Calcitonin gene-related peptide: endocrine distribution and characterization of circulating forms.

This study reports the application of a highly sensitive and specific extraction-based radioimmunoassay for determining levels of calcitonin gene-related peptide (CGRP) in endocrine tissues and plasma of normal rats. In addition, we have characterized the immunoreactive material found in plasma using gel filtration and high performance liquid chromatography. The CGRP content of the thyroid was 100- to 1000-fold higher than that of the adrenals, pituitary or pancreas, while the gonads and kidneys contained appreciably smaller amounts of CGRP. There was no age- and sex-related difference in tissue content, with the exception of a subgroup of 450 g rats. These had a significantly raised thyroidal CGRP content compared to other rats of the same group. A significant correlation between thyroidal content and plasma levels was noted only in old (450 g) rats. CGRP-like immunoreactivity in plasma pooled from young adult (150 g) rats was found to be heterogeneous. Only 8% of the total immunoreactivity recovered from a Sephadex G50 column was found to coelute with synthetic rat CGRP when freeze-dried plasma was chromatographed and a significantly greater proportion (approximately 67%) eluted in the void volume. The void volume peak was markedly reduced when acid-methanol extracts of plasma were chromatographed under similar conditions. A major proportion of the immunoreactive material co-eluting with synthetic CGRP on gel filtration was also found to elute at a position corresponding to the monomer on HPLC: this was consistent with, though not proof of, structural identity.

Osteoporosis with hip fracture: changes in calcium regulating hormones.

It has been suggested that there are two distinct types of involutional osteoporosis in women. Loss of ovarian function causes spine and wrist fractures in middle-aged women whilst a decline in renal endocrine function and bone formation is responsible for fractures, especially of the hip, in elderly women. We have investigated calcium regulation in post-menopausal women with hip fractures in their seventh decade together with non-osteoporotic controls of similar age. The major findings were reduced secretion of calcitonin (P less than 0.01) and 1,25 dihydroxy-vitamin D (P less than 0.025) in the osteoporotics as compared with controls. No differences in plasma levels of calcium, parathyroid hormone and 25 hydroxyvitamin D were observed. Our results show some similarities with those reported in both proposed types of osteoporosis, suggesting considerable overlap between them. Secondary hyperparathyroidism does not appear to be a usual feature of osteoporotic women with hip fracture up to age 75 years. Our hormonal findings are more in keeping with marked oestrogen deficiency, suggesting that loss of ovarian function is a major determinant of the osteoporosis in these women.

The origin of circulating calcitonin gene-related peptide in the rat.

It is known that in addition to the calcitonin precursor the calcitonin gene also encodes a novel peptide, calcitonin gene-related peptide (CGRP). This potent vasodilator has been found in the circulation of man. This present study demonstrates that CGRP is also found in the circulation of the rat and that plasma CGRP comes from two different sources: the thyroid, a major source in old rats, and the perivascular nerves probably at all ages.

Calcium absorption in rheumatoid arthritis.

Calcium absorption, assessed by a double isotope method, was found to be impaired in postmenopausal women with rheumatoid arthritis of recent onset (mean 14.2 months) compared with controls. Circulating levels of 1,25-dihydroxyvitamin D (calcitriol) were higher than in controls, suggesting a primary malabsorption of calcium in these patients. The reduction in calcium absorption correlated with several measures of disease activity, suggesting that the disease process was responsible for the intestinal defect, but an effect from non-steroidal anti-inflammatory agents cannot be excluded. A primary reduction in calcium absorption may increase the risk of osteoporosis in women with rheumatoid arthritis.

Measurement of 1,25-dihydroxyvitamin D3 receptors in breast cancer and their relationship to biochemical and clinical indices.

Both normal and malignant breast tissue contain the specific receptor for 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). A recent study has shown its presence in 80% of surgically removed breast tumors, although only at low levels. We have measured the 1,25-(OH)2D3 receptor in breast tumors from 68 patients and have found it at similar frequency (75%) but at much higher concentrations (range, less than 1 to 30 fmol/mg protein). This receptor has the same characteristics as that measured in classic 1,25-(OH)2D3 target tissues and was distinguished by sucrose gradient centrifugation from plasma contaminants. Complete case histories and follow-up were available on 56 of these patients, and 1,25-(OH)2D3 receptor status (less than 8 or greater than or equal to 8 fmol/mg protein) was not related to the level of estrogen receptors, menopausal status, T-stage or histology of tumors, or presence of 99mTc phosphate hot spots on bone scans. The lack of relationship between the level of 1,25-(OH)2D3 receptors and other prognostic indicators suggests its potential as a new independent variable for assessing breast cancer patients. However, at this stage, 1,25-(OH)2D3 receptor status did not result in any significant difference in probability of survival or metastasis-free survival. Assessment of the importance of this variable for treatment or outcome must await an increased number of patients and a longer time since surgery.

