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 Blood samples were withdrawn after 28, 56 and 84 days of treatment and analysed for vitamin D3 metabolites, prolactin and growth hormone, and serum ionized (free) and total calcium (Ca).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+.