INHIBITION OF ENDOGENOUS ACETYLCHOLINE-RELEASE BY BLOCKADE OF VOLTAGE-DEPENDENT CALCIUM CHANNELS IN ENTERIC NEURONS OF THE GUINEA-PIG COLON

The effects on acetylcholine release from the guinea-pig colon of the N-type calcium channel blocker omega-conotoxin GVIA (omega-conotoxin), the L-type calcium channel blocker nifedipine and the putative blocke r of T-type channels, flunarizine, have been investigated. Endogenous basal acetylcholine release and electrically (1 Hz, 1 ms, 456 mA)-evok ed overflow in the presence of cholinesterase inhibitor were studied. omega-Conotoxin (1-10 nM) and nifedipine (0.03-3 muM) dose-dependently inhibited basal and electrically-evoked acetylcholine release. Maxima l inhibition of basal or electrically-evoked acetylcholine release was about 40% for nifedipine and about 75% for omega-conotoxin. The poten cy of nifedipine was inversely related to the external calcium concent ration: its EC50 value in low-calcium medium (0.5 mM) was as low as 12 nm. Flunarizine inhibited acetylcholine release only at concentration s higher than 0.2 muM. Our results are consistent with an involvement of N- and L-type calcium channels in the control of the endogenous ace tylcholine release from the guinea-pig colon.