Role of calcium in the mechanism of action of the prostaglandins E1 and F2 alpha as well as of their antagonist polyphloretin phosphate (PPP).

The contractile effect of PGE1 on guinea-pig ileum is not changed either by increase (to 4 mM) or by decrease (to 1 mM) of the Ca++ content in Krebs solution. The decrease of Ca++ in the medium intensifies the contractile effect of PGE2 alpha. Reduction of Ca++ weakens the antagonistic effect of PPP (100 micrograms/ml) against PGF2 alpha. PPP has a marked aftereffect with respect to the effects of PGF2 alpha--in cases of normal Ca++ content in the Krebs solution this aftereffect appears upon longer (10 min) contact of PPP with the intestinal segment, while in cases of changed Ca++ content it appears already after 2-min PPP action. It is concluded that in order to induce contractions in the guinea-pig ileum, PGE1 and PGF2 alpha utilize the internal Ca++. It is assumed that PGF2 alpha increases the reactivity of the anionic sites of the smooth-muscle membrane for Ca++. As a result of this, the complex compounds of Ca++ at its higher concentration make more difficult the realization of the contractile process. It is assumed that Ca++ is needed for the maximum manifestation of the antagonistic effect of PPP.