Endogenous prolactin modulated the calcium absorption in the jejunum of suckling rats

Prolactin has been reported to stimulate intestinal calcium absorption in young and mature, but not aging rats. The present study was performed on suckling rats to elucidate the actions of endogenous prolactin on calcium absorption in various intestinal segments. Before measuring the calcium fluxes, 9-day-old rats were administered for 7 days with 0.9% NaCl, s.c. (control), 3 mg/kg bromocriptine, i.p., twice daily to abolish secretion of endogenous prolactin, or bromocriptine plus exogenous 2.5 mg/kg prolactin, s.c. Thereafter, the 16-day-old rats were experimented upon by instilling the 45Ca-containing solution into the intestinal segments. The results showed that, under a physiological condition, the jejunum had the highest rate of calcium absorption compared with other segments (1.4 +/- 0.35 micromol.h-1.cm-1, p < 0.05). The duodenum and ileum also manifested calcium absorption, whereas the colon showed calcium secretion. Lack of endogenous prolactin decreased lumen-to-plasma and net calcium fluxes in jejunum from 2.07 +/- 0.31 to 1.19 +/- 0.12 and 1.40 +/- 0.35 to 0.88 +/- 0.18 micromol.h-1.cm-1 (p < 0.05), respectively, and exogenous prolactin restored the jejunal calcium absorption to the control value. Endogenous prolactin also had an effect on the duodenum but, in this case, exogenous prolactin did not reverse the effect of bromocriptine. However, neither ileal nor colonic calcium fluxes were influenced by prolactin. Because luminal sodium concentration has been demonstrated to affect calcium absorption in mature rats, the effect of varying luminal sodium concentrations on calcium fluxes in suckling rats was evaluated. The jejunum was used due to its highest rate of calcium absorption. After filling the jejunal segments with 124 (control), 80, 40 mmol/L Na+-containing or Na+-free solution, increases in calcium absorption were found to be inversely related to luminal sodium concentrations in both control and bromocriptine-treated rats. The plasma concentration of 45Ca under luminal sodium free condition was also higher than that of the control condition (2.26% +/- 0.07% vs. 2.01% +/- 0.09% administered dose, p < 0.05). However, 3H-mannitol, a marker of the widening of tight junction that was introduced into the lumen, had a stable level in the plasma during an increase in plasma 45Ca, suggesting that the widening of tight junction was not required for enhanced calcium absorption. In conclusion, calcium absorption in suckling rats was of the highest rate in the jejunum where endogenous prolactin modulated calcium absorption without increasing the paracellular transport of mannitol.     

Age- and sex-dependent stimulation of calcium uptake in duodenal cells by prolactin in vitamin D-deficient rats

Administration of ovine-prolactin (O-PRL) stimulated Ca2+ uptake in isolated duodenal cells prepared from vitamin D-deficient rats. The time course of this effect was biphasic: uptake activity reached a peak in 2.5 hrs followed by a decrease at 5 hrs to original levels. This stimulatory effect of O-PRL was observed in vitamin D-deficient male, but not in female rats. This stimulatory effect was observed in 16- and 26-week old, but not 9 week old, animals. Increase in Ca2+ uptake in duodenal cells was not due to a decrease in intracellular Ca2+ efflux. We measured serum Ca concentration in vitamin D-deficient female rats and found that serum Ca increased in D-deficient female rats between 16 and 52 weeks whereas a minimal increase was observed in D-deficient male rats. Although prolactin was shown to stimulate duodenal Ca2+ uptake, it appears that the source of the increase in levels of serum Ca in D-deficient female rats was not derived from an increase in Ca2+ uptake by prolactin in duodenum. The increase in serum calcium with time may explain why female D-deficient rats survive longer then male.     

