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-u ntreated renal epithelial A6 cells by measuring short-circuit current (I-sc). Biphasic increases in I-sc, a transient phase followed by a su stained one, were elicited in response to 1 mM 3-isobutyl-l-methylxant hine (IBMX, an inhibitor of phosphodiesterase) which increased cytosol ic cAMP concentration. IBMX increased the apical C1(-) conductance. Th e sustained phase of I-sc induced by IBMX was reduced by 50 mu M bumet anide (Na+/K+/2 C1(-) cotransporter inhibitor) or 100 mu M 4,4'-diisot hiocyanostilbene-2,2' -disulfonic acid (DIDS, an inhibitor of C1(-)/HC O3- exchanger). Under the normal condition, the inhibitory effect of b umetanide was much larger than that of DIDS. On the other hand, under a low C1(-) condition, the effect of DIDS was more effective than that of bumetanide. Further, under a C1(-)-free condition Na+/HCO3- sympor ter contributed to the IBMX-generated I-sc. Taken together, our observ ations suggest that in A6 cells (i) IBMX stimulates C1(-) secretion as sociated with an increase in apical C1(-) conductances, (ii) the ionic components to generate the IBMX-induced I-sc are mainly maintained by bumetanide-sensitive Na+/K+/2 C1(-) cotransporter and DIDS-sensitive C1(-)/HCO3- exchanger, (iii) C1(-)/HCO3- exchanger coupled to Na+/HCO3 - symporter under a low-C1(-) condition or Na+/HCO3- symporter under a C1(-)-free condition compensating tion contributes to the IBMX-induce d I-sc for diminishment of the Na+/K+/2C1(-) cotransporter-mediated C1 (-) secretion, (iv) LBMX increases C1(-) and HCO3- conductances in the apical membrane.