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 alpha1 subunit, as this domain has the potential to be the target of protein kinases but may be truncated as a result of post-translational processing. In the present studies, the alpha1C and beta2 subunits were identified by subunit-specific antibodies after partial purification from heart membranes, or immunoprecipitation from cardiac myocytes. Both the beta2 and the full-length alpha1C subunits were found to be 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 alpha1C 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.