Renal function and plasma levels of arginine vasotocin during free flight in pigeons

We examined urinary water loss and plasma levels of arginine vasotocin (AVT) in free-flying, tippler pigeons trained to fly continuously for up to 5 h. First, we used [³H]polyethyleneglycol ([³H]PEG) as a glomerular filtration marker by implanting an osmotic minipump into each bird. In two flights (10 birds in winter at an ambient temperature of 13-15°C and seven in summer at 23°C), we measured pre-flight (hydrated, resting control birds) and post-flight [³H]PEG activity and osmolality in blood and ureteral urine. For comparison, we measured these variables in 10 birds in winter before and after controlled dehydration (24h at 25 or 30°C). Second, we measured plasma levels of AVT in 6-8 birds before and immediately after each of three different summer flights. Urine osmolality increased significantly by up to three times the control level in both post-flight and dehydrated pigeons; urine:plasma osmolality ratios did not exceed 2. Compared with controls, glomerular filtration rate (GFR) was significantly lower after flight in summer, but did not change in either post-flight or dehydrated winter pigeons. In winter, mean post-flight urine flow rate (UFR) decreased significantly to less than half the control level, while in summer, post-flight UFR did not differ from control levels. In general, mean filtered water reabsorption (Fr(H₂O)) increased from 95 % in controls to 98 % in post-flight and dehydrated birds. Plasma levels of AVT increased after flight to between three and eight times the preflight levels. The data from this first study of kidney function during flight are consistent with previous studies of dehydration in birds and exercise in mammals in which both increased Fr(H₂O) and decreased GFR contribute to renal conservation of water.