Philip F. Hirsch

Is calcitonin an important physiological substance

That calcitonin (CT) at supraphysiological doses is hypocalcemic led to the mistaken conclusion that it was important for calcium homeostasis and this idea has persisted to this day. Despite these findings, there is no readily apparent pathology due to CT excess or deficiency and there is no evidence that circulating CT is of substantial benefit to any mammal. Mammalian CT at physiological doses is not essential and very likely the CT gene has survived because of the gene’s alternate mRNA pathway to produce calcitonin-gene-related peptide found in neural tissues. CT is not involved in calcium homeostasis or any other important physiological function, except, possibly, as an adjunct to protection of the skeleton under conditions of calcium stress, and appears to be in the process of becoming vestigial. CT produced in other tissues has paracrine actions that modulate functions such as proton transport, acid-base balance, prolactin secretion, and gastrointestinal motility. C-cells in mammals evolved from the ultimobranchial body that secretes CT in all lower vertebrates. It is highly probable that changes in amino acid sequence during evolution are responsible for the loss of activity, as fish CT is about 40 times as potent as human CT. CT may have been very important to survival in seawater fish, but the presence of the parathyroid gland and other evolutionary changes occurring in tetrapods suggest that the function of CT is no longer important.

Interrelationships of bone ash and whole bone properties in the lactating and parous rat

SummarySeveral investigators have studied the mechanical properties of long bones of rats and have found that bone strength may be associated with bone mineral content. In this study we examined further the interrelationships of whole bone properties and bone ash weight of the rat femur. Using a 3-point bending test, the bones were evaluated during lactation and after one to three pregnancy— lactation periods—experiments in which bone ash was significantly reduced. A relationship was established between the time of lactation and the ash weight, stiffness, strength, and toughness but not the ductility. Using all of the experimental data, a highly significant relationship was observed between ash weight and stiffness. A similar significant correlation was revealed between bone ash and strength or toughness, although drawing individual lines for each experiment rather than a common line for all the experiments appeared more appropriate. Given the inherent inaccuracies in the material property measurements of bone, the results suggest that routine measurements of whole bone properties provide an important and sensitive way to evaluate bone quality and that these properties correlate significantly with the bone ash weight.

Calcium-lowering action of glucocorticoids in adrenalectomized-parathyroidectomized rats: Specificity and relative potency of natural and synthetic glucocorticoids

The specificity and potency of glucocorticoids to lower serum calcium (Ca) in rats after parathyroidectomy (PTX) and adrenalectomy (ADX) were examined. Rats fasted overnight were given sc injections of various steroids immediately after the operations. The fall in serum calcium 5h after PTX-ADX in rats given hypo-calcemic doses of corticosterone was compared to that after injection of a test steroid. At high doses, progesterone, estradiol, testosterone, and aldosterone were inactive, whereas glucocorticoids were consistently hypocalcemic. These results indicate that the Ca-lowering effect is specific for steroids with glucocorticoid activity. Potency estimates were made by comparing the dose-response of natural and synthetic glucocorticoids to that of corticosterone, the major glucocorticoid in rats. The mean potency of hydrocortisone was 8.2 times that of corticosterone. Prednisolone was about 9.6, triamcinolone 33, betamethasone 109, and dexamethasone 301 times as potent as corticosterone. Thus, the use of the calcium-lowering action as a bioassay has provides a specific and rapid in vivo method to compare potencies of glucocorticoids consistent with those obtained by anti-inflammatory and glycogen deposition assays. The importance of this interesting calcitonin-like action of glucocorticoids in normal physiology of calcium metabolism is not yet established.

Importance of calcitonin in physiology, clinical pharmacology, and medicine

This certainly is a great occasion. Thank you, Harold, for bringing together 20 or so eminent worthies in the history of calcitonin. Phil Hirsch and I are more than delighted to join with you all in this celebration. Our paper is in three parts: 1) our role in the discovery of calcitonin; 2) importance of calcitonin; 3) our current research on a second set of hypocalcemic hormones, the glucocorticoids .

Discovery and pharmacologic evaluation of thyrocalcitonin

Abstract The discovery of thyrocalcitonin in 1963 grew out of an investigation of the greater fall of serum calcium levels in rats after parathyroidectomy by hot wire cautery than by surgical excision. It was found that the differential effect could be explained by a cautery-stimulated release of a hypocalcemic substance (thyrocalcitonin) from the thyroid gland into the circulation. A bioassay method used to monitor steps in the purification procedure helped to make possible the recent isolation of pig thyrocalcitonin as a homogeneous polypeptide. This method is based on the extent of hypocalcemia produced in calcium-depleted, young intact Holtzman rats one hour after subcutaneous injection of test preparations. The method was substantially improved in precision and convenience by feeding the rats the Harvard low calcium diet for only one day instead of four days before the assay and by using rats five weeks rather than six and a half weeks of age. Feeding the Harvard low calcium diet, adequate in phosphate, for one day was found to be superior to fasting or use of a commercial diet low in both calcium and phosphate. Injection of inorganic phosphate greatly enhanced the hypocalcemic effect of thyrocalcitonin in fasted rats. In strains of rats relatively less responsive to rat and pig thyrocalcitonin, the thyrocalcitonin content of the thyroid glands was higher than in the more responsive Holtzman rats. Older rats were found to be less responsive to injected thyrocalcitonin than younger rats. Thyrocalcitonin and parathyroid hormone are antagonistic in their effects on plasma calcium. Thyrocalcitonin is active in the absence of parathyroid hormone and its effect is not dependent on the presence of the pituitary gland, kidney or gastrointestinal tract. It acts directly on bone in vitro to inhibit resorption. In the absence of the thyroid gland, recovery from induced hypercalcemia is slower than normal, and this deficiency can be corrected by administration of thyrocalcitonin. However, a more important function of thyrocalcitonin in normal physiology than protection against hypercalcemia may be its role in the development and maintenance of the skeleton.

