Mechanisms of HCO3- secretion in the rabbit connecting segment

The connecting tubule (CNT) contains alpha-(H+-secreting) and beta-(HCO3--s ecreting) intercalated cells and is therefore likely to contribute to acid- base homeostasis. To characterize the mechanisms of HCO3- transport in the rabbit CNT, in which there is little definitive data presently available, w e microdissected the segments from the superficial cortical labyrinth, perf used them in vitro, measured net HCO3- transport (J(HCO3-)) by microcalorim etry, and examined the effects of several experimental maneuvers. Mean +/- SE basal J(HCO3-) was -3.4 +/- 0.1 pmol.min(-1).mm(-1) (net HCO3- secretion ), and transepithelial voltage was -13 +/- 1 mV (n = 47). Net HCO3- secreti on was markedly inhibited by removal of luminal Cl- or application of basol ateral H+-ATPase inhibitors (bafilomycin or concanamycin), maneuvers that i nhibit beta-intercalated cell function. Net HCO3- secretion was not affecte d by inhibitors of alpha-intercalated cell function (basolateral Cl- remova l, basolateral DIDS, or luminal H+-ATPase inhibitors). Net HCO3- secretion was stimulated by isoproterenol and inhibited by acetazolamide. These data indicate that 1) CNTs secrete HCO3- via an apical DIDS-insensitive Cl-/HCO3 - exchanger, mediated by a basolateral bafilomycin- and concanamycin-sensit ive H+-ATPase; 2) inhibition of cytosolic carbonic anhydrase decreases HCO3 - secretion; and 3) stimulation of beta-adrenergic receptors increases HCO3 - secretion. The failure to influence net HCO3- transport by inhibiting alp ha-intercalated cell apical H+-ATPases or basolateral Cl-/HCO3- exchange su ggests that the CNT has fewer functioning alpha-intercalated cells than the cortical collecting duct. These are the first studies to examine the rate and mechanisms of HCO3- secretion by the rabbit CNT; this is clearly an imp ortant segment in mediating acid-base homeostasis.