Desmopressin-induced dog ciliary artery relaxation

The effect of vasopressin on choroidal blood flow, intraocular pressure, and orbital venous pressure in rabbits

PURPOSE

To investigate the effects of arginine-vasopressin (AVP) on intraocular pressure (IOP), orbital venous pressure (OVP), and choroidal blood flow (ChorBF) regulation in anesthetized rabbits.

METHODS

Mean arterial pressure (MAP), IOP, and OVP were measured by direct cannulation of the central ear artery, the vitreous, and the orbital venous sinus, respectively. Laser Doppler flowmetry was used to record ChorBF. To change the perfusion pressure (PP), MAP was manipulated mechanically with occluders around the aorta and vena cava. In the first group of animals (n = 11) the dose-response relationship was measured. In the second group of animals (n = 8) pressure-flow relationships were determined at baseline and in response to intravenous application of a low (0.08 ng/kg/min) and a high (1.33 ng/kg/min) infusion rate of AVP.

RESULTS

AVP caused a dose-dependent increase of MAP and choroidal vascular resistance (ChorR), whereas IOP, OVP, ChorBF, and heart rate (HR) were decreased. In contrast to the high infusion rate, the low infusion rate of AVP had no effect on baseline ChorBF. However, the pressure-flow relationship was shifted downward significantly by both infusion rates at PP below baseline.

CONCLUSIONS

AVP reduces IOP and OVP significantly and is a potent vasoconstrictor in the choroidal vascular bed. In the choroid, the effect of AVP is not only dose-dependent, but also PP-dependent, which is indicated by the reduced perfusion relative to control with low-dosed AVP at low PP.

A comparison between haemodynamic effects of vasopressin analogues

Some analogues of arginine vasopressin (AVP) reportedly possess hypotensive properties, and two such peptides are Cys(1)-Tyr(2)-Phe(3)-Val(4)-Asn(5)-Cys(6)-Pro(7)- d-Arg(8)-Gly(9)-NH(2) (VD-AVP) and d(CH(2))(5)-Cys(1)- d-Tyr(Et)(2)-Arg(3)-Val(4)-Asn(5)-Cys(6)-Lys(7)-Lys(8)-ethylenediamine(9) (TA-LVP). In the present investigation we examined the effects of TA-LVP (0.3, 1.0 and 3.0 microg/kg/min), VD-AVP (0.3, 1.0 and 3.0 microg/kg/min) and AVP (1.0, 3.0, 10 ng/kg/min) on haemodynamics, blood volume (BV) and plasma troponin levels in anaesthetised rats. Infusion of TA-LVP significantly ( P<0.05) reduced blood pressure (-45+/-3%; n=8; mean +/- SEM), mean circulatory filling pressure ( P(mcf); -41+/-3%), and cardiac output (CO; -59+/-4%). The reduction in CO at a lower dose of TA-LVP was due to reduced venous tone, while at higher doses the reduction was predominantly the result of reduced BV (-35+/-4%). The large decrease in BV during the infusion of TA-LVP, substantially increased resistance to venous return (50+/-11%), which was the main contributor in reducing CO. Administration of AVP significantly increased blood pressure (41+/-4%) and arterial resistance (98+/-16%) without any impact on P(mcf) and BV, while significantly reducing CO (-26+/-5%). Infusion of VD-AVP did not produce hypotension, but produced a modest but significant reduction in CO (-18+/-5%) and insignificant but moderate increases in peripheral resistance (30+/-12%) and resistance to venous return (28+/-8%). Plasma troponin levels were not affected by any of the peptides. The hypotensive action of TA-LVP was due to a reduction in CO as a result of a reduced pre-load, while the pressor effect of AVP increased after-load sufficiently to impede flow, reducing CO. VD-AVP was devoid of any hypotensive effects, suggesting that V(2)-vasopressin receptors are most likely to play a limited role in the control of cardiac and vascular function in these animals.

Vasopressin V2 (SR121463A) and V1a (SR49059) receptor antagonists both inhibit desmopressin vasorelaxing activity

Although [Arg(8)]vasopressin is a potent vasoconstrictor, it possesses vasorelaxant properties manifested either after vasopressin V1 receptor blockade or directly in some vascular beds. The nature of the receptor involved in the vasorelaxant effect of [deamino-Cys(1) D-Arg(8)]vasopressin (desmopressin), a vasopressin V2 receptor agonist, was studied on rat precontracted aortic rings by the use of highly selective new non-peptide vasopressin receptor antagonists. The present study demonstrates for the first time that desmopressin relaxant effect is antagonized by the vasopressin V2 receptor antagonist SR121463A, but also by the vasopressin V1A receptor antagonist SR49059, suggesting that desmopressin-induced relaxation is mediated by a receptor subtype sharing both V1A and V2 pharmacological profiles.

Functional role of nitric oxide in regulation of ocular blood flow

Nitric oxide generated by three distinct enzyme systems appears to play a critical role in many diverse physiological processes. Using both conventional and immunohistochemical techniques, nitric oxide synthases have been identified throughout the body, including all regions of the eye. A large number of in vitro and in vivo preparations have been utilized showing nitric oxide to have an important role in regulation of regional ocular blood flow. Nitric oxide-mediated control of basal ocular blood flow is demonstrated by vasoconstriction seen in experiments where vascular endothelial cells are removed, or when nitric oxide synthase is inhibited. The endogenous source of nitric oxide in the eye appears to be both endothelial and neural. In addition, administration of drugs that can 'donate' nitric oxide produces vasodilation of the eye vasculature. Local vasodilation in response to illumination of the retina is controlled by generation and release of nitric oxide, whereas most other physiological adjustments of ocular blood flow (i.e., autoregulation and responses to altered blood gas levels) seem to be relatively independent of nitric oxide mechanisms. Nitric oxide is implicated in a variety of ocular pathophysiological states including uveitis, retinal ischemic disease, diabetes and glaucoma.