Carole J. VanderWiel

Calcitonin and phosphate

This report summarizes the relationship of calcitonin to phosphate. The hypocalcemic action of calcitonin is dependent upon phosphate, while the hypophosphatemic action is independent of calcium. Calcitonin moves phosphate into bone cells and bone fluid in contrast to reducing the movement of calcium from bone to blood. Calcitonin acts rapidly and at low doses on the osteocytes and lining cells at bone surfaces. Morphological changes can be identified within 7 min. This action causes the accumulation of an electron-dense material both in bone lining cells and their microenvironment. It is postulated that both the hypocalcemic action of calcitonin and its ability to cause an accumulation of material at bone surfaces may result from the movement of phosphate into these areas. The biochemical action which could produce the phosphate movement is unknown. The possibility is suggested that calcitonin increases phosphate transport into bone cells.

The influence of calcitonin on the plasma and urine phosphate changes produced by parathyroid hormone

SummaryThe influence of calcitonin (CT) on parathyroid hormone-induced changes in plasma phosphate, urine phosphate, plasma32P and32P specific activity (32P-S.A.) was studied using thyroparathyroidectomized rats. Radiophosphorus was injected at either 10 h (32P-10 h) or 8 days (32P-8 d) before commencing hormonal treatment. In order to produce consistent plasma32P-S.A. changes in rats treated with parathyroid hormone (PTH) alone, we maintained all animals for 48 h prior to hormone injection on a closely regulated carbohydrate intake (both quantity of food and time of feeding). Rats were then fasted 10 h prior to injection of PTH (0.01 U/g body weight/h), or CT (0.1 mU/g body weight/h), or both hormones injected simultaneously. Under these conditions, PTH produced a rise (relative to controls) in plasma32P-S.A. (-8 d) during an 8-h experimental period, while plasma32P-S.A. (-10 h) fell. Calcitonin treatment alone did not affect plasma32P-S.A., regardless of when32P was administered.When both hormones were injected concurrently, CT augmented the PTH-induced hypophosphatemia and magnified the fall in plasma32P levels, but the addition of CT did not affect PTH-induced changes in either renal calcium or phosphate excretion. However, the PTH-induced changes in plasma32P-S.A. were abolished.These data could not be explained by the opposing effects of the two hormones on rates of bone resorption. We interpret the results as adding support to the postulate that calcitonin moves phosphate into and prevents its loss from bone and bone fluid.

Ultrastructural and physiological evidence for calcitonin-induced postprandial calcium storage in bones of rats.

SummaryThe postulate tested by these experiments is that calcitonin directs a portion of the calcium absorbed from food into temporary storage at surfaces of bone. This study utilized young adult rats in which the parathyroid glands had been autotransplanted. Many rats were also thyroidectomized (TX). All were trained to a 0900 h feeding schedule with predetermined calcium content of food. Calcitonin was injected only during the first 4 h postprandially to some of the TX animals. Urine calcium was monitored and, after sacrifice, changes in the tibia were recorded. The following results were obtained: (a) In thyroid-intact (TI) rats, renal calcium excretion was reduced if the daily intake of calcium was less than 90 mg. In TX rats calcium excretion rose each day during the time of intestinal absorption of calcium. Calcitonin injection to TX rats reduced urinary calcium content on the day it was injected. However, on the following 2 days, renal calcium excretion rose to twice that of TX controls. (b) Electron micrographs of bone tissue from TI rats and TX rats injected with calcitonin showed greater pyroantimonate reaction within bone fluid than did tissue from TX controls. Similarly, when prepared by an anhydrous procedure, bone surfaces of tibia taken from rats with endogenous or exogenous calcitonin contained dense material accumulation (following a calcium-containing meal) not present in bones from TX rats. (c) When tibia shaft fragments of rats sacrificed 4 h after consuming a calcium-containing meal were washed in acidic saline, more calcium accumulated during the first 15 min in the media containing bones from TI than from TX rats. The final equilibration level between media and bone fragments was not affected by the calcitonin state of the rat. These experiments demonstrate both physiological and bone morphological differences between TI and TX rats which can be negated by postprandial calcitonin injection to TX animals. They support the postulate that the secretion of calcitonin postprandially aids in the conservation and storage of ingested calcium.

