Down-Regulation of Prolactin Receptors in Rabbit Mammary Gland: Differential Subcellular Localization

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Abstract
A one-step discontinuous sucrose gradient has been utilized to separate plasma membrane and Golgi fractions in rabbit mammary glands. This procedure produces an enriched plasma membrane fraction (F1) as followed by 5′-AMPase activity and a fraction containing the major portion of the Golgi-rich components, using the enzyme marker galactosyltransferase. There is five times greater specific binding of prolactin compared to insulin in the mammary gland, but the distribution of the two receptors in the various fractions is similar. The intravenous injection of 3 mg ovine prolactin to lactating rabbits leads to an occupation of free, and a down-regulation of total prolactin receptors in the plasma membrane rich fraction (F1) in biopsies removed at the earliest periods after the injection of prolactin, subsequently, followed by a down-regulation of Golgi-associated prolactin receptors. These studies lend support to the view that the down-regulation of prolactin receptors occurs initially at the cell periphery reinforcing the theory of the internalization of the hormone-receptor complexes and their subsequent degradation.

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