Characterization of receptors mediating contraction of the rat isolated small mesenteric artery and aorta to arginine vasopressin and oxytocin

1. The exact nature of the receptor subtype(s) involved in the action of arg-vasopressin (AVP) on the rat aorta and small mesenteric artery (SMA) is controversial. Therefore, we have studied the effects of the selective V_1A receptor antagonists, OPC 21268 and SR 49059, and the oxytocin (OT) receptor antagonist, atosiban, on the AVP- and OT-induced contractions of the two vessels.
2. AVP and OT displayed similar intrinsic activities in the rat aorta and SMA, but AVP was ∼130 fold and ∼500 fold more potent than OT, respectively. In the rat aorta, Hill slopes (n_H) were similar for OT and AVP. However, in rat SMA, the OT concentration-effect (E/[A]) curve was significantly steeper than the AVP E/[A] curve (n_H,=3.3±0.20, 2.3±0.15; P<0.001).
3. In the aorta OPC 21268, SR 49059 and atosiban competitively antagonized the AVP and OT E/[A] curves. Except for atosiban and SR 49059 against AVP, competitive antagonism was also observed in the SMA. Atosiban caused concentration-dependent steepening of the AVP E/[A] curve, whereas SR 49059 decreased the upper asymptote.
4. Schild analysis yielded affinities indicative of V_1A receptor involvement in both vessels: pK_B/pA₂=9.20–9.48, 7.56–7.71 and 6.19–6.48 for SR 49059, OPC 21268 and atosiban, respectively.
5. Neither AVP nor OT relaxed U46619 pre-contracted aorta or SMA in the presence of SR 49059, suggesting no interference of a vasodilatory component.
6. Despite predominant involvement of V_1A receptors in both vessels, the different Hill slopes of AVP and OT E/[A] curves as well as the steepening of the AVP E/[A] curves by atosiban are indicative of receptor heterogeneity in the rat SMA.