Vasoactive intestinal polypeptide inhibits electromyographic activity of the reticulo-omasal orifice in meal-fed sheep in warm and cold environments

Abstract

The effects of intragastric arterial infusions (1 mL min−1) of physiological saline, vasoactive intestinal polypeptide (VIP) or VIP-antagonist [4Cl-D-Phe6, Leu17] VIP (1 nmol mL−1) on electromyographic (EMG) activities of the reticulum and reticulo-omasal orifice (ROO) in conscious, solid, meal-fed ewes kept in either warm (18 °C) or cold (1 °C) environments were studied. Infusions were of 15 min duration before, during and after feeding in both environments. In each ruminoreticular cycle the ROO showed long bursts of EMG activity coincidental with reticular EMG activity. Quiescence of the ROO was never fully observed until the reticulum relaxed. The frequencies of reticular bursts in both environments were higher during the period of active eating than during the post- and prefeeding periods. Infusion of VIP in the warm but not in the cold environment reduced frequency of reticular EMG. Compared with saline and antagonist, VIP increased the mean duration of the ROO quiescence to 13 s in the cold environment and to a significantly lower mean of 9 s in the warm environment. We conclude that 1) VIP mediates relaxation of the ROO in solid, meal-fed sheep, 2) ROO EMG activity is influenced by different environments and by phases of the feeding cycle, and 3) the inactivity of the ROO (relaxation) is not coincidental with the reticular EMG bursts in solid-meal-fed sheep. Key words: Sheep, VIP, reticulo-omasal orifice, EMG activity, warm and cold environments