Actions of rat growth hormone-releasing factor and norepinephrine on cytosolic free calcium and inositol trisphosphate in rat C-cells

Regulation of calcitonin gene expression by hypocalcemia, hypercalcemia, and vitamin D in the rat

High calcium leads to the secretion of calcitonin, and the administration of 1,25-dihydroxyvitamin D3 leads to a decreased transcription of the calcitonin gene. We now report the effect of chronic hypercalcemia, hypocalcemia, and vitamin D deficiency on calcitonin gene expression in vivo in the rat. Hypercalcemia was created by calcium infusions for 6 h, a high-calcium diet given to weanling rats for 3 weeks, and the transplantation of the Walker carcinosarcoma 256 cell line. Despite serum calcium as high as 22 mg/dl, there was no difference in calcitonin mRNA levels among these rats. The control genes studied, actin and somatostatin, which is specific for C cells in the thyroparathyroid tissue, also did not differ among the different groups of rats. Injected 1,25-(OH)2D3 decreased calcitonin mRNA levels at 6 h, as previously reported. Hypocalcemia, created by feeding diets deficient in calcium and vitamin D to weanling rats for 3 weeks, had no effect on calcitonin mRNA levels, in contrast to the large increases in PTH mRNA levels. These results demonstrate that calcitonin gene expression in vivo in the rat is regulated by administered 1,25-(OH)2D3 but not by changes in serum calcium.

Inhibitory effect of somatostatin on cAMP accumulation and calcitonin secretion in C-cells: involvement of pertussis toxin-sensitive G-proteins

The effect of somatostatin on cAMP accumulation and calcitonin secretion in C-cells of the rat medullary thyroid carcinoma cell line rMTC 6-23 was investigated. Intracellular cAMP accumulation as well as calcitonin secretion could be dose-dependently stimulated by rat growth hormone releasing factor (rGRF). The long-acting somatostatin analogue octreotide inhibited rGRF-stimulated cAMP accumulation and calcitonin secretion dose dependently but failed to block 8-bromo-cAMP-stimulated calcitonin secretion. The inhibitory effect of octreotide on rGRF-induced calcitonin secretion was partially abolished by pretreating the cells with pertussis toxin. The octreotide effect was not due to changes in the degradation of cAMP, as it was similarly seen in the presence of isobutylmethylxanthine. Thus we conclude that pertussis toxin-sensitive G-proteins are involved in the cAMP-mediated regulation of calcitonin secretion in C-cells.

Cytosolic calcium responses of single rMTC 44-2 cells to stimulation with external calcium and potassium

Few endocrine tissues can detect changes in the extracellular Ca2+ concentration within the physiological range and modify their hormone secretion accordingly. A rat cell line of C-cell origin (rMTC 44-2) secretes calcitonin and neurotensin in response to small increases in external Ca2+. To better understand the mechanism of extracellular Ca2+ sensing in this cell type, we studied single fura-2-loaded rMTC 44-2 cells perfused with increasing concentrations of Ca2+ and K+. In the basal state (Ca2+ = 0.5 mM), cytosolic Ca2+ levels were 53 nM, with 27% of the cells having spikes or oscillations. With elevation of the external Ca2+ to between 0.5 and 4 mM, 84% of the cells showed a rapid (less than 5 s) rise in cytosolic Ca2+ to values 2- to 10-fold higher than basal levels. Most of the responding cells exhibited complex patterns of cytosolic Ca2+ fluctuations, including oscillations with frequencies varying from less than 1/min to as many as 6/min. When averaged over time, the cytosolic Ca2+ of individual cells showed a dose-dependent response with changes in external Ca2+, resembling the relationship between extracellular Ca2+ and calcitonin secretion. With continued or repeated stimulation, the spike amplitude often declined. These cytosolic Ca2+ responses were attenuated in the presence of the Ca(2+)-channel blockers cadmium and nifedipine. Cytosolic Ca2+ responses to perfusion with elevated K+ (20 mM) were similar in waveform to those seen with Ca2+ stimulation. Most cells displayed cytosolic Ca2+ changes in response to both ionic secretagogues when stimulated with external Ca2+ or K+.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Reversible desensitization of calcitonin secretion by repetitive stimulation with calcium

The extracellular ionized calcium concentration (Ca2+) is a main regulator of calcitonin (CT) release. Calcium-induced CT secretion differs for acute versus long-term alterations of Ca2+. Using the rat C cell line rMTC 6-23 we have investigated the effect of repetitive stimulation by Ca2+ on CT release. After a Ca-induced initial rise of CT secretion, repetitive Ca stimulation led to a decline of CT release to unstimulated levels (after about 4 h). Reversing the high Ca2+ concentration (2.0 mM) to basal (1.1 mM) for 2 h and then increasing Ca2+ again resulted in a restored stimulatory action of Ca2+ (about 100% increase above the control). In contrast, repetitive stimulation with the dihydropyridine Ca channel agonist Bay K-8644 showed an unchanged stimulatory effect, as observed for the cAMP analog 8-bromo-cAMP, too. The results indicate that the reversible desensitization of Ca-induced CT secretion might be due to a modification of the voltage-dependent Ca channels proximal to or at the site of Bay K-8644 action.