CHARACTERIZATION OF ALPHA(1)-ADRENOCEPTORS WHICH MEDIATE CHRONOTROPY IN NEONATAL RAT CARDIAC MYOCYTES

1. In the present study, we investigated the effect of culture on alph a1-adrenoceptors that mediate chronotropy and on alpha1-adrenergic sig nal transduction in neonatal rat cardiac myocytes. 2. The spontaneous beating rate of neonatal rat myocytes after 3 or 7 days in culture was 37.4 +/- 4.2 or 102.0 +/- 4.3 beats min-1, respectively. The alpha1-a drenoceptor-mediated chronotropic effect of norepinephrine was positiv e at day 3 of culture. In contrast to day 3 of culture, the neonatal m yocytes exhibited a negative chronotropic response to norepinephrine o n day 7 of culture. Both of these effects of norepinephrine were compl etely abolished by prazosin. 3. The affinity (K(d)) and/or density (B( max)) of alpha1-adrenoceptors labeled with [H-3]prazosin in membranes from cultured myocytes were not significantly different between day 3 and day 7 of culture. 4. The expression of G(s), G(i), G(q) and G(o) a lpha-subunits in membranes from cultured myocytes was found to be sign ificantly increased with the passage of culture time by immunoblot ana lysis. In contrast, no significant differences in G(beta)-subunit expr ession were observed between day 3 and day 7 of culture. 5. Norepineph rine-stimulated inositol 1,4,5-trisphosphate production by radio-bindi ng protein in neonatal myocytes after 7 days of culture was significan tly higher than that of the day 3 counterpart. 6. No significant chang es in phospholipid and cholesterol contents in membranes from neonatal myocytes were observed with longer culture times. 7. These results su ggest that changes in the responsiveness to alpha1-adrenergic stimulat ion from positive to negative chronotropy during culture of cardiac my ocytes are mediated, at least in part, by functional alterations in th e alpha1-adrenergic signal transduction systems, including both G-prot ein expression and inositol 1,4,5-trisphosphate production.