Regulation of calcium-regulating hormones by exogenous sex steroids in early postmenopause.

A major function of calcitonin in humans appears to be maintenance of the skeleton. There is a marked sex difference in circulating calcitonin levels: women have much lower levels. This has led to speculation that calcitonin lack may be one factor involved in the pathogenesis of postmenopausal bone loss. We have measured levels of calcitonin and the other major calcium-regulating hormones in healthy women during the early menopause, and studied the effects of reversing their oestrogen deficiency with natural and synthetic oestrogen. The major findings were that calcitonin levels were increased by oestrogen administration (P less than 0.02-less than 0.001) and that the levels of the bone-resorbing hormones, parathyroid hormone and 1,25 dihydroxyvitamin D, were not higher in postmenopausal than in premenopausal women. We suggest that loss of oestrogen at the menopause accelerates the natural age-related decline of calcitonin secretion, thus further decreasing the calcitonin levels. This leads to increased sensitivity of the skeleton to the actions of the bone-resorbing hormones. It seems likely that the well-known effect of oestrogen in preventing postmenopausal bone loss is achieved, at least in part, by enhancement of calcitonin secretion.

Katacalcin: a new plasma calcium-lowering hormone.

A second potent plasma calcium-lowering peptide, katacalcin (PDN-21), flanks calcitonin within the human calcitonin precursor. Plasma katacalcin was present in 57 healthy volunteers. Concentrations were higher in males than in females and approximately equimolar with calcitonin. Plasma katacalcin doubled within 5 min of calcium infusion. Plasma katacalcin was markedly raised in 20 patients with medullary carcinoma of the thyroid. Measurement of plasma katacalcin concentrations may prove useful in the diagnosis and follow-up of this condition. Katacalcin, like calcitonin, may be involved in both plasma calcium regulation and skeletal maintenance and thus may prove useful in the treatment of bone disease.

Effect of rifampicin and isoniazid on vitamin D metabolism.

Rifampicin, 600 mg, and isoniazid, 300 mg daily for 14 days, reduced circulating levels of 25-hydroxy vitamin D (25-OHD) and 1 alpha, 25-dihydroxy vitamin D (1,25(OH)2D) by 34% (P less than 0.01) and 23% (P less than 0.05) in eight healthy subjects. This was accompanied by a rise in parathyroid hormone (PTH) of 57% (P less than 0.01), but not by a fall in serum calcium or phosphate levels. There was induction of endogenous cortisol oxidation in all subjects, but only in four fast acetylators was there a concomitant increase in antipyrine elimination. In the four slow acetylators antipyrine metabolism was inhibited after the first dose of the drugs. In nine tuberculous patients followed serially there was a fall in 25-OHD and 1,25 (OD)2D and a rise in PTH at the end of 1 mo (P less than 0.05). After 6 mo therapy 25-OHD concentration was further reduced (P less than 0.01), but there was no significant change in 1,25 (OH)2D or PTH levels. Combination treatment with rifampicin and isoniazid perturbs vitamin D metabolism, but less than might have been predicted from reports on each drug given alone. Nevertheless, tuberculous patients with already compromised calcium homeostasis receiving this combination of drugs should be carefully monitored.

Effect of isoniazid on vitamin D metabolism and hepatic monooxygenase activity.

isoniazid, 300 mg daily for 14 days, reduced serum calcium and phosphate levels (P less than 0.001) in eight healthy subjects. After a single dose of isoniazid the concentration of 1 alpha-,25-dihydroxyvitamin D, the most active metabolite of vitamin D, fell by 47% (P less than 0.01) and was reduced throughout the study. Levels of 25-hydroxyvitamin D, the major circulating form of the vitamin, declined in all subjects and to below normal range in six (P less than 0.01). Parathyroid hormone levels rose by 36% (P less than 0.01) in response to the relative hypocalcemia produced. Isoniazid inhibited hepatic mixed-function oxidase activity, as evidenced by a reduction in antipyrine and cortisol oxidation, and a similar inhibition of the hepatic 25-hydroxylase and renal 1 alpha-hydroxylase would explain the reduction in the corresponding vitamin D metabolites. This perturbation of vitamin D metabolism differs from the vitamin D wasting effects after rifampicin. Patients with tuberculosis treated with isoniazid and rifampicin may show changes similar to those shown here in calcium and phosphate homeostasis and thus may be at risk of developing metabolic bone disorders.

