Properties of three COOH-terminal splice variants of a human cardiac L-type Ca2+-channel alpha(1)-subunit

There is growing evidence for diversity of cardiac-type (class C) voltage-dependent calcium-channel alpha(1)-subunits arising from the alternative splicing of a primary transcript. In this study, we show the existence of carboxy-terminal variability in the human cardiac alpha(1)-gene by genomic cloning. We found that the genomic DNA segment encoding the COOH-terminal tail of the protein is composed of nine invariable and two alternative exons. The alternative utilization of these latter two exons gives rise to the formation of three message variants for this region. Reverse transcription followed by polymerase chain reaction and radioanalytic quantitation of the reverse transcription-polymerase chain reaction products showed significant variations in the distribution of these isoforms (hHt alpha(1), rHt alpha(1), fHt alpha(1)) in distinct parts of the heart, the aorta, and fibroblasts. Expression of the three alpha(1)-isoforms in Xenopus oocytes or in HEK-293 cells and analysis of the kinetics and voltage dependence of the induced calcium-channel currents revealed only insignificant differences in the behavior of these isoforms. When the alpha(1)-isoforms were coexpressed with a human beta-subunit, no alpha(1)-specific divergences were observed, but the effects of beta-subunit coexpression on alpha(1)-isoform biophysical properties were confirmed. The differential abundance of the three isoforms and the influence of an accessory subunit are of potential physiological significance.