Relationship between extra and intracellular sources of calcium and the contractile effect of thiopental in rat aorta

To evaluate the relationship between the vasocontractile effect of thiopent al and the extra and intracellular sources of Ca2+, we analyzed both the co ntractile effect of the barbiturate on rat aortic rings and its ability to modify the intracellular calcium concentration in cultured rat aorta smooth muscle cells. Thiopental (10-310 mug/mL) contracted aortic rings only in t he presence of extracellular Ca2+, and this effect was not blocked by verap amil or diltiazem. On the contrary, Ca2+ (0.1-3.1 mM) evoked contractions o nly when thiopental (100 mug/mL) was present. Although in calcium-free solu tion thiopental (100 mug/mL) did not contract aortic rings, it abolished th e contractile effect of either phenylephrine (10(-6) M) or caffeine (10 mM) . Finally, thiopental augmented the intracellular calcium concentration in cultured smooth muscle cells incubated either in the presence or absence of calcium. In conclusion, thiopental's vasocontractile effect depends on ext racellular calcium influx, which is independent of L-calcium channels. The increase in intracellular Ca2+ concentration elicited by thiopental in Ca2-free solution and its ability to block the effect of phenylephrine and caf feine suggest that this barbiturate can deplete intracellular pools of calc ium. Therefore, the calcium entry pathway associated with the contractile e ffect of thiopental may correspond to the capacitative calcium entry model.