Met-enkephalin-induced vasoconstriction in the microcirculation of rat striated muscle

Sprague-Dawley rats were anesthetized with pentobarbital (50 mg/kg) and the arterioles of the cremaster muscle were observed using television microscopy. Methionine (Met)-enkephalin was added to give bath concentrations of 10(-11) to 10(-7) M. Met-enkephalin caused a decrease in the diameter of first-order (127 +/- 7 microns), second-order (57 +/- 7 microns), and third-order (18 +/- 2 microns) arterioles, with the third-order arterioles showing the greatest reactivity. The effects of Met-enkephalin were attenuated by the opiate antagonist, naloxone, by the alpha-adrenergic receptor antagonists, phentolamine and prazosin, and by chemical sympathectomy with 6-hydroxydopamine. These results suggest that Met-enkephalin causes vasoconstriction in the striated muscle microcirculation by stimulating norepinephrine release from local nerve terminals.