The Action of Prostaglandin E1 on Frog Skeletal Muscle

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Abstract
The effects of prostaglandin E1 (PGE1) were investigated in frog semitendinosus and sartorius muscle preparations in vitro, PGE1 augmented and prolonged the twitch response, and the effect was equally developed in preparations stimulated through the nerve or directly. The threshold concentration was between 0.03 and 0.14 mM, and definite changes occurred with concentrations between 0.7 and 1.4 mM. Enhancement of twitch responses developed gradually, and a peak was reached within 5–20 min following PGE1 application. Prolonged exposure of the tissue to PGE1 showed that the twitch height increases first to a maximum and then declines to control values and below, despite the continuous presence of the compound in the bath. This decrease of twitch height was coupled with a progressive lengthening of the response. Twitch depression and prolongation were not reversible after transfer to normal Ringer but 2 mM caffeine consistently restored the depressed response to control levels, or above. PGE1, unlike caffeine, did not elicit sarcomeric oscillations. No substantial changes of membrane potentials at junctional or extrajunctional regions resulted from PGE1 treatment. It is concluded that PGE1 acts at a muscle site possibly by lowering the mechanical threshold of the contracting element.

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