Effect of somatotropin and dietary protein concentration on hormone and metabolite responses to single injections of hormones and glucose.

Four lactating Holstein cows averaging 155 d postpartum were used in a crossover design to examine the influence of somatotropin administration (33 mg/d for 28 d) and dietary CP (11 vs. 16% CP) concentration on physiological responses to single intravenous injections of glucagon, insulin, glucose, and somatotropin. Dietary CP was without influence on plasma hormone or metabolite responses to the challenges but increased basal concentrations of FFA. Basal concentrations of glucagon, glucose, and insulin were not significantly increased by somatotropin treatment; however, there was a tendency for insulin concentrations to be higher in somatotropin-treated cows. Somatotropin treatment increased basal concentrations of somatotropin and FFA. Increases in glucose concentrations after glucagon challenge occurred more quickly, reached a greater peak, and lasted longer for cows treated with somatotropin than for control cows. Insulin concentrations were decreased after somatotropin challenge in control cows, but not in somatotropin-treated cows. Responses of metabolites and hormones to glucose or somatotropin challenges were not influenced by somatotropin treatment. Changes in the metabolism of glucose and insulin suggest that cows treated with somatotropin have a greater ability to synthesize and conserve glucose than control cows. Data obtained support the concept of the homeorhetic action of somatotropin.