Na ϩ -H ϩ exchanger isoform 3 (NHE-3) within the luminal cell membrane and out across the peritubular cell membrane via the Na ϩ , K ϩ -ATPase.…”

Proximal tubular bicarbonate reabsorption and PCO2 in chronic metabolic alkalosis in the rat.

“…The loops were confirmed as being the last accessible proximal convolutions by the failure of a small injected oil droplet to reappear in any subsequent proximal loop on the surface of the kidney. Timed collections (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) Pco2 measurements. The Pco2 microelectrodes used in these studies were constructed as described previously (8), and had outer tip diameters of [4][5][6][7][8][9][10] The ability of our microelectrode system to detect small differences in Pco2, with the recorder set at 1 mV = 5 mm, was tested by using it to measure Pco2 in three solutions equilibrated with known gas mixtures (Pco2 values of 30.9, 32.4, and 35.9 mmHg as determined by Scholander analysis).…”

Emergency Department Management of the Salicylate-Poisoned Patient

“…Chronic potassium depletion causes increased reabsorption of bicarbonate in the proximal renal tubules and difficulty achieving an alkaline urine. The effects of acute potassium depletion on urinary excretion of bicarbonate are uncertain [78]. It seems reasonable to infuse potassium and NaHCO3 simultaneously, especially in patients who are already hypokalemic.…”