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

RESUMO

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 containig 1.25 mM Ca. Lowering the temperature to 25 degrees Celsius increased the sensitivity and maximum strength of the adrenaline concentration-effect curves. These changes were reversed by warming to 37 degrees Celsius. 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 serotomin 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 eletric stimulation at 25 degrees Celsius exhibited a pronounced increase of strength and a small increase of sensitivity. -log Kb for prazosin against adrenaline was encreased by cooling (8.7 and 9.1 at 37 degrees Celsius and 25 degrees Celsius 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 Celsius respectively). Receptor reserve and occupancy at EC(50) also remained unchanged at 25 degrees Celsius. 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 in volved 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.