Cocaine-induced relaxation of isolated rat aortic rings and mechanisms of action: Possible relation to cocaine-induced aortic dissection and hypotension

Cocaine HCl is well known for its toxic effects on the cardiovascular system, but little is known about its effects on different regional blood vessels. We designed experiments to determine if cocaine HCl could influence the tension of isolated aortic rings, i.e., induce contraction or relaxation. Surprisingly, cocaine HCl (1×10^-5 to 6×10^-3 M) relaxed isolated aortic rings precontracted by phenylephrine in a concentration-dependent manner. No significant differences were found between intact or denuded isolated aortic rings (P>0.05). The maximal % relaxations of intact vs. denuded isolated aortic rings were 108.9±24.3% vs. 99.5±8.3% (P>0.05). Cocaine HCl, 2×10^-3 M, was found to inhibit contractions by phenylephrine; EC₅₀s were increased (P<0.01) and E_max's were decreased (51.3±16.4% vs. 89.8±10.6%, P<0.01). A variety of amine antagonists could not inhibit the relaxant effects of cocaine HCl (P>0.05). The cyclooxygenase-1 inhibitor, indomethacin, also failed to inhibit relaxations induced by cocaine HCl (P>0.05). Neither L-arginine, N(G)-monomethyl-L-arginine (L-NMMA), nor methylene blue could inhibit the relaxations induced by cocaine HCl (P>0.05), suggesting cocaine HCl does not relax isolated aortic rings by inducing the synthesis or release of nitric oxide (NO) or prostanoids from either endothelial or vascular muscle cells. Inhibitors of cAMP, cGMP and protein kinase G (PKG) also failed to inhibit cocaine-induced relaxations. Cocaine HCl (1×10 ^-5 to 6×10^-3 M) could also relax isolated aortic rings precontracted by phenylephrine in high K⁺ depolarizing buffer. Surprisingly, calyculin A, an inhibitor of myosin light chain (MLC) phosphatase, inhibited cocaine-induced relaxations in a concentration-dependent manner, suggesting the probable importance of cocaine-induced MLC phosphatase activation in rat aortic smooth muscle cells. It was also found that cocaine HCl could dose-dependently inhibit Ca²⁺-induced contractions of isolated aortic rings in high K⁺-Ca²⁺-free buffer, suggesting that cocaine HCl may inhibit Ca²⁺ influx and/or intracellular release.