Effect of apamin on release of vasoactive intestinal polypeptide (VIP) from the cat intestines

The effect of apamin, a polypeptide from bee venom, on the release of vasoactive intestinal polypeptide (VIP) during active neurogenic vasodilatation in the intestines was studied in vivo in anesthetized cats. Three non-adrenergic, non-cholinergic mechanisms were investigated, i.e. the vasodilatation seen upon transmural electrical field stimulation, pelvic nerve activation and stimulation of the intramural nerves with 5-hydroxytryptamine (5-HT) infused i.a. Apamin given close i.a. abolished the three vasodilator responses. Concomitantly, the increase of VIP release was also markedly diminished although apamin increased the rate of VIP release seen in the "resting" control period. The results are in agreement with the hypothesis that VIP is the neurotransmitter in the three investigated vasodilator mechanisms.