Plasma Calcium, Plasma and Thyroidal Calcitonin, and Histomorphometric Bone Changes in Parathyroidectomized Rats

Abstract This study examined physiological and bone morphological changes resulting from parathyroidectomy (PTX) in male rats. Two diets were provided: one contained 1.2% calcium, 1% phosphorus; the other 0.4-0.6% calcium, 0.6% phosphorus. However, at 51/2 months post-PTX all rats were transferred to a diet containing 0.3% calcium. The daily ration for each rat was 16 g. The parameters examined were: (1) Plasma calcium concentrations; (2) plasma and thyroidal calcitonin levels; and (3) histomorphometry of metaphyseal trabecular bone. When maintained on the lower calcium diet, fasted plasma calcium concentrations of PTX rats stabilized between 5 and 6 mg/dl. In contrast, in rats fed the higher calcium diet, these values gradually rose to between 7 and 8 mg/dl (5 months post-PTX). After transfer to the 0.3% calcium diet, plasma calcium values fell to >6 mg/dl. Thyroidal calcitonin content following PTX rose to values three times those of age-matched controls regardless of the daily calcium intake. The volume of trabecular bone in the tibial metaphysis increased threefold by 61/2 months after PTX; however, there was a decrease in osteoid on these bone surfaces. The static parameters of bone resorption and formation in PTX rats were not statistically different from controls; however, the ratio of osteoblasts to lining cells on trabecular surfaces increased following PTX. The cause of the increase in thyroidal calcitonin following PTX is as yet unknown, but appears to be unrelated to the changes in plasma calcium in rats fed a high calcium diet. This rise in plasma calcium is attributed to accumulation of calcium in the bone surface exchangeable compartment which is reversible. The increase in volume of trabecular bone may be due to slight changes in rates of bone turnover which are not detectable in analysis of static parameters. There was no evidence that the epiphyseal growth plate, or the rate of enchondral bone formation was affected by PTX. The effect of high dietary calcium on post-PTX plasma calcium values points out the need for close control of calcium content of rat chow if the rat is to be used as a model for studying calcium homeostasis or hormonal effects on bone remodeling.

Factors affecting parathyroid hormone-induced hypophosphatemia and32P specific activity in thyroparathyroidectomized rats

SummaryPlasma changes in calcium, phosphate, and their radionuclides were studied in thyroparathyroidectomized (TPTX) rats treated with parathyroid hormone (PTH) for 8 h, this treatment starting 10 h after injection of45Ca and32P. Prior to intravenous infusion or hourly injections of PTH (10 mU/g/h), rats were maintained in one of three ways: on an extended fast (24 h); on a partial fast (10 h); or provided with 10% glucose and 1% calcium lactate overnight as a substitution for solid food. The pattern of change for plasma calcium,45Ca, and45Ca specific activity (S.A.) produced by PTH was not affected by these dietary conditions. The changes in phosphate were as follows: During the experimental (8 h) period, the rate of loss of32P from plasma in control rats was proportional to the length of the fast. This suggests that32P was released into plasma during the experimental period proportional to the ready availability of soft tissue glucose. In rats on an extended fast, PTH was phosphaturic, hypophosphatemic, and increased the rate of loss of32P from plasma without affecting32P S.A. values. In rats fasted for only 10 h, PTH produced similar effects on plasma phosphate and plasma32P values, but also caused a significant fall in plasma32P S.A. After glucose and calcium lactate treatment, PTH-induced phosphaturia was temporarily lost and the marked hypophosphatemia was replaced with a slight hyperphosphatemia. Plasma32P values also rose slightly; therefore, no effect on32P S.A. was produced. It is concluded from these studies that as the result of the phosphaturia caused by PTH, the hypophosphatemia which is produced automatically changes the phosphate gradient between various body compartments, causing phosphate entry into plasma. The authors postulate that this phosphate entering plasma is withdrawn primarily from bone fluid and bone.

