Effects of temperature on α adrenoceptors in limb veins: Role of receptor reserve

In cutaneous veins of the dog, cooling augments the response to sympathetic nerve stimulation and exogenous norepinephrine (NE). The postjunctional α adrenoceptors in this blood vessel belong to both the α₁ and α₂ subtypes. Cooling augments α₂-adrenergic responses (presumably because of an increased receptor affinity), but depresses α₁-adrenergic responses (presumably because of a direct inhibitory effect on the contractile process). When agonists of high efficacy such as NE or phenylephrine are used, an α₁ adrenoceptor reserve is present that buffers the response from the inhibitory effect of cooling. This allows the potentiating effect of cold on the α₂-adrenergic component of the response to catecholamines to predominate, and the contractile response to exogenous NE and sympathetic nerve stimulation is augmented. By contrast, in deep veins of the limb, cold reduces the contractions evoked by α₁- and α₂-adrenergic activation. This can be explained best by the absence of a receptor reserve for α₁-adrenergic agonists of high efficacy, combined with a reduced density of postjunctional α₂ adrenoceptors.