The influence of oral corticosterone replacement on plasma prolactin levels of adrenalectomized female rats

The administration of 80 μg of corticosterone/ml of drinking solution to adrenalectomized (ADX) rats resulted in a 24 hour serum corticosterone pattern similar to that of intact animals except that the magnitude of the afternoon-nocturnal surge was one third. Basal plasma prolactin levels and the estrogen-induced afternoon prolactin surge were similar for intact and for ADX animals receiving corticosterone in the drinking solution. Adrenalectomized animals receiving 0.9% NaCl to drink, however, had an afternoon prolactin surge that was significantly lower than that of intact animals while basal levels were similar.     

High-calcium diet modulates effects of long-term prolactin exposure on the cortical bone calcium content in ovariectomized rats

High physiological prolactin induced positive calcium balance by stimulating intestinal calcium absorption, reducing renal calcium excretion, and increasing bone calcium deposition in female rats. Although prolactin-induced increase in trabecular bone calcium deposition was absent after ovariectomy, its effects on cortical bones were still controversial. The present investigation, therefore, aimed to study the effect of in vivo long-term high physiological prolactin induced by either anterior pituitary (AP) transplantation or 2.5 mg/kg prolactin injection on cortical bones in ovariectomized rats. Since the presence of prolactin receptors (PRLR) in different bones of normal adult rats has not been reported, we first determined mRNA expression of both short- and long-form PRLRs at the cortical sites (tibia and femur) and trabecular sites (calvaria and vertebrae) by using the RT-PCR. Our results showed the mRNA expression of both PRLR isoforms with predominant long form at all sites. However, high prolactin levels induced by AP transplantation in normal rats did not have any effect on the femoral bone mineral density or bone mineral content. By using (45)Ca kinetic study, 2.5 mg/kg prolactin did not alter bone formation, bone resorption, calcium deposition, and total calcium content in tibia and femur of adult ovariectomized rats. AP transplantation also had no effect on the cortical total calcium content in adult ovariectomized rats. Because previous work showed that the effects of prolactin were age dependent and could be modulated by high-calcium diet, interactions between prolactin and these two parameters were investigated. The results demonstrated that 2.0% wt/wt high-calcium diet significantly increased the tibial total calcium content in 9-wk-old young AP-grafted ovariectomized rats but decreased the tibial total calcium content in 22-wk-old adult rats. As for the vertebrae, the total calcium contents in both young and adult rats were not changed by high-calcium diet. The present results thus indicated that the adult cortical bones were potentially direct targets of prolactin. Moreover, the effects of high physiological prolactin on cortical bones were age dependent and were observed only under the modulation of high-calcium diet condition.     

Effect of high calcium diet on magnesium, catecholamines, and blood pressure of stroke-prone spontaneously hypertensive rats

To test the effect of a high dietary calcium intake on blood pressure, we fed stroke-prone spontaneously hypertensive (SHR-SP) and Wistar-Kyoto rats (WKY) diets containing (a) 0.25% Ca/0.08% Mg, (b) 4.0% Ca/0.02% Mg, and (c) 4.0% Ca/0.08% mg, beginning at 6 weeks of age. SHR-SP and WKY rats receiving 4% Ca with the lower Mg content had lower blood pressures, hypomagnesemia, and hypomagnesuria, and grew poorly. SHR-SP receiving 4% Ca and the higher Mg diet had blood pressures no different from those of rats receiving the 0.25% Ca diet, in spite of having lower body weights. Rubidium flux studies in erythrocytes were not influenced by Ca or Mg in the diets. Plasma phosphate values were moderately reduced in rats receiving 4% Ca diets. Epinephrine and norepinephrine values were higher in SHR-SP than in WKY rats. Norepinephrine increased with stress in both strains, independent of diet. Epinephrine values were lower in SHR-SP receiving the 4% Ca diets and showed less of an increase with stress compared to SHR-SP receiving the 0.25% Ca diet. After 26 weeks of diets, SHR-SP and WKY rats were given 0.9% NaCl in their drinking water. NaCl increased blood pressure in SHR-SP irrespective of Ca content of the diet. These data suggest that a high Ca diet influences Mg homeostasis and adrenal medullary function in SHR-SP. Further, SHR-SP appear resistant to any blood pressure lowering effect of Ca irrespective of NaCl intake.     

