Modulation of Cardiac Na + Current Phenotype by β 1 -Subunit Expression

Na+ current (INa) is smaller, activates and inactivates more slowly, and displays less negative voltage dependence of inactivation in the neonatal rat than in the adult rat. We have observed very similar changes when INa is recorded as a function of time in culture in mouse atrial tumor (AT-1) cells. The differences between mature and immature INa are reminiscent of those observed when skeletal muscle Na+ channel alpha subunits are expressed alone (immature) or with the beta1 subunit (mature). In the present experiments, we tested the hypothesis that suppression of beta1-subunit expression by antisense oligonucleotides would prevent the development of a mature INa. The mouse beta1 subunit was cloned from an AT-1 cDNA library and found to be identical to that in the rat at 216/218 amino acids. AT-1 cells exposed to anti-beta1 antisense oligonucleotides displayed an immature INa at day 8 in culture, whereas untreated cells or cells exposed to sense oligonucleotides displayed a mature INa. This result was observed with 2 different oligonucleotides, and neither affected the rapidly activating component of the delayed rectifier K+ current, another current recorded in AT-1 cells. These findings indicate that in these cells, the gating of INa is modulated by beta1 expression and that alpha-beta1 coexpression is required for the development of a mature cardiac INa phenotype.