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

Abstract

The .alpha. adrenoceptor-mediated vasoconstriction in isolated perfused tail arteries from spontaneously hypertensive (SHR) and age matched Wistar Kyoto (WKY) normotensive rats was 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. 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 potent antagonist of the responses induced by periarterial field stimulation and by methoxamine. Apparently, .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.