Evidence for the coexistence and co-release of [Met]enkephalin and noradrenaline from sympathetic nerves of the bovine vas deferens.

The localization and neurosecretion of methionine-enkephalin was studied in sympathetic nerves of the bovine vas deferens. Immunostaining showed methionine-enkephalin-like immunoreactivity in a network of varicose nerve fibres in the smooth muscle layers of the vas deferens. When vas deferens homogenates were subjected to differential and sucrose density gradient centrifugation, methionine-enkephalin was found to parallel the distribution of noradrenaline in the more dense region of the gradient, where "heavy" or large dense-cored vesicles are present. Electron microscopic immunochemistry confirmed this finding and showed methionine-enkephalin-like immunoreactivity in large dense-cored vesicles. The release of methionine-enkephalin upon electrical stimulation was studied in superfusion experiments. The methionine-enkephalin secretion was shown to be Ca2+-dependent and was inhibited by adding the adrenergic neuron blocking drug guanethidine to the superfusion medium. We conclude that in the bovine vas deferens methionine-enkephalin is only present in large dense-cored vesicles of adrenergic neurons and that the peptide is released from these vesicles together with noradrenaline by a Ca2+-dependent mechanism.