INVESTIGATIONS INTO THE NATURE OF α₂‐ADRENOCEPTORS IN RAT TAIL ARTERIES

Abstract

In rat isolated perfused tail arteries, dose-response curves were established for the vasopressor effects of phenylephrine (alpha 1-adrenoceptor agonist), clonidine (alpha 1- and alpha 2-adrenoceptor agonist), clonidine in the presence of 10(-7) mol/l prazosin (alpha 2-agonist), and BHT-920 (alpha 2-agonist). The ED50 values were: phenylephrine 1.85 X 10(-10) mol; clonidine 6.3 X 10(-10) mol; clonidine + prazosin 3.2 X 10(-6) mol; BHT-920 6.1 X 10(-6) mol. The arterial reactivity to BHT-920 was stable only after 4-5 h of perfusion. Responses to BHT-920 were not antagonized by yohimbine (alpha 2-adrenoceptor antagonist) but were antagonized by low concentrations of prazosin (alpha 1-adrenoceptor antagonist). These data constitute conflicting evidence regarding the existence of alpha 2-adrenoceptors in rat tail arteries. The data are consistent with the proposal that there are two recognition sites on alpha 1-adrenoceptors; phenylephrine and BHT-920 may stimulate different sites on alpha 1-adrenoceptors.

α2‐ADRENOCEPTORS IN RAT TAIL ARTERY

“…This point notwithstanding, Weiss et al (1983) showed that in Sprague-Dawley rat tail artery preparations (helical strips), steady-state responses to clonidine were antagonized by low concentrations (10-100 nmol/l) of yohimbine, indicating the presence of ar2-adrenoceptors; this was confirmed in binding studies using tritiated clonidine as ligand. These data stand in direct conflict with the finding of Marwood et al (1985) that yohimbine appeared actually to potentiate rather than antagonize the vasoconstrictor effect of clonidine in the same vessel. Marwood et al (1985) explained the potentiating effect of yohimbine as resulting from an underlying time-dependent increase in sensitivity to the vasoconstrictor effects of clonidine.…”

“…These data stand in direct conflict with the finding of Marwood et al (1985) that yohimbine appeared actually to potentiate rather than antagonize the vasoconstrictor effect of clonidine in the same vessel. Marwood et al (1985) explained the potentiating effect of yohimbine as resulting from an underlying time-dependent increase in sensitivity to the vasoconstrictor effects of clonidine. It is likely that such an increase in sensitivity to agonists with time is related to the short period of exposure of the tissue to the agonists, since in previous studies in which agonist responses were allowed to stabilize, no such time-dependent increases in sensitivity were noted (Medgett & Langer 1984; Fig.…”

“…The authors cite two recent papers (Medgett & Langer 1984; Hicks et al 1984) in which positive evidence was obtained for the existence of a2-adrenoceptors on smooth muscle cells in proximal segments of the tail artery of both Sprague-Dawley rats and SHR. The apparent discrepancy between the results of the previous studies and those of Marwood et al (1985) may be explained by some major shortcomings in the methodology used in the study of Marwood et al (1985). The most significant criticism is that equilibrium conditions with regard to agonist concentration were not achieved in the latter study.…”

“…In the case of either an agonist or a competitive antagonist, the free concentration in the external solution should be maintained at a steady level and should be known", to quote from the recent comprehensive review by Kenakin (1984). In the study of Marwood et al (1985) bolus injections of agonists in 0.1 ml of solution were made into the perfusion stream; taking into account agonist dilution and the volume of the vessel segment, a conservative upper estimate for the contact time of the agonist with the luminal surface of the vessel is about 10 s. In the previous studies, on the other hand, as well as allowing the agonist to make contact with both the intra-and extraluminal surfaces, a constant concentration of agonist was allowed to remain in contact with the preparation until the response had stabilized; with nonselective agonists (such as adrenaline and nonadrenaline) and for a1 -selective agonists (such as phenylephrine and methoxamine) this took about 1 min whereas for ar2-selective agonists (such as clonidine and TL99) up to 2 min were required. It is well known that responses to ar2-adrenoceptor agonists develop more slowly than those to al-adrenoceptor agonists in a variety of in vivo and in vitro preparations (see the review by McGrath 1982).…”

“…It should also be noted that the presence of an antagonist may slow the rate at which a response develops as well as reducing its magnitude. Marwood et al (1985) may not have made the wisest choice of agonists for their experiments. Clonidine has been regarded for some time as not being sufficiently selective for a*-adrenoceptors to be of use in receptor subclassification studies (McGrath 1982).…”