Relaxant effect of prostaglandin E1 on the isolated intestine of the toad (bufo melanostictus)

The pharmacological actions of prostaglandins E₁, F₁, E₂ and A₁ were studied on the longitudinal and circular muscles of the isolated small intestine of the toad. Prostaglandin E₁ caused relaxation, whereas prostaglandin A₁ was inactive and prostaglandins F₁ and E₂ caused contractions. Other substances which relaxed the toad intestine were adrenaline, noradrenaline and isoprenaline. On a molar basis, prostaglandin E₁ was seven times less potent than isoprenaline, but it was three times more potent than adrenaline and seventeen times more potent than noradrenaline. Phentolamine and propranolol blocked the relaxant effects of catecholamines in concentrations which did not alter the relaxant effect of prostaglandin E₁. The results suggested that prostaglandin E₁ acted directly on the intestinal smooth muscle of the toad rather than by the local release of catecholamines.