Cooling augments alpha 2-adrenoceptor-mediated contractions in rat tail artery

Experiments were performed to assess the effects of acute moderate cooling on postjunctional alpha 1- and alpha 2-adrenoceptors in isolated rings of tail arteries from male Sprague-Dawley rats. Rings were contracted with norepinephrine (NE; 10(-9) to 10(-4) M) alone or in the presence of prazosin (Pz; 3 x 10(-7) M) or rauwolscine (Rw; 10(-7) M). NE concentration-response curves were inhibited by alpha 1-blockade (Pz) but not significantly affected by alpha 2-blockade (Rw). In all rings, cooling caused an increase in the slope of the dose-response curve and a significant increase in the concentration of agonist required to evoke contractions, as assessed by that concentration of NE required to evoke a contraction equal to 10% of maximal (EC10). Cooling inhibited contractions evoked by the selective alpha 1-adrenergic agonist phenylephrine (PE) as assessed by EC10 but had no significant effect on the weak contractions elicited by the selective alpha 2-adrenergic agonist B-HT 920. Prior elevation of tone with either KCl or prostaglandin F2 alpha enhanced alpha 2-mediated contractions. These contractions were augmented by cooling, whereas those caused by either KCl or prostaglandin F2 alpha alone were not significantly affected. Our results suggest that alpha 2-adrenoceptor-mediated responses in this blood vessel are dependent on the level of preexisting tone and are potentiated by cooling.