L-TYPE CALCIUM-CHANNEL EXPRESSION DEPENDS ON THE DIFFERENTIATED STATEOF VASCULAR SMOOTH-MUSCLE CELLS

Despite intensive interest in understanding the differentiation of vas cular smooth muscle cells (VSMC), no information is available about di fferential regulation of ion channels in these cells. Since expression of the L-type Ca2+ channel can be influenced by differentiation in ot her cell types, we tested the hypothesis that the L-ype (C class) chan nel is a specific differentiation marker of VSMC and that expression o f these channels depends on the state of cell differentiation. We used rat aortic (A7r5) VSMC, which express functional L-type Ca2+ channels , and induced dedifferentiation by cell culture in different media. Tr eatment with retinoic acid was used to redifferentiate the VSMC. We ch aracterized the differentiated state of the cells by using immunohisto chemistry and Western blot analysis for smooth muscle (SM) alpha-actin and SM-myosin heavy chain (MHC). The number of functional Ca2+ channe ls was significantly decreased in dedifferentiated VSMC and increased upon differentiation with retinoic acid. Ca2+ channel function was ass essed by whole-cell voltage clamp techniques. Using Western blot and d ihydropyridine binding analysis, we found that the expression of the C a2+ channel alpha(1) subunit, and to a lesser extent the beta(2) subun it, was directly correlated with the expression of SM alpha-actin and SM-MHC. We conclude that expression of L-type Ca2+ channel alpha(1) su bunits, and thus a functional Ca2+ channel, is highly coordinated with expression of the SM-specific proteins required for specialized smoot h muscle cell functions. Furthermore, our results demonstrate that the L-type Ca2+ channel is a novel marker for differentiation of VSMC. Th e data suggest that regulation of ion channel expression during differ entiation may have physiological importance for normal smooth muscle f unction and may influence VSMC behavior under pathophysiological condi tions.