Effect of vasoactive intestinal peptide on ion transport across dog tracheal epithelium

Under short-circuit conditions, vasoactive intestinal peptides (VIP) did not alter net Na⁺ movement but selectively stimulated net Cl^- secretion across dog tracheal epithelium with a high affinity (K(m) ≃ 10^-8 M). The increase in Cl^- secretion was not different from the rise in short-circuit current (I(sc)). However, stimulation of Cl^- secretion was not maximal, because the addition of isoproterenol (10^-6 M) to VIP-treated tissues further increased the I(sc) by 54%. The effect of exogenous VIP was not blocked by a combination of atropine, phentolamine, propranolol (10^-5 or 10^-6 M) or tetrodotoxin (10^-6 M). Under open-circuit conditions, VIP caused an increase in the net secretion of Cl^- and Na⁺, but the changes did not reach statistical significance. We conclude that VIP acts directly on receptors on the surface of epithelial cells to stimulate active Cl^- secretion. The abundance of VIP nerves in the submucosa suggests that VIP may be important in regulation of fluid movement across the epithelium.