Vasoactive intestinal peptide receptor antagonists in rat seminal vesicle membranes

Abstract

Vasoactive intestinal peptide (VIP) receptor coupled to activation of adenylate cyclase have been previously identified in seminal vesicle membranes of rat. In the present study we demonstrate that the synthetic peptides [4-Cl-D-Phe 6, Leu 17]VIP and the growth hormone releasing factor (GRF) analog [Ac-Tyr 1,D-Phe 2]GRF 1–29-NH 2 inhibit in a competetive manner the specific 125I-VIP binding to the same membrane preparation. The order of potency of the two peptides compared to VIP was: VIP (IC 50 = 1.3± 0.5 nM) > [4-Cl-D-Phe 6, Leu 17]VIP (IC 50 = 160.0 ± 45.0 nM) > [Ac-Tyr 1, D-Phe 2]GRF 1–29-NH 2 (IC 50 = 290.0 ± 59.4 nM). Whereas VIP showed a stimulatory activity upon adenylate cyclase with a potency (ED 50 = 7.0 ± 0.7 nM) compatible with the affinity of the VIP binding sites previously described, the other two peptides tested showed no effect at that level. The behavior as antagonists of both [4-Cl-D-Phe 6, Leu 17]VIP and [Ac-Tyr 1,D-Phe 2]GRF 1–29-NH 2 was confirmed by: (a) the parallel shifts of the VIP dose-response curves for stimulation of adenylate cyclase activity in the presence of the antagonists; (b) the close agreement between the binding affinity and the inhibition of adenylate cyclase activity for the two peptides; and (c) the lack of effect of the two antagonists upon the adenylate cyclase activity stimulated by the β-adrenoceptor agonist isoproterenol which indicates the specificity of the interaction. The present results suggest that [4-Cl-D-Phe 6, Leu 17]VIP and [Ac-Tyr 1,D-Phe 2] GRF 1–29-NH 2 competitively antagonize VIP actions at the receptor level in rat seminal vesicle and provide a useful tool in the study of the physiological role of VIP in the genitourinary tract.