Enhanced calcitonin secretion in the rat after parathyroidectomy and during chronic calcium deprivation

Calcitonin, the forgotten hormone: does it deserve to be forgotten?

Calcitonin is a 32 amino acid hormone secreted by the C-cells of the thyroid gland. Calcitonin has been preserved during the transition from ocean-based life to land dwellers and is phylogenetically older than parathyroid hormone. Calcitonin secretion is stimulated by increases in the serum calcium concentration and calcitonin protects against the development of hypercalcemia. Calcitonin is also stimulated by gastrointestinal hormones such as gastrin. This has led to the unproven hypothesis that postprandial calcitonin stimulation could play a role in the deposition of calcium and phosphate in bone after feeding. However, no bone or other abnormalities have been described in states of calcitonin deficiency or excess except for diarrhea in a few patients with medullary thyroid carcinoma. Calcitonin is known to stimulate renal 1,25 (OH)2 vitamin D (1,25D) production at a site in the proximal tubule different from parathyroid hormone and hypophosphatemia. During pregnancy and lactation, both calcitonin and 1,25D are increased. The increases in calcitonin and 1,25D may be important in the transfer of maternal calcium to the fetus/infant and in the prevention and recovery of maternal bone loss. Calcitonin has an immediate effect on decreasing osteoclast activity and has been used for treatment of hypercalcemia. Recent studies in the calcitonin gene knockout mouse have shown increases in bone mass and bone formation. This last result together with the presence of calcitonin receptors on the osteocyte suggests that calcitonin could possibly affect osteocyte products which affect bone formation. In summary, a precise role for calcitonin remains elusive more than 50 years after its discovery.

Calcium: taste, intake, and appetite

This review summarizes research on sensory and behavioral aspects of calcium homeostasis. These are fragmented fields, with essentially independent lines of research involving gustatory electrophysiology in amphibians, ethological studies in wild birds, nutritional studies in poultry, and experimental behavioral studies focused primarily on characterizing the specificity of the appetite in rats. Recently, investigators have begun to examine potential physiological mechanisms underlying calcium intake and appetite. These include changes in the taste perception of calcium, signals related to blood calcium concentrations, and actions of the primary hormones of calcium homeostasis: parathyroid hormone, calcitonin, and 1,25-dihydroxyvitamin D. Other influences on calcium intake include reproductive and adrenal hormones and learning. The possibility that a calcium appetite exists in humans is discussed. The broad range of observations documenting the existence of a behavioral limb of calcium homeostasis provides a strong foundation for future genetic and physiological analyses of this behavior.

Calcium intake by rats: influence of parathyroid hormone, calcitonin, and 1,25-dihydroxyvitamin D

We examined the contribution of the primary hormones of calcium homeostasis to the control of calcium intake in the rat. Male Sprague-Dawley rats with 50 mM CaCl2 solution as their only source of calcium received subcutaneous hormone infusions for 13 days. Parathyroid hormone (PTH; 40, 80, or 160 ng/h) produced sustained dose-related decreases in CaCl2 intake. High doses of calcitonin (CT; 32 or 64 ng/h) increased CaCl2 intake transiently, and low doses (4, 8, or 16 ng/h) had no effect. 1,25-Dihydroxyvitamin D [1,25(OH)2D] in doses > 1 ng/h initially increased CaCl2 intake, but the effects of moderate doses (2 or 4 ng/h) tended to dissipate, and the sustained effect of high doses (8 or 16 ng/h) was to reduce CaCl2 intake. Infusions of combinations of the hormones had effects consistent with their individual actions: there was no evidence for synergy. Based on changes in plasma hormone concentrations, it appeared that most of the infusions had effects within the physiological range. Consistent with hypotheses that calcium appetite is mediated by circulating calcium, PTH and CT infusions produced reciprocal changes in plasma calcium concentrations and CaCl2 intake. However, the finding that 1,25(OH)2D elevated both plasma calcium concentrations and CaCl2 intake raises the possibility that one or more of the hormones may mediate calcium appetite directly.

Effects of 17 beta-estradiol on calcitonin secretion and content in a human medullary thyroid carcinoma cell line

The presence of a direct estrogen effect on calcitonin secretion is controversial. Because most of the data available were obtained from complex in vivo systems, we chose an in vitro approach to assess the problem. Using a human C cell carcinoma cell line (TT cells) with well-documented estrogen receptors, we investigated the effect of 17 beta-estradiol (E2) on basal and stimulated calcitonin secretion, on calcitonin content, and on total cellular protein. After short (30 and 180 minutes) and long-term (24 h to 6 days) incubation of the cells with different concentrations of E2 (from 0.01 to 100 nM) we observed no stimulatory but a transient dose-dependent inhibitory effect on CT secretion and content. The nadir of the effect on CT secretion appeared at 24 h, demonstrating a reduction to 80.5 +/- 7.8% of control at 1 nM and to 59.1 +/- 15% of control at 100 nM E2. After 72 h, the CT levels of the E2-exposed groups returned to control levels. The acute stimulation of the cells with TPA plus forskolin after preincubation with E2 up to 6 days showed no difference in the increment of CT release compared to the control groups. Additionally, E2 had a dose-dependent stimulatory effect on cell protein content. The data demonstrate the absence of a direct stimulatory effect of E2 on CT secretion, revealing a dose-dependent inhibitory effect on CT secretion and content.

Role of voltage-dependent calcium channels in secretion of calcitonin from human medullary thyroid carcinoma cells

Extracellular calcium concentration is an important regulator of calcitonin secretion. We used primary cell cultures of human medullary thyroid carcinoma to study the role of voltage dependent calcium channels for stimulus secretion coupling. Increasing extracellular calcium concentration (1.6-5.0 mM) in the medium caused a dose dependent release of calcitonin. The calcium channel activator BAY K 8644, a dihydropyridine, stimulated calcitonin secretion in a dose dependent manner (10(-7)-10(-5) M). This effect was completely inhibited by equimolar concentrations of the calcium channel blocker nifedipine and abolished in the absence of extracellular calcium. Similarly, nifedipine suppressed the stimulatory action of extracellular calcium. The effects of calcium and BAY K 8644 with and without nifedipine suggest that calcium influx via voltage dependent calcium channels plays an important role in calcitonin secretion. The primary cell culture of human medullary thyroid carcinoma is a good model for the study of stimulus secretion coupling.