Glucocorticoids possess calcitonin-like antihypercalcemic properties in rats

AbstractThe interaction among parathyroid hormone (PTH), calcitonin (CT), and glucocorticoids on blood calcium (Ca) was examined. Prior studies had shown that adrenalectomy (ADX) reduced the fall in blood calcium in rats after parathyroidectomy (PTX). Convincing evidence was provided showing that the ADX effect in PTX rats was due to the loss of corticosterone, the major glucocorticoid in rats; restoring physiological blood levels of corticosterone abolished the ADX effect in PTX rats.The initial attempt of the present study was to explain the failure of ADX or exogenous glucocorticoids to alter serum Ca levels in rats with intact thyroid and parathyroid glands or in thyroidectomized rats with functional parathyroid transplants (PTT). We found, as previously reported, that the 5-h level of serum Ca in rats with parathyroid glands was not affected by sc hydrocortisone (cortisol) or by ADX. It was also not affected by thyroparathyroidectomy (TPTX) or after both ADX and TPTX in rats with PTT.These results suggested to us that the glucocorticoid effect to lower serum was inhibited by endogenous parathyroid hormone (PTH) from the parathyroid gland and/or by normal levels of blood Ca. Both of these proposed mechanisms were examined and failed to explain the absence of the ADX effect as well as the glucocorticoid effect in normocalcemic parathyroid-intact rats, because an ADX effect was observed in TPTX rats given hypercalcemic doses of rat or bovine PTH 1-34 or calcitriol. Also, administered cortisol restricted the increased hypercalcemia induced by PTH in ADX-TPTX rats. Expanding on the results in TPTX rats with induced hypercalcemia, we found that neither the ADX effect nor the glucocorticoid effect occurred in thyroid-intact rats with or without functional PTT. These as well as previous results show that:1.Glucocorticoids, like CT, restrict hypercalcemia in TPTX rats.2.The ADX effect and its reversal by glucocorticoids in rats with induced hypercalcemia occur only in the absence of the thyroid gland (removal of CT).3.Gucocorticoids, like CT, lower serum calcium during the hypocalcemia after PTX, an effect that occurs in the presence or absence of the thyroid gland. This study did not reveal why neither ADX nor exogenous glucocorticoids altered serum calcium levels in normocalcemic rats with either intact parathyroid glands or PTT. We conclude that under appropriate conditions, glucocorticoids act in a fashion similar to that of CT in restricting hypercalcemia and in lowering blood Ca.

Alterations in Thyroidal Calcitonin Content of Rats Fed Diets of Varying Ca:P Ratios

Summary The thyroidal content of CT was measured by bioassay and radioimmu-noassay in intact, parathyroidectomized, and adrenalectomized rats fed diets with varying Ca:P ratios for 4 to 8 weeks. There was a good correlation between values obtained with the two assay methods. In intact, parex, and adrex rats, thyroidal CT content was higher 4 and 8 weeks after feeding a diet with a Ca:P ratio of 1:8. However, there was a little difference in overall CT content between intact, parex, and adrex rats despite marked differences in the levels of serum calcium. CT content appeared to be related to the length of time the rats were fed the high phosphate diet. In intact rats fed diets with Ca:P ratios ranging from 8:1 to 1:8 and prepared by varying the calcium content (P constant at 0.4%), the CT content of thyroid gland and serum was elevated only in those animals fed the 1:8 diet. In other intact rats fed with diets having the same range of ratios but prepared by varying the phosphorus content (Ca constant at 0.4%), thyroidal CT increased progressively with the diets having a Ca:P ratio below 1:1. The results indicate that changes in thyroidal CT stores produced by feeding diets with varying Ca:P ratios cannot be explained solely on the basis of changes in the levels of serum calcium.

Purification and Chemical Properties of Porcine Thyrocalcitonin

Abstract Thyrocalcitonin has been isolated from porcine thyroid tissue following a 40,000-fold purification. Specific biological activity is 200 MRC units per mg. The purified peptide was shown to be homogenous by multiple criteria and to consist of 32 amino acids: Arg2, His1, Thr2, Ser4, Glu1, Pro2, Gly3, Ala1, Val1, Met1, Asn4, Leu3, Phe3, Tyr1, Trp1, half-Cys2. The purified hormone is resolved by chromatography on carboxymethyl cellulose or cellulose thin layer plates into two components. The only difference between these components is that one form of the hormone contains methionine as the sulfoxide; the other component, methionine in the reduced form. Both forms have equal specific activity. The peptide contains no covalently bound carbohydrate, iodine, or other substituted amino acids. The 2 half-cystine residues are present in the hormone as an intrachain disulfide bond. Since 1 half-cystine residue is the NH2-terminal amino acid, the amino-terminal portion of the molecule is in the form of a 23-membered ring. The lack of a high degree of ordered structure in the molecule is reflected in the complete solvent accessibility of the methionine, tryptophan, and tyrosine residues without prior denaturation of the polypeptide.