A study of the action of calcitonin by its effects on lead-induced hypercalcemia

SummaryIntravenous (i.v.) injection of lead acetate produces an immediate elevation in total plasma calcium and phosphate levels. This is due to the formation of calcium-phosphate compounds which can be removed by centrifugation at 25,000 ×g. For this study, the effect of salmon calcitonin (CT) on these lead-induced plasma changes was studied. Intact male rats (175–250 g) were injected i.v. with lead acetate 10–30 mg/kg. Sodium acetateinjected rats served as controls. CT (0.1–0.2 mU/g) injected 30 min prior to lead modified the lead-induced plasma changes as follows: The concentration of lead remaining in plasma was statistically reduced. This was accompanied by a decrease in the amount of colloidal calcium-phosphate removed by ultracentrifugation and a corresponding decrease in the lead-induced elevation of total plasma calcium and phosphate levels. This action of CT was still effective in actuely nephrectomized rats. However, a 15-day pretreatment with a diphosphonate (20 mg EHDP/kg/day) abolished the hypocalcemic effect of CT and also abolished the ability of CT to affect the lead-induced plasma changes. Finally, CT was ineffective if injected in as short a time period as 30 min after lead injection.It is concluded from these studies that CT causes a rapid sequestering of lead from plasma into specific sites in bone. Previous reports show that within minutes after injection, CT causes the formation of calcium-phosphate complexes in fluid at the surfaces of bone and in osteocyte lacunae. It is postulated that lead reacts with these colloids (complexes) in a manner similar to its attachment to calcium and phosphate in plasma, thereby removing this lead from blood.

Reduction of lead-induced hypercalcemia by calcitonin: Comparison between thyroid-intact and thyroidectomized rats

SummaryThis report examines the ability of either exogenous or endogenous calcitonin to reduce the degree of hypercalcemia and hyperphosphatemia which follows an intravenous injection of lead acetate (20 mg/kg body weight). Blood samples were obtained prior to and 1 h after lead injection. The experimental groups were normal young adult rats; rats with autotransplanted parathyroid glands and functional thyroids (TI); and rats bearing transplanted parathyroids which were also thyroidectomized (TX). The normal rats were fed ad libitum and injected with calcitonin (40 pg/g body weight) after an overnight fast. Rats with parathyroid transplants were trained to a 0900 h feeding schedule and studied at sequential times related to feeding. TX rats were compared to TI animals and to TX rats injected postprandially with calcitonin. The following results were obtained: (a) When lead was injected into fasted normal rats, the ability of calcitonin to reduce the lead-induced hypercalcemia developed within 1 h after hormone administration. By 4 h after calcitonin injection this effect of the hormone had essentially disappeared. (b) In TI rats trained to a 0900 h feeding schedule, the degree of lead-induced hypercalcemia was less than that in TX rats for most time periods after consuming either a calcium-containing or a calcium-free meal. (c) Calcitonin injected postprandially into TX rats brought the response in these rats back in line with TI animals but only for the projected biological life of the hormone. It is concluded that the reduction in lead-induced hypercalcemia seen in TI rats is indicative of the presence of circulating endogenous calcitonin. It is suggested that this effect of calcitonin is a reflection of its biochemical action in bone cells and may be related to accumulation of phosphate induced by the hormone.

An ultrastructural study of postprandial changes in bone lining cells of lead-injected thyroidectomized and thyroid-intact rats

SummaryThis report is an electron microscopic study demonstrating intracellular differences in endosteal bone lining cells in tissue removed from thyroid-intact (TI) and thyroidectomized (TX) rats. Rats with functional parathyroid gland transplants, many of which were thyroidectomized, were trained to a 0900 h feeding schedule. Some of the TX rats (TX + CT) were injected postprandially with calcitonin (80 pg/g body weight). One hour before sacrifice all rats were injected with lead acetate (20 mg/kg body weight). Sections of the diaphysealmetaphyseal junction of the tibia were prepared for morphological examination using two procedures designed for the study of nondecalcified bone tissue. The following results were obtained: (a) Following pyroantimonate fixation, the electron-dense precipitate in sections obtained from TI and TX + CT rats was arranged in a distinct intracellular organization within bone lining cells. Following an anhydrous tissue preparation (in ethylene glycol), the mitochondria of these cells contained distinct electron-lucent areas. A similar result was found in tissue from TI rats regardless of the relationship of the time of sacrifice to the last feeding. (b) In TX rats, the major difference in the appearance of these bone lining cells was in tissue removed 4 h following a calcium-containing meal. The electron-dense precipitate (resulting from pyroantimonate fixation) was diffusely distributed throughout the lining cells rather than organized. The mitochondria of the cells (anhydrous procedure) contained distinct electrondense areas. These marked differences in tissues from TX rats were transient. With time following feeding, the histological appearance of these cells returned to that seen in cells from TI rats and TX rats injected postprandially with calcitonin. These results, utilizing the unique reaction of lead with bone cells, demonstrate an additional calcitonin-dependent response of rats to oral intake of calcium. They support the concept that endosteal bone tissue contains target cells for calcitonin and that this hormone has a role in postprandial conservation and storage of calcium.