Biphasic response of circular muscle to prostaglandin E2 in guinea-pig colon

The effects of prostaglandin E2 (PGE2) on mechanical and electrical activities of the colonic circular muscle strip of guinea-pig were examined. PGE2, at doses over 10(-7) M, produced an initial contraction, followed by a relaxation, while PGF2 alpha (10(-6) M) produced a tonic contraction. The biphasic response of circular muscle strip to PGE2 was not influenced by atropine, tetrodotoxin, propranolol and indomethacin. PGE2 (10(-6) M) initially increased spike discharges, followed by their inhibition in the colonic circular muscle strip. Verapamil (10(-5) M) blocked PGE2-induced initial contraction but did not block the following relaxation. At low temperature (22 degrees C-27 degrees C) and in the presence of tetraethylammonium (5 x 10(-3) M), PGE2 (10(-6) M) produced a contractile response without showing any relaxation. The present results suggest that PGE2-induced initial contraction may depend on an increase in spike discharges and that the following relaxation may depend on an increase in K-ion permeability in the colonic circular muscle. The reason why PGE2 showed a biphasic response on the colonic circular muscle strip is considered to be a summation of rapid small contraction and of late large relaxation.