Relations between Na+, Ca2+ and K+ extracellular concentrations and the effects of PGE2 on the longitudinal muscle of the guinea pig caecum.

Prostaglandin E2 (PGE2) stimulated the electrical and contractile activities of the isolated longitudinal muscle of the guinea pig caecum. Pretreatment of the smooth muscle strips with Na+-deficient solutions decreased the stimulant effects of PGE2. The action of PGE2 was abolished on the background of high K+ and low Na+ solutions. A reduction of extracellular Ca2+ to 0.2 mM did not change significantly the responses to PGE2. The addition of D-600 depressed both spike potential activity and contractions and only a weak depolarization occurred after PGE2. However, the effects of PGE2 were not completely inhibited after removing Ca2+ from the solution. These observations show that the stimulant effects of PGE2 on the longitudinal muscle of the guinea pig caecum depend mainly on the extracellular concentration of Na+ and are less sensitive to the lack of Ca2+. It is suggested that under certain conditions Na+ may transfer currents through the calcium ion channels in the smooth muscle membrane.