MECHANISM OF CYCLIC AMP-INDUCED HYPERPOLARIZATION IN CANINE COLON

The mechanism of forskolin (FSK)-induced hyperpolarization was investi gated in strips of canine colonic circular muscle. FSK responses were compared to those of the K+ channel opener lemakalim (LEM). Both FSK ( 10 mu M) and LEM (10 mu M) hyperpolarized cells near the myenteric bor der by 10 to 20 mV. Responses to both agents were abolished by 35 mM e xternal K+, indicating a probable mediation by K+ channels. FSK increa sed the open probability of Ca++-activated K+ channels in isolated col onic myocytes. However, in muscle strips charybdotoxin (100 nM) and te traethylammonium (10 mM) failed to reduce FSK- and LEM-induced hyperpo larizations whereas tetrapentylammonium (50 mu M) and 4-aminopyridine (10 mM) blocked both responses. Phencyclidine (100 mu M), Ba++ (1 mM) and the antagonist of ATP-sensitive K+ currents glybenclamide (10 mu M ) blocked LEM- but not FSK-induced hyperpolarizations. Delayed rectifi er current in isolated myocytes was activated near -20 mV and was bloc ked by (order of potency): nifedipine > tetrapentylammonium > phencycl idine > 4-aminopyridine > tetraethylammonium. Charybdotoxin (100 nM), Ba++ (1 mM) and glybenclamide (10 mu M) were without effect. Ca++-actt iated K+ current was activated near +30 mV and was blocked by: charybd otoxin > tetraethylammonium > tetrapentylammonium much greater than ph encyclidine = 4-aminopyridine. These data suggest that LEM induces mem brane hyperpolarization by activation of a K+ current with a pharmacol ogy similar to ATP-sensitive K+ current whereas cyclic AMP-induced hyp erpolarization appears to involve activation of a current other than d elayed rectifier current, Ca++-activated K+ current or ATP-sensitive K + current.