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 thiopental and the extra and intracellular sources of Ca²⁺, we analyzed both the contractile 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 μg/mL) contracted aortic rings only in the presence of extracellular Ca²⁺, and this effect was not blocked by verapamil or diltiazem. On the contrary, Ca²⁺ (0.1-3.1 mM) evoked contractions only when thiopental (100 μg/mL) was present. Although in calcium-free solution thiopental (100 μg/mL) did not contract aortic rings, it abolished the 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 extracellular 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 caffeine suggest that this barbiturate can deplete intracellular pools of calcium. Therefore, the calcium entry pathway associated with the contractile effect of thiopental may correspond to the capacitative calcium entry model.