Voltage-dependent, slowly activating K+ current (IKs) and its augmentation by carbachol in rat pancreatic acini

Acetylcholine-stimulated exocrine secretion of Cl- and water requires the concomitant activation of K+ channels. However, there has not been much investigation of the carbachol- (CCH-) activated K+ channel of rodent pancreatic acini. Here, in a study of rat pancreatic acini, we characterize a voltage-dependent, slowly activating outward current (I-Ks) that is augmented by CCH. Intact acini were obtained by enzymatic digestion and fast-whole-cell patch-clamp was applied. With symmetrical [Cl-] (32 mmol/l) in the pipette and bath solution, acinar cells had resting membrane voltages of -45+/-0.8 mV (n=97) under current-clamp conditions. CCH (10 mu mol/l), which is known to activate Cl- channels via a Ca2+-mediated pathway, sharply depolarized the membrane to -4+/-0.5 mV, which was more negative than E-Cl (0 mV), and reversed it to -41+/-0.9 mV (n=83) by washout. A clamp voltage of 0 mV activated I-Ks under control conditions (91+/-8.6 pA, n=83). During CCH application an increase of outward current was observed at 0 mV, and at -50 mV a marked increase of inward Cl- current occurred. In the presence of CCH the slow activation of I-Ks was rarely distinguishable because of interference by the huge Cl- conductance. During CCH washout and decrease of inward current, a persistent augmentation of I-Ks was revealed (486+/-36.3 pA, n=83). I-Ks and its augmentation were abolished by substituting K+ in the pipette solution with Cs+. Augmentation of I-Ks was mimicked by applying ionomycin (0.1 mu mol/l), a Ca2+ ionophore. Pharmacological blockers were tested. The chromanol 293B and clotrimazole blocked I-Ks at micromolar concentrations (IC50=3 mu mol/l and 9 mu mol/l, respectively) and Ba2+ was a poor blocker (IC50=3 mmol/l). In the presence of CCH (0.2 mu mol/l), the membrane was depolarized to around -20 mV and the addition of 293B (10 mu mol/l) further depolarized the membrane by 11+/-3 mV (n=5). These data suggest the presence of I-Ks channels in rat pancreatic acini and their muscarinic activation.