Contractile effects of PGD2 and PGF2α on mechanical and electrical activities in longitudinal and circular muscles of the guinea-pig colon

Contractile effects of PGD₂ and PGF_2α on mechanical and electrical activities were investigated in longitudinal and circular muscles of the guinea-pig isolated proximal colon.PGD₂ and PGF_2α (1 μM)-induced contractions were reduced with a decrease in external Ca ion concentration and were prevented at zero Ca (EGTA 1mM) in both muscles.In electrical activities recorded by the single sucrose gap method, PGD₂ and PGF_2α (1 μM) increased the frequency of spike generation on the slow rates in both muscles.Verapamil (10 μM), a Ca antagonist, blocked spontaneous and PG-induced spike activities with little effect on the slow waves in both muscles. Verapamil markedly reduced the contractions to PGs in the longitudinal muscle, but not in the circular muscle.The results suggest that PGD₂ and PGF_2α produced contractions which may depend largely on an increase in the inward Ca current of the spikes in longitudinal muscle, and may depend both on a release of Ca from the Ca sequestration in the cell, and on a spike-dependent Ca influx in circular muscle of the guinea-pig colon.