Long-term prolactin exposure differentially stimulated the transcellular and solvent drag-induced calcium transport in the duodenum of ovariectomized rats

Prolactin, having been shown to stimulate transcellular active and solvent drag-induced calcium transport in the duodenum of female rats, was postulated to improve duodenal calcium transport in estrogen-deficient rats. The aim of the present study was, therefore, to demonstrate the effects of long-term prolactin exposure produced by anterior pituitary (AP) transplantation on the duodenal calcium transport in young (9-week-old) and adult (22-week-old) ovariectomized rats. We found that ovariectomy did not alter the transcellular active duodenal calcium transport in young and adult rats fed normal calcium diet (1.0% w/w Ca) but decreased the solvent drag-induced duodenal calcium transport from 75.50 +/- 10.12 to 55.75 +/- 4.77 nmol.hr(-1).cm(-2) (P < 0.05) only in adult rats. Long-term prolactin exposure stimulated the transcellular active calcium transport in young and adult AP-grafted ovariectomized rats fed with normal calcium diet by more than 2-fold from 7.56 +/- 0.79 to 16.54 +/- 2.05 (P < 0.001) and 9.78 +/- 0.72 to 15.99 +/- 1.75 (P < 0.001) nmol.hr(-1).cm(-2), respectively. However, only the solvent drag-induced duodenal calcium transport in young rats was enhanced by prolactin from 95.51 +/- 10.64 to 163.20 +/- 18.03 nmol.hr(-1).cm(-2) (P < 0.001) whereas that in adult rats still showed a decreased flux from 75.50 +/- 10.12 to 47.77 +/- 5.42 nmol.hr(-1).cm(-2) (P < 0.05). Because oral calcium supplement has been widely used to improve calcium balance in estrogen-deficient animals, the effect of a high-calcium diet (2.0% w/w Ca) was also investigated. The results showed that stimulatory action of long-term prolactin on the transcellular active duodenal calcium transport in both young and adult rats was diminished after being fed a high-calcium diet. The same diet also abolished prolactin-enhanced solvent drag-induced duodenal calcium transport in young and further decreased that in adult AP-grafted ovariectomized rats. We concluded that the solvent drag-induced duodenal calcium transport in adult rats was decreased after ovariectomy. Long-term prolactin exposure stimulated the transcellular active duodenal calcium transport in both young and adult rats whereas enhancing the solvent drag-induced duodenal calcium transport only in young rats. Effects of prolactin were abolished by a high-calcium diet.     

Digoxin-like immunoreactive factor in rat plasma: effect of sodium and calcium intake

Studies have been performed in rats to determine whether an endogenous material capable of binding to digoxin antibodies is present in the plasma. Such a material has been shown in other species and has been hypothesized to represent an endogenous ligand for the receptor on Na-K ATPase through which cardiac glycosides act. In rats consuming a normal rodent chow (1% calcium by weight) and drinking deionized water, endogenous binding of digoxin antibody in radioimmunoassay amounted to 23.1 +/- 4.6 fM digoxin equivalents/100 microliter of plasma (mean +/- SEM, n = 8). Since a hypothetical role for such an endogenous ligand is the regulation or renal sodium excretion by inhibition of renal Na-K ATPase, the effect of increased sodium intake on plasma levels of this digoxin-like immunoreactive factor (DLIF) was studied. Animals consuming the same chow, but drinking 0.5% NaCl solution in place of water for a 4 week period showed significantly greater DLIF in plasma which was measured at 109.2 +/- 20.3 fM digoxin equivalents/100 microliter of plasma (p less than 0.001). Because DLIF has been implicated in the pathogenesis of hypertension we also studied the effects of calcium intake on plasma levels of DLIF. In previous studies we have shown that rats allowed to drink 0.5% saline develop a moderate hypertension which can be reversed with calcium supplementation. In the present studies, 3 dietary calcium subgroups (0.01% Ca, 1.0% Ca and 4% Ca) were formed among animals drinking water or 0.5% saline for 4 weeks. No effect of low calcium intake on plasma DLIF was found either in water or saline drinkers. However, calcium supplementation produced a significant reduction in plasma DLIF in both water and saline drinking animals.     