Interrelations of calcium-regulating hormones during normal pregnancy.

Profound changes in calcium metabolism occur during pregnancy. The mother has to make available extra calcium for fetal requirements while ensuring that her plasma and bone calcium concentrations are satisfactorily maintained. In a cross-sectional study plasma concentrations of the major calcium-regulating hormones--namely, calcitonin, parathyroid hormone, 25-hydroxyvitamin D (25-OHD), and 1,25-dihydroxyvitamin D (1,25-(OH)2D)--were measured to establish their interrelations during normal pregnancy. The major changes observed were increases in the circulating concentrations of 1,25-(OH)2D and calcitonin. Concentrations of parathyroid hormone and 25-OHD remained within the normal range. The increased concentrations of 1,25-(OH)2D enable the increased physiological need for calcium to be met by enhancing intestinal absorption of this element. The simultaneous rise in calcitonin opposes the bone-resorbing activities of 1,25-(OH)2D, thereby protecting the integrity of the maternal skeleton. Maternal calcium homeostasis is thus maintained yet the requirements of the fetus are fulfilled.

Calcitonin and the calcium-regulating hormones in postmenopausal women: effect of oestrogens.

In man, the major function of calcitonin appears to be prevention of excessive or unwanted bone resorption. There is a striking sex difference in circulating levels, with a relative deficiency in women. Calcitonin secretion in young adults is increased by oestrogens and therefore long periods of oestrogen lack, such as after the menopause, may be associated with a more pronounced calcitonin deficiency. This exaggerated deficiency could be an important factor in the pathogenesis of postmenopausal bone loss, especially since the latter may be due to excessive bone resorption. In a study of the effects of oestrogen treatment on circulating levels of calcitonin, parathyroid hormone, and vitamin-D metabolites in postmenopausal women, the most striking change was a sharp rise in plasma-calcitonin. Oestrogens prevent postmenopausal bone loss, and it is suggested that this effect could be mediated, at least in part, through control of calcitonin secretion. Calcitonin may prove effective in the prevention of postmenopausal bone loss, and it is suggested that this effect could be mediated, at least in part, through control of calcitonin secretion. Calcitonin may prove effective in the prevention of postmenopausal bone loss. Its place in the treatment of postmenopausal osteoporosis warrants further evaluation.

PIP: The major function of calcitonin in man appears to be prevention of excessive or unwanted bone resorption. There is a striking sex difference in circulating levels, with a relative deficiency existing in women. Calcitonin secretion in young adults in increased by estrogens and therefore long periods of estrogen lack, such as after menopause, may be associated with a more pronounced calcitonin deficiency. This exaggerated deficiency could be an important factor in the pathogenesis of postmenopausal bone loss, especially since the latter may be due to excessive bone resorption. In a study of the effects of estrogen treatment on circulating levels of calcitonin, parathyroid hormone, and vitamin D metabolites in postmenopausal women, the most striking change was a sharp rise in plasma-calcitonin. Estrogens prevent postmenopausal bone loss, and it is suggested that this effect could be mediated, at least in part, through control of calcitonin secretion. Calcitonin may prove effective in the prevention of postmenopausal bone loss. Its place in the treatment of postmenopausal osteoporosis warrants further evaluation.

Effects in postmenopausal women of natural and synthetic estrogens on calcitonin and calcium-regulating hormone secretion. Relevance to postmenopausal osteoporosis.

The major function of calcitonin in man appears to be the prevention of excessive or unwanted bone resorption. There is a marked sex difference in circulating levels, with a relative deficiency in women. As calcitonin secretion in young women is known to be increased by estrogens, long periods of estrogen lack - such as the postmenopause - are possibly associated with a more pronounced calcitonin deficiency. This exaggerated deficiency could be an important factor in the pathogenesis of postmenopausal bone loss, especially as the latter is believed to be due to excessive bone resorption. We have studied the effects of three exogenous estrogens on the circulating levels in postmenopausal women of calcitonin, parathyroid hormone, the vitamin D metabolites 1,25 dihydroxyvitamin D and 25 hydroxyvitamin D, and calcium and phosphate. The most striking finding was a sharp rise in the plasma levels of calcitonin. Estrogens are known to prevent postmenopausal bone loss but the mechanism of action remains obscure. We suggest that the beneficial effects of estrogens could be mediated, at least in part, through control of calcitonin secretion. Thus calcitonin may prove effective in the prevention of postmenopausal bone loss and its role in the treatment of established osteoporosis warrants further evaluation.