IDENTIFICATION AND SUBCELLULAR-LOCALIZATION OF THE SUBUNITS OF L-TYPECALCIUM 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 s ubunit composition of native cardiac L-type calcium channels has not b een completely defined, Furthermore, a very important question exists regarding the status of the C-terminal domain of the pore-forming alph a(1) subunit, as this domain has the potential to be the target of pro tein kinases but may be truncated as a result of posttranslational pro cessing, In the present studies, the alpha(1C) and beta(2) subunits we re identified by subunit-specific antibodies after partial purificatio n from heart membranes, or immunoprecipitation from cardiac myocytes, Both the beta(2) and the full-length alpha(1C) subunits were found to be expressed and co-localized in intact cardiac myocytes along T-tubul e membranes. Using a quantitative antibody binding analysis, we demons trated that the majority of the alpha(1C) subunits in intact cardiac m yocytes appear to be full-length, In addition, we observed that adenyl yl cyclase is localized in a pattern similar to the channel subunits i n cardiac myocytes, Taken together, our results provide new insights i nto the structural basis for understanding the regulation of L-type ca lcium channels by a cAMP-mediated signaling pathway.