The effect of PGE 2 on contraction delay and velocity of the field stimulated guinea-pig vas deferens

Abstract

The shape of mechanical responses elicited by continuous field stimulation of the guinea-pig vas deferens depends on the frequency of stimulation. When the frequency was increased from 5 to 50 Hz, the maximum of contractions became higher and a shorter delay and a greater velocity of contractions could be observed. Prostaglandin E 2 (PGE 2) markedly retarded the onset and depressed the velocity of mechanical responses. Inhibition caused by this drug was inversely proportional to the nerve stimulation frequency. Above a certain concentration (10 ng/ml) PGE 2 failed to produce any further depression, even a reduction of the inhibitory effect could be observed. The action of PGE 2 on velocity and onset of contractions at a given frequency (5 or 10 Hz) increased with concentration up to 10 ng/ml. When the number of shocks was plotted semi-logarithmically against tension of the tissue a typical “S” shaped curve was obtained. This shock-number-effect curve could be shifted with PGE 2 to the right without a significant change in slope. The interaction between PGE 2 and number of shocks did not fulfill the criterion for competitive antagonism. The dual (pre- and postjunctional) action of PGE 2 may be the reason for the noncompetitive nature of antagonism. In the rat vas deferens, the shape of responses elicited by continuous stimulation with different frequencies (5, 10 and 50 Hz) was very similar. Prostaglandin E 2 was ineffective at any frequency of stimulation and in any concentration applied (10, 100 and 1000 ng/ml).