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+ (Ca2+i) in vascular smooth muscle cells (VSMCs). To elucidate intercellular distribution and function of the Ca2+ channel in SR in cultured VSMCs, we observed Ca2+i transients by digital two-dimensional imaging with a fluorescent Ca2+ 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 Ca2+i 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 microM), which inactivates the caffeine-sensitive channel, and with 1 mM Ca2+ chelator (EGTA) instead of Ca2+ in the incubation medium to block the CA2+ entry from outside, angiotensin II (10 nM) induced the Ca2+i 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+i response to vasoactive substances indicate that VSMCs are functionally divided into subgroups with different Ca2+ channel predominance on SR, necessitating reevaluation of the previous studies obtained from multiple VSMCs.