Synergism between neuropeptide Y and norepinephrine highlights sympathetic cotransmission: studies in rat arterial mesenteric bed with neuropeptide Y, analogs, and BIBP 3226.

Although abundant literature supports the notion that neuropeptide Y (NPY) synergizes in vivo and in vitro, the vasomotor activity elicited by norepinephrine (NE), the converse interaction (i.e., the adrenergic modulation of the NPY vasomotor response) has been less characterized. To assess whether NE synergizes the vasomotor effect of NPY, the rat arterial mesenteric bed was chosen as a model experimental system. Mesenteries were precontracted with NE and few minutes later were perfused with exogenous NPY. Under these conditions, NPY contracted the arterial mesenteric bed with an EC50 value of 0.72 +/- 0.06 nM. NPY was unable to contract this vascular territory without an agonist-induced precontraction. Other agonists, such as endothelin-1, a synthetic analog of prostaglandin F2alpha, or 5-hydroxytryptamine, also were effective primers because in their presence, NPY was a potent vasoconstrictor. In contrast, mesenteries precontracted with KCl failed to evidence the NPY-induced rise in perfusion pressure. Two structural analogs of NPY, PYY and [Leu31, Pro34]NPY, mimicked the activity of NPY. The NPY fragment 13-36 did not elicit such a response. All NPY analogs exhibited less efficacy and potency relative to NPY. The NPY- and related structural analog-induced vasoconstriction was competitively and reversibly antagonized by BIBP 3226; the pA2 of the NPY interaction was 7.0. The application of 0.1 to 1 microM BIBP 3226 or 0.1 to 10 nM prazosin at the peak of the NPY vasomotor response elicited a gradual blockade of the vasoconstriction. Although BIBP 3226 blocked the increase in perfusion pressure elicited by NPY, leaving unaffected the NE-induced tone, 10 nM prazosin blocked the full response, including the NE-induced component. Tissue preincubation with 200 nM nifedipine abolished the NPY-induced vasoconstriction; likewise, the acute application of 10 to 100 nM nifedipine blocked gradually the maximal NPY-induced contraction. Removal of the mesenteric endothelial layer increased the potency of NPY by 2-fold; it also slightly potentiated the antagonist activity of BIBP 3226. The synergism between NPY and NE backs the principle of sympathetic cotransmission.