Primary Action of Endothelin on Ca Release in Bovine Coronary Artery Smooth Muscle Cells

Intracellular free Ca concentrations (Cai) were determined by fura-2 microfluorometry in single freshly dispersed cells to differentiate endothelin (ET)-induced Ca release from Ca influx through voltage-gated Ca channels (VGCC). In physiological solution ET (10(-8) M) significantly (P less than 0.05) increased Cai 23 +/- 3% (+/- SE) above baseline; this increase was not significantly attenuated by 2 x 10(-4) M lanthanum, a blocker of VGCC, or Ca-free solution. When the sarcoplasmic reticulum was depleted of Ca by prolonged treatment with 5 x 10(-3) M caffeine, depolarization with 80 mM K (80K; or 30K) plus ET did not increase Cai above that induced by 80K (or 30K) in caffeine alone. In contrast, 10(-6) M BAY K 8644, instead of ET in the protocol, significantly (P less than 0.05) increased Cai above that induced by 80K (or 30K). ET released Ca from the caffeine-sensitive internal store but was not rapid and transient like caffeine-induced release, which elicited a peak Cai increase in less than 1 min; instead, release was more gradual and prolonged with Cai peaking in greater than 2 min, thus resembling the response to 10(-5) M ryanodine. With two ET exposures, either a transient nonrepeatable increase in Cai or a delayed, but sustained, increase in Cai resulted, similar to the response to ryanodine. These data indicate that in freshly dispersed bovine cells the predominant mechanism by which ET increases Cai is release of Ca from the sarcoplasmic reticulum; if any increase in L-type voltage-gated Ca influx occurred, it was minimal and matched by efflux.