Sustained negative inotropism mediated by alpha-adrenoceptors in adult mouse myocardia: developmental conversion from positive response in the neonate.

1. Inotropic responses to alpha-adrenoceptor stimulation and the effects of antagonists were examined in isolated ventricular preparations from neonatal and adult mice. 2. Phenylephrine, in the presence of propranolol, produced positive inotropic responses in neonates up to 1 week after birth, while it produced negative inotropic responses in mice older than 3 weeks. 3. Both positive and negative responses to phenylephrine in neonates and adults, respectively, were antagonized by prazosin, WB4101 (2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane) and 5-methylurapidil, but not by atropine, yohimbine or chlorethylclonidine. 4. Noradrenaline (NA) produced positive inotropic responses both in the neonate and adult; the responses were observed in a lower concentration-range in the neonate than in the adult. WB4101 produced a significant leftward shift of the concentration-response curve for noradrenaline in adult preparations while only a slight rightward shift was observed in the neonate. 5. Our results demonstrate the presence of alpha-adrenoceptor-mediated inotropic responses in the mouse ventricular myocardia. The response to phenylephrine changes from a positive to a negative effect during postnatal development. The responses are mediated by alpha 1-adrenoceptors, and modulate the overall inotropic response to NA in the adult.