Effect of prolactin on 86Rb+ uptake, potassium content and [G-3H]ouabain binding of lactating rabbit mammary tissue.

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Abstract
Ouabain-sensitive 86Rb+ uptake in slices of lactating rabbit mammary gland significantly increased after 20 min or 1 h of incubation with ovine prolactin (NIH-P-S12; 1 .mu.g/ml). Total K+ content of the tissue significantly increased at 20 min of incubation with prolactin. Neither vasopressin nor oxytocin, at concentrations of 2, 20 or 40 .mu.IU/ml, had a significant effect on ouabain-sensitive 86Rb+ uptake or total K+ of the tissue after 30 min or 1 h of incubation. Tissue slices incubated in cycloheximide at 10 .mu.g/ml for up to 260 min showed a reduction in ouabain-sensitive 86Rb+ uptake and total K+ content, with half-lives [t1/2] of 115 and 63 min, respectively. No consistent in vitro effect of prolactin on (Na+ + K+)-activated ATPase activity in homogenates, crude microsomal fractions or NaI-activated membrane fractions from lactating rabbit mammary gland was found. After 3 h of incubation of tissue slices in the presence or absence of prolactin (5 .mu.g/ml), no significant differences in the number of [G-3H]ouabain molecules bound per cell (5.2 .times. 104 and 6.2 .times. 104, respectively) or in Kd for ouabain binding (5.4 .times. 10-7 M and 5.9 .times. 10-7 M, respectively) were observed. Incubation of the tissue with cycloheximide (10 .mu.g/ml) for 1-6 h caused an exponential decrease in [G-3H]ouabain binding with a t1/2 of 3 h. Prolactin stimulates the activity of the (Na+ + K+)-activated ATPase in slices of lactating rabbit mammary gland within 1 h but over this period does not affect the number of ouabain-binding sites, which are apparently turned over with a t1/2 of 1-3 h.