SECRETION OF HCO3- OH- IN CORTICAL DISTAL TUBULE OF THE RAT/

Secretion of bicarbonate has been described for distar nephron epithel ium and attributed to apical Cl-/HCO3- exchange in beta-intercalated c ells. We investigated the presence of this mechanism in cortical dista l tubules by perfusing these segments with acid (pH 6) 10 mM phosphate Ringer. The kinetics of luminal alkalinization was studied in station ary microperfusion experiments by double-barreled pH (ion-exchange res in)/1 M KCI reference microelectrodes. Luminal alkalinization may be d ue to influx (into the lumen) of HCO3- or OH-, or efflux of H+. The ma gnitude of the Cl-/HCO(3)(-)xchange component was measured by perfusin g the lumen with solutions with or without chloride, which was substit uted by gluconate. This component was not different from zero in contr ol and alkalotic (chronic plus acute) Wistar rats. Homozygous Brattleb oro rats (BRB), genetically devoid of antidiuretic hormone, were used since this hormone has been shown to stimulate H+ secretion, which cou ld mask bicarbonate secretion. In these rats, no evidence for Cl-/HCO3 - exchange was found in control BRB and in early distal segments of al kalotic animals, but in late distal tubule a significant component of 0.14 +/- 0.033 nmol/cm(2) sec was observed, which, however, is small w hen compared to the reabsorptive flow found in control Wistar rats, of 0.95 +/- 0.10 nmol/cm(2) sec. In addition, 5 x 10(-4) M SITS had no e ffect on distal bicarbonate reabsorption in controls as well as on sec retion in alkalotic Wistar and Brattleboro rats, which is compatible w ith the absence of effect of this drug on the apical Cl- /HCO3- exchan ge in other tissues. It is concluded that most distal allcalinization is not Cl- dependent, and that Cl-/HCO3- exchange may be found in cort ical distal tubule, but its magnitude is, even in alkalosis, markedly smaller than the reabsorptive flux, which predominates in the rats stu died in this paper, keeping luminal pH lower than that of blood.