INCREASED SENSITIVITY OF NEONATAL MOUSE MYOCARDIA TO AUTONOMIC TRANSMITTERS

1 Chronotropic and inotropic responses to noradrenaline and acetylchol ine were examined in isolated right atrial and ventricular preparation s from neonatal and adult mice. 2 Noradrenaline and acetylcholine prod uced positive and negative chronotropic responses, respectively, in th e atria from both ages. Noradrenaline produced positive inotropic resp onses in ventricular preparations from both ages. In all cases, the se nsitivity, expressed in terms of pD(2) values, was higher in neonatal preparations. 3 In the ventricle, desipramine produced a leftward shif t of the concentration-response curve for noradrenaline in the adult, but no such shift was observed in the neonate. The sensitivity to isop renaline of ventricular preparations was higher in the neonate than in the adult. 4 Our results demonstrated developmental decreases in sens itivities to autonomic transmitters in mouse myocardia. As for the ino tropic response to noradrenaline of ventricular muscle, both pre- and postjunctional mechanisms were responsible for the developmental decre ase in sensitivity.