Dihydropyridine-sensitive voltage-gated (l-type) Ca²⁺channels play an essential role in cardiac and smooth muscle excitation-contraction coupling. Transcripts for the two pore-forming subunits ofl-type Ca²⁺channels,α_1Candα_1Dhave been detected in heart and lung; however, distribution, structure and regulated expression of these channel subunit mRNAs have not been examined in detail. Here we use RNase protection and RT-PCR assays to identify cardiac-specific features of expression of the two channel mRNAs. First, expression ofα_1DmRNA is found in lung, aorta and atrium, but is not detected in ventricle. Limited expression ofα_1DmRNA is also seen in enriched preparations of cardiac myocytes: it is significant in atrial myocytes, but not in ventricular myocytes. Second, RT-PCR analyses indicate that atrialα_1Dchannels exclusively contains the 15-amino acid insertion between third and fourth segments in repeat IV. Finally, expression ofα_1CmRNA, but notα_1DmRNA, is up-regulated by glucocorticoids in atrium and ventricle: adrenalectomy and subsequent injection of the glucocorticoid agonist dexamethasone decreased and increased the channel message, respectively. Dexamethasone also significantly increased the number of dihydropyridine-binding sites in ventricle. In contrast,α_1CmRNA levels were glucocorticoid-insensitive in lung and aorta. Thus, basal and glucocorticoid-induced expression, and splicing of the twol-type Ca²⁺channelα₁subunit transcripts are tissue specifically controlled in atria and ventricles of rat heart.