In vivo demonstration of a paracrine, inhibitory action of Met-enkephalin on adrenomedullary catecholamine release in the rat.

The present study was an attempt to assess the inhibitory effect of methionine-enkephalin (Met-Enk) on adrenal catecholamine release under in vivo conditions employing a microdialysis system. One adrenal gland of intact male rats was implanted with a microdialysis system. One day after surgery, the adrenal dialysis system was connected to a perfusion pump and Ringer solution or a Ringer solution containing Met-Enk, naloxone (Nal), or a combination of Met-Enk and Nal was used for dialysis; dialysate fractions were collected at 5-min intervals. Catecholamine secretion was stimulated by an iv injection of 1.4 mumol (200 micrograms) acetylcholine (Ach). Met-Enk-immunoreactive material in adrenal medulla extracts and dialysate fractions was analyzed by reverse phase HPLC combined with Met-Enk RIA. Under resting conditions, adrenal release rates of norepinephrine and epinephrine into the Ringer solution were constant. After Ach application secretion of both catecholamines increased about 2.7-fold. Within 10 min after the injection, catecholamine levels returned to baseline levels. Intra-adrenal application of Met-Enk reduced Ach-stimulated epinephrine, but not norepinephrine, secretion significantly; application of Nal did not affect Ach-stimulated catecholamine secretion in the initial fraction after Ach injection, but significantly prolonged amine secretion after the cholinergic stimulus. Application of Nal followed by a combined application of Met-Enk and Nal was without an effect on the amount of catecholamines released in the initial fraction after Ach injection compared to that in the control group. Thus, naloxone prevented the inhibitory effect of Met-Enk on Ach-stimulated CA release. HPLC analysis of adrenal medulla extracts followed by Met-Enk RIA revealed that several forms of Met-Enk are present in chromaffin cells, whereas in adrenal dialysates only one, albeit broad, signal of Met-Enk immunoreactivity was detectable. We demonstrate in vivo a paracrine or autocrine action of Met-Enk on Ach-stimulated catecholamine release by applying the peptide directly into the adrenal gland via a microdialysis system. We conclude that neuropeptides endogenous to the adrenal gland not only reduce the amplitude of catecholamine secretion in response to an Ach stimulus but, in addition, modulate the duration of catecholamine secretion. This is demonstrated by a prolonged catecholamine secretion if opiate receptors are blocked by Nal.