Effect of Bovine Somatotropin on the Distribution of Immunoreactive Insulin-Like Growth Factor-I in Lactating Bovine Mammary Tissue

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Abstract
The distribution pattern of immunoreactive insulin-like growth factor-I in normal lactating bovine mammary tissue and in tissue obtained after bovine somatotropin treatment was determined by indirect immunofluorescence. In normal tissue, insulin-like growth factor-I immunoreactivity was observed almost exclusively associated with stromal elements. Intralobular stromal cells, small blood vessels, and capillaries all expressed moderate to high immunoreactivity. In contrast, mammary epithelial cells displayed only sparse cytoplasmic immunoreactivity. Immunoreactive material was also present in the periductular connective tissue area, possibly asociated with the basal plasma membrane of epithelial cells. Somatotropin treatment of animals resulted in elevated serum insulin-like growth factor-I concentrations and altered the distribution of insulin-like growth factor-I-stainable material in mammary tissue. After somatotropin treatment, immunoreactivity was still detected in mammary stroma; however, prominent staining was also observed in the cytoplasm of mammary epithelial cells. Given the possible role of insulin-like growth factor-I in the regulation of bovine mammary epithelial cell growth and function, our findings raise the possibility that somatotropin may induce insulin-like growth factor-I production in mammary tissue, or other tissues, to influence indirectly the growth or function of the epithelial cells. This offers a possible mechanism for bovine somatotropin stimulation of lactation.

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