Growth-Dependent Alterations of Intracellular Ca2'-Handling Mechanisms of Vascular Smooth Muscle Cells PDGF Negatively Regulates Functional Expression of Voltage-Dependent, 1P3-Mediated, and Ca2'-Induced Ca21 Release Channels

To examine the alterations of intracellular Ca2' ([Ca2"]i)-handling mechanisms in cultured vascular smooth muscle cells (VSMCs) of rat aorta (Shin et al Circ Res 1991;69:551-556), we stimulated VSMCs by extracellular high K', caffeine, and angiotensin II and evaluated Ca2' influx through voltage-dependent Ca2+ channels, Ca2+-induced Ca2+ release, and inositol trisphosphate-dependent Ca2+ release from internal stores. Percentage of VSMCs responding to each stimulant (responder ratio) and degree of [Ca2+]i increase in the responding cells were analyzed by a two-dimensional fura-2 imaging system. The responder ratios to the three stimulants were high (70-90%o) in the quiescent phase (days 1-2), although some cells selectively responded to one or two of the stimulants. Responder ratios prominently decreased to approximately 20% in the proliferating phase (days 2.5-3). In the subconfluent (days 3.5-4) and postconfluent (days 5-6) phases, the responder ratio to high K+ and angiotensin II recovered to the same level as during the quiescent phase, whereas that to caffeine remained low (-10-20%). In responding cells, the degree of [Ca2+]i increase by caffeine and angiotensin II was stable (-100%) during culturing, whereas that to high K' was small (-30-40%Yo) in the quiescent and proliferating phases and rapidly increased threefold in the subconfluent and postconfluent phases. Furthermore, arrest of cell growth in serum-free medium prevented the reduction of responder ratios in the proliferating phase and restored the decreased ratio of the caffeine responder. Acceleration of VSMC proliferation by platelet-derived growth factor decreased the ratios in all phases. These results imply that 1) the functional expressions of [Ca2+1J-handling mechanisms in response to these vasoactive stimuli are influenced by cell growth and cytodifferentiation of VSMCs or platelet-derived growth factor and 2) they are regulated independently from each other. (Circulation Research 1991;69:1327-1339)