Effects of Prolactin on Na+K+-ATPase Activity in the Nephron during Maturation in the Rat

Abstract

ABSTRACT. The effect of prolactin (PRL) on renal Na+K+-ATPase was investigated in 7-d-old neonatal rats. Animals were treated by bromocriptine (Br; a blocker of endogenous PRL secretion), and the enzyme activity was compared with that of untreated controls. Na+K+-ATPase was determined in renal sections in the medullary thick ascending limb of Henle's loop and in the distal tubule by cytochemistry. In the distal tubule, Na+K+-ATPase activity was significantly lower in Br-treated animals than in controls ( Mechanisms by which fluid and salt balances are preserved in the fetal and neonatal periods are not completely understood. The development of fetal and neonatal sodium excretion ability (1, 2) and its relationship with hormones involved in volume regulation such as the renin-angiotensin-aldosterone system (3, 4) vasopressin (5, 6), and, more recently, atrial natriuretic factor (6) have been investigated and are still controversial. PRL is a hormone with apparently little effect in the control of fluid volume in mature animals (7, 8); however, its role has been recently questioned in fluid and electrolyte homeostasis throughout vertebrate phylogeny (9) and its elevated levels (10) during gestation and the neonatal period.Renal Na+Kt-ATPase, the driving force of tubular reabsorption, plays a key role in fluid and electrolyte homeostasis; in a previous study (1 1), we reported in mature animals a parallelism in enzyme activity and PRL levels in vivo and PRL stimulation of the sodium pump in vitro in a range of concentrations similar to those observed during maturation, suggesting that the hormone may control the enzyme activity at this period. In the 20 present study, we first investigated the effect of in vivo changes in PRL secretion on renal Na+K+-ATPase in 7-d-old rats. Groups of neonatal rats were treated with Br, a drug that blocks PRL secretion, for 5 d and renal Na+K+-ATPase activity in these treated animals was compared with that of untreated animals. The hormone effects were investigated in vitro by determining the enzyme activity in kidney sections from Br-treated and control animals after incubation with increasing concentrations of ovine PRL. MATERIALS AND METHODSAnimals. Pregnant Sprague-Dawley rats (Charles River Breeding Laboratories, Wilmington, MA) were obtained and allowed to deliver naturally at our research facilities. They were observed at least twice daily to determine the exact parturition time; day birth counted as d 1. Pups were kept with their dam. All animals had free access to the standard laboratory diet (Soufirat, UAR, Epinay sur Seine, France) and tap water. Litters (n = 3) were reduced to five pups, and treatment for a given litter was determined randomly. No animals died. Ten male adult rats (300 g) were used as a reference group (adult control).Br Treatment. On d 7, three pups from each litter (Br) were treated with Br (2-Br-ergocriptine mesylate; Sandoz Pharmaceuticals, Hanover, NJ). Because in preliminary experiments intraperitoneal and S.C. injections induced necrosis, the drug...