Daily Rhythms in Rates of Glomerular Filtration and Cloacal Excretion in Captive and Wild Song Sparrows (Melospiza melodia)

Abstract

We infused polyethylene glycol (PEG) into laboratory-housed song sparrows (Melospiza melodia) via intraperitoneally implanted osmotic minipumps. The plasma concentration of PEG was significantly higher at the end of the dark phase of the photoperiod than at the end of light, indicating a nighttime reduction in the rate of glomerular filtration (GFR, the route of PEG elimination). The rate of cloacal voiding of PEG also varied through the day. The rate was similar to the rate of PEG infusion from the minipump during most of the daylight hours. However, PEG excretion was signifcantly reduced during the dark phase and elevated during the first hours of light, a pattern that resulted from sequestering urine in the lower intestine during the night. To examine patterns of osmoregulation in the field, we also implanted minipumps into free-living sparrows. One to fiue days later we recaptured the birds and measured concentrations of PEG in plasma and urine. As in the laboratory, the GFR (and urine flow rate) tended to be lowest in the early morning and higher later in the day. This nocturnal reduction in GFR and cloacal voiding should contribute to minimizing water losses during the night.