1. The effects of the non-peptide arginine vasopressin V1 receptor antagonist (OPC-21268) and the non-peptide V2 receptor antagonist (OPC-31260) on vasopressin-induced contraction of human internal mammary arteries and rat mesenteric resistance arteries were investigated.

2. In human internal mammary arteries, the non-peptide V1 receptor antagonist, OPC-21268, failed to antagonize vasopressin-induced contraction at low concentrations and potentiated the contraction at higher concentrations (300 nmol/l, P < 0.05). A peptide selective V1 receptor antagonist {[d(CH2)5, sarcosine7]arginine vasopressin} potently inhibited the vasopressin-induced contraction, indicating the presence of functionally constrictor V1 receptors in human internal mammary arteries. Both peptide (desGly-NH29[d(CH2)5, D-Ile2, Ile4]arginine vasopressin) and non-peptide ‘selective’ V2 receptor antagonists (OPC-31260, 3 μmol/l) significantly antagonized vasopressin-induced contraction (P < 0.01), indicating partial V1 receptor antagonist activity.

3. The vasopressin-induced contraction in human internal mammary arteries was reversed by high concentrations of the non-peptide V2 receptor antagonist, OPC-31260, but not by the non-peptide V1 receptor antagonist, OPC-21268.

5. In rat mesenteric resistance arteries, both OPC-21268 (10 nmol/l) and OPC-31260 (1 μmol/l) antagonized vasopressin-induced contraction (P < 0.01).

6. The results of this study in vitro indicate that in human internal mammary arteries, the non-peptide OPC-21268 is a partial V1 receptor agonist with no V1 receptor antagonist activity, whereas the non-peptide OPC-31260 acts as a V1 receptor antagonist. Both OPC-21268 and OPC-31260 have V1 receptor antagonistic activity in vitro in the rat mesenteric resistance arteries.

7. These findings illustrate the complexity of the vasopressin receptor system and highlight the variability in results with peptide or non-peptide vasopressin analogues, between studies in vivo or in vitro, between species and across vascular beds.

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