Cloning of a T-type Ca²⁺ channel isoform in insulin-secreting cells

The T-type Ca²⁺ channel is an important determinant of electrical activity and of Ca²⁺ influx in rat and human pancreatic β-cells. We have identified and sequenced a cDNA encoding a T-type Ca²⁺ channel α₁-subunit derived from INS-l, the rat insulin-secreting cell line. The sequence of the cDNA indicates a protein composed of 2,288 amino acids that shares 96.3% identity to α₁G, the neuronal T-type Ca²⁺ channel subunit. The transmembrane domains of the protein are highly conserved, but the isoform contains three distinct regions and 10 single amino acid substitutions in other regions. Sequencing rat genomic DNA revealed that the α₁-subunit we cloned is an alternative splice isoform of α₁G. By using specific primers and reverse transcription-polymerase chain reaction, we demonstrated that both splice variants are expressed in rat islets. The isoform deduced from INS-1 was also expressed in brain, neonatal heart, and kidney. Functional expression of this α₁G isoform in Xenopus oocytes generated low voltage- activated Ba²⁺ currents. These results provide the molecular biological basis for studies of function of T-type Ca²⁺ channels in β-cells, which is where these channels may play critical roles in diabetes.