Zinc: an inhibitor of prolactin (PRL) secretion in humans

Biochemical Studies in Perfundates and Homogenates of Isolated Porcine Kidneys after Flushing with Zinc or Zinc-Prolactin Modified Preservation Solution Using a Static Cold Storage Technique

Zinc is an effective anti-inflammatory and antioxidant trace element. The aim of this study was to analyse the protective effect of zinc and zinc–prolactin systems as additives of preservation solutions in the prevention of nephron damage caused during ischemia. The study used a model for storing isolated porcine kidneys in Biolasol^®. The solution was modified with the addition of Zn at a dose of 1 µg/L and Zn: 1 µg/L with prolactin (PRL): 0.1 µg/L. After 2 h and 48 h of storage, the levels of alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, sodium, potassium, creatinine and total protein were determined. Zinc added to the Biolasol^® composition at a dose of 1 µg/L showed minor effectiveness in the protection of nephrons. In turn, Zn²⁺ added to Biolasol + PRL (PRL: 0.1 µg/L) acted as a prolactin inhibitor. We do not recommend the addition of Zn(II) (1 µg/L) and Zn(II) (1 µg/L) + PRL (0.1 µg/L) to the Biolasol solution.

Relationships among plasma zinc, plasma prolactin, milk transfer, and milk zinc in lactating women

The objective was to analyze interrelationships among plasma zinc (PZ), plasma prolactin (PRL), milk transfer, and milk zinc from a longitudinal study of lactating women with a range of zinc intakes. Sixteen of 26 women received a 15 mg/d zinc supplement, resulting in a significant increase in total zinc intake compared with nonsupplemented participants (P<.001). No significant differences in key variables were observed between the zinc-supplemented and non-zinc-supplemented women; data were thus combined. An inverse trend between PRL and PZ was observed at 0.5 (r=-.36, P=.08) and 7 months (r=-.38, P=.06). Correlations between PRL at 0.5 months and at 3, 5, and 7 months were significant (r>.50, P<.01). PRL concentrations and milk transfer were positively correlated at 5 (r=.67, P<.001) and 7 months (r=.53, P<.01), consistent with PRL role in lactation maintenance. Studies including zinc-deficient lactating women are required to clarify the interrelationships between zinc status and PRL secretion.

Zinc supplementation does not inhibit basal and metoclopramide-stimulated prolactinemia secretion in healthy men

Dopamine (DA) and zinc (Zn++) share common mechanisms in their inhibition of prolactin (PRL) secretion. Both substances are present in the same brain areas, where Zn++ is released together with DA, suggesting a modulatory effect of Zn++ on dopaminergic receptors. The aim of the present study was to evaluate the effect of Zn++ supplementation on basal and PRL secretion stimulated by metoclopramide (MCP), a dopaminergic antagonist. Seven healthy men were evaluated in controlled study, where MCP (5 mg) was given intravenously, before and after 3 months of oral Zn++ (25 mg) administration. Our results indicate that chronic Zn++ administration does not change basal or MCP-stimulated plasma PRL secretion suggesting that, in humans, Zn++ does not interfere on PRL secretion mediated through dopaminergic receptors.

Total body zinc depletion and its relationship to the development of hyperprolactinemia in chronic renal insufficiency

Modulation of free plasma zinc levels has been implicated in the increase in plasma prolactin levels seen in patients with chronic renal insufficiency (CRI). The relative importance of this mechanism in comparison to others, however, has not been elucidated. Zinc equilibrium between plasma and red blood cells is partly dependent upon red blood cell carbonic anhydrase (CA). In the present paper, we have investigated the interrelationships among total plasma zinc, leukocyte zinc, prolactin, and erythrocyte CA in patients with CRI. Uremic patients were shown to have significantly increased levels of plasma prolactin and erythrocyte CA activity when compared to normal controls. Moreover, red blood cell CA total concentration and isoenzyme-I and-II levels, as well as plasma zinc were found to be significantly decreased in uremic patients in comparison to normal controls. In patients with CRI, a negative correlation was demonstrated between erythrocyte CA catalytic activity and plasma zinc, as well as between plasma zinc and plasma prolactin levels. Moreover, leukocyte zinc content, which is a reliable indicator of total body zinc stores, was found to be significantly decreased in uremic patients when compared to normal controls. A strong negative correlation between leukocyte zinc content and plasma prolactin levels was documented in CRI patients. Our results suggest that alterations in erythrocyte CA levels, enzymatic activity or isoenzyme profile are most probably mechanistically and etiologically unrelated to the high plasma prolactin levels in CRI patients. Contrariwise, depletion of total body zinc stores, rather than redistribution of this trace metal among extracellular compartments, may represent one of the major contributing mechanisms leading to uremic hyperprolactinemia.

Endocrine interaction between zinc and prolactin. An interpretative review

Zinc plays a very important role in animal and human metabolism. Nowadays, it is one of the most extensively studied trace element, since its sphere of action has been demonstrated to be very broad. From the biochemical standpoint, it controls more than 300 different enzymes, many of them involved with intermediary metabolism, DNA and RNA synthesis, gene expression, and immunocompetence. It also plays a significant role in hormonal homeostasis, since it can interact with almost all hormones. Zn2+ is closely related to the thyroid and steroid hormones, insulin, parathormone, and pituitary hormones, particularly prolactin (PRL). Zn2+ can inhibit PRL secretion within a range of physiologically and pharmacologically relevant concentrations. This property has raised the possibility of clinical applications of zinc. In this article, we review the literature on the subject in an attempt to provide a comprehensible general view.

1,25(OH)2D response to combined zinc and phosphorus depletion in rats

To investigate the role of zinc depletion in the response of calcitriol to a single provocative stimulus, we studied vitamin D metabolites, parathyroid hormone (PTH), and mineral balance during phosphorus depletion in animals with different zinc nutritional status. Male Lewis rats (66) were pair fed normal phosphorus (NP), zinc-replete (+) or -deplete (-) diets for 2 wk. Thereafter, one-half of each paired group underwent 1 wk of phosphorus depletion (LP). Zinc-deplete animals had lower plasma zinc, and phosphorus-deplete animals had lower plasma phosphorus concentrations than respective nutritionally replete controls. Plasma calcium, PTH, and 25-hydroxycholecalciferol concentrations were similar in all four groups at end of experiment. Mean plasma 1,25(OH)2D concentration increased 80% from 35 +/- 3 to 63 +/- 9 pg/ml (P less than 0.007) when values were compared before and after phosphorus restriction in zinc-replete animals (LP + group). The 7.9% increase from 35 +/- 4 to 38 +/- 4 pg/ml in 1,25(OH)2D concentration in animals with combined zinc and phosphorus depletion (LP-) was not significant. External calcium balance, however, was maintained in the combined zinc and phosphorus-deplete group. We conclude that zinc depletion limits the increase in plasma 1,25(OH)2D concentration associated with phosphorus depletion. The mechanism is unknown but may involve an effect of zinc on renal 25(OH)D 1-alpha-hydroxylase synthetic activity.