Prolactin is an important regulator of intestinal calcium transport

Prolactin has been shown to stimulate intestinal calcium absorption, increase bone turnover, and reduce renal calcium excretion. The small intestine, which is the sole organ supplying new calcium to the body, intensely expresses mRNAs and proteins of prolactin receptors, especially in the duodenum and jejunum, indicating the intestine as a target tissue of prolactin. A number of investigations show that prolactin is able to stimulate the intestinal calcium transport both in vitro and in vivo, whereas bromocriptine, which inhibits pituitary prolactin secretion, antagonizes its actions. In female rats, acute and long-term exposure to high prolactin levels significantly enhances the (i) transcellular active, (ii) solvent drag-induced, and (iii) passive calcium transport occurring in the small intestine. These effects are seen not only in pregnant and lactating animals, but are also observed in non-pregnant and non-lactating animals. Interestingly, young animals are more responsive to prolactin than adults. Prolactin-enhanced calcium absorption gradually diminishes with age, thus suggesting it has an age-dependent mode of action. Although prolactin's effects on calcium absorption are not directly vitamin D-dependent; a certain level of circulating vitamin D may be required for the basal expression of genes related to calcium transport. The aforementioned body of evidence supports the hypothesis that prolactin acts as a regulator of calcium homeostasis by controlling the intestinal calcium absorption. Cellular and molecular signal transductions of prolactin in the enterocytes are largely unknown, however, and still require investigation.     

Study of acute pharmacologic effects of prolactin on calcium and water transport in the rat colon by an in vivo perfusion technique

We investigated the acute effect of intraperitoneally administered prolactin on calcium and water transport in colon of sexually mature female Wistar rats using an in vivo perfusion technique. Test solution containing (in mM) NaCl, 100; KCl, 4.7; MgSO4, 1.2; CaCl2, 20; D-glucose, 11; sodium ferrocyanide (Na4Fe(CN)6), an index of net water transport, 20; and 0.7 (microCi 45CaCl2 (1 Ci = 37 GBq) was perfused througth the 8-cm colonic loop for 60 min at perfusion rates of 0.5 or 1.0 mL x min(-1). Calcium and water transport was also studied under a no flow condition to stimulate the condition often found in the colon by in vivo ligated colonic loop for 30 min. Control results showed no correlation between calcium transport and water flux. Flow of luminal solution at 0.5 and 1.0 mL x min(-1) was found to reverse net calcium absorption from 0.04+/-0.01 nmol x g(-1) dry weight x h(-1) to net calcium secretion of 0.04+/-0.04 and 0.9+/-0.02 nmol x g(-1) dry weight x h(-1), respectively. Neither 0.4, 0.6, nor 1.0 mg x kg(-1) prolactin had any effect on calcium fluxes in the colon. On the other hand, at a perfusion rate of 1 mL x min(-1), 0.4 mg x kg(-1) prolactin significantly decreased net water absorption from 3.86+/-0.90 to 0.88+/-0.64 mL x g(-1) dry weight x h(-1) (P < 0.001), and the higher doses of 0.6 and 1.0 mg x kg(-1) prolactin reversed net water absorption to net water secretion of 2.20+/-0.63 and 2.33+/-0.89 mL x g(-1) dry weight x h(-1), respectively (P < 0.001). The stimulatory effect of prolactin on water transport was completely abolished by reducing the perfusion rate from 1.0 mL x min(-1) to zero. The stimulatory effect of prolactin on water secretion at perfusion rate of 1.0 mL x min(-1) was also abolished when luminal [Na+] was reduced from 180 to 80 mM. We concluded that, unlike in the small intestine, calcium fluxes in the colon are not related to water transport and did not respond at all to prolactin. Water transport, on the other hand, was reversed from net absorption to secretion by prolactin. We propose that this prolactin-induced water secretion is probably mediated by recycling of luminal sodium in the vicinity of tight junctions.     

