Increase in atrial contractility induced by PGE2 in diabetic rats

Contractile response to exogenous prostaglandin E2 (PGE2) was studied in auricles from normal and acutely-diabetic (streptozotocin-treated) rats. In normal atria, PGE2 induced a biphasic inotropic effect negative at low concentrations and positive at higher ones. In diabetic, PGE2 only elicited a positive inotropic action which was greater in efficacy and potency than in normal controls. Incubation of diabetic atrial preparations with alpha-adrenoceptor antagonists (phentolamine, phenoxybenzamine or Prazosin) diminished the prostaglandin effect. However, blockade of beta-adrenoceptors with propranolol did not modify the response. Blockers of arachidonic acid metabolism via cyclo-oxygenase (indomethacin and acetylsalicylic acid) or via lipoxygenase(s) (nordihydroguaiaretic acid and dithizone) were able to reduce the positive inotropism of PGE2. A significant blockade of the stimulant action of PGE2 was seen in the presence of inhibitors of thromboxane synthesis (L-8027 and imidazole). These results suggest that in diabetic atria PGE2 effect could be associated to an involvement of cardiac alpha-adrenergic stimulation which promotes endogenous arachidonic acid release with diversification of its metabolism towards cyclo- and lipoxygenase(s)- pathway and direct to an increased thromboxane formation which could account for the positive inotropic effect induced by PGE2.