Influences of season and of temperature, photoperiod, and subcutaneous melatonin infusion on the glomerular filtration rate of ewes

Abstract

Influences of season and of temperature, photoperiod, and subcutaneous melatonin infusion on glomerular filtration rate (GFR) were measured in ewes. There was a seasonal difference of GFR between summer (June-August) and winter (December-February) in Tokyo (35 degrees, 35' N); GFR was significantly (P < 0.05) higher in summer (4.2 +/- 0.3 ml/min/kg) than in winter (3.0 +/- 0.2 ml/min/kg). GFR was measured after exposure to three types of photoperiod, 24L:0D, 12L:12D, and 0L:24D, for 7 to 8 days. The value for GFR obtained at 20 degrees C was significantly lower (P < 0.05) with 0L:24D than with the other two photoperiodic conditions. GFR obtained during subcutaneous melatonin infusion (20 micrograms/hr for 16 hr/day for 7 days) with 24L:0D conditions was significantly (P < 0.05) lower (2.5 +/- 0.1 ml/min/kg) than without infusion (3.8 +/- 0.3 ml/min/kg) and was about the same as that obtained in animals under 0L:24D conditions. At 30 degrees C, GFR exhibited no difference between the 3 photoperiodic conditions and was always lower than that found at 20 degrees C. Possible influences of melatonin on GFR are discussed.