Sinusoidal electromagnetic field of 50 hz helps in retaining calcium in tibias of aged rats

Effect of 50Hz sinusoidal electromagnetic field (SEMF) on normal bone physiology was evaluated in young and old female and male Wistar rats. Exposure to SEMF resulted in increased 45Ca retention in tibias of aged animals only. Levels of serum calcium in young female and male rats were significantly less than in respective aged rats. These were further decreased after 4 weeks of SEMF exposure. SEMF exposure did not change the serum calcium levels in aged rats, and inorganic phosphates in young and aged animals. Similarly, the levels of tartrate resistant acid and alkaline phosphatase were significantly decreased in young rats, whereas the levels remained unchanged in aged rats of either sex. The results revealed that SEMF of 1mT can prevent bone calcium loss due to aging in animals.     

Increased Na+ intake during gestation in rats is associated with enhanced vascular reactivity and alterations of K+ and Ca2+ function

Gestation is associated with decreased blood pressure and resistance to the effects of vasoconstrictor agents. A recent study showed that pregnant rats, on increased sodium intake, present physiological changes that resemble those observed in preeclampsia. We investigated the effects of sodium supplementation on reactivity and on potassium and Ca(2+) channel activity in blood vessels during gestation. Sodium supplements, 0.9% or 1.8% NaCl as drinking water, were given to nonpregnant and pregnant rats for 7 days (last week of gestation). Reactivity to phenylephrine (PE), KCl, arginine vasopressin (AVP), and tetraethylammonium (TEA) was measured in aortic rings under modulation of potassium and calcium channels. TEA, a nonselective K(+) channel inhibitor, induced concentration-dependent responses in aortic rings from nonpregnant but not in those from pregnant rats. The response to TEA was restored in rings from pregnant rats after preincubation with 10 mmol/l KCl. Sodium supplementation did not affect the response to TEA in the aortas of pregnant animals. After sodium supplementation, maximum responses to PE and AVP were decreased and increased in aortic rings from nonpregnant and pregnant rats, respectively. Cromakalim (an ATP-sensitive K(+) channel activator)-induced inhibition of the responses to the three vasoconstrictors was more striking in aorta from nonpregnant than pregnant rats on regular diet, whereas it produced similar inhibition in tissues from both groups of animals on 0.9% and 1.8% NaCl. NS-1619 (a Ca(2+)-sensitive K(+) activator) elicited heightened effects in the aortas of pregnant animals receiving 0.9% NaCl supplementation. Nifedipine (a Ca(2+) channel blocker) caused greater inhibition of the contractile responses in tissues from nonpregnant rats on regular diet, and its action was increased in pregnant rats on sodium-supplemented diets. These data demonstrate that augmented sodium intake during gestation in the rat is linked with the reversal of gestational-associated resistance to vasopressors and indicate that this is an experimental model showing some features of gestational hypertension.     

Determination of Vitamin D-dependent calcium absorption by 45Ca gavage in the rat

Precise determination of Vitamin D-dependent intestinal calcium absorption in longitudinal studies is problematic. We have assessed Vitamin D-dependent intestinal calcium absorption by ⁴⁵Ca gavage. Rats were gavaged with a 1 mL solution containing ⁴⁵Ca (CaCl₂, 9.3 MBq/mL) maintained at 37 °C. Total Ca concentration of the gavage fluid was optimised by comparing the absorption curves for fluids made up to 0.025, 2.025, 4.025 and 40.025 mmol/L with ⁴⁰CaCl₂. The effect of varying dietary Ca on fractional Ca absorption was determined in rats fed semi-synthetic diets containing either 0.05%, 0.2%, 0.4% or 1.0% Ca for 50 days. Serum 1,25 dihydroxyvitamin D (1,25D) was determined by radioimmunoassay. Total gavage Ca of 0.025 mmol/L achieved the highest peak fractional absorption and was adopted for all future experiments. Fifty days after allocation to the diets both fractional Ca absorption and 1,25D were highest in rats fed 0.05% Ca and lowest in those fed 1.0% Ca (absorption, P < 0.05 and 1,25D, P < 0.05). There was a strong logarithmic relationship between 1,25D and fractional Ca absorption (R² 0.69, P < 0.001). Weekly repetition of the procedure did not cause a fall in haematocrit over 7 weeks. Radiocalcium (⁴⁵Ca) absorption by gavage provides a simple measure of Vitamin D-dependent Ca absorption for repetitive use in longitudinal studies.     

