Positive inotropic effect of carbachol and inositol phosphate levels in mammalian atria after pretreatment with pertussis toxin

The m-cholinoceptor agonist carbachol elicits a negative inotropic effect in mammalian atria. Pretreatment with pertussis toxin converts the negative to a positive inotropic effect. In this study we investigated the time course of the effects of carbachol on force of contraction and phosphoinositide products in electrically driven left auricles from guinea pig hearts after pretreatment with pertussis toxin (180 micrograms/kg i.v.; 24 hr). Inositol phosphates and phosphatidylinositols were labeled with [3H]inositol and separated with high-performance liquid chromatography and thin-layer chromatography, respectively. All experiments were performed in the presence of LiCl (10 mmol/l). The positive inotropic effect of carbachol (10 mumol/l) began within 2 min and was maximal within 15 min. Inositol 1,4,5-trisphosphate rose within 1 min followed by an increase in inositol 1,3,4,5-tetrakisphosphate, inositol 1,3,4-trisphosphate, inositol 1,4-bisphosphate and inositol 1-phosphate beginning within 2 min. It is concluded that the carbachol-induced positive inotropic effect is associated with an increase in the presumed second messengers inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Since the increase in inositol 1,4,5-trisphosphate precedes the increase in force of contraction, it may initiate the positive inotropic effect. The increase in inositol 1,3,4,5-tetrakisphosphate may be involved in maintaining the positive inotropic effect of carbachol.