Differential growth hormone responses to glucose and growth hormone releasing hormone in lactating dairy cows

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Abstract
Dairy cows in early lactation were reported to secrete growth hormone in response to declining glucose concentrations at day 5 but not day 30 post-partum, whereas GH responses to TRH were reported to be enhanced after day 30 post partum. The present study examined GH response following glucose infusion and the effect of GHRH as well as effects of SRIH on GHRH-stimulated GH release. Declining plasma glucose concentrations after glucose infusion stimulated GH release at day 5 post partum but not in nonpregnant, nonlactating cows or in cows at days 30 and 90 post partum. GHRH stimulated GH release on all days tested, but the response was highest at day 30 post partum when compared to other days. Somatostatin infusion inhibited GHRH effects on GH concentrations only at day 30 post partum and in nonpregnant, nonlactating cows. Thus, a differential response of the GH regulatory system could be demonstrated between days 5 and 30 post partum utilizing different stimuli. Evaluation of plasma glucose and free fatty acid concentrations on days 5, 10, 20, and 30 post partum revealed a progressive decrease in FFA but not glucose as lactation progressed. Decreased plasma FFA concentrations were paralleled by a decrease in basal GH, somatomedin-C and epinephrine. Thus, a decline in FFA may be responsible for the disparity between effects of GHRH and glucose on GH release between days 5 and 30 post partum.