Effects of beta-endorphin, vasoactive intestinal polypeptide and cholecystokinin octapeptide on cat carotid...

The effects of beta-endorphin, vasoactive intestinal polypeptide (VIP) and cholecystokinin octapeptide (CCK-8) on carotid chemoreceptor activity have been investigated in cats anaesthetized with pentobarbitone. Spontaneous chemoreceptor discharge was decreased by intracarotid injection of beta-endorphin and by low doses of VIP, whereas it was increased by CCK-8 and higher doses of VIP, these effects being relatively long-lasting and often associated with changes in systemic blood pressure. The chemoexcitation evoked by acetylcholine and sodium cyanide was reduced during intracarotid infusion of any of the three peptides studied, and that caused by CO2-saturated Locke solution was reduced by beta-endorphin, largely unaltered by VIP and variably affected by CCK-8. The inhibitory effect of beta-endorphin was greatly reduced by naloxone, implying that it probably involved actions at naloxone-sensitive opiate receptors in the carotid body. Substance P was unable to overcome the chemoinhibitory effect of methionine enkephalin. Possible functions of polypeptides in the carotid body are discussed.