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 β 2-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 mg cm −2 h −1). Subcutaneous administration of propranolol produced local response. Intramuscular injection of salbutamol (selective β 2-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 as (42°C). The inhibition of β 1-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 β 1- and β 2-adrenergic receptors. We suggest that the β 1-adrenergic effect is restricted mainly to the CNS, while the β 2-adrenergic effect takes place at the effector level. We postulate this level to be either the cutaneous microvasculature or the epidermal layer.