Effect of 1,25-dihydroxyvitamin D3 and nifedipine on prolactin release in normal man

Medication-induced hyperprolactinemia

Medication use is a common cause of hyperprolactinemia, and it is important to differentiate this cause from pathologic causes, such as prolactinomas. To ascertain the frequency of this clinical problem and to develop treatment guidelines, the medical literature was searched by using PubMed and the reference lists of other articles dealing with hyperprolactinemia due to specific types of medications. The medications that most commonly cause hyperprolactinemia are antipsychotic agents; however, some newer atypical antipsychotics do not cause this condition. Other classes of medications that cause hyperprolactinemia include antidepressants, antihypertensive agents, and drugs that increase bowel motility. Hyperprolactinemia caused by medications is commonly symptomatic, causing galactorrhea, menstrual disturbance, and impotence. It is Important to ensure that hyperprolactinemia in an Individual patient is due to medication and not to a structural lesion in the hypothalamic/pituitary area; this can be accomplished by (1) stopping the medication temporarily to determine whether prolactin levels return to normal, (2) switching to a medication that does not cause hyperprolactinemia (in consultation with the patient's psychiatrist for psychoactive medications), or (3) performing magnetic resonance imaging or computed tomography of the hypothalamic/pituitary area. If the patient's hyperprolactinemia is symptomatic, treatment strategies include switching to an alternative medication that does not cause hyperprolactinemia, using estrogen or testosterone replacement, or, rarely, cautiously adding a dopamine agonist.

Nongenomic effects of 1alpha,25-dihydroxyvitamin D(3)

The hormonally active form of vitamin D, 1alpha,25-dihydroxyvitamin D(3), is the key molecule of the vitamin D endocrine system, which produces biological effects in about 30 target cell systems. Growing experimental evidence supports the hypothesis that these biological effects can be generated both by a signal transduction mechanism involving a nuclear receptor (nVDR) that modulates gene transcription, and via a nongenomic receptor located in the plasma membrane (mVDR), which modulates a complex signaling system involving the rapid opening of Ca(2+) channels. Some data reviewed herein also indicate that crosstalk between genomic and nongenomic pathways operates in several cell types, and suggest that the physiological role of the rapid, nongenomic actions might involve the regulation of hormone-mediated gene activation.

Relationship between circulating vitamin D3 metabolites and prolactin or growth hormone levels in rat

Previous studies (Haug & Gautvik 1985) have demonstrated specific receptors for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in a clonal (GH3) strain of rat pituitary tumour cells. It was discovered that 1,25(OH)2D3 affected the production of prolactin and growth hormone in these cells in a calcium dependent manner. These findings were the basis for a hypothesis that vitamin D3 could be involved in the regulation of pituitary hormones in vivo. To further investigate this contention, female rats were given subcutaneous injections of 1,25(OH)2D3, 25-hydroxyvitamin D3 (25(OH)D3) or 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) three times a week for up to 12 weeks. Blood samples were withdrawn after 28, 56 and 84 days of treatment and analysed for vitamin D3 metabolites, prolactin and growth hormone, and serum ionized (free) and total calcium (Ca). Between treatment group comparisons of serum prolactin and growth hormone levels did not show significant vitamin D3 induced alterations. However, correlation matrix analyses on all variables revealed that serum level of growth hormone was significantly (P < 0.05) and inversely related to corresponding total Ca. Prolactin, on the other hand, may be subject to a complex regulation by 1,25(OH)2D3 and free Ca2+.

Culture requirements for optimal expression of 1 alpha,25-dihydroxyvitamin D3-enhanced thyrotropin secretion

An effect of the hormone, 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] on hormone secretion by normal rat pituitary cells was investigated in vitro. Based on previous findings using GH4C1 cells, dispersed anterior pituitary cell cultures were prepared and maintained in serum-free conditions for up to 6 d. Under these circumstances, there was no effect of 1,25(OH)2D3 to alter medium or cell-associated levels of thyrotropin (TSH), prolactin (PRL), or growth hormone (GH). Cultures maintained under these conditions had lower medium and cell-associated hormone levels and lesser responses to agonists than cultures maintained in serum-supplemented medium. In the presence of 10% charcoal-treated fetal bovine serum, treatment with 10(-8) M 1,25(OH)2D3 for 24 h selectively increased TRH (10(-10) to 10(-7) M)-induced TSH secretion (P less than 0.001), with maximal enhancement observed at 10(-9) M TSH-releasing hormone (TRH). Enhancement of TSH secretion by 1,25(OH)2D3 was detected after 15 min exposure to TRH. There was no effect on agonist-induced PRL or GH secretion or on cell-associated hormone levels. The effect was evident after 24 h treatment with 1,25(OH)2D3, and decreased thereafter. Several other steroid hormones had no effect on 10(-9) M TRH-induced TSH secretion. These data contrast with the effect of 1,25(OH)2D3 in GH cells. They suggest that 1,25(OH)2D3 may act selectively in the normal pituitary to modulate TSH secretion.