The effect of α₂-adrenergic receptors on cutaneous water evaporation in the rock pigeon (Columba livia)

The role of β-adrenergic receptors in regulating cutaneous water evaporation (CWE) in the rock pigeon (Columba livia) is well documented. Here, we studied the involvement of the α₂-adrenergic receptors in this cooling mechanism of the heat-acclimated (HAc) pigeon. Systemic α₂-adrenergic activation [clonidine, 50 μg kg^-1, intramuscular (i.m.)] was found to increase CWE in heat-acclimated pigeons at an ambient temperature (T_a) of 25°C. Subcutaneous administration of the drug had no significant effect. Preinjection of an α₂- adrenergic antagonist (yohimbine, 10 mg kg^-1, i.m.) completely prevented clonidine-induced CWE and attenuated propranolol-induced CWE by 53%. Pretreatment with a β-adrenergic agonist (isoproterenol, 4 mg kg ^-1, i.m.) abolished the effect of clonidine. None of the above treatments was found to elicit significant CWE in nonacclimated (NAc) pigeons. These findings, in addition to previously reported data, indicate a complex regulatory pathway of CWE in the heat-acclimated pigeon consisting of α₂- and β₂-adrenergic receptors. The possible hierarchical pattern of these receptors is discussed.