Sympathetic purinergic vasoconstriction and thermosensitivity in a canine cutaneous vein

In canine cutaneous veins, cooling augments the contractile responses evoked by sympathetic nerve stimulation despite a cooling-induced reduction in the release of norepinephrine. With exogenous norepinephrine, the increased responsiveness observed during cooling results from enhanced sensitivity of the postjunctional alpha-2 adrenoceptors. The present experiments were performed to analyze the mechanism of the increased neurogenic response during cooling. Rings of canine saphenous vein were suspended for isometric tension recording in organ chambers filled with modified Krebs-Ringer bicarbonate solution, gassed with 95% O2-5% CO2. Cooling (from 37-24 degrees C) increased the contractile response evoked by nerve stimulation under control conditions, after alpha-1 adrenergic blockade with prazosin, alpha-2 adrenergic blockade with rauwolscine or the combination of both antagonists. The influence of cooling to enhance the neurogenic response was inhibited only by combined alpha adrenergic and purinergic-receptor blockade (alpha,beta-methylene ATP). Electrical stimulation failed to evoke a contractile response (either at 37 or 24 degrees C) in the presence of tetrodotoxin or after acute sympathetic denervation with 6-hydroxydopamine. alpha,beta-Methylene ATP abolished the contractile response evoked by ATP but did not affect the concentration-effect curves to alpha-1 (phenylephrine) or alpha-2 (UK 14,304) adrenergic stimulation. Cooling augmented the contractile responses evoked by ATP. The results suggest that ATP released from sympathetic neurons in the vessel wall contributes to the cooling-induced augmentation of contractile responses to sympathetic nerve stimulation in canine cutaneous veins. This may explain the increased prominence of purinergic mechanisms in cutaneous blood vessels.