Suppression of clonidine-induced vasoconstriction by cooling

Using the cannula inserting method, we investigated whether vascular responses to norepinephrine, phenylephrine, clonidine, tyramine and KCl were altered by cooling (37 degrees C to 27 degrees C) in isolated canine ear arteries. Vasoconstrictor responses to norepinephrine, phenylephrine and tyramine were slightly depressed or unchanged, whereas those to clonidine and KCl were significantly suppressed by cooling. It is suggested that activation of Ca channels via alpha-2 adrenoceptors may be depressed by cooling in dog ear arteries.