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

The effects of selective and cion-selective beta-adrenergic agents on cutan eous water evaporation (CWE) were studied in hand-reared rock pigeons (Colu mba livia). CWE was measured by the vapor diffusive resistance method, usin g a transient porometer. Intramuscular and subcutaneous injections of a non -selective beta-adrenergic antagonist (propranolol) or a selective beta(2)- adrenergic antagonist (ICI-118 551) to heat-acclimated (HAc) pigeons at amb ient temperature (T-a) of 24 degrees C resulted in intensive CWE. The CWE v alues that were triggered by propranolol and ICI-118 551 (18.59 +/- 0.73 an d 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 administra tion of propranolol produced local response. Intramuscular injection of sal butamol (selective beta(2)-adrenergic agonist) to HAc pigeons drastically d iminished CWE induced by either propranolol, metoprolol or heat exposure. S uch manipulations also enhanced panting at relatively low T(a)s (42 degrees C). The inhibition of beta(1)-adrenergic receptors by metoprolol increased CWE, while inhibition by atenolol produced no change from basal values. Th is 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 beta(1)- and beta(2)-adrenergic receptors. We suggest t hat the beta(1)-adrenergic effect is restricted mainly to the CNS, while th e beta 2-adrenergic effect takes place at the effector level. We postulate this level to be either the cutaneous microvasculature or the epidermal lay er. (C) 2000 Elsevier Science Inc. All rights reserved.