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 alpha 1-adrenoceptors that mediate chronotropy and on alpha 1-adrenergic signal 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 alpha 1-adrenoceptor-mediated chronotropic effect of norepinephrine was positive at day 3 of culture. In contrast to day 3 of culture, the neonatal myocytes exhibited a negative chronotropic response to norepinephrine on day 7 of culture. Both of these effects of norepinephrine were completely abolished by prazosin. 3. The affinity (Kd) and/or density (Bmax) of alpha 1-adrenoceptors labeled with [3H]prazosin in membranes from cultured myocytes were not significantly different between day 3 and day 7 of culture. 4. The expression of Gs, Gi, Gq and Go alpha-subunits in membranes from cultured myocytes was found to be significantly increased with the passage of culture time by immunoblot analysis. In contrast, no significant differences in G beta-subunit expression were observed between day 3 and day 7 of culture. 5. Norepinephrine-stimulated inositol 1,4,5-trisphosphate production by radio-binding protein in neonatal myocytes after 7 days of culture was significantly higher than that of the day 3 counterpart. 6. No significant changes in phospholipid and cholesterol contents in membranes from neonatal myocytes were observed with longer culture times.(ABSTRACT TRUNCATED AT 250 WORDS)