Growth hormone, insulin and sugar in the blood plasma of bulls. Interrelated diurnal variations

Plasma growth hormone (GH) of eight young, sexually mature, pedigree bulls, observed at hourly intervals, varied during the day in a manner indicating intermittent secretion in peaks or bursts. The diurnal GH averages were about 10 ng/ml. GH averages for 2-3 h intervals showed minima following or during the periods of morning and afternoon feeding. A third minimum occurred between 10 and 12 p.m. Peak activity, estimated by the frequency of GH values greater than 10 ng/ml was significantly reduced during two of these low-GH-periods (afternoon and late night). The minima in GH followed after (morning) or coincided with (afternoon) maxima in plasma insulin (two materials, GH/insulin, 11 a. m.-10 p.m.: r=-0.31 and -3.34, P less than 0.01). This means that the two hormones behaved after food intake much in the same ways as in man in spite of the fact that plasma sugar decreased after feeding (GH/sugar, 11 a.m.-10 p.m.: r=0.27, two materials combined, P less than 0.001). The possibility of GH involvement in the hour-to-hour metabolic homoeostasis of the animals is discussed.