Regulation of serum 1,25-dihydroxyvitamin D3 in lactating rats.

To characterize further the mechanism(s) underlying the increased serum 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] concentration associated with lactation in the rat, we examined hormone biosynthesis [i.e., renal 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) activity] and hormone disappearance in groups of lactating Holtzman rats and age- and sex-matched nonlactating controls. 1 alpha-Hydroxylase activity was significantly greater in kidneys from lactating rats (4.0 +/- 0.42 fmol.mg-1.min-1) on a basal diet than in those from nonmated females (1.4 +/- 0.08 fmol.mg-1.min-1), an increment sufficient to account for the observed fourfold elevation of 1,25(OH)2D3 in the dams. The increase occurs despite the lower serum 1,25(OH)2D3 levels in lactating than in nonlactating rats at 12 and 24 h after a bolus injection of 1,25(OH)2D3 (2 ng/g body wt). Elevation of serum 1,25(OH)2D3 is not a requisite consequence of lactation, however, because dams receiving supplemental calcium from food (1.6%) and water (0.3%) exhibited no increase of either serum 1,25(OH)2D3 or 1 alpha-hydroxylase activity compared with controls. In contrast, lactating rats that received a diet with only 0.1% calcium had 5-fold higher serum 1,25(OH)2D3 levels and 20-fold higher 1 alpha-hydroxylase activity than nonlactating rats on the same diet. We conclude that other factors in conjunction with lactation, but not the lactating state per se, promote the changes in 1,25(OH)2D3 metabolism observed.

Parathyroid hormone is not required for normal milk composition or secretion or lactation-associated bone loss in normocalcemic rats.

To determine if parathyroid hormone (PTH) is essential for lactation in rats, the parathyroid glands were removed surgically during the first week of lactation and the rats were given a diet containing a high calcium-phosphorus ratio to maintain a normal serum calcium concentration. Lactating rats were placed on diet containing 1.2% calcium (Ca) and 0.8, 0.6, or 0.4% phosphorus (P) on day 2 postpartum (PP) and were parathyroidectomized (PTX) at 4-6 days PP. At 10 days PP serum Ca was 10.5 +/- 0.2 mg/dl (mean +/- SEM) for PTX rats and 10.4 +/- 0.3 mg/dl in sham-operated lactating rats when the diet contained 0.6% P. When the diet P was 0.8%, the litters gained little or no weight and serum Ca fell to 6.9 +/- 0.6 mg/dl by day 10 PP in PTX rats compared with 10.2 +/- 0.2 mg/dl in sham rats. PTX rats fed the diet containing 1.2% Ca and 0.6% P maintained a normal serum Ca level until at least day 18 PP, but their serum P levels fell gradually from approximately 5 mg/dl at 10 days to 3 mg/dl at 18 days PP. In spite of this hypophosphatemia, the litters of PTX and sham rats had gained the same amount of weight by age 16 days, indicating equal milk production in the two groups. Milk Ca, P, and total solids were not significantly different between PTX and sham rats on day 11 PP.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypercalcemia fails to suppress elevated serum parathyroid hormone concentrations during lactation in rats.

We have previously reported that increased serum immunoreactive parathyroid hormone (iPTH) in the lactating (L) rat is generally accompanied by hypocalcemia when diets containing 0.4% calcium (Ca) or less are fed. However, instances were also observed in which elevated iPTH levels did not coincide with a hypocalcemic signal. To test the hypothesis that iPTH levels can remain elevated even in the presence of hypercalcemia in lactation, a diet containing 1.2% Ca and 0.4% phosphorus (P) was fed to lactating rats in three experiments (A, B, and C) to achieve serum ionized calcium (ICa) levels approximately 10% above levels for nonmated (NM) controls. The serum ICa of NM controls fed the 1.2% Ca diet was slightly, but significantly, elevated, and serum iPTH (determined by an N-terminal specific assay) was significantly suppressed compared with NM controls fed a 0.4% Ca diet. In experiment A, L rats fed a 1.2% Ca diet had 81% higher serum iPTH levels than NM controls fed the same diet in spite of a mean (+/- SEM) ICa level of 1.77 +/- 0.05 mM for L rats versus 1.46 +/- 0.01 mM for NM controls; NM controls fed a 0.4% Ca diet had serum ICa of 1.37 +/- 0.01 mM. This novel finding of significantly higher iPTH and ICa in L compared with NM rats fed a 1.2% Ca and 0.4% P diet was confirmed in experiment B with eight rats in each group of L or NM rats fed either the 1.2% or the 0.4% Ca diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Adverse effects of a high-glucose diet on body weight and plasma calcium and 1,25-dihydroxyvitamin D3 levels in calcium-deficient growing rats.

We tested the hypothesis that dietary calcium would lead to greater impairment of body weight gain and calcium homeostasis if rats are fed a diet with a high glucose content compared with our standard diet in which the carbohydrate is supplied by whole wheat flour. Groups of female rats at 21 days of age were given either of two equivalent calcium-deficient diets with carbohydrate supplied either by glucose (LCaG) or by wheat flour (LCaW). Control rats were fed the wheat-flour diet containing 0.4% calcium. Since previous studies indicated divergent effects of glucose-based and flour-based diets on body weight in vitamin D-deficient rats, we designed a parallel study with vitamin D-deprived rats. Compared with rats fed the LCaW diet, the rats fed the LCaG diet had inferior body weight gain and more severe hypocalcemia (1-2 mg/ml lower) over a 40-day period, and no significant elevation of the plasma 1,25(OH)2D3 level at 61 days of age. Rats fed the LCaW diet maintained a 3-fold elevation of plasma 1,25(OH)2D3 relative to the level of control rats fed the 0.4% calcium diet. The dry weight and percent ash of tibias were similarly reduced in the two calcium-restricted groups compared to the control group. Among the vitamin D-deprived rats, those fed the glucose diet had poorer weight gain than those fed the wheat flour diet. However, both groups had similarly depressed serum calcium level, tibia ash content and 1,25(OH)2D3 level. Thus, a glucose diet combined with calcium restriction or vitamin D deprivation appears to accentuate the impairment of body weight gain and, when combined with calcium restriction, it also accentuates the impairment of calcium homeostasis and interferes with the adaptive increase in plasma 1,25(OH)2D3.

