Dihydropyridines (DHPs) block the vascular smooth muscle L-type Ca²⁺ channel at lower concentrations than the cardiac Ca²⁺ channel, although their α₁ subunit, which binds the DHPs, is derived from the same gene. This α_1C gene gives rise to several splice variants, among which the α_1C-b variant is affected by lower concentrations of nisoldipine than the α_1C-a variant. Functional expression of chimeras of α_1C-a and α_1C-b subunits demonstrated that the transmembrane segment IS6 is responsible for the different dihydropyridine sensitivity. Northern blot analysis showed that transcripts coding for the IS6 segment of the α_1C-a subunit were expressed in heart but not in aorta, whereas the IS6 segment of the α_1C-b subunit was expressed predominantly in vascular smooth muscle. In situ hybridization of rat heart sections confirmed this expression pattern of IS6 α_1C-a and IS6 α_1C-b in ventricular and smooth muscle myocytes, respectively. These results suggest that the different dihydropyridine sensitivities of cardiac and vascular L-type Ca²⁺ channels are caused at least partially by the tissue-specific expression of alternatively spliced IS6 segments of the α_1C gene.