Endokrine Reaktion von Bullen auf einen Stoffwechselbelastungstest

Five AI-bulls (5.5-6 years of age) fitted with permanent vena cava or aorta abdominalis canulae were subjected to an one week lasting food reduction used as a metabolic stress test. Frequent bleedings were performed every 30 minutes for 10 hour periods from 5 days prior to until 35 days after start of the challenge. Reproductive and metabolic hormones (LH, testosterone, growth hormone, insulin and insulin like growth factor-I) were determined by radio-immunoassays. Average concentrations of LH remained unchanged during food withdrawal, increased up to twofold within the following two weeks and returned to precontrol levels thereafter. Mean testosterone started to decrease after the first three days of food reduction and remained on a lower plateau for another four weeks. Growth hormone increased in four bulls during the first twelve days after the challenge, thereafter descending below precontrol values. In the fifth bull the growth hormone levels sank right from the beginning of the food withdrawal phase continuously. Only in three bulls the concentrations of the insulin like growth factpr-I were reduced after the start of the stress test, in the other two bulls the values increased already during the food withdrawal, in order to reach significantly higher levels than the precontrol ones. All bulls showed a very homogenous reaction concerning insulin with an immediate decrease after start of food withdrawal, precontrol levels were reached again within three days after realimentation. As a result there was a significant inverse relation between the relative breeding values for non return and the individual increase of LH after realimentation. The levels of insulin prior to and after the stress test were positively correlated with the breeding values for milk quantity. The endocrine consequences of a one week's metabolic stress lasted more than four weeks, all investigated hormone systems were affected. While a homogenous reaction of LH and insulin could be stated - which, however, depended in the individual range on the relative breeding values for fertility or milk quantity - two bulls showed a different individual reaction concerning testosterone, growth hormone and insulin like growth factor-I. The detailed mechanisms leading to these distinct adaptation results are not yet clear and need further investigations.