GROWTH HORMONE, INSULIN AND SUGAR IN THE BLOOD PLASMA OF BULLS. INTERRELATED DIURNAL VARIATIONS

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Abstract
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 −0.34, P < 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 < 0.001). The possibility of GH involvement in the hour-to-hour metabolic homoeostasis of the animals is discussed.