1. The effect of vasopressin receptor antagonists varies between analogues (peptide, non-peptide) and across species. In this study the effect of the novel non-peptide vasopressin V1a receptor antagonist SR 49059 on human internal mammary arteries was investigated.

2. SR 49059 produced a potent, concentration-dependent, inhibitory effect on vasopressin-induced contraction of human coronary bypass graft internal mammary arteries. Both SR 49059 (1 μmol/l) and a peptide selective V1a antagonist {[d(CH2)5sarcosine7]arginine vasopressin} (1 μmol/l) abolished vasopressin-induced contraction. The non-peptide V1a receptor antagonist OPC-21268 (1 μmol/l) had no effect on vasopressin-induced contraction.

3. The effect of SR 49059 was specific to vascular vasopressin receptors as noradrenaline-induced contraction was not influenced by SR 49059.

4. The results of this study in vitro indicate that the non-peptide SR 49059 is a potent, specific vasopressin V1a receptor antagonist in the human internal mammary artery and suggest that it may be a useful tool for studying the pathophysiological role of vasopressin in man.

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