Relationship between Na+ and Ca++ extracellular levels and prostaglandin F2 alpha action on longitudinal muscle of guinea-pig caecum.

The response of the longitudinal muscle of guinea-pig caecum have been studied in a medium with different concentrations of Na+, K+ and Ca++. Some parameters of the electrical and mechanical activity have been followed simultaneously by the sucrose gap method. Prostaglandin F2 alpha (PGF2 alpha) stimulates the electrical and mechanical activity of the preparations isolated from this smooth muscle. The PGF2 alpha-induced depolarization of the membrane of the smooth-muscle cells, spike potentials and concentrations decrease proportionally to the Na+ reduction in the extracellular medium to 50 mM. K+ excess eliminates the effects of PGF2 alpha. Reduction of Ca++ within the limits of 1.0 to 0.2 mM leads to increase of the PGF2 alpha-induced depolarization and spike potentials, but the contractile response to PGF2 alpha decreases at Ca++ 0.2 mM. In Ca-free solution (containing (EGTA) and on the background of D 600 the spike potentials and the contraction upon treatment with PGF2 alpha are considerably decreased. These results suggest participation of Na+ and Ca++ in the effects of PGF2 alpha and it may be concluded that the extracellular concentration of Na+ and Ca++ modulates the effect of PGF2 alpha.