Increased sensitivity of neonatal mouse myocardia to autonomic transmitters

1 Chronotropic and inotropic responses to noradrenaline and acetylcholine were examined in isolated right atrial and ventricular preparations from neonatal and adult mice. 2 Noradrenaline and acetylcholine produced positive and negative chronotropic responses, respectively, in the atria from both ages. Noradrenaline produced positive inotropic responses in ventricular preparations from both ages. In all cases, the sensitivity, expressed in terms of pD₂ values, was higher in neonatal preparations. 3 In the ventricle, desipramine produced a leftward shift of the concentration–response curve for noradrenaline in the adult, but no such shift was observed in the neonate. The sensitivity to isoprenaline of ventricular preparations was higher in the neonate than in the adult. 4 Our results demonstrated developmental decreases in sensitivities to autonomic transmitters in mouse myocardia. As for the inotropic response to noradrenaline of ventricular muscle, both pre- and postjunctional mechanisms were responsible for the developmental decrease in sensitivity.