Control of arteriovenous anastomoses in rabbit ear model of digital perfusion

Abstract

The arteriovenous anastomoses (AVA) of the cutaneous microcirculation of the hands and feet are fundamental determinants of thermoregulatory blood flow and may be involved in cold intolerance. These direct microvascular studies are an initial characterization of adrenergic receptor subtypes participating in control of AVA in the ears of anesthetized male New Zealand White rabbits. Adrenergic alpha 1-stimulation with phenylephrine produced AVA constriction, whereas terazosin (an alpha 1-antagonist) produced dilation and attenuated the responses to phenylephrine. Adrenergic alpha 2-stimulation with UK-14304 produced constriction of the AVA, whereas atipamezole (an alpha 2-antagonist) produced dilation and attenuated the responses to UK-14304. When equimolar concentrations of antagonists were studied, the AVA dilation produced by alpha 2-blockade was greater than that produced by alpha 1-blockade. Norepinephrine (a mixed alpha 1- and alpha 2-agonist) also produced vasoconstriction, which was attenuated by both prazosin (an alpha 1-antagonist) and atipamezole. In summary, 1) AVA contain a heterogeneous mixture of both alpha 1- and alpha 2-receptors, and 2) alpha 2-receptors may have a greater influence than alpha 1-receptors on overall tone in AVA.

Role of Alpha-1 Adrenoceptor Subtypes Mediating Constriction of the Rabbit Ear Thermoregulatory Microvasculature

“…The present study revealed that phenylephrine induced significant vasoconstriction among arterioles, arteriovenous anastomoses, and venules. The arteriovenous anastomoses were identified as the part of the ear microvasculature most sensitive to extramural aIadrenoceptor stimulation: this is in agreement with prcvious studies (9,16). In arterioles, a significanl rightward shift of the phenylephrine concentrationresponse curve was noted in the ear vasculature treatcd with 5-methyl-urapidil or BMY7378.…”

“…This experiment utilized an acute in vivo preparation of the microvasculature of the rabbit ear as previously described (16). Experimental protocols were approved by the Animal Care and Use Committee of the University and followed the National Institutes of Health guidelines for the care and use of laboratory animals.…”

“…a non-subtype-selective u1 agonist, alone (control) or in the prcsence of specific a,-subtype antagonists. The baseline diameters of the vessels in the ear microvasculature were measured following a 30-minute stabilization period with the ear equilibrating in the tissue bath (16). Phenylcphrine was then added at 5-minute intervals, in sequentially increasing concentrations (10-11-10-4 M ; all drug concentrations described were the final concentrations in the tissue bath), to the tissue bath to establish phenylephrine dose-response curves for the arterioles.…”

“…The vasoconstrictor responses induced by exogenous norepinephrine, norepinephrine-like compounds, or sympathetic activation are mediated by alpha, (a1) and a,-adrenoceptors. Previous studies in our laboratory demonstrated that the resistance arterioles and arteriovenous anastomoses have different adrenoceptor control mechanisms (9,16): a,-adrenoceptors dominate arteriolar constriction, whereas the arteriovenous anastomoses are dually regulated by a, and a2-adrenoceptors (9,16). These findings provide a possible explanation of how blood flow in the ear is distributed between two parallel perfusion circuits to guarantee a sufficient nutritional supply while simultaneously maintaining a constant core body temperature by increasing or decreasing flow in the arteriovenous anastomoses.…”

Effect of cooling on cutaneous microvascular adrenoceptors in vivo in the rabbit ear

“…Following a cold thermal stress, the auricular circulation in normal rabbits is characterized by an overall reduction in blood flow in the ear and cutancous perfusion similar to that observed in human digits (22). Recent in vivo studies of the rabbit have identified specific receptor innervation of the ear microvasculature, with a,-adrenoceptor control predominating over arteriovenous anastomosis regulation (18). Therefore, it was hypothesized that in the rabbit car, cold exposure would decrcase the arteriovenous-shunt flow to a greater cxtent than nutritional flow by enhancing the microvascular a2-adrenoceptor response to norepinephrine.…”