PTHrP secretion is stimulated by CT and inhibited by CgA peptides

We studied the regulation of the secretion in vitro of the parathyroid hormone-related protein (PTHrP) associated with the hypercalcemia of malignancy by Chromogranin A (CgA)-derived peptides and by human calcitonin (CT) in the BEN human lung tumor cell line. The amino terminal peptide of CgA, CgA1-40, inhibited the secretion of PTHrP, whereas other peptides had no such effect. Human CT stimulated the secretion of PTHrP, whereas other hormones had no such effect. Both effects occurred in a dose-dependent manner. These studies reveal novel regulatory pathways among peptides and proteins that are commonly associated with each other and can have paracrine interactions. CgA may be processed at its multiple dibasic sites to peptides that regulate the secretion of its co-resident hormones, in this case PTHrP. In addition to a paracrine effect, CT may be clinically useful as a provocative agent for PTHrP secretion. Complex interactions are present among the calcium-regulating hormones and their associated proteins.

Effects of calcitonin, calcitonin analogues, and calcitonin gene-related peptide on basal in vitro renin secretion

We investigated the relation of calcitonin as a calcium-active hormone to its more recently described effects on peripheral vascular tone. Basal renal renin secretion in vitro in rat kidney slices was studied in the presence of salmon calcitonin (SCT, 4400 U/mg), amino acid substituted analogues of SCT, 16-alanine SCT (6200 U/mg) and 12,16,19 tri-alanine SCT (350 U/mg), and of rat calcitonin gene-related peptide (rCGRP). All calcitonin species at the same hypocalcemic activity (1 U/mL) modestly but significantly suppressed renin secretion from control levels (9.79 +/- 0.44 to 7.51 +/- 0.53, 7.70 +/- 0.72, and 7.78 +/- 0.90 Goldblatt units/g/h for SCT, 16-ala SCT, and tri-ala SCT, P less than .05 for all calcitonins v control), whereas rCGRP had no effect. Thus, on a molar basis, the renin suppressing effects of the various calcitonin species paralleled their bioassay-defined calcium sequestering activity, 16-ala SCT greater than SCT much greater than tri-ala SCT. Lower concentrations of SCT (10(-2) U/mL and 10(-4) U/mL, approximately 6 X 10(-10) and 6 X 10(-12) mol/L, respectively) had virtually identical effects. Moreover, verapamil (5 X 10(-6) mol/L) blocked the SCT-induced suppression of renin secretion (9.79 +/- 0.44 v 9.36 +/- 1.05 GU/g/h, P = NS). We conclude that the juxtaglomerular apparatus is a calcitonin-responsive system, in which calcitonin and its analogues act to suppress basal renin secretion in vitro. This effect seems to depend on and may be mediated by modulating cellular calcium uptake, and suggests a wider, calcium-related role for calcitonin than had previously been suspected.

Prostaglandins promote osteoclastlike cell formation by a mechanism involving cyclic adenosine 3',5'-monophosphate in mouse bone marrow cell cultures

We have developed a mouse bone marrow culture system in which multinucleated osteoclast (OC)-like cells are formed within 8 days. Using this culture system, we examined the effect of prostaglandins (PGs), potent bone-resorbing agents, on OC-like cell formation. Four PGs (PGE1 and PGE2 at 10(-8)-10(-5) M, 6-keto-PGF1 alpha at 10(-5) M, and PGF2 alpha at 10(-6)-10(-5) M) significantly stimulated the formation of OC-like cells. The potency of the PGs in inducing OC-like cell formation was the highest in PGE1 and PGE2, followed by PGF2 alpha and 6-keto-PGF1 alpha in that order, and the order was highly correlated with the order of the potency in increasing the production of cyclic adenosine 3',5'-monophosphate (cAMP) in bone marrow cells. Addition of dibutyryl-cAMP also induced OC-like cell formation. Moreover, isobutylmethylxanthine (IBMX), a potent inhibitor of phosphodiesterase, potentiated the OC-like cell formation induced by PGE2, whereas salmon calcitonin greatly inhibited it. Calcitonin induced cAMP production in cultures treated with PGE2, but not in cultures with vehicle. When bone marrow mononuclear cells were cultured on dentine slices in the presence of PGE2, multinucleated OC-like cells were similarly formed and they resorbed calcified dentine, resulting in so-called Howship's lacunae. These results suggest that PGs stimulate resorption of calcified tissues by promoting osteoclast formation. The activity of PGs in inducing OC-like cell formation is considered mediated mainly by a mechanism involving cAMP.

