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, methionineenkephalin 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.