Effects of prostaglandins F2alpha and E1 on the longitudinal and circular smooth muscle of the guinea pig caecum in relation to the concentration of extracellular calcium.

Responses of the longitudinal and circular smooth muscle of the guinea-pig caecum were studied in media with different concentrations of extracellular calcium. A sucrose-gap method was used to record the electrical and mechanical parameters of smooth muscle activity. In the longitudinal smooth muscle no difference in the action of PGF2alpha and PGE1 was seen at low (0.8 mM), Normal (2.5 mM) or high (7.5 mM) concentrations of Ca2+. Both PG's stimulated the longitudinal strip and the action of acetylcholine at normal Ca2+ concentration was slightly inhibited immediately after the superfusion with the PG's and augmented thereafter. In the circular strip no significant difference between the stimulatory effect of PGF2alpha, and PGE1 in low and high Ca2+ solution was found. However at 2.5 mM Ca2+ PGF2alpha evoked much greater stimulation of the circular strip than did PGE1. After the end of the superfusion with either PG the effect of acetylcholine was inhibited by either PG at low Ca2+ level and potentiated at high Ca2+ level the effect of acetylcholine was inhibited by PGE1 but not by PGF2alpha. Thus, the effects of PGF2alpha and PGE1 on the circular smooth muscle differed from each other in their dependence on the concentration of extracellular calcium.