Roy V. Talmage

Are calcium intakes and physical activity patterns during adolescence related to radial bone mass of white college-age females?

The determinants of bone mass, i.e., size and density, in young adult women after cessation of growth in length of the bones are not well understood. Usual dietary calcium (Ca) intakes and physical activity (PA) patterns during the post-pubertal half-decade have been considered as two important factors contributing to bone mass. In the present hypothesis-generating cross-sectional study, radial bone mineral content and density were measured by single-photon absorptiometry at two sites containing different proportions of trabecular and cortical bone tissue in 705 healthy, Caucasian college women (18–22 years). Ca intake during high school and college, as estimated by milk and cheese intake only, was categorized into low, moderate and high groups; and physical activity, estimated during the same time frame, was also categorized into low, moderate and high groups. Bone measurements were related to both long-term dietary Ca intake from milk and cheese and long-term PA in sports, dance or other exercises, as assessed by recall. By univariate analyses, both distal and mid-radial bone mineral content (BMC) and areal bone mineral density (BMD) were found to be positively related to gynecological age (GA) (p<0.01). Also, independent effects of long-term Ca intake (p<0.05) on distal BMC and BMD, and of long-term PA (p<0.05) on distal and mid-BMC and BMD were observed. When adjusted for GA, body mass index (BMI) and PA, the relationship between long-term Ca no longer remained significant; but when we adjusted for GA, BMI and long-term Ca, the significance of PA on radial bone remained essentially unchanged. The combined effects of both long-term moderate Ca and long-term moderate PA versus long-term low Ca and long-term low PA in college-age women showed that distal radial bone parameters were significantly greater in the moderate/moderate individuals when compared with subjects with low levels of both variables. The high/high combination also significantly improved distal bone variables, but in general not much more so than the moderate/moderate pair. Mid-radial bone values showed only statistical trends, when analyzing high/high versus low/low comparisons. We conclude that adequate levels (moderate or high of either) of both Ca and PA during adolescence and early adulthood are positive and independent determinants of both radial BMC and BMD, including much trabecular bone tissue, but that long-term exercise exerts a greater overall beneficial effect on distal radial bone measurements than does long-term Ca consumption.

Relationship of Blood Concentrations of Calcium, Phosphate, Gastrin and Calcitonin to the Onset of Feeding in the Rat

Summary Daily fluctuations in plasma calcium concentrations in rats trained to a closely regulated feeding pattern have been compared to corresponding plasma gastrin and calcitonin concentrations. The time period studied was that extending from 4 hr prior to the start of the feeding period through the first 2 hr of feeding. Both plasma calcium and phosphate levels fell prior to the start of the feeding period and remained low at least for the first 2 hr of feeding. This pattern was also observed in rats in which food was withheld for 2 hr past the regular feeding time. Plasma 45Ca and 32P concentrations (radionuclide injected at least one week prior to sampling) did not follow the pattern of their stable counterparts. Instead, these values rose or remained constant until after feeding had commenced, after which they fell precipitously. Both plasma calcitonin and gastrin levels rose rapidly after the start of the feeding period. The primary point of emphasis is that calcitonin secretion was produced in these rats by an intestinal related stimulus and not by a rise in plasma calcium concentration. The technical assistance of Dorothy B. Raneri, Blanche P. Holloway and Johnny F. Obie is acknowledged and appreciated. Thanks are expressed to Merlene Moore for clerical assistance.

An interpretation of acute changes in plasma45Ca following parathyroid hormone administration to thyroparathyroidectomized rats

