EFFECT OF SYMPATHECTOMY ON INOTROPIC RESPONSIVENESS TO ALPHA-ADRENOCEPTOR STIMULATION IN DEVELOPING MOUSE MYOCARDIA

Effects of postnatal sympathectomy on inotropic responsiveness to alph a-adrenoceptor stimulation were examined in mouse myocardia to determi ne whether the developmental conversion of alpha-adrenoceptor-mediated inotropic responses from positive to negative is triggered by sympath etic innervation. Sympathectomy was performed chemically by consecutiv ely administering 6-hydroxydopamine for 14 days after birth and confir med by the absence of inotropic responses to tyramine. In newborn myoc ardia, phenylephrine, in the presence of propranolol, produced concent ration-dependent positive inotropic responses. Three weeks after birth , phenylephrine, in the presence of propranolol, produced concentratio n-dependent negative inotropic responses, both in control and in sympa thectomized myocardia; no difference was observed between the two grou ps of mice in the maximum decrease in contractile force produced by ph enylephrine. The sensitivity (pD(2) value) to phenylephrine was signif icantly higher in sympathectomized myocardia. In conclusion, sympathet ic innervation of the mouse ventricular myocardium is not required for the developmental conversion of the or-adrenoceptor-mediated inotropi c response from positive to negative.