Brief Communication Subpopulations of Rat Vascular Smooth Muscle Cells as Discriminated by Calcium Release Mechanisms From Internal Stores

Transsarcolemmal influx and release from the sarcoplasmic reticulum (SR) through specific Ca2 ' channels are the two main pathways to elevate cytosolic Ca2 ' (Ca24i) in vascular smooth muscle cells (VSMCs). To elucidate intercellular distribution and function of the Ca'4 channel in SR in cultured VSMCs, we observed Ca'4i transients by digital two-dimensional imaging with a fluorescent Ca'4 indicator, fura-2, and found an alternative response to either caffeine or angiotensin II under the condition that selectively enabled Ca2 ' release from SR. Caffeine (20 mM) increased the Ca24. by 292+36 % (mean±SEM) over the basal level in one third of the VSMC population (n=19), while the remaining cells in the same observation field showed no or very weak response (110±4%). In contrast, after the treatment with caffeine plus ryanodine (30,uM), which inactivates the caffeine-sensitive channel, and with 1 mM Ca24 chelator (EGTA) instead of Ca'4 in the incubation medium to block the Ca24 entry from outside, angiotensin II (10 nM) induced the Ca21j elevation (287+26%) in previously caffeine-nonresponsive cells, although caffeine-responsive cells retained quiescence (112 ±2%). These responses did not differ when the order of the reagent application was reversed. These heterogeneities of VSMCs in the Ca2 % response to vasoactive substances indicate that VSMC