The effect of warming on adrenergic neurotransmission in canine cutaneous vein

The effect of warming on adrenergic neurotransmission was examined in canine cutaneous veins. Isometric tension was recorded from rings of saphenous veins of the dog in organ chambers filled with physiological salt solution. During contractions caused by potassium or prostaglandin F(2α), warming from 37 to 41° C caused an augmentation. During contractions caused by stimulation of the adrenergic nerves, and by exogenous norepinephrine, warming caused a relaxation. The relaxation with warming was not altered by the β-adrenergic antagonists, propranolol, or by inhibitors of extraneuronal and neuronal uptake of norepinephrine. During contractions evoked by the α₂-adrenergic agonists, α-methyl norepinephrine and B-HT 920, warming caused a relaxation, whereas during contractions due to the α₁-adrenergic agonists, cirazoline, methoxamine, ST 587, and phenylephrine, it caused an augmentation. The relaxation caused by warming during norepinephrine-induced contractions was prevented by the preferential α₂-antagonists yohimbine and rauwolscine, but not by the preferential α₁-antagonist, prazosin. In strips of saphenous vein incubated with [³H]norepinephrine, warming did not affect the release of labeled transmitter evoked by nerve stimulation. These experiments indicate that warming directly enhances contractility of vascular smooth muscle, while depressing the responsiveness of cutaneous vessels to sympathetic nerve activation by a selective inhibitory effect on postjunctional α₂-adrenoceptors. Relaxation with warming is greater during nerve stimulation than during administration of exogenous norepinephrine, which may be due to a predominance of postjunctional, α₂-adrenoceptors in the neuromuscular junction.