EFFECTS OF VASOACTIVE INTESTINAL CONTRACTOR ON VOLTAGE-ACTIVATED CA2+CURRENTS IN FELINE PARASYMPATHETIC NEURONS

Intracellular current-clamp and single-electrode voltage-clamp techniq ues were used to study in vitro action potentials and the action of va soactive intestinal contractor (VIC; 0.03-1 muM) on the high-voltage-a ctivated Ca2+ currents (I(Ca)) of neurons in feline colonic parasympat hetic ganglia. In the current-clamp recording mode, action potential a mplitude was depressed by cobalt (1 mM) and omega-conotoxin (300 nM) o r in nominally Ca2+-free Krebs solutions. In the single-electrode volt age-clamp recording mode, the I(Ca) was isolated by blocking the volta ge-gated Na+ current with tetrodotoxin (1-3 muM) and by Krebs solution s containing a low Na+ concentration. The voltage-activated K+ current s were blocked by intracellular injection of cesium through a recordin g electrode filled with 2 M CsCl and external application of tetraethy lammonium (30-50 mM) and barium (2 mM). The Ca2+-dependent Cl- current was blocked by replacement of Ca2+ (2 mM) with equimolar barium. Anom alous rectification was blocked by external application of 2 mM cesium . The I(Ca) was evoked by depolarizing step commands more positive tha n -40 mV from holding potentials ranging between -80 and -60 mV. I(Ca) was depressed by cobalt (1 mM), cadmium (100 muM), and omega-conotoxi n (500 nM) but not by nifedipine (10 muM), nicardipine (10 muM), and v erapamil (10 muM). BAY K 8644 (3-10 muM) also did not affect the I(Ca) . VIC (0.1-1 muM), one of the endothelin (ET) isopeptides, caused an i nward current followed by an outward current. The VIC-induced inward a nd outward currents were associated with an increase and decrease in m embrane conductance, respectively. VIC also caused an initial depressi on followed by a long-lasting augmentation of the I(Ca). ET-1, ET-2, a nd ET-3 equally mimicked the action of VIC on both holding current and I(Ca). These data suggest that VIC activates a receptor-operated chan nel and modulates the omega-conotoxin-sensitive voltage-activated Ca2 channels through ET(B) receptor subtypes of neurons in feline colonic parasympathetic ganglia.