Cross talk of bumetanide-sensitive and HCO3--dependent transporters activated by IBMX in renal epithelial A6 cells.

We studied cAMP-dependent regulation of ion transport in aldosterone-untreated renal epithelial A6 cells by measuring short-circuit current (Isc). Biphasic increases in Isc, a transient phase followed by a sustained one, were elicited in response to 1 mm 3-isobutyl-1-methylxanthine (IBMX, an inhibitor of phosphodiesterase) which increased cytosolic cAMP concentration. IBMX increased the apical Cl- conductance. The sustained phase of Isc induced by IBMX was reduced by 50 microM bumetanide (Na+/K+/2 Cl- cotransporter inhibitor) or 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, an inhibitor of Cl-/HCO3- exchanger). Under the normal condition, the inhibitory effect of bumetanide was much larger than that of DIDS. On the other hand, under a low Cl- condition, the effect of DIDS was more effective than that of bumetanide. Further, under a Cl--free condition Na+/HCO3- symporter contributed to the IBMX-generated Isc. Taken together, our observations suggest that in A6 cells (i) IBMX stimulates Cl- secretion associated with an increase in apical Cl- conductances, (ii) the ionic components to generate the IBMX-induced Isc are mainly maintained by bumetanide-sensitive Na+/K+/2 Cl- cotransporter and DIDS-sensitive Cl-/HCO3- exchanger, (iii) Cl-/HCO3- exchanger coupled to Na+/HCO3- symporter under a low-Cl- condition or Na+/HCO3- symporter under a Cl--free condition contributes to the IBMX-induced Isc, compensating for diminishment of the Na+/K+/2Cl- cotransporter-mediated Cl- secretion, (iv) IBMX increases Cl- and HCO3- conductances in the apical membrane.