Neural activation of alpha-2 adrenoceptors in cat cutaneous vasculature.
The present study was designed to assess the relative contribution of postjunctional alpha-1 and alpha-2 adrenoceptors in neurally evoked cutaneous vasoconstrictor responses in anesthetized cats. Preganglionic stimulation of the thoracic sympathetic nerve trunk produced an intensity-related decrease of digital skin blood flow as measured by laser-Doppler flowmetry. Sympathetic-cholinergic sudomotor (electrodermal) responses were recorded simultaneously as an additional index of neuronal activation. Vasoconstrictor responses were not altered by pretreatment with i.v. propranolol (1 mg/kg) or atropine (1 mg/kg) and were refractory to low doses (2 mg/kg) of hexamethonium. As expected, atropine abolished evoked skin potential responses which were also sensitive only to higher doses (20 mg/kg) of hexamethonium. Pretreatment with either phentolamine (1 mg/kg i.v.), yohimbine (0.5 mg/kg i.v.) or prazosin (0.3 mg/kg i.v.) produced significant reduction of sympathetic vasoconstriction with blockade by phentolamine and yohimbine being far greater than that seen with prazosin. In animals previously given prazosin (0.3 mg/kg), subsequent administration of yohimbine (0.5 mg/kg) almost totally blocked the remaining evoked cutaneous vasoconstriction. These results demonstrate that both alpha-1 and alpha-2 adrenoceptors are present in the cutaneous bed of the cat with the predominant effect mediated by innervated alpha-2 adrenoceptors.