Hormonal regulation of biliary calcium excretion in rats: inhibition of calcitonin action by alpha 1-adrenergic stimulation

The effect of synthetic [Asu1,7] eel calcitonin (CT) and other hormones on biliary calcium excretion was investigated in rats cannulated bile duct. Administration of CT (80 mU/100 g body weight) produced a significant increase in liver calcium and a corresponding elevation of bile calcium content. The increase in bile calcium content was also caused by administration of insulin (0.1 U/100 g), epidermal growth factor (10 micrograms/100 g), glucagon (10 micrograms/100 g), epinephrine (10 micrograms/100 g), norepinephrine (10 micrograms/100 g), 4 beta-phorbol 12-myristate-13-acetate (10 micrograms/100 g) and ATP (1.0 mg/100 g), suggesting that this increase may be a receptors-mediated response. Of these hormones and drugs, norepinephrine, a alpha-receptor mediator, clearly prevented CT effect on biliary calcium excretion. Moreover, phenylephrine, a alpha 1-receptor agonist, caused an inhibition of the CT effect, while the agonist significantly increased biliary calcium excretion. The present study clearly demonstrates that biliary calcium excretion is stimulated by various hormones which increase calcium influx into liver cells, and suggests that the CT action may be inhibited by alpha 1-adrenergic stimulation.

Inhibition by phenytoin of in vitro secretion of calcitonin from rat thyroid glands and cultured rat C cells

Baby rat thyroid glands and cultured rat medullary carcinoma C cells were incubated acutely with phenytoin (38-100 microM), and the calcitonin (CT) secreted into the serum-free medium was measured by radioimmunoassay (RIA). Phenytoin did not alter CT release from glands or C cells incubated in 1 mM Ca, but, when Ca was raised to 1.75 or 2.5 mM, a marked inhibitory effect of phenytoin was apparent. The inhibitory effect could be negated by including 10 microM BAY-K-8644 in the medium. Inhibitory effects on CT release also were obtained with 100 microM trifluoperazine or 100 microM nitrendipine, and these inhibitory effects also were counteracted by 10 microM BAY-K-8644. The results show that clinically relevant amounts of phenytoin can inhibit CT release, perhaps by interfering with C-cell Ca channels or by inhibiting calmodulin-dependent processes.

Influence of the interaction of calcitonin and 1,25(OH)2-vitamin D3 on prolactin secretion

Calcitonin inhibits and 1,25(OH)2-Vitamin D3 (1,25(OH)2D3) stimulates prolactin and thyrotropin secretion. During concurrent administration of calcitonin and 1,25(OH)2D3 the action of the two calciotropic substances on thyrotropin secretion antagonizes each other. In the submitted investigation the effect of a single dose of synthetic salmon calcitonin on the prolactin secretion at rest and TRH-induced secretion was compared in untreated healthy subjects and in subjects given for four days previously 1,25(OH)2D3 in doses which have a stimulating effect on prolactin secretion (3 micrograms/d). Calcitonin, 50 U, administered by the i.v. route by infusion inhibited the secretion of prolactin at rest (p less than 0.05) and the TRH-stimulated secretion during the 20th, 30th, 40th and 60th minute after TRH administration (p less than 0.01 at all time intervals) in untreated subjects but also the secretion at rest (p less than 0.01) and TRH - stimulated secretion at times 20, 30, 40 and 60 mins following TRH administration (p less than 0.01, p less than 0.05, p less than 0.05 and p less than 0.05, respectively) in subjects pretreated with 1,25(OH)2D3. Compared with the thyrotropic system, the inhibiting effect of calcitonin on the lactotropic system is not cancelled by the administration of 1,25(OH)2D3. It is assumed that differences in the response of the lactotropic and thyrotropic system to the concurrent administration of calcitonin and 1,25(OH)2D3 are conditioned by the different ratio of the number of receptors for calcitonin and 1,25(OH)2D3 in the lactotropic and thyrotropic cell.

Effects of calcitonin, 1,25(OH)2 vitamin D3 and trifluoperazine on thyrotropic secretion and their mutual interactions

The effects of pharmacological doses of calcium ionophores, calcitonin and 1,25 (OH2)D3, and of the calmodulin inhibitor, trifluoperazine, on thyrotropin secretion at rest and TRH stimulated secretion were investigated as well as their mutual interactions in five groups of clinically healthy volunteers. The thyrotropic secretory response to TRH was inhibited by a single intravenous dose of 50 U synthetic salmon calcitonin or trifluoperazine administered by mouth seven days prior to the TRH test, 6-12 mg/day, and stimulated by 1,25(OH)2D3 administered by mouth four days before the TRH test -3 micrograms/day. The stimulating action of 1,25(OH)2D3 was abolished by calcitonin, administered by the intravenous route - 50 U - closely before the TRH test but not by the concomitant administration of trifluoperazine. The author expressed the hypothesis, that calcitonin and 1,25(OH)2D3 but not trifluoperazine and 1,25(OH)2D3 are mutual antagonists which act at the same subcellular level of the thyrotropic cell and that synthesis and activation of calmodulin is not the trigger of thyrotropic secretion after stimulation with 1,25(OH)2D3. An altered thyrotropin secretion must be foreseen during the therapeutic administration of the investigated calciotropic substances, the mutual interactions of which could be used in practice.

