Distribution of neurally activated postjunctional adrenoceptors in cat forelimb vasculature

We assessed the relative contribution of postjunctional alpha-adrenoceptor subtypes in neurally evoked vasoconstrictor responses in the forelimb of anesthetized cats. Preganglionic stimulation of the thoracic sympathetic nerve trunk produced frequency-related decreases in blood flow of the entire forelimb as measured by ultrasonic flowmetry as well as vasoconstriction in the digital cutaneous bed as measured by laser-Doppler flowmetry. Vasoconstrictor responses were not altered significantly by intravenous (i.v.) treatment with propranolol (1 mg/kg) or atropine (1 mg/kg). In the entire limb, prazosin, (3-100 micrograms/kg i.v.) was a more potent antagonist of neurally evoked responses as compared with rauwolscine. In contrast, rauwolscine (10-300 micrograms/kg i.v.) was a more effective antagonist in the cutaneous bed. Combined treatment with both prazosin and rauwolscine was far more effective than either antagonist given alone in blocking vasoconstriction regardless of the measurement site. Moreover, basal cutaneous blood flow was increased by rauwolscine but not by prazosin. These results suggest that both subtypes of postsynaptic alpha-adrenoceptors are activated by sympathetic nerve stimulation. In the cutaneous bed, alpha 2-adrenoceptors appear to predominate. In addition, cutaneous vascular tone also appears to be regulated by hormonal alpha 2-adrenoceptor activation in cats.