Acute changes in plasma calcium and45Ca were studied in young adult male thyroparathyroidectomized (TPTX) rats injected with moderate doses of parathyroid hormone (PTH). For plasma calcium changes, comparison was made between rats fasted or fed prior to PTH injection. For plasma45Ca changes, the effect of the time of administration of the radionuclide was also studied; this included rats injected with PTH 1 h after radionuclide (“1 h45Ca”), 18 h later (“18 h45Ca”) and more than 6 days later (“6 day45Ca”). The results can be summarized as follows: (1) Plasma calcium changes were greater when PTH was injected into “fed” rather than into “fasted” rats. (2) PTH always produced a relative increase (compared to controls tested concurrently) in plasma45Ca concentrations. This increase was the same in the “1 h45Ca” and the “18 h45Ca” groups. (3) Plasma45Ca rose at least temporarily following PTH injection in the “18 h45Ca” group. (4) The45Ca rise following PTH was always greater in “fed” than in “fasted” groups. (5) Plasma45Ca specific activities (S.A.) tended to rise in the “6 day45Ca” group and to fall in the “18 h45Ca” group, following PTH injection. However, the45Ca S.A. was always higher in fed than fasted groups. (6) In a few experiments in which32P was injected with45Ca, specific activity changes in plasma45Ca following PTH injection werenot accompanied by similar changes in32P specific activity.These results could not be adequately explained by PTH effects on bone resorption, but the data supported the postulate that PTH controls plasma calcium concentrations by increasing transport of calcium through the osteocyte-lining cell (osteoblast) bone cell complex from the bone fluid compartment to the ECF.

Daily fluctuations in plasma calcium, phosphate, and their radionuclide concentrations in the rat

Daily fluctuations in plasma calcium, phosphate and their radionuclides (injected 5 or more days previously) were determined in rats maintained on a closely controlled feeding and light schedule. Male rats (150–300 g) were trained to a 7 a.m.–7 p.m. ‘light’ schedule with food available 9 p.m. to 9 a.m. All parameters dropped rapidly at the start of each feeding period and then rose during the day. The daily changes in radionuclide concentrations were several orders of magnitude greater than for the stable ions. Continuous access to food produced an earlier fall (5 p.m.) in all plasma values. In thyroidectomized rats (with parathyroid transplants) plasma calcium and phosphate remained relatively constant during the 24 h period. If the time of availability of food was moved 6 h earlier (no change in light cycle), the drop in these plasma values also occurred 6 h earlier. Closer examination of these daily changes indicated that all values fell at least 1 h prior to feeding. In fasted rats, plasma calcium and phosphate concentration fell as usual; however,45Ca and32P rose instead of falling. It was concluded that, in normal rats, changes in endogenous calcitonin secretion may actually increase rather than decrease daily perturbations of plasma calcium and phosphate. Daily changes in these values are influenced by a conditioned reflex developed due to the feeding habits of the rats. Finally, it is suggested that intestinal hormones rather than calcium may be the primary control of calcitonin secretion, and that the gastrointestinal tract plays a major role, in addition to absorptive activity, in both calcium and phosphate homeostasis.

Effect of fasting and parathyroid hormone injection on plasma45Ca concentrations in rats

Young male rats were administered45Ca 5 days to 2 weeks prior to use. All rats were either parathyroidectomized (PTX) or thyroparathyroidectomized (TPTX) and given several days to recover from surgery. The first group of rats were maintained on a 12 h dark-fed and 12 h light-fasted daily cycle. The remainder of the rats were used for parathyroid hormone (PTH) studies (0.1–0.6 U/g body weight) following which blood samples were obtained from the tail for 1 to 6 h. Two groups of these rats were bilaterally nephrectomized 18 h before PTH injection. Two contrasting results were obtained: in PTX (or TPTX) rats maintained on the closely regulated food and light regime, plasma45Ca concentrations rose markedly each day at the start of the fasting period and then fell slowly. Total plasma calcium values fell throughout the fasting period. A similar rise and fall was also observed in45Ca values of rats experimentally fasted after being maintained with food continuously available. In contrast, in all PTX or TPTX rats, PTH injection was followed by an equal rise in both plasma calcium and45Ca values so that for the first few hours plasma45Ca specific activity was unchanged. These data are consistent with the concept of a bone fluid compartment (BFC) separated by a cellular interface from the primary extracellular fluid space (ECF). It is postulated that through this cellular interface calcium is actively ‘pumped’ from the BFC to the ECF. The rise in plasma45Ca values at the start of fasting is explained on the basis of decreased entry of stable calcium from the gastrointestinal tract and a continued movement of calcium and45Ca from the BFC to the ECF. The concomitant increase in plasma calcium and45Ca during the first few hours after PTH injection is explained by a rapid action of PTH to increase the rate of calcium movement from BFC to ECF by its action at the cellular interface, without altering45Ca specific activity until such time as dissolution of bone crystals is required as a supply of calcium.

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.

