Smooth muscle alpha-2 adrenoceptors mediate vasoconstrictor responses to exogenous norepinephrine and to sympathetic stimulation to a greater extent in spontaneously hypertensive than in Wistar Kyoto rat tail arteries.

The alpha adrenoceptor-mediated vasoconstriction in isolated perfused tail arteries from spontaneously hypertensive (SHR) and age matched Wistar Kyoto (WKY) normotensive rats has been examined. Responses induced by periarterial field stimulation, exogenous norepinephrine or the selective alpha-1 adrenoceptor agonist methoxamine were preferentially antagonized by prazosin in both SHR or WKY tail arteries. However, in SHR only, the alpha-2 adrenoceptor antagonist idazoxan (RX 781094) at low concentrations, significantly antagonized responses to periarterial field stimulation and to exogenous norepinephrine. Except at rather high concentrations, idazoxan was inactive as an antagonist of responses induced by methoxamine. The alpha-1 adrenoceptor blocking agent prazosin was a very potent antagonist of the responses induced by periarterial field stimulation and by methoxamine. These results indicate that alpha-2 adrenoceptors predominate in both SHR and WKY tail arteries, but a significant subpopulation of smooth muscle alpha-2 adrenoceptors is present in tail arteries of SHR but not of WKY rats. In contrast to WKY normotensive rats, postjunctional alpha-2 adrenoceptors may also be involved in the vasoconstrictor responses to sympathetic nerve stimulation in tail arteries of SHR.