Monomeric serum calcitonin and bone turnover during anticonvulsant treatment and in congenital hypothyroidism

Decreased basal and calcium-stimulated calcitonin serum levels have been found in children with congenital hypothyroidism and in those receiving anticonvulsant drugs. The purpose of our investigation was to confirm these results using a new technique for calcitonin measurement and to study the effect on bone turnover. Calcitonin serum levels were measured with two different antibodies before and after a low-dose Ca infusion in patients receiving phenytoin, primidone, carbamazepine, or valproate and in patients with congenital hypothyroidism receiving L-thyroxine. In comparison with control values, basal and Ca-stimulated extractable calcitonin, representing the monomeric and biologically active form of the hormone, were moderately decreased in patients with epilepsy receiving phenytoin and primidone, and severely decreased in patients with hypothyroidism. Ca and bone metabolism were normal, except for an elevated renal threshold for phosphate (indicating phosphate conservation) in patients receiving phenytoin and primidone, and increased fasting urinary excretion of Ca and hydroxyproline (indicating increased bone resorption) in patients with hypothyroidism. The secretory capacity of the C cells for monomeric calcitonin is decreased in children receiving treatment with some, but not all, anticonvulsant drugs, and lacking in patients with hypothyroidism. Patients with calcitonin deficiency may be prone to osteopenia if the tendency to increased osteoclastic activity is aggravated by secondary hyperparathyroidism in patients with epilepsy receiving phenytoin and primidone or by inappropriate thyroid replacement therapy in patients with hypothyroidism.

Inhibition by prostaglandin E2 of renal effects of calcitonin in rats

To investigate the possible role of prostaglandin E2 (PGE2) in modulating the actions of PTH and calcitonin (CT) in the kidney, the effects of PGE2 were examined on the in vivo conversion of [3H]25-hydroxyvitamin D3 to [3H]1,25-dihydroxyvitamin D3 ([3H]1,25-(OH)2D3) in vitamin D-deficient thyroparathyroidectomized (T-PTX) rats and on the urinary excretion of phosphate (Pi) in vitamin D-replete T-PTX rats in the presence of either PTH or CT. Plasma accumulation of [3H] 1,25-(OH)2D3 increased from 12.2 +/- 0.6 pmol/100 ml in controls to 19.5 +/- 1.1 (P less than 0.01) by 20 micrograms/h PGE2 to 29.8 +/- 1.8 (P less than 0.001) by 7.5 U/h PTH, and to 23.3 +/- 0.7 (P less than 0.01) by 500 mU/h CT. Administration of PGE2 inhibited CT-stimulated accumulation of 1,25-(OH)2D3 to levels not different from those by PGE2 alone (17.8 +/- 1.0 pmol/100 ml). In contrast, PGE2 had no effect on PTH-stimulated 1,25-(OH)2D3 accumulation. PTH and CT caused an increase in urinary Pi excretion and a decrease in plasma Pi levels. PGE2 abolished the effects of CT, but not of PTH, on both urinary Pi excretion and plasma Pi levels. Administration of PGE2 alone caused no significant changes in plasma Pi levels and only minimal increase in urinary Pi excretion. PGE2 did not suppress urinary cAMP excretion stimulated by CT. These results demonstrate that PGE2 specifically suppresses the effects of CT to stimulate synthesis of [3H]1,25-(OH)2D3 from [3H]25-hydroxyvitamin D3 and to inhibit tubular reabsorption of Pi without affecting urinary cAMP excretion. Since CT appears to stimulate 1 alpha-hydroxylase and inhibit Pi reabsorption in proximal tubules, nephron segments devoid of CT-sensitive adenylate cyclase, these data suggest that PGE2 modulates the actions of CT, but not of PTH, on proximal tubular functions.

