Regulation of Cl- transport by IBMX in renal A6 epithelium

We studied regulation of Cl- transport by cAMP and Ca2+ in renal epithelial A6 cells. Stimulation of A6 cells by 1 mM 3-isobutyl-1-methylxanthine (IBMX, an inhibitor of phosphodiesterase), which increased cytosolic cAMP, elicited biphasic increases in short-circuit current (Isc), i.e., a transient phase followed by a sustained one. Apical application of 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, a Cl- channel blocker) markedly and dose-dependently inhibited the IBMX-induced Isc. Pretreatment with nifedipine (100 microM, a Ca2+ channel blocker) or 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra-(acetoxymethyl)-ester (BAPTA/AM, 10 microM, a Ca2+ chelator) partially but markedly inhibited the Isc. On the other hand, a cAMP-dependent protein kinase inhibitor, H89 (0.5 microM for 1 h), also reduced the IBMX-induced Isc to a level similar to that following nifedipine or BAPTA pretreatment. Nifedipine had no synergistic effects on the IBMX-induced Isc in cells treated with H89. Ionomycin (a Ca2+ ionophore) could mimic the transient increase dose dependently, and H89 did not block the ionomycin-induced Isc. Taken together, our observations suggest that: (1) part of the IBMX-stimulated Cl- release is regulated by an increased cytosolic Ca2+ through nifedipine-sensitive Ca2+ influx; (2) cAMP-dependent phosphorylation may be required for elevation of the cytosolic Ca2+ concentration but not for activation of Cl- channels, which are directly activated by cytosolic Ca2+; and (3) the IBMX-induced sustained Cl- release requires cAMP elevation in addition to an increase in the cytosolic Ca2+ concentration.