Cutaneous water evaporation in the heat-acclimated rock pigeon(Columba Livia) - physiological and biochemical aspects

The most conspicuous phenomenon in the process of heat acclimation of the rock pigeon (Columbia livia) is the remarkable increase in its capacity to evaporate water from its skin. This cooling route becomes the chief thermoregulatory means in the heat-acclimated (HAc) pigeon and is responsible for its ability to maintain normal body temperatures even at extremely high ambient temperatures of 60°C. Since the avian skin lacks sweat glands or any other homologous functional structure, cutaneous water evaporation (CWE) must occur along a different pathway than that known in mammals. The aim of this review is to characterize the CWE mechanism in the rock pigeon from three aspects: the regulatory pathway, the driving force, and the water passageway to the skin surface. CWE is controlled by the adrenergic system at various levels, both peripherally and at higher levels. It was found that nonspecific β-adrenergic inhibition (by propranolol) increases CWE in the HAc pigeon, but not in the non-acclimated (NAc) pigeon. This effect was found to be mainly peripheral, β₂-related Systemic α₂-adrenergic stimulation (by clonidine) also increased CWE. However, this response showed no local effect. Skin blood flow (SkBF) was measured at various ambient temperatures and during CWE-stimulating adrenergic manipulations in the effort to understand the possible role of the cardiovascular system as the driving force in the mechanism. Two different methods were integrated-laser-Doppler flowmetry and ultrasonic flowmetry. As expected, heat exposure (50°C) significantly increased SkBF. This increase was found to be stronger in HAc pigeons. Injection of propranolol increased SkBF in HAc pigeons, but decreased SkBF in NAc pigeons. Injection of clonidine decreased SkBF in both acclimation states. Based on these results, we concluded that SkBF by itself plays no crucial rule in the CWE mechanism. Therefore, apparently other vaso-dynamic changes, apart from SkBF, are involved in CWE. Ultrasonic measurements showed that pharmacologically-induced CWE (propranolol or clonidine) in HAc pigeons is accompanied by an increase in barterial flow, and by a decrease in venous flow. No significant changes (arterial or venous) were found in NAc pigeons. The above results suggest a reduction in arterial resistance along with an increase in venous resistance. This would lead to an increase in capillary hydrostatic pressure, and may therefore serve as the main driving force in the process of CWE. We also hypothesize that in addition to the driving force, there is another component-an adjustable gating mechanism-which functions as a resistance modulator for water movement towards the skin surface.