Purinergic-induced ion current in monolayers of C7-MDCK cells: Role of basolateral and apical ion transporters

This study examines purinergic modulation of short-circuit current (I-SC) in monolayers of C7- and C11-MDCK cells resembling principal and intercalated cells from collecting ducts. In C7 monolayers, basolateral and apical application of ATP led to similar elevation of I-SC, consisting of a transient phase with maximal I-SC increment of similar to10 muA/cm(2) terminating in 2-3 min, and a sustained phase with maximal I-SC less than 2 muA/cm(2) and terminating in 10 min. ATP-induced I-SC was insensitive to the presence of Na+, Cl- and inhibitors of K+ (Ba2+, charibdo-toxin (ChTX), clotrimazole (CLT), apamin) and Na+ (amiloride) channels in the mucosal solution. Inhibitors of Cl- channels. DIDS and NPPB, added to apical membranes at a concentration of 100 pm, did not affect ATP-induced I-SC, whereas at 500 muM. NPPB inhibited it by 70-80%. Substitution of Cl- with NO3- in serosal medium decreased ATP-induced I-SC by 2-3-fold and elevation of [K+](o) from 6 to 60 mm changed its direction. Basolateral NPPB inhibited I-SC by 10-fold with ED50 of similar to30 muM, whereas ChTX (50 nM) and CLT (2 mum) diminished this parameter by 30-50%. Suppression of Na+, K+, Cl- cotransport with bumetanide did not affect the transient phase of ATP-induced I-SC and slightly diminished its sustained phase. ATP increased ouabain- and bumetanide-resistant K+ (Rb-86) influx across the basolateral membrane by 7-8-fold, which was partially inhibited with ChTX and CLT. ATP-treated C11 cells exhibited negligible I-SC and their presence did not affect I-SC triggered by ATP in C7 cells. Thus, our results show that transcellular ion current in ATP-treated C7 cells is mainly caused by the coupled function of apical and basolateral anion transporters providing transient Cl- secretion. These transporters possess different sensitivities to anion channel blocker NPBB and are under the control of basolateral K+ channels(s) inhibited by ChTX and CLT.