Schuster V L
J Clin Invest. 1985 Jun;75(6):2056-64. doi: 10.1172/JCI111925.
We studied the effects of cyclic AMP (cAMP) on HCO-3 transport by rabbit cortical collecting tubules perfused in vitro. Net HCO-3 secretion was observed in tubules from NaHCO3- loaded rabbits. 8-Bromo-cAMP-stimulated net HCO-3 secretion, whereas secretion fell with time in control tubules. Both isoproterenol and vasopressin (ADH) are known to stimulate adenylate cyclase in this epithelium; however, only isoproterenol stimulated net HCO-3 secretion. The mechanism of cAMP-stimulated HCO-3 secretion was examined. If both HCO-3 and H+ secretion were to occur simultaneously in tubules exhibiting net HCO-3 secretion, cAMP might increase the net HCO-3 secretory rate by inhibiting H+ secretion, by stimulating HCO-3 secretion, or both. These possibilities were examined using basolateral addition of the disulfonic stilbene (4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS). In acidifying tubules from NH4Cl-loaded rabbits, DIDS eliminated HCO-3 reabsorption, a result consistent with known effects of DIDS as an inhibitor of H+ secretion. In contrast, cAMP left acidification (H+ secretion) intact. DIDS applied to HCO-3 secretory tubules failed to increase the HCO-3 secretory rate, indicating minimal H+ secretion in HCO-3 secreting tubules. Thus, inhibition of H+ secretion by cAMP could not account for the cAMP-induced stimulation of net HCO-3 secretion. cAMP-stimulated HCO-3 secretion was reversibly eliminated by 0 Cl perfusate, whereas luminal DIDS had no effect. Bath amiloride (1 mM) failed to eliminate cAMP-stimulated HCO-3 secretion when bath [Na+] was 145 mM or 5 mM. cAMP depolarized the transepithelial voltage. The collected fluid [HCO-3] after cAMP could be accounted for by electrical driving forces, suggesting that cAMP stimulates passive HCO-3 secretion. However, cAMP did not alter HCO-3 permeability measured under conditions expected to inhibit transcellular HCO-3 movement (0 Cl- solutions and bath DIDS). This measured HCO-3 permeability was not high enough to account, by passive diffusion, for the HCO-3 fluxes observed in Cl-containing solutions. We conclude the following: cAMP increased net HCO3- secretion by stimulating HCO3- secretion and not by inhibiting H+ secretion; this HCO3- secretion may have occurred by Cl-HCO3- exchange; Na+-H+ exchange appeared not to play a role in basolateral H+ extrusion under these conditions; and the stimulation of HCO3- secretion by isoproterenol, but not ADH, suggests the existence of separate cell cAMP pools or cellular heterogeneity in this cAMP response.
我们研究了环磷酸腺苷（cAMP）对体外灌注的兔皮质集合管中HCO₃⁻转运的影响。在灌流含NaHCO₃的兔肾小管中观察到了净HCO₃⁻分泌。8-溴-cAMP刺激了净HCO₃⁻分泌，而对照肾小管中的分泌随时间下降。已知异丙肾上腺素和血管加压素（抗利尿激素，ADH）均可刺激该上皮细胞中的腺苷酸环化酶；然而，只有异丙肾上腺素刺激了净HCO₃⁻分泌。我们研究了cAMP刺激HCO₃⁻分泌的机制。如果在表现出净HCO₃⁻分泌的肾小管中同时发生HCO₃⁻和H⁺分泌，cAMP可能通过抑制H⁺分泌、刺激HCO₃⁻分泌或两者兼而有之来提高净HCO₃⁻分泌速率。我们通过在基底外侧添加二磺酸芪（4,4'-二异硫氰酸芪-2,2'-二磺酸盐，DIDS）来研究这些可能性。在灌流含NH₄Cl的兔肾小管使其酸化时，DIDS消除了HCO₃⁻重吸收，这一结果与DIDS作为H⁺分泌抑制剂的已知作用一致。相反，cAMP使酸化（H⁺分泌）保持不变。将DIDS应用于HCO₃⁻分泌的肾小管未能提高HCO₃⁻分泌速率，表明在HCO₃⁻分泌的肾小管中H⁺分泌极少。因此，cAMP对H⁺分泌的抑制不能解释cAMP诱导的净HCO₃⁻分泌的刺激作用。0 Cl⁻灌流液可使cAMP刺激的HCO₃⁻分泌可逆性消除，而管腔DIDS则无作用。当浴液[Na⁺]为145 mM或5 mM时，浴液中加入氨氯吡咪（1 mM）未能消除cAMP刺激的HCO₃⁻分泌。cAMP使跨上皮电压去极化。cAMP作用后收集的液体中的[HCO₃⁻]可由电驱动力来解释，这表明cAMP刺激了HCO₃⁻的被动分泌。然而，在预期抑制跨细胞HCO₃⁻转运的条件下（0 Cl⁻溶液和浴液DIDS）测量时，cAMP并未改变HCO₃⁻通透性。这种测量的HCO₃⁻通透性不够高，无法通过被动扩散来解释在含Cl⁻溶液中观察到的HCO₃⁻通量。我们得出以下结论：cAMP通过刺激HCO₃⁻分泌而非抑制H⁺分泌来增加净HCO₃⁻分泌；这种HCO₃⁻分泌可能通过Cl⁻-HCO₃⁻交换发生；在这些条件下，Na⁺-H⁺交换似乎在基底外侧H⁺排出中不起作用；异丙肾上腺素而非ADH对HCO₃⁻分泌的刺激表明在这种cAMP反应中存在独立的细胞cAMP池或细胞异质性。
