The nonselective cation channel in the basolateral membrane of rat exocrine pancreas.

Nonselective Ca-sensitive cation channels in the basolateral membrane of isolated cells of the rat exocrine pancreas were investigated with the patch clamp technique. With 1.3 mmol/l Ca on the cytosolic side, the mean openstate probability P of one channel was about 0.5. In insideout oriented cell-excised membrane patches the substances diphenylamine-2-carboxylic acid (DPC), 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) and 3′,5-dichlorodiphenylamine-2-carboxylic acid (DCDPC) were applied to the cytosolic side. These compounds inhibited the nonselective cation channels by increasing the mean channel closed time (slow block). 100 μmol/l of NPPB or DPC decreased P from 0.5 (control conditions) to 0.2 and 0.04, respectively, whereas 100 μmol/l of DCDPC blocked the channel completely. All effects were reversible. 1 mmol/l quinine also reduced P, but in contrast to the abov mentioned substances, it induced fast flickering. Ba (70 mmol/l) and tetraethylammonium (TEA; 20 mmol/l) had no effects. We investigated also the stilbene disulfonates 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS), 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) and 4,4′-dinitro-2,2′-stilbenedisulfonate (DNDS). 10 μmol/l SITS applied to the cytosolic side increased P from 0.5 to 0.7 and with 100 μmol/l SITS the channels remained nearly permanently in its open state ( P≅1). A similar activation of the channels was also observed with DIDS and DNDS. These effects were poorly reversible. The stilbene disulfonates acted by increasing the channel mean open time. When the channel was inactivated by decreasing bath Ca concentration to 0.1 μmol/l, addition of 100 μmol/l of SITS had no effect. Similarly, reducing bath Ca concentration from 1.3 mmol/l in presence of 100 μmol/l SITS (channels are maximally activated) to 0.1 μmol/l, inactivated the channels completely. These results demonstrate, that SITS can only activate the channels in the presence of Ca. SITS had no effects, when applied to the extracellular side in outside out patches. In summary, the substances DPC, NPPB and DCDPC inhibit nonselective cation channels, where DCDPC has the most potent and NPPB the smallest effect; whereas SITS, DIDS and DNDS activate the channel when applied from the cytosolic side in the presence of Ca ions. [ABSTRACT FROM AUTHOR]