RESUMO
Many adult mammals and birds respond to high surrounding temperatures with thermal tachypnea - an increase in breathing frequency accompanied by shallow tidal volume, with minimal increase in oxygen consumption (VÌO2 ). This pattern favors heat dissipation by evaporative water loss (EWL) through the respiratory tract. We asked to what extent this response was apparent at the earliest stages of development, when pulmonary ventilation initiates. Measurements of pulmonary ventilation (VÌE; barometric technique), VÌO2 (open-flow methodology) and EWL (water scrubbers) were performed on chicken embryos at the earliest appearance of pulmonary ventilation, during the internal pipping stage. Data were collected, first, at the normal incubation temperature (37.5°C); then, ambient and egg temperatures were increased to approximately 44°C over a 2â h period. Other embryos of the same developmental stage (controls) were maintained in normothermia for the whole duration of the experiment. During heat exposure, the embryo's VÌO2 and carbon dioxide production increased little. In contrast, VÌE more than doubled (â¼128% increase), entirely because of the large rise in breathing frequency (â¼132% increase), with no change in tidal volume. EWL did not change significantly, probably because, within the egg, the thermal and water vapor gradients are almost nonexistent. We conclude that chicken embryos respond to a major heat load with tachypnea, like many adult mammals and birds do. Its appearance so early in development, although ineffective for heat loss, signifies that thermal tachypnea represents an important breathing response necessary to be functional from hatching.
Assuntos
Embrião não Mamífero/fisiologia , Resposta ao Choque Térmico , Taquipneia , Animais , Dióxido de Carbono/metabolismo , Embrião de Galinha , Consumo de Oxigênio , Ventilação Pulmonar/fisiologia , Volume de Ventilação PulmonarRESUMO
We investigated whether or not the preferred ambient temperature (Tapref) of the 1-day old chicken hatchling, a precocial neonate with excellent locomotory capacity, clearly identifiable thermogenesis and independence from maternal care, coincides with the lower critical temperature (LCT) of thermoneutrality and minimal oxygen consumption (VÌ(O(2))). Tapref of single chicks measured in a thermocline (N=16) averaged 33.5±0.3 °C (mode, 33.3±0.4 °C). The same value was obtained in hatchlings studied in pairs. LCT was computed from the ambient temperature (Ta)-VÌ(O(2)) relationship, constructed by slowly decreasing the Ta of a respirometer from 38 to 29 °C over 2.5h, while continuously measuring VÌ(O(2)) by an open-flow methodology; LCT averaged 36.4 °C±0.3 or 36.8 °C±0.4, depending on the method of computation. In all hatchlings Tapref was lower than LCT (P<0.001), by a magnitude that depended on the method of computation of the two variables, 2.8 °C±0.3 (P<0.001) or 3.9 °C±0.5. The Tapref-LCT difference implied that, at Tapref, VÌ(O(2)) was higher than at thermoneutrality. We conclude that in the chicken hatchling thermal preference does not coincide with thermoneutrality, probably because during development what seems optimal from a thermoregulatory viewpoint may not necessarily be so for other regulatory functions.