[Ca2+]i reduction increases cellular proliferation and apoptosis in vascular smooth muscle cells -: Relevance to the ADPKD phenotype

Cardiovascular complications are the leading cause of morbidity and mortality in autosomal dominant polycystic kidney disease. Pkd2(+/-) vascular smooth muscle cells (VSMCs) have an abnormal phenotype and defective intracellular Ca2+ ([Ca2+](i)) regulation. We examined cAMP content in vascular smooth muscles from Pkd2(+/-) mice because cAMP is elevated in cystic renal epithelial cells. We found cAMP concentration was significantly increased in Pkd2(+/-) vessels compared with wild-type vessels. Furthermore, reducing the wild-type VSMC [Ca2+](i) by Verapamil or BAPTA-AM significantly increased cellular cAMP concentration ( mainly by phosphodiesterase [PDE] inhibition), the rate of VSMC proliferation ( determined by direct cell counting, H-3-incorporation, FACS analysis of cells entering S phase, and quantitative Western PCNA and ERK1/2 analyses), and the rate of apoptosis ( by Hoechst staining, FACS analysis of the Annexin-V positive cells, and quantitative Western Bax, cytochrome c, and activated caspase 9 and 3 analyses). The low [Ca2+](i) induced VSMC proliferation was independent of cAMP/B-Raf signaling, while that of apoptosis was promoted by cAMP. In summary, Pkd2(+/-) VSMCs have elevated cAMP levels. This elevation can also be induced by reducing [Ca2+](i) in wild-type VSMCs. The [Ca2+](i) reduction and cAMP accumulation can cause an increase in both cellular proliferation and apoptosis, resembling Pkd mutant phenotype.