Microelectrode determination of pH and PCO2 in rat proximal tubule after benzolamide: evidence for hydrogen ion secretion.

Microelectrode determination of pH and Pco 2 in rat proximal tubule after benzolamide: Evidence for hydrogen ion secretion. Previous micropuncture studies supporting hydrogen secretion as the mechanism of bicarbonate reabsorption have relied on the demonstration of an acid disequilibrium pH in the proximal tubule after systemic administration of a carbonic anhydrase inhibitor. Previous calculations of disequilibrium pH, however, have involved the necessary assumption that Pco 2 in the proximal convoluted tubule was equal to arterial blood Pco 2 . This assumption can no longer be supported in view of the recent demonstration that the Pco 2 in proximal and distal tubular fluid exceeded arterial blood by approximately 25 mm Hg. The purpose of the present study was to determine directly pH and Pco 2 with micro-electrodes in both the early and late segments of the accessible proximal tubule of nine Sprague-Dawley rats before and after administration of benzolamide (2.0 mg/kg/hr, i.v.). In the early proximal tubule, pH decreased significantly after benzolamide administration from 6.98 ± 0.03 to 6.62 ± 0.03 pH U (P 2 also decreased from 65.1 ± 1.2 to 59.3 ± 1.4 mm Hg(P 2 decreased significantly (64.3 ± 1.5 to 57.7 ± 1.6) (P (1) the fall in both pH and Pco 2 in the early proximal tubule indicates that carbonic acid, not carbon dioxide accumulates after inhibition of luminal carbonic anhydrase; (2) although Pco 2 also decreased in the late proximal tubule, unlike the early segment, pH was unchanged after benzolamide administration, perhaps as a result of increased bicarbonate delivery; and (3) Pco 2 in vivo was significantly greater than was systemic arterial Pco 2 before and after benzolamide administration in both the early and late proximal convoluted tubule. These findings lend support to the view that bicarbonate reabsorption in the proximal convoluted tubule occurs, in part, by hydrogen secretion. Determination du pH et de la Pco 2 dans le tube proximal du rat apres benzolamide: Preuve de la secretion de l'ion hydrogene. Les travaux de microponctions anterieurs en faveur de la secretion d'ions hydrogene comme mecanisme de la reabsorption de bicarbonate ont repose sur la demonstration d'un pH acide de desequilibre dans le tube proximal apres l'administration systemique d'un inhibiteur de l'anhydrase carbonique. Cependant les calculs anterieurs du pH de desequilibre ont implique l'hypothese necessaire selon laquelle la Pco 2 du tube contourne proximal est egale a celle du sang arteriel. Cette hypothese ne peut plus etre soutenue du fait de la demonstration recente que la Pco 2 du liquide tubulaire proximal et distal est superieure d'environ 25 mm Hg a celle du sang. Le but de ce travail etait de determiner directement le pH et la Pco 2 au moyen de microelectrodes dans les segments precoces et terminaux du tube proximal accessible de neuf rats Sprague-Dawley avant et apres l'administraton de benzolamide (2,0 mg/kg/hr, i.v.). Dans le tube proximal precoce le pH diminue significativement apres benzolamide de 6,98 ± 0,03 a 6,62 ± 0,03 U pH (P 2 diminue aussi de 65,1 ± 1,2 a 59,3 ± 1,4 mm Hg (P 2 diminue significativement (64,3 ± 1,5 a 57,7 ± 1,6) (P (1) que la chute simultanee de pH et Pco 2 dans le tube proximal precoce indique que acide de carbonique, et non carbon dioxide, s'accumule apres l'inhibition de l'anhydrase carbonique; (2) bien que la Pco 2 diminue aussi dans le tube contourne proximal tardif le pH, a la difference du segment precoce, n'est pas modifie apres benzolamide, probablement du fait d'une augmentation du debit de bicarbonate; et (3) que la Pco 2 in vivo est significativement superieure a la Pco 2 arterielle systemique avant et apres benzolamide, a la fois dans les segments precoce et tardif du tube contourne proximal. Ces constatations plaident en faveur du fait que la reabsorption de bicarbonate dans le tube contourne proximal est effectuee, en partie, par la secretion d'ion hydrogene.