Diurnal Variations in Plasma Insulin and Growth Hormone Associated with Two Stages of Lactation in High Producing Dairy Cows*

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Abstract
Plasma concentrations of GH, insulin, glucose, and FFA were measured in high yielding dairy cows at periods of peak milk production and later during periods of increased feed intake. During each period, five cows were prepared with indwelling juHgular catheters, and blood was sampled at 10-min intervals for 24 h, followed by hourly sampling for an additional 24 h. During early lactation (30 days post partum), the plasma GH concentration was elevated (13.2 ng/ml) compared to that in later lactation (90 days post partum; 9.8 ng/ml). This increased GH status was due to a greater magnitude of individual secretory spikes (27.3 vs. 20.2 ng/ml) rather than a difference in the frequency of spikes or in baseline plasma levels of GH. As lactation progressed from 30–90 days post partum, milk yield decreased, feed intake increased, and overall plasma concentration of insulin increased (17.3 vs. 29.3 μU/ml),refkecting both an elevated magnitude of hormone secretory spikes (30.2 vs. 55.2 μU/ml) and an elevated baseline concentration (16.8 vs. 27.3 μU/ml). The short term repeatability of overall mean and total 24-h GH and insulin secretion in lactating cows was demonstrated as well as the uniqueness of individual cow GH secretory patterns. Significant differences were not found in either glucose or FFA concentrations between lactation periods. Increased GH and decreased insulin during early lactation are likely to promote the mobilization of adipose tissue stores needed to supplement dietary energy consumption.