Mechanism of down‐regulation of L‐type Ca2+ channel in the proliferating smooth muscle cells of rat aorta

Abstract

The mechanism of down‐regulation of L‐type Ca²⁺ channel (L‐VOC) was investigated in rat aortic smooth muscle cells in primary culture. On culture days 3–5, the cells actively incorporated the 5‐bromo‐2′‐deoxy‐uridine (BrdU), and did not respond to K⁺ depolarization nor express α_1C subunit of L‐VOC. At confluence on day 8, BrdU incorporation decreased, and the cells up‐regulated α_1C subunit mRNA, expressed α_1C subunit protein at cell periphery, and responded to K⁺ depolarization. Treating the proliferating cells on day 3 with serum‐free media or 10 μM PD98059, a MAP kinase kinase inhibitor, for 2 days induced the expression of α_1C subunit protein and the responsiveness to K⁺ depolarization. However, the serum starvation, but not PD98059, decreased the BrdU incorporation and increased the α_1C subunit mRNA. It is concluded that the expression of L‐VOC is substantially suppressed in the proliferating cells due to two mechanisms; a MAP kinase‐mediated post‐transcriptional down‐regulation and the transcriptional down‐regulation by additional mitogenic signals. J. Cell. Biochem. 87: 242–251, 2002.