n-Secreting Cells

The T-type Ca + channel is an important determinant of electrical activity and of Ca + influx in rat and human pancreatic -cells. We have identified and sequenced a cDNA encoding a T-type Ca + channel 1s u b u n i t derived from INS-1, 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 1G, the neuronal T-type Ca 2 + 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 1s u bunit we cloned is an alternative splice isoform of 1G . 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 1G isoform in X e n o p u s oocytes generated low voltage–activated Ba + currents. These results provide the molecular biological basis for studies of function of T-type Ca +