Comparison of vasopressin and oxytocin receptors in the rat uterus and vascular tissue

The In Vitro Contractile Response of Canine Pregnant Myometrium to Oxytocin and Denaverine Hydrochloride

In pregnant bitches, the response to oxytocin and denaverine hydrochloride in dystocia management is usually poor. To better understand the effect of both drugs on myometrial contractility, the circular and longitudinal muscle layers were examined in an organ bath. For each layer, three myometrial strips were stimulated twice, each with one of three oxytocin concentrations. The effect of denaverine hydrochloride was studied once in direct combination with oxytocin and alone with subsequent oxytocin administration. Contractions were recorded and evaluated for average amplitude, mean force, area under the curve (AUC), and frequency. Effects of different treatments were analyzed and compared within and between layers. In the circular layer, oxytocin significantly increased amplitude and mean force compared to untreated controls regardless of stimulation cycles or concentrations. In both layers, high oxytocin concentrations caused tonic contractions, while the lowest concentration created regular rhythmic contractions. Longitudinal layer tissue responded to oxytocin with a significantly decreased contractility when stimulated twice, presumably a sign of desensitization. Denaverine hydrochloride neither affected oxytocin induced contractions nor showed a priming effect to subsequent oxytocin. Thus, no benefit of denaverine hydrochloride on myometrial contractility was found in the organ bath. Our results suggest a better efficiency of low-dose oxytocin in canine dystocia management.

Possible neural mediation of the central effects of oxytocin on uterine motility

The central nervous system contains the nuclei at the origin of autonomic and neuroendocrine pathways to the uterus. Although the anatomical basis of these pathways is known, the conditions of their recruitment and their interactions in the context of copulation remain to be explored. We tested the hypothesis that some central mechanisms could simultaneously recruit both pathways to the uterus. In this aim, we recorded intrauterine pressure changes in anesthetized female rats at the estrus stage after intracerebroventricular (ICV) administration of oxytocin (OT). Doses of 0.3–300 ng elicited increases of frequency and amplitude of uterine contractions. These effects were partly mimicked by the OT agonist [Thr4,Gly7]OT but not by arginine vasopressin. They were blocked by the OT receptor antagonist atosiban delivered either ICV or intravenously. The latter suggests that ICV OT activated the systemic release of OT. The effects of OT were also blocked by hexamethonium, a ganglionic blocking agent, by atropine, a muscarinic receptor antagonist, and by Nω-nitro-l-arginine methyl ester, an inhibitor of nitric oxide synthesis. The results reveal that ICV OT recruits autonomic efferent pathways to the uterus. These results support our hypothesis that the activation of central nuclei can promote uterine contractility, and that OT may be a central coordinator of autonomic and neuroendocrine pathways. The hypothalamus, the source of direct OT-ergic projections to the pituitary, the brain stem, and the spinal cord, may be a target of central OT.

Oxytocin does not directly affect vascular tone in vessels from nonpregnant and pregnant rats

Recent evidence suggests oxytocin (OT) may regulate vascular tone. OT and its receptor (OTR) have been identified in the rat heart and great vessels. Expression of OT and OTR is increased in some tissues during pregnancy. We hypothesized that the OT/OTR system may be a physiological regulator of vascular tone and mediate the decreased vascular resistance noted during pregnancy. Using a wire myograph system, we measured changes in vascular tone in response to OT in small mesenteric arteries, uterine arcuate arteries, and thoracic aorta from nonpregnant and pregnant rats. Additionally, we used reverse transcriptase-polymerase chain reaction (RT-PCR) to measure mRNA for OTR in these vascular tissues. Although OTR mRNA was identified by RT-PCR, OT did not elicit a vasodilatory effect in any of the vessels studied. High concentrations of OT (>10(-8) M) caused vasoconstriction that was eliminated by a specific vasopressin V(1a) receptor antagonist. Although it may have an indirect effect in regulation of peripheral resistance, we conclude that OT is unlikely to play a direct role in the physiological regulation of vascular tone.

Spinal proerectile effect of oxytocin in anesthetized rats

The spinal cord contains the neural network that controls penile erection. This network is activated by information from peripheral and supraspinal origin. We tested the hypothesis that oxytocin (OT), released at the lumbosacral spinal cord level by descending projections from the paraventricular nucleus, regulated penile erection. In anesthetized male rats, blood pressure and intracavernous pressure (ICP) were monitored. Intrathecal (it) injection of cumulative doses of OT and the selective OT agonist [Thr4,Gly7]OT at the lumbosacral level elicited ICP rises whose number, amplitude, and area were dose dependent. Thirty nanograms of OT and one-hundred nanograms of the agonist displayed the greatest proerectile effects. Single injections of OT also elicited ICP rises. Preliminary injection of a specific OT-receptor antagonist, hexamethonium, or bilateral pelvic nerve section impaired the effects of OT injected it. NaCl and vasopressin injected it at the lumbosacral level and OT injected it at the thoracolumbar level or intravenously had no effect on ICP. The results demonstrate that OT, acting at the lumbosacral spinal cord, elicits ICP rises in anesthetized rats. They suggest that OT, released on physiological activation of the PVN in a sexually relevant context, is a potent activator of spinal proerectile neurons.

