Vasoactive intestinal peptide stimulates blood flow and secretion of avian salt glands

The neuromodulatory role of vasoactive intestinal peptide (VIP) in avian salt gland secretion and blood flow was investigated in conscious saltwater-acclimated Pekin ducks. Glandular blood flow was measured by laser-Doppler flowmetry or the radioactive microspheres technique. Osmolal excretion was closely related to salt gland blood flow during salt loading. At threshold conditions of salt gland secretion, VIP infused intracarotidally induced both osmolal excretion and arteriolar vasodilation dose dependently (30-240 pmol.min-1.kg body wt-1). The VIPergic effect on the secretory process for NaCl was enhanced by simultaneous intracarotid application of acetylcholine (5 nmol.min-1.kg body wt-1), whereas the intrinsic vasodilatory potency of acetylcholine appeared to be nonadditive in coinfusion experiments. Ongoing secretion induced by systemic infusion of hypertonic saline could be suppressed by muscarinic antagonists, with salt gland blood flow being sustained at the reduced level of atropine-resistant vasodilation. Subsequent intracarotid infusion of VIP stimulated glandular blood perfusion and also, to a minor extent, osmolal excretion, suggesting an independent, functional VIP system in efferent salt gland control.