KINETIC-STUDIES AND PRODUCTION-RATE OF MELATONIN IN PONY MARES

The aims of the present study were to determine basic kinetic parameters and the nycthemeral production rate of melatonin in the horse. Seven pony mares were used for the kinetic studies. Five other pony mares were used under long and short days for the production rate studies. Melatonin was administered by intravenous, oral, and intragastric routes at different dose levels. The plasma melatonin clearance was 1.02 +/- 0.31 l . kg(-1). h(-1), and the volume of distribution was 0.89 +/- 0.53 l/kg for the 0.4 mu g/kg melatonin dose. The systemic availability after oral and intragastric administration was consistently low (less than or equal to 10%). The hourly melatonin production rates during short (36 +/- 33.6 ng . kg(-1). h(-1)) and long nights (42 +/- 24.0 ng . kg(-1). h(-1)) were not significantly different. It is concluded that the oral dose of exogenous melatonin required to control reproductive pattern in horses would need to be 10 times higher than the nocturnal melatonin production (0.5 mu g/kg) to obtain a physiological plasma melatonin concentration.