Identification and subcellular localization of the subunits of l-type calcium channels and adenylyl cyclase in cardiac myocytes

The properties of cardiac L-type channels have been well characterized electrophysiologically, and many such studies have demonstrated that the channels are regulated by a cAMP-dependent pathway. However, the subunit composition of native cardiac L-type calcium channels has not been completely defined. Furthermore, a very important question exists regarding the status of the C-terminal domain of the pore-forming α₁ subunit, as this domain has the potential to be the target of protein kinases but may he truncated as a result of post-translational processing. In the present studies, the α(1C) and β₂ subunits were identified by subunit-specific antibodies after partial purification from heart membranes, or immunoprecipitation from cardiac myocytes. Both the β₂ and the full-length α(₁C) subunits were found to he expressed and co-localized in intact cardiac myocytes along T- tubule membranes. Using a quantitative antibody binding analysis, we demonstrated that the majority of the α(₁C) subunits in intact cardiac myocytes appear to be full-length. In addition, we observed that adenylyl cyclase is localized in a pattern similar to the channel subunits in cardiac myocytes. Taken together, our results provide new insights into the structural basis for understanding the regulation of L-type calcium channels by a cAMP-mediated signaling pathway.