Cooling and α₁- and α₂-adrenergic responses in cutaneous veins: Role of receptor reserve

Experiments were designed to determine the effects of cooling on α₁- and α₂-adrenergic responses in isolated canine cutaneous veins. Rings of saphenous veins were suspended for isometric tension recording in physiological salt solution. Cooling (from 37 to 24°C) augmented contractions to norepinephrine under control conditions and after α₁-adrenergic blockade (prazosin) but not following α₂-adrenergic blockade (rauwolscine). Cooling augmented contractions evoked by the α₂-adrenergic agonists B-HT 920 and UK 14304 but did not affect responses to the full α₁-adrenergic agonist phenylephrine. These experiments suggest that cooling augments α₂-adrenergic responsiveness without affecting α₁-adrenergic responsiveness. However, the contractions evoked by the partial α₁-adrenergic agonist St 587 were virtually abolished by cooling. Moreover, following partial irreversible blockade of the α₁-adrenoceptors with phenoxybenzamine, cooling also reduced the contractions evoked by phenylephrine. Therefore, cooling reduces α₁-adrenergic responsiveness in canine cutaneous veins, but in the case of full α₁-adrenergic agonists such as norepinephrine and phenylephrine the inhibitory effect of cooling is buffered by an α₁-adrenoceptor reserve. With norepinephrine, this permits the potentiating effect of cooling on the α₂-adrenergic component of the response to predominate.