G-PROTEIN REGULATION OF AN L-TYPE CALCIUM-CHANNEL CURRENT IN CANINE JEJUNAL CIRCULAR SMOOTH-MUSCLE

Calcium entry into smooth muscle cells is essential to maintain contra ctility. In canine jejunal circular smooth muscle cells the predominan t calcium entry pathway is through L-type calcium channels. The aim of this study was to determine the G-protein regulation of L-type calciu m channel current (I-CaL) in isolated canine jejunal circular smooth m uscle cells. Barium (80 mM) was used as the charge carrier. GTP-gamma S and (GTP increased maximal inward current from 118.7 +/- 12 pA to 22 7.5 +/- 21.5 pA (n = 8) and 174.6 +/- 10.1 pA (n = 6) respectively. Th e increase in inward current was blocked by nifedipine suggesting it w as through L-type calcium channels. Pertussis toxin did not alter base line I-CaL while cholera toxin increased I-CaL from 125 +/- 19 pA in c ontrols (It = 6) to 347 a 30 pA (n = 4). Staurosporine inhibited the i ncrease in current evoked by GTP-gamma S and calyculin further increas ed I-CaL over the increase evoked by GTP-gamma S. The results suggest that cholera. toxin sensitive G-proteins activate L-type calcium chan nels in isolated canine jejunal circular smooth muscle cells through p rotein phosphorylation.