We studied cAMP-dependent regulation of ion transport in aldosterone-untreated renal epithelial A6 cells by measuring short-circuit current (I sc ). Biphasic increases in I sc , 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 I sc induced by IBMX was reduced by 50 μm bumetanide (Na+/K+/2 Cl− cotransporter inhibitor) or 100 μm 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS, an inhibitor of C1−/HCO 3 − 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+/HCO 3 − symporter contributed to the IBMX-generated I sc . 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 I sc are mainly maintained by bumetanide-sensitive Na+/K+/2 Cl− cotransporter and DIDS-sensitive Cl−/HCO 3 − exchanger, (iii) Cl−/HCO 3 − exchanger coupled to Na+/HCO 3 − symporter under a low-Cl− condition or Na+/HCO 3 − symporter under a Cl−-free condition contributes to the IBMX-induced I sc , compensating for diminishment of the Na+/K+/2Cl− cotransporter-mediated Cl− secretion, (iv) IBMX increases Cl− and HCO 3 − conductances in the apical membrane.
The Journal of Membrane Biology – Springer Journals
Published: Mar 31, 2009
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