Comparison of reserpine-induced supersensitivity of newborn and adult rabbit ventricular muscles to cardiotonic agents.

The myocardium of the neonate has incomplete sympathetic innervation and decreased sensitivity to cardiotonic agents. It is not known whether the neonatal myocardium exhibits the tendency to develop reserpine-induced supersensitivity to cardiotonic agents. Using isolated ventricular muscles from 1-week-old and adult rabbits, concentration-response relationships of inotropic tension to isoproterenol, ouabain, norepinephrine and calcium were determined with and without reserpine pretreatment. Ventricular muscles from the newborn animals exhibited the same degree of reserpine-induced supersensitivity to isoproterenol and ouabain as that seen in the adult animals. Although the newborn demonstrated reserpine-induced supersensitivity to calcium, the adult did not. Neither age group demonstrated reserpine-induced supersensitivity to norepinephrine. These findings indicate that newborn animals are capable of producing reserpine-induced supersensitivity, despite the immaturity of their cardiac sympathetic innervation, contractile machinery and beta-adrenergic mechanisms. These results also indicate the non-selective nature of reserpine-induced supersensitivity.