Bicarbonate Secretion by Rabbit Cortical Collecting Tubules in Vitro

Abstract

A B S T R A C T We previously reported that rabbit renal cortical collecting tubules can secrete bicarbonate in vitro (i.e., there can be net transport from bath to lumen, causing the concentration in the lumen to increase). Net bicarbonate secretion was observed most often when rabbits had been pretreated with NaHCO3 and were excreting alkaline urine before being killed for experiments. The purpose of the present studies was to elucidate the mechanism involved by testing the effects of ion substitutions and drugs on collecting tubules that were secreting bicarbonate. Acetazolamide inhibited net bicarbonate secretion, suggesting that the process is dependent upon carbonic anhydrase. Net bicarbonate secretion also decreased when sodium in the perfusate and bath was replaced by choline, but not when chloride was replaced by nitrate or methylsulfate. Ouabain had no significant effect. Amiloride caused net bicarbonate secretion to increase. The rate of net secretion did not correlate with transepithelial voltage. The results are compared to those in turtle urinary bladders that also secrete bicarbonate. There are no direct contradictions between the results in the two tissues, i.e., in turtle bladders acetazolamide also inhibited bicarbonate secretion and ouabain had no effect. Nevertheless, it seems unlikely that net secretion of bicarbonate by collecting tubules involves specific exchange for chloride, as has been proposed for turtle bladders, because replacement of chloride by other anions did not inhibit bicarbonate secretion by collecting tubules. It was previously shown that the collecting tubules in vitro also may absorb bicarbonate, especially when the rabbits have been treated with NH4C1 and are excreting acid urine before being killed.

Cyclic adenosine monophosphate-stimulated bicarbonate secretion in rabbit cortical collecting tubules.

“…Thus, changes in VT are not necessarily accompanied by changes in HCO3 flux, which would be expected for passive diffusion. Others have also found VT and HCO3 flux to be poorly correlated (1). Third, under conditions that we would expect to inhibit transcellular HCO3 transport (0 Cl-solutions plus bath DIDS), cAMP produced no significant change in the HCO3 permeability coefficient (Table IV).…”

“…CVP volumes ranged from 23-33 n1. In all periods, 3-4 collections for HCO-concentration (total 1 Table II. There were highly significant differences in all three parameters between the two groups.…”

“…Net HCO3 secretion has been described in cortical collecting tubules dissected from alkali-loaded rats and rabbits (1)(2)(3)(4), but little is known about the hormonal control of this process. Although initial studies by Knepper et al suggested that chronic mineralocorticoid administration stimulates HCO3 secretion by the cortical collecting tubule (5), subsequent investigation has shown that this stimulation is secondary to the accompanying metabolic alkalosis rather than a direct hormonal affect on HCO-transport (6).…”

Hydrogen ion secretion by the collecting duct as a determinant of the urine to blood PCO2 gradient in alkaline urine.

“…These possibilities as recently outlined by Stinebaugh et al (7), and Warnock and Rector (31) include bicarbonate secretion into buffer containing fluid (32), mixture of acid and alkaline tubule fluid from different nephron populations (13), water abstraction from tubule fluid resulting in concentration of H2CO3 and HCO-in conjunction with CO2 loss (24), and finally the dissociation of HCO-to COm and H2CO3 ("ampholyte" effect) (5,6). Bicarbonate secretion has been observed in various types of acidifying epithelia including the rabbit cortical collecting tubule (32). The contribution of secretion to the final concentration of bicarbonate in urine has not been established, however.…”

Regulation of intracellular pH in the rabbit cortical collecting tubule.

“…Intercalated cells are interspersed between principal cells, make up 35-40% of the total cells of the CCT (1,2), and both secrete (18) and reabsorb HCO- (19). At least two subtypes of intercalated cells are present.…”