Absence of circadian rhythm in the disposition of melatonin in the cow.

Five adult pasture-bred Prim Holstein cows were used to document the time invariance of melatonin disposition in the cow. Melatonin was administered by an intravenous route as a bolus (5 micrograms/kg of body weight) at either 1430 or 2230. Data were analysed according to a three-compartment open model with melatonin elimination from the central compartment. The mean (+/- SD) kinetic parameters describing melatonin disposition, during day and night, respectively, were plasma clearances: 0.0267 +/- 0.0084 l/kg per min and 0.0229 +/- 0.0031 l/kg per min; steady state volume of distribution: 0.826 +/- 0.229 l/kg and 0.780 +/- 0.193 l/kg; terminal half lives: 58.9 +/- 23.7 min and 64.1 +/- 33.7 min; mean residence times: 32.0 +/- 8.8 min and 33.9 +/- 6.2 min. No statistical difference was found between diurnal and nocturnal values for these parameters. These results support the concept of time invariance for melatonin kinetics in cattle and validate the use of diurnal experiments for the study of melatonin kinetics.