Potentiation by vasopressin of adrenergic vasoconstriction in the rat isolated mesenteric artery

1. The aim of the present study was to investigate in rat mesenteric artery rings whether low concentrations of vasopressin could modify the contractile responses to noradrenaline and electrical stimulation of perivascular nerves.
2. Vasopressin (10⁻¹⁰–10⁻⁷ M) caused concentration-dependent contractions (pD₂=8.36±0.09). The V₁-receptor antagonist d(CH₂)₅Tyr(Me)AVP (10⁻⁹–10⁻⁸ M) produced parallel rightward shifts of the control curve for vasopressin. Schild analysis yielded a pA₂ value of 9.83 with a slope of 1.10±0.14.
3. Vasopressin (3×10 ⁻¹⁰ and 10⁻⁹ M) caused concentration-dependent potentiation of the contractions elicited by electrical stimulation (2–8 Hz; 0.2 ms duration for 30 s) and produced leftward shifts of the concentration-response curve for noradrenaline. The V₁-receptor antagonist induced concentration-dependent inhibitions of potentiation induced by vasopressin. The selective V₁-receptor agonist [Phe^*, Orn⁸]-vasotocin (3×10 ⁻¹⁰ and 10⁻⁹ M) induced potentiation of electrical stimulation-evoked responses which was also inhibited in the presence of the V₁ antagonist (10⁻⁸ M). In contrast, the V₂-receptor agonist deamino-8-D-arginine vasopressin (desmopressin 10⁻⁸–10⁻⁷ M) did not modify the electrical stimulation-induced responses and the V₂-receptor antagonist [d(CH₂)₅, D-Ile^*, Ile⁴, Arg⁸]-vasopressin (10⁻⁸–10⁻⁷ M) did not affect the potentiation evoked by vasopressin.
4. In artery rings contracted by 10⁻⁶ M noradrenaline in the presence of 10⁻⁶ M guanethidine and 10⁻⁶ M atropine, electrical stimulation (2, 4 and 8 Hz) produced frequency-dependent relaxations which were unaffected by 10⁻⁹ M vasopressin but abolished by 10⁻⁶ M tetrodotoxin.
5. Vasopressin also potentiated contractions elicited by KCl and contractions induced by addition of CaCl₂ to KCl depolarized vessels. The augmenting effects were inhibited by the V₁ antagonist.
6. In the presence of the calcium antagonist nifedipine (10⁻⁶ M), vasopressin failed to enhance the contractile responses to electrical stimulation, noradrenaline and KCl.
7. The results demonstrate that low concentrations of vasopressin strongly potentiate the contractions to adrenergic stimulation and KCl depolarization. This effect appears to be mediated by V₁ receptor stimulation which brings about an increase in calcium entry through dihydropyridine-sensitive calcium channels.