Parathyroid Hormone Effects in Rats Treated with Diphosphonate

The ability of disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP; 40 milligrams per kilogram of body weight per day) to reduce the hypercalcemic effect of parathyroid hormone in thyroparathyroidectomized rats was confirmed. However, treatment with this large dose of EHDP enhanced the hypophosphatemic effect of a low dose of parathyroid hormone (10 international units per100 grams of body weight), apparently by promoting the renal excretion of phosphate. The data suggest that EHDP may have a direct effect on the renal action of parathyroid hormone and, in this way, may also affect vitamin D metabolism by the kidney.

Effect of salmon calcitonin infusion on plasma concentrations of recently administered45Ca

SummaryThe effect of intravenous salmon calcitonin (SCT) infusion on the net rate of loss from plasma of recently injected45Ca has been studied in conscious fasted rats, some of which had been previously thyroparathyroidectomized (TPTX). The45Ca was injected 10 h prior to or 1 or 2 h after the initiation of the infusion. Salmon calcitonin (0.05–0.2 MRC mU/g body weight/h) infusion produced the expected hypocalcemia and hypophosphatemia. The drop in plasma calcium levels produced by the hormone was lowered further by the addition of phosphate to the infusate.Plasma45Ca changes, during SCT infusion, followed a biphasic pattern when the radionuclide injection preceded SCT infusion by 10 h. During the first part of the infusion, the rate of loss of45Ca from plasma was greater in SCT-treated than in control rats. Later, this pattern reversed, and the rate of loss of45Ca from plasma became slower in SCT-treated animals. If45Ca was injected after SCT infusion was initiated only the second phase of the biphasic effect was observed; i.e., disappearance of45Ca from plasma was slower in SCT-infused than in control rats.These data are considered to support the hypothesis that rapid and opposing calcium fluxes exist between bone fluid and plasma, and that calcitonin acts by decreasing the calcium efflux from bone fluid. The calcium moved in this efflux system includes some that has only recently entered bone fluid from plasma. This is the explanation given for the initial decrease in plasma45Ca concentrations following SCT infusion. The reverse effect in the second phase can be explained as the result of a secondary reduction in calcium influx to bone fluid from plasma.

Influence of calcitonin on the initial uptake of lead and mercury by bone.

Summary Lead acetate (20 to 40 mg/kg body weight) or mercuric chloride (3 mg/kg body weight) was injected intravenously into normal rats and those pretreated with calcitonin (0.1 mU/g body wt/hr for 1-4 hr). Rats were sacrificed at intervals from 5 min to 24 hr after heavy metal injection. Tibias were prepared histologically, using Okada and Mimuras vital staining technique. The metaphyses and diaphyses were examined under the light microscope to study the effect of calcitonin on the uptake and distribution of these two metals in bone. Calcitonin dramatically reduced the initial uptake of lead and mercury by bone. This reduction, at least for lead, occurred in both trabecular and compact bone. The mechanism(s) by which calcitonin influences the distribution of lead and mercury in bone has not yet been determined.

A reevaluation of the cause of acute hypercalcemia following intravenous administration of lead acetate

SummaryThe effect of intravenous (i.v.) injection of lead acetate (15 or 30 mg/kg) was studied in young adult male rats. The reaction of lead with rat plasma to produce colloidal material containing both calcium and phosphate was demonstrated both in vitro and in vivo. This material could be centrifuged down at 25,000 ×g from plasma aliquots to which lead had been added or from plasma samples obtained as early as 5 min after i.v. lead injection. The hypercalcemia and hyperphosphatemia reached their peak rapidly after lead injection, and even at the higher dose level these lasted less than 6 h.45Ca and32P were injected at times varying from 1 to 8 days prior to lead administration. Plasma radioactivity levels rose with their stable counterparts without affecting plasma nuclide specific activity. Lead uptake on bone surfaces could be detected within 5 min of injection, and continued to accumulate for at least 6 h. It is concluded that although lead attaches to bone surfaces, the hypercalcemia and hyperphosphatemia are caused by the direct interaction of lead with calcium and phosphate in solution in plasma. The formation of the colloidal material lowers ionic calcium and phosphate. In vivo, calcium and phosphate are immediately withdrawn from extravascular sources returning plasma concentrations to their initial value. The source of at least the calcium is believed to be bone. The attachment of lead to bone surfaces does not in itself release significant amounts of calcium and phosphate into blood.