Abstract
Summary
Serum GH concentrations were determined in mammary arterial and venous samples from six early (5-12 weeks postpartum) and six late (37-57 weeks postpartum) lactating Holstein cows. Basal serum GH averaged 4.0 ng/ml in cows during early and late lactation. Treatment of five cows in early lactation with 80 mg CB-154 or five cows in late lactation with 80 mg ergocryptine was without effect on basal serum GH concentrations. Administration of 200μg TRH to early or late lactating cows increased GH concentrations to maxima of 18 and 14 ng/ml at 12 min post-injection, respectively. Serum GH of cows in early lactation treated with CB-154 and cows in late lactation treated with ergocryptine, averaged 25 and 15 ng/ml respectively, 12 min after administration of 500 μg TRH. Mammary arterial concentrations of GH were greater than venous concentrations only after being acutely increased by TRH. We conclude, that in comparison with late lactating cows, early lactating cows tended to release more GH and mammary uptakes of GH were greater in response to TRH. These responses were unaffected by either CB-154 or ergocryptine. Mammary arteriovenous differences in GH occurred when serum concentrations were acutely increased.
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