The role of β-adrenergic receptors in the cutaneous water evaporation mechanism in the heat-acclimated pigeon (Columba livia)

The effects of selective and non-selective β-adrenergic agents on cutaneous water evaporation (CWE) were studied in hand-reared rock pigeons (Columba livia). CWE was measured by the vapor diffusive resistance method, using a transient porometer. Intramuscular and subcutaneous injections of a non-selective β-adrenergic antagonist (propranolol) or a selective β₂-adrenergic antagonist (ICI-118 551) to heat-acclimated (HAc) pigeons at ambient temperature (T(a)) of 24°C resulted in intensive CWE. The CWE values that were triggered by propranolol and ICI-118 551 (18.59±0.73 and 16.48±0.70 mg cm^-2 h^-1, respectively) were close to those induced by heat exposure (17.62±1.40 mgcm^-2h^-1). Subcutaneous administration of propranolol produced local response. Intramuscular injection of salbutamol (selective β₂-adrenergic agonist) to HAc pigeons drastically diminished CWE induced by either propranolol, metoprolol or heat exposure. Such manipulations also enhanced panting at relatively low T(a)s (42°C). The inhibition of β₁-adrenergic receptors by metoprolol increased CWE, while inhibition by atenolol produced no change from basal values. This difference may be attributed to their distinctive nature in penetrating the blood-brain barrier. Our findings indicate a regulatory pathway for CWE consisting of both β₁- and β₂-adrenergic receptors. We suggest that the β₁-adrenergic effect is restricted mainly to the CNS, while the β₂-adrenergic effect takes place at the effector level. We postulate this level to be either the cutaneous microvasculature or the epidermal layer. Copyright (C) 2000 Elsevier Science Inc.