Comparison of vasopressin binding sites in human uterine and vascular smooth muscle cells

Several studies indicate that oxytocin and vasopressin receptors in the human uterus are heterogeneous. We have investigated whether oxytocin and vasopressin bind to separate receptors or one class of receptors in human uterine smooth muscle cells. [3H]d(CH2)5Tyr(Me)AVP, the vasopressin V1A receptor selective radioligand, was used for comparison of vasopressin binding sites in human uterine and vascular smooth muscle cell membranes. Both membrane preparations exhibited one class of high-affinity binding sites with Kd values of 6.44 and 0.47 nM, Bmax values of 166 and 34.8 fmol/mg protein for uterine and vascular smooth muscle cells, respectively. In vascular preparations, the selective vasopressin V1A receptor antagonist, SR 49059 ((2S) 1-[(2R 3S)-(5-chloro-3-(2-chlorophenyl)- -(3.4-dimethoxybenzenesulfonyl)-3-hydroxy-2,3-dihydro-1H-indole-2- carbonyl]-pyrrolidine-2-carboxamide), showed high affinity with Ki value of 0.98 nM, confirming that these receptors belong to the vasopressin V1A receptor subtype. On the contrary, in uterine preparations, binding of [3H]d(CH2)5Tyr(Me)AVP was more effectively displaced by oxytocin and the oxytocin receptor selective antagonist, L-371257, (1-[1-[4-[ N-Acetyl-4-piperidinyl)oxy]2-methoxybenzoyl]piperidin-4-yl]- 4H-3,1-benzoxazin-2(1H)-one), than vasopressin and SR 49059, suggesting that binding may be due to cross-reaction with the oxytocin receptors. These results suggest that human uterine smooth muscle cells express only a high density of oxytocin receptors.

Human vascular endothelial cells express oxytocin receptors

Pharmacological studies in humans and animals suggest the existence of vascular endothelial vasopressin (AVP)/oxytocin (OT) receptors that mediate a vasodilatory effect. However, the nature of the receptor subtype(s) involved in this vasodilatory response remains controversial, and its coupled intracellular pathways are unknown. Thus, we set out to determine the type and signaling pathways of the AVP/OT receptor(s) expressed in human vascular endothelial cells (ECs). Saturation binding experiments with purified membranes of primary cultures of ECs from human umbilical vein (HUVEC), aorta (HAEC), and pulmonary artery (HPAEC) and [3H]AVP or [3H]OT revealed the existence of specific binding sites with a greater affinity for OT than AVP (Kd = 1.75 vs. 16.58 nM). Competition binding experiments in intact HUVECs (ECV304 cell line) with the AVP antagonist [125I]4-hydroxyphenacetyl-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-NH2 or the OT antagonist [125I]D(CH2)5[O-Me-Tyr-Thr-Orn-Tyr-NH2]vasotocin, and various AVP/OT analogs confirmed the existence of a single class of surface receptors of the classical OT subtype. RT-PCR experiments with total RNA extracted from HUVEC, HAEC, and HPAEC and specific primers for the human V1 vascular, V2 renal, V3 pituitary, and OT receptors amplified the OT receptor sequence only. No new receptor subtype could be amplified when using degenerate primers. DNA sequencing of the coding region of the human EC OT receptor revealed a nucleotide sequence 100% homologous to that of the uterine OT receptor reported previously. Stimulation of ECs by OT produced mobilization of intracellular calcium and the release of nitric oxide that was prevented by chelation of extra- and intracellular calcium. No stimulation of cAMP or PG production was noted. Finally, OT stimulation of ECs led to a calcium- and protein kinase C-dependent cellular proliferation response. Thus, human vascular ECs express OT receptors that are structurally identical to the uterine and mammary OT receptors. These endothelial OT receptors produce a calcium-dependent vasodilatory response via stimulation of the nitric oxide pathway and have a trophic action.

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 V1A 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 approximately 130 fold and approximately 500 fold more potent than OT, respectively. In the rat aorta, Hill slopes (nH) 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 (nH, = 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 V1A receptor involvement in both vessels: pKB/ 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 V1A 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.

Quantitative reverse transcription and polymerase chain reaction analysis of oxytocin and vasopressin receptor mRNAs in the rat uterus near parturition

Oxytocin receptors (OT-R) are known to be involved in the course of labor since a massive increase in OT-binding sites is observed in the uterus just before parturition. Vasopressin (AVP)-binding sites have also been observed and have been shown to mediate uterotonic responses. To determine exactly which subtypes of OT/AVP receptors are expressed in the rat uterus near parturition, we carried out absolute quantitations of the neurohypophysial hormone receptor (OT-R and the vasopressin receptors V1a-R, V1b-R and V2-R) mRNAs with an assay based on reverse transcription-polymerase chain reaction (RT-PCR) using in vitro transcribed mutated cRNAs as internal standards. The number of mRNA molecules/ng of total RNA was 35 +/- 6, 220 +/- 33 and 39 +/- 9 for OT-R (P < 0.01) and 16 +/- 1, 25 +/- 8 and 31 +/- 5 for V1a-R (P > 0.05) on day (D) 21, 22 and 23 of gestation (post-parturient), respectively. We did not detect V1b-R and V2-R mRNAs in the pregnant uterus. Therefore, the heterogeneity of OT and AVP receptors in the rat uterus can only be assigned to the presence of OT-R and V1a-R neurohypophysial hormone receptor subtypes, whereas V1b-R and V2-R can not be invoked. Only OT-R mRNA levels change in the uterus near parturition.