Effect of calcium stress on the skeleton mass of intact and ovariectomized rats

Female rats were ovariectomized (Ovx) or sham-operated (control) at 18 weeks and the entire skeleton obtained at 24 weeks (baseline) or after an additional 31 day (28 week) interval on a normal (1.0%) or deficient (0.02%) calcium diet. Ovx rats showed a 42% increase in whole body bone resorption (3H-tetracycline loss) in the absence of calcium stress (1.0% calcium diet) and a 70% increase in resorption with morphological evidence of dramatic loss of cancellous bone mass when placed on calcium-deficient (0.02%) diets. Ovx rats kept on the 1.0% calcium diet showed a significant increase in both their body weight (30.2%) and total bone mass (11.6%) compared to baseline sham-operated controls. However, the total skeleton mass of these animals was significantly reduced (-20%) from that predicted by calculations based on body weight. Maintaining animals on calcium-deficient diets had no significant effect on the total skeleton mass of either control or Ovx rats in comparison with age-matched controls on 1.0% diets. It was further determined that an increase in bone mass between 24 and 28 weeks in rats receiving 1.0% dietary calcium occurred in both the axial and appendicular skeleton and was proportionately similar between control and Ovx groups. However, in animals subjected to dietary calcium stress during this interval, the decreased skeletal growth noted was confined primarily to the axial skeleton. The data indicate that ovariectomy or ovariectomy plus calcium stress does not result in loss of total bone mass during the interval of dramatically increased resorption and rapid loss of cancellous bone. The results suggest that the deterioration in individual bone structural and mechanical integrity due to ovariectomy or dietary calcium deficiency may not be attributed to overt loss in total bone mass but may involve a redistribution of bone mass.     

Absence of effect of the incorporation of a milk phosphopeptide on the utilization of calcium and phosphorus in the young pig

Casein phosphopeptides are known to influence calcium absorption. A 50-day study was performed in 6-week old pigs fed either a control diet or a 5% casein phosphopeptide-containing diet (PP group). Both diets provided similar amounts of Ca (0.8%), P (0.5%), proteins, energy and vitamins. PP diet provided near 1/2 of total Ca, 1/3 of total P and 1/5 of proteins in the form of casein phosphopeptide. Ca and P excretion, absorption and retention were evaluated during a 10-day balance study. Bones were collected at slaughter to determine density, bending moment and bone mineral content. Calcium absorption and bone parameters (urinary hydroxyproline included) were not influenced by the type of diet. P absorption, but not retention, was slightly higher in the control group. Urinary Ca was higher and urinary P lower in PP pigs than in controls. These changes might result from the different kinds of dietary phosphorus, inorganic versus phosphopeptide, rather than from the difference between dietary proteins.     

Circadian Neuroendocrine Role in Age-Related Changes in Body Fat Stores and Insulin Sensitivity of the Male Sprague-Dawley Rat

A role for circadian neuroendocrine rhythms in the age-related development of obesity and insulin resistance was investigated in the male Sprague-Dawley rat. The phases and amplitudes of the plasma rhythms of several metabolic hormones (i.e. corticosterone, prolactin, insulin, and triiodothyronine) differed in lean, insulin-sensitive (3-week-old rats). insulin-resistant (8-week-old rats) and obese, insulin-resistant (44-week-old rats) animals. Simulation of the daily rhythms of endogenous corticosterone and prolactin by daily injections of the hormones at times corresponding to the peak levels found in 3-week-old rats reversed age-related increases in insulin resistance and body fat in older (5-6-month-old) rats. Ten such daily injections of corticosterone and prolactin in 12-14-week-old rats produced long-term reductions in body fat stores (30%). plasma insulin concentration (40%″). and insulin resistance (60%) (determined by a glucose tolerance test) measured 11-14 weeks after the treatment. Alterations in circadian neuroendocrine rhythms may account for age-related changes in carbohydrate and lipid metabolism in the male Sprague-Dawley rat, and resetting of these rhythms by appropriately timed daily injections of corticosterone and prolactin may help maintain metabolism characteristic of younger animals.     

