Cooling and alpha 1- and alpha 2-adrenergic responses in cutaneous veins: role of receptor reserve

Experiments were designed to determine the effects of cooling on alpha 1- and alpha 2-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 degrees C) augmented contractions to norepinephrine under control conditions and after alpha 1-adrenergic blockade (prazosin) but not following alpha 2-adrenergic blockade (rauwolscine). Cooling augmented contractions evoked by the alpha 2-adrenergic agonists B-HT 920 and UK 14304 but did not affect responses to the full alpha 1-adrenergic agonist phenylephrine. These experiments suggest that cooling augments alpha 2-adrenergic responsiveness without affecting alpha 1-adrenergic responsiveness. However, the contractions evoked by the partial alpha 1-adrenergic agonist St 587 were virtually abolished by cooling. Moreover, following partial irreversible blockade of the alpha 1-adrenoceptors with phenoxybenzamine, cooling also reduced the contractions evoked by phenylephrine. Therefore, cooling reduces alpha 1-adrenergic responsiveness in canine cutaneous veins, but in the case of full alpha 1-adrenergic agonists such as norepinephrine and phenylephrine the inhibitory effect of cooling is buffered by an alpha 1-adrenoceptor reserve. With norepinephrine, this permits the potentiating effect of cooling on the alpha 2-adrenergic component of the response to predominate.