Hypocalcemia during restraint stress in rats. Indication that gastric ulcer prophylaxis by exogenous calcium interferes with calcitonin release

The influence of restraint stress on serum calcium (Ca) and phosphate was studied in normal and thyroidectomized rats. In addition the response of gastric stress ulcer index, blood gastrin and glucagon to exogenous Ca was investigated. In intact as well as in thyroidectomized animals serum total, ionised and previously injected radioactive Ca decrease during an 8h stress period, whereas inorganic phosphate increases. Together with a constant specific activity these findings are consistent with hypoparathyroidism and calcitonin independent hypocalcemia during stress. Intragastric infusion of 45 mg/kg Ca-gluconate per 8h proves to be a potent anti-stress ulcer regimen in intact and neck-sham operated, but not in thyroidectomized rats without and with additional adrenal demedullation. Gastrin and glucagon were not correlated with calcemia during either stress alone or stress combined with intragastric Ca infusion. It is suggested that the development of gastric stress ulcerations can be prevented by a Ca-mediated release of endogenous calcitonin.

Influence of methyl xanthine treatment on calcitonin effect

The presented results clearly demonstrate that theophylline, caffeine and theobromine have a blocking action on calcitonin effect as determined by the elevation of serum calcium level, although different degree of potency was found with each drugs. There is no difference in respect of the serum calcium elevation after a single dose of methyl xanthines or after a 9-10 day lasting treatment. The castration had no additional enhancing effect of the calcitonin resistance of methyl xanthine treated animals. The administration of androgens simultaneously with the methyl xanthine treatment failed to produce improvement of calcitonin sensitivty preventing the elevation of serum calcium after calcitonin injection. So the calcitonin resistance of methyl xanthine treated animals appears to be different to the calcitonin resistance of sex hormone deprived conditions because the latter could be abolished by androgen administration. Theophylline treatment resulted in a slightly but significantly elevated serum calcium comparing to controls without exogeneous calcitonin. This magnitude of elevation remains between the normocalcaemic limits.

Evidence for interference of 25 (OH) vitamin D3 with phosphaturic action of calcitonin

The present study evaluated the effect of 25(OH)vitamin D3[25(OH)vit D3] on the phosphaturic action of calcitonin in anesthetized parathyroidectomized rats. In group 1, calcitonin was given intravenously over six clearance periods. In group 2, after three periods of calcitonin given intravenously, 25(OH)vit D3 was added and given together with calcitonin for three additional periods. During calcitonin infusion, Cp/CIn 0.18 +/- 0.02 (mean +/- SE) in group 1 was not different from the corresponding Cp/CIn 0.18 +/- 0.03 in group 2; but when 25(OH)vit D3 was added, Cp/CIn 0.12 +/- 0.01 in group 2 was lower (P less than 0.001) than the corresponding Cp/CIn 0.26 +/- 0.02 in group 1. With intravenous calcitonin the urinary excretion of cycle AMP (UcAMP) 97 +/- 29 in group 1 did not differ from the corresponding UcAMP 86 +/- 27 pmol/min in group 2, but when 25(OH)vit D3 was added UcAMP 41 +/- 12 in group 2 was lower (P less than 0.001) than the corresponding UcAMP 131 +/- 14 pmol/min in group 1. This study demonstrated that 25(OH)vit D3 blocks the phosphaturic action of calcitonin. The associated fall in Uctamp suggests that25 (OV)vit D3 acts possibly by inhibiting the calcitonin-induced activation of adenylate cyclase in the kidney. However, other alternative mechanisms of action have not been excluded.

Effects of oestrogen on thyrocalcitonin activity in male and female rabbits

Thyrocalcitonin is an effective hypocalcaemic agent in both the male and female Dutch Belted rabbit. In this strain of rabbit, a level of oestradiol (1 μg/kg of body weight) which is not hypercalcaemic and levels of oestradiol that are hypercalcaemic can effectively block the action of thyrocalcitonin in both the male and female.

Influence of gonadal hormones on the action of calcitonin in the rat

The influence of short-term and long-term treatment with gonadal hormones on the response to calcitonin was investigated in the rat. Oestrogen-treatment, short-term as well as long-term, resulted in a reduced responsiveness to calcitonin. Long-term treatment with androgens enhanced the hypocalcaemic effect of calcitonin in castrated rats of either sex, but reduced the effect in intact animals. No sex differences could be registered in the sensitivity to calcitonin, when intact animals were compared according to age, while marked differences were observed, when the animals were compared according to weight. There was a linear decrease in the response to calcitonin with increasing age in rats of both sexes. An intraperitoneal calcium load was followed by an acute rise in the serum calcium levels. The adult animals counteracted the hypercalcaemia more slowly than the young ones. Significant differences also occurred between male and female rats, the rise in the serum calcium concentration being much more pronounced in the latter group. The hypocalcaemic activity of thyroid tissue from rats of both sexes and of various ages showed considerable variations, but no differences correlated to age or sex.