Lead induces endothelium- and Ca²⁺-independent contraction in rat aortic rings

The contractile effect of lead on rat aortic rings was examined. Lead (0.1-3.1 mM) elicited concentration-dependent but endothelium-independent contractions, which were unaffected by prazosin (1 μM). The contractile effects of lead were similar when the aortic rings were bathed either in the absence or presence of external Ca²⁺. Lanthanum (1 mM) but not verapamil (1 μM) inhibited the lead contractions; hence non-L-calcium channels are involved in such effect. In addition, lead induced contractions on aortic rings incubated in Ca²⁺-free EGTA-containing solution for 70 min., an experimental condition in which intracellular Ca²⁺-stores are depleted. Finally, the contractile effect of lead was not modified by calphostin C (an inhibitor of protein kinase C). In conclusion, the present results suggest that in rat aorta, the lead-induced contraction is independent of extra- and intracellular calcium stores. In addition, the effect of lead is independent of either catecholamines or protein kinase C. It is likely that in rat aorta, lead enters into the smooth muscle cells through non-L-calcium channels, and when acting like calcium on the contractile machinery it produces contraction. The differences observed between our results and those obtained by other authors may indicate that the mechanism of the contractile effect of lead varies among the different blood vessels.