Circadian Neuroendocrine Role in Age-Related Changes in Body Fat Stores and Insulin Sensitivity of the Male Sprague-Dawley Rat

A role for circadian neuroendocrine rhythms in the age-related development of obesity and insulin resistance was investigated in the male Sprague-Dawley rat. The phases and amplitudes of the plasma rhythms of several metabolic hormones (i.e. corticosterone, prolactin, insulin, and triiodothyronine) differed in lean, insulin-sensitive (3-week-old rats). insulin-resistant (8-week-old rats) and obese, insulin-resistant (44-week-old rats) animals. Simulation of the daily rhythms of endogenous corticosterone and prolactin by daily injections of the hormones at times corresponding to the peak levels found in 3-week-old rats reversed age-related increases in insulin resistance and body fat in older (5-6-month-old) rats. Ten such daily injections of corticosterone and prolactin in 12-14-week-old rats produced long-term reductions in body fat stores (30%). plasma insulin concentration (40%'). and insulin resistance (60%) (determined by a glucose tolerance test) measured 11-14 weeks after the treatment. Alterations in circadian neuroendocrine rhythms may account for age-related changes in carbohydrate and lipid metabolism in the male Sprague-Dawley rat, and resetting of these rhythms by appropriately timed daily injections of corticosterone and prolactin may help maintain metabolism characteristic of younger animals.     

Calcium metabolism, serum thyroid-stimulating hormone, prolactin, and growth hormone in spontaneously hypercholesterolemic rats

We have shown previously that spontaneously hypercholesterolemic (SHC) rats exhibit abnormal bone metabolism with advanced bone resorption, which develops with age. In this study, we measured serum levels of growth hormone, thyroid-stimulating hormone, and prolactin in addition to several parameters of calcium metabolism and renal function in young (6-week) and old (24-week) SHC rats and compared these with age-matched Sprague-Dawley rats. In young SHC rats, urinary excretion of hydroxyproline and serum levels of calcium were significantly elevated and excretion of protein into urine and urea nitrogen in the serum were normal, suggesting that calcium metabolism was abnormal without kidney dysfunction at this age. Serum growth hormone and thyroid-stimulating hormone levels were markedly higher (20- to 30-fold and 4- to 5-fold, respectively) in young and old SHC rats, whereas serum prolactin levels were similar. A high level of serum thyroid-stimulating hormone was associated with elevated levels of thyroxine and triiodothyronine in young SHC rats, but not old ones. These results demonstrate that the rat exhibits abnormalities in endocrine function as well as calcium metabolism preceding the occurrence of renal dysfunction.     

Decreased calcium and magnesium urinary excretion during prostaglandin synthesis inhibition in the rat

The effect of endogenous renal prostaglandins on calcium and magnesium reabsorption was investigated. Renal tubular handling of calcium and magnesium was studied by clearance methods in anesthetized Sprague-Dawley and Brattleboro rats, either intact or thyroparathyroidectomized (ATPTX), before and during prostaglandin synthesis inhibition by meclofenamate, indomethacin, or piroxicam infusion. These three inhibitors had similar effects on calcium and magnesium excretion: A significant decrease in absolute and fractional excretions of both cations was observed in intact Sprague-Dawley rats, and in ATPTX rats of both strains, but not in intact Brattleboro rats. These results suggest an inhibitory effect of prostaglandins on vasopressin-, glucagon-, but not PTH-mediated calcium and magnesium reabsorption. This effect is likely to occur in the thick ascending limb of Henle, which is both a target site for these polypeptidic hormones, and a segment where the bulk of calcium and magnesium is reabsorbed.     

