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ORIGINAL RESEARCH ARTICLE

Prolactin, Cortisol, and Insulin Regulation of Nucleoside Uptake Into Mouse Mammary Gland Explants

Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan 48201

Nucleosides are essential components of milk that are used for the nourishment of newborns. Effects of the three primary lactogenic hormones, including prolactin (PRL), insulin (I), and cortisol (H), on nucleoside uptake and incorporation into cultured mammary tissues taken from 12- to 14-day pregnant mice were determined; most experiments focused on the regulation of uridine uptake. Insulin alone, as well as PRL in the presence of insulin and cortisol, was shown to stimulate uridine uptake and incorporation into RNA in mammary explants taken from 12- to 14-day pregnant mice. The PRL effects were expressed at concentrations of 25 ng/ml and above, which are physiological plasma concentrations. In the absence of sodium, uridine uptake and incorporation were diminished, suggesting the presence of a sodium-dependent uridine transporter. In kinetic studies the apparent K_m for uridine uptake was calculated to be 312 µM, and the V_max 2.90 µmol/hr/L cell water; PRL had no effect on the K_m but increased the V_max to 5.88 µmol/hr/L cell water. When assessing uridine uptake in the presence of the other nucleosides at 0.1 mM, only cytidine competed with uridine uptake. The fact that distribution ratios of greater than 15:1 were achieved with uridine indicates that uridine uptake may be via an active transporter. These studies show that PRL enhances uridine update in mammary tissues by stimulating the activity, and probably synthesis, of a sodium-dependent, active uridine and cytosine transporter.

Key Words: prolactin • cortisol • insulin • mammary gland • nucleic acids • uridine • adenosine

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