Osmoregulation and body fluid compartmentalization in dehydrated heat-exposed pigeons

Osmoregulatory and volume-regulatory responses of heat-acclimated pigeons (Columbia livia) were studied during normal hydration and dehydration combined with heat exposure. Dehydrated heat-exposed pigeons (exposure to 50°C following 48 h of water deprivation; 16-18% mass loss) could recover 97% of their initial body mass within 30 min of free drinking at the end of heat exposure. At the end of heat exposure, body temperature increased by 3°C and hematocrit increased by 12.5%. Serum electrolyte and protein concentrations increased by 33-53% (P < 0.001). Serum osmolality reached an outstanding mean value of 436.7 ± 28.5 mosmol/kg (n = 11), 30.5% higher than the normal mean value. Serum glutamic-pyruvic transaminase and glutamic-oxaloacetic transaminase concentrations did not change during dehydration, suggesting no impairment in circulatory function. Blood urea nitrogen increased sixfold, indicating a total shutdown of the kidney. Relative plasma volume was maintained during dehydration at the expense of extravascular spaces and with a decreased vascular permeability as indicated by the increase in Evans blue-labeled albumin half-life (control, 104 ± 53 min; dehydration, approaching infinity). Altogether, extracellular fluid volume and intracellular fluid volume contributed 53 and 47% of the evaporative water loss, respectively. It is concluded that plasma volume regulation may play an important role in the effective thermoregulatory responses of heat-exposed dehydrated pigeons. This regulation is achieved by preferential shifts of body water reserved among the various body water compartments coinciding with a remarkable tolerance to high osmotic pressures.