Fluctuations in Peripheral Serum Testosterone Levels within a Day, with Age and by Sexual Stimulation in Male Beagle Dogs Bred Indoors

Abstract

Fluctuations in serum levels of testosterone (T) within a day, both with age and as a result of sexual stimulation, were examined in male beagle dogs. Eighty male dogs aged between 3 months and 16 years and bred in our laboratory were used under strictly controlled breeding conditions (temperature: 22±1°C, relative humidity: 55±5%, lighting time: 8 : 00 a.m.-8: 00 p.m.) . The level of T was measured by an RIA method. In order to examine the fluctuation in T level within a day, blood samples were obtained at 0 : 00, 6 : 00, 12 : 00 and 18 : 00 in each of five dogs aged 1.7 and 2.1 years. T levels fluctuated with a regular pattern that was lowest at 12 : 00, and increased to a peak at 18 : 00-6 : 00, thereafter decreasing until 12 : 00. In order to examine the change in T level with age, blood samples were obtained at 9 : 00, 12 : 00 and 16 : 00 from 70 dogs aged between 3 months and 16 years. The regular diurnal pattern of T level was usually seen, and the levels at 12 : 00 were always low and did not fluctuate at any age except for 6 months, and 13, 14 and 16 years. The T level at 9 : 00 increased to reach a peak at 4 years, whereas that at 16 : 00 did so at 2 years. T levels at 9 : 00 were significantly higher at 4-12 and 14years than at 3 months, and were higher at 4 years than at 9 months. T levels at 16 : 00 were higher at 2-12 and 14 years than at 3 months, and those at 2, 4 and 12 years were higher than at 6 and 9 months. Upon sexual stimulation with female estrous secretion from the vagina, T levels in male dogs aged 2 years were increased significantly at 3h.