Abstract
Two distinct isoforms of the T-type Ca(2+) channel, Cav3.1 and Cav3.2, play a pivotal role in the generation of pacemaker potentials in nodal cells in the heart, although the isoform switches from Cav3.2 to Cav3.1 during the early neonatal period with an unknown mechanism. The present study was designed to investigate the molecular system of the parts that are responsible for the changes of T-type Ca(2+) channel isoforms in neonatal cardiomyocytes using the whole-cell patch-clamp technique and mRNA quantification. The present study demonstrates that PKC activation accelerates the Ni(2+)-sensitive beating rate and upregulates the Ni(2+)-sensitive T-type Ca(2+) channel current in neonatal cardiomyocytes as a long-term effect, whereas PKC inhibition delays the Ni(2+)-sensitive beating rate and downregulates the Ni(2+)-sensitive T-type Ca(2+) channel current. Because the Ni(2+)-sensitive T-type Ca(2+) channel current is largely composed of the Cav3.2-T-type Ca(2+) channel, it is accordingly assumed that PKC activity plays a crucial role in the maintenance of the Cav3.2 channel. The expression of Cav3.2 mRNA was highly positively correlated with PKC activity. The expression of a transcription factor Nkx2.5 mRNA, possibly corresponding to the Cav3.2 channel gene, was decreased by an inhibition of PKCβII. These results suggest that PKC activation, presumably by PKCβII, is responsible for the upregulation of Ca(V)3.2 T-type Ca(2+) channel expression that interacts with a cardiac-specific transcription factor, Nkx2.5, in neonatal cardiomyocytes.