Receptors for Vasoactive Intestinal Polypeptide on Crude Smooth Muscle Membranes from Porcine Uterus*

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Abstract
Vasoactive intestinal polypeptide (VIP) is a neuropeptide that causes relaxation of the myometrium. Receptors for VIP were studied on crude smooth muscle membranes (20,000 X g pellet) from porcine myometrial homogenate. The binding of 70 pM 125I-labeled VIP could be accounted for by 80% receptor bound and 20% nondisplaceable by 1 /M VIP. The receptor binding was dependent on temperature and pH, with optimum at 20 C and pH 7.4. At 37 C, the binding was low and transient due to rapid degradation of VIP and of the receptor. Association of 125I-labeled VIP at 20 C showed steady state after about 60 min. Dissociation of [125I]iodo-VIP from the receptors by addition of 1 μ VIP showed a time course that was biexponential, with 30% in a rapid and 70% in a slow dissociating state. A calculation of Kd as the ratio between the rate constants of dissociation and association gave values of 0.23 and 9.5 nM. At steady state, the binding of [125I]iodo-VIP was displaced by unlabeled VIP, with half-maximum at a concentration of 3 nM. The Scatchard plot was curvilinear with an upward concavity, suggesting that two classes of noninteracting receptors with KdS of 0.4 and 10 nM are present in amounts of 0.2 and 1.5 pmol/mg membrane protein, respectively. Alternatively, the receptor binding may be influenced by negatively cooperative interactions. Secretin was 102 times, somatostatin 5 × 102 times, and glucagon more than 104 times less potent than VIP. Substance P, ritodrine chloride, leu-enkephalin and met-enkephalin, neurotensin, oxytocin, and prostaglandin-F2α and prostaglandin-E2 and no effect in concentrations up to 20 μM. The presence of specific receptors for VIP in myometrial membranes supports the hypothesis that VIP has a physiological role as smooth muscle relaxant in the uterus.