Influence of moderate cooling (37 degrees C-25 degrees C) on the reactivity of isolated rat tail artery.

The aim of the investigation was to examine the effects of cooling on the tail artery regarding the scarceness of such studies in spite of the essential thermoregulatory role played by this vessel. Segments of the proximal portion were suspended isometrically in medium containing 1.25 mM Ca. Lowering the temperature to 25 degrees C increased the sensitivity and maximum strength of the adrenaline concentration-effect curves. These changes were reversed by warming back to 37 degrees C. Cocaine attenuated the increase of sensitivity without changing the increase of the maximum response. Either the sensitivity and strength of the responses to phenylephrine and serotonin were increased by cooling. Clonidine evoked weak contractions in 18 out of 38 experiments. After cooling, the responses persisted only in 7 arteries and the strength was almost halved. Responses to field electric stimulation at 25 degrees C exhibited a pronounced increase of strength and a small increase of sensitivity. -log Kb for prazosin against adrenaline was increased by cooling (8.7 and 9.1 at 37 degrees C and 25 degrees C, P < 0.01). After partial receptor inactivation using phenoxybenzamine, the dissociation-constant (KA) indicated a moderate affinity for phenylephrine that was not changed by cooling (4.1 and 4.2 x 10(-6) at 37 degrees and 25 degrees C respectively). Receptor reserve and occupancy at EC50 also remained unchanged at 25 degrees C. It can be concluded that: 1) cooling increases the tail artery reactivity, partly as a consequence of the inhibition of adrenergic neuronal uptake; 2) responsiveness to alpha 2-agonists is not involved in the effects of cooling whereas the role of alpha 1-adrenoceptor could not be properly clarified; 3) cooling may facilitate some steps of the contractile activation beyond the agonist-receptor interaction.