Enhanced trabecular-bone calcium deposition in female rats with a high physiological dose of prolactin diminishes after ovariectomy

Although an increase in trabecular-bone calcium deposition has been shown to be regulated by prolactin during lactation, the physiological significance of prolactin in bone calcium metabolism in nonlactating rats remains unclear. This investigation sought to demonstrate the effects of endogenous prolactin and a high physiological dose of exogenous prolactin on bone turnover and bone calcium deposition in normal female rats, using the 45Ca-labeling technique. Our results showed that suppression of endogenous prolactin with 6 mg/kg bromocriptine for 15 days significantly enhanced bone formation, but not bone resorption, in primarily trabecular sites, resulting in a significant increase in calcium deposition in the sternum and vertebrae, from -0.20+/-0.07 to 0.40+/-0.09 (p<0.05) and -0.07+/-0.11 to 0.34+/-0.06 (p<0.05) mmol Ca.(g dry mass)-1, respectively. Similarly, 2.5 mg/kg prolactin, a high physiological dose, increased sternal and vertebral calcium deposition, from -0.20+/-0.07 to 0.24+/-0.09 (p<0.05) and -0.07+/-0.11 to 0.25+/-0.18 (p<0.05) mmol Ca.(g dry mass)-1, respectively, by increasing bone formation more than bone resorption. However, as expected, prolactin had no effect on the tibia or femur, which are primarily cortical sites. Because several actions of prolactin have been known to be estradiol-dependent, we further investigated the dependence of prolactin action on 17beta-estradiol. We found that 2.5 mg/kg prolactin did not increase sternal calcium deposition in ovariectomized rats. However, 10 microg/kg 17beta-estradiol supplementation restored the action of prolactin. Ovariectomized rats given 17beta-estradiol plus prolactin also manifested slightly but significantly higher sternal total calcium content than sham-operated rats, (4.58+/-0.12 vs. 4.36+/-0.11 mmol Ca.(g dry mass)-1 (p<0.05)). We concluded that a high physiological dose of prolactin promoted calcium deposition in primarily trabecular sites of nonlactating rats. This effect was diminished after ovariectomy. In addition, we showed that basal endogenous prolactin played a role in the maintenance of normal trabecular-bone turnover.     

Calcium bioavailability and kinetics of calcium ascorbate and calcium acetate in rats

The objective was to investigate the bioavailability and mechanism of calcium absorption of calcium ascorbate (ASC) and calcium acetate (AC). A series of studies was performed in adult Sprague-Dawley male rats. In the first study, each group of rats (n = 10/group) was assigned to one of the five test meals labeled with (45)Ca: (i) 25 mg calcium as heated ASC or (ii) unheated ASC, (iii) 25 mg calcium as unheated AC, (iv) 3.6 mg Ca as unheated ASC, or (v) unheated AC. Femur uptake indicated better calcium bioavailability from ASC than AC at both calcium loads. A 5-min heat treatment partly reduced bioavailability of ASC. Kinetic studies were performed to further investigate the mechanism of superior calcium bioavailability from ASC. Two groups of rats (n = 10/group) received oral doses of 25 mg Ca as ASC or AC. Each dose contained 20 micro Ci (45)Ca. Two additional groups of rats (n = 10/group) received an intravenous injection (iv) of 10 micro Ci (45)Ca after receiving an unlabeled oral dose of 25 mg calcium as ASC or AC. Sequential blood samples were collected over 48 hrs. Urine and fecal samples were collected every 12 hrs for 48 hrs and were analyzed for total calcium and (45)Ca content. Total calcium and (45)Ca from serum, urine, and feces were fitted by a compartment kinetics model with saturable and nonsaturable absorption pathways by WinSAAM (Windows-based Simulation Analysis and Modeling). The difference in calcium bioavailability between the two salts was due to differences in saturable rather than passive intestinal absorption and not to endogenous secretion or calcium deposition rate. The higher bioavailability of calcium ascorbate was due to a longer transit time in the small intestine compared with ASC.