Effects of high dietary contents of calcium and phosphorus on mineral metabolism and growth of vitamin D-deficient suckling and weaned rats.

We have tested the hypothesis that normalization of the plasma calcium (Ca) and phosphorus (P) concentrations by dietary means in vitamin D-deficient rat pups will prevent rickets. From day 6 of pregnancy rats were given a vitamin D-free diet containing 1.6% Ca and 1.4% P (-D 1.6) which normalized plasma Ca during lactation. Pups weaned from these mothers, and continuing on the -D 1.6 diet until 56 days of age, had a mean plasma Ca value of 8.6 +/- 0.2 mg/dl and were not significantly different from pups fed a vitamin D-replete diet with 0.4% Ca and 0.4% P in the following parameters: body weight (mean +/- SE for -D 1.6 rats: 197 +/- 4 g), percent bone ash (53 +/- 0.5), and tibia epiphyseal cartilage width (385 +/- 26 micron). In contrast, pups consuming the vitamin D-free diet with 0.4% Ca and 0.4% P had plasma Ca of 4.9 +/- 0.2 mg/dl, body weight of 156 +/- 4 g, reduced bone ash (45 +/- 0.5%) and abnormally wide epiphyseal cartilage (727 +/- 113 micron). Thus, elevating the plasma Ca level of vitamin D-deficient rat pups by dietary means can normalize body weight, epiphyseal cartilage width and bone mineral content.

Circulating parathyroid hormone concentrations in normal and vitamin D-deprived rat pups determined with an N-terminal-specific radioimmunoassay.

Plasma PTH concentrations were determined in rats with an antiserum to human PTH (1-34) that cross reacts with rat PTH in order to evaluate the effects of growth, vitamin D deficiency and dietary modulations of plasma calcium. Displacement curves for synthetic rat PTH (1-34) and diluted rat plasma were parallel with curves for the hPTH (1-34) standards and hPTH (1-84). Parathyroidectomy in 33-day-old rats resulted in a fall in iPTH from (mean +/- SE) 8.1 +/- 0.5 to 5.4 +/- 0.5 pg/ml in an assay in which the lowest detectable concentration was 3 pg/ml. Elevation of plasma Ca by 0.7 mg/dl by dietary calcium supplementation in 25-day-old pups led to a plasma iPTH level of 4.4 +/- 0.7 pg/ml compared with 9.9 +/- 0.7 pg/ml in controls. There were no significant changes in iPTH in control rats fed a complete diet containing 0.4% Ca and 0.4% P over the age range 14-56 days. Rat pups (-D) suckling vitamin D-deprived mothers had plasma Ca and iPTH values of 7.76 +/- 0.16 mg/dl and 173 +/- 27 pg/ml, respectively, at 25 days of age and 5.8 +/- 0.2 mg/dl and 677 +/- 85 pg/ml, respectively, at 56 days. The -D pups that had access to their mothers' calcium-supplemented diet (1.6% Ca and 1.4% P) had a mean plasma Ca value that was 0.5 mg/dl above that of the control group and a mean plasma iPTH level of 6.7 +/- 0.8 pg/ml. We suggest that the RIA for PTH measures mostly, if not entirely, bioactive PTH. We conclude that plasma iPTH (a) remains essentially unchanged in normal rats during the rapid growth period, (b) can increase up to 75-fold in hypocalcemic vitamin D-deficient rats, and (c) can be suppressed to barely detectable levels when plasma Ca is raised less than 1 mg/dl above the level of normal control rats.

Effects of high doses of vitamin D3 and 1,25-dihydroxyvitamin D3 in lactating rats on milk composition and calcium homeostasis of the suckling pups.

Changes in serum Ca and phosphorus and in kidney Ca were determined in lactating rats and their suckling pups after the mothers received high doses of vitamin D3 or 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. High dietary vitamin D3 intake (300 IU/g diet) or daily oral doses of vitamin D3 (1 microgram/g BW) to vitamin I)-replete (+D) lactating rats for 8 or 12 days caused significant increases in serum Ca in the mothers (1-2 mg/dl) and in their suckling pups (1.5 mg/dl). Daily oral doses of 1,25-(OH)2D3 (2 ng/g BW) to +D lactating rats caused a similar increase in serum Ca in the mothers, but did not affect the serum Ca of the pups. The administration of a high dose of 1,25-(OH)2D3 to vitamin D-deficient lactating rats or high doses of vitamin D3 to +D rats, caused no change in milk Ca, Mg, or phosphorus. Milk from +D rats given high doses of [3H]vitamin D3 (1 microgram/g BW) contained mostly [3H]vitamin D3 (85%) and a small amount of [3H]25-hydroxyvitamin D3 (6%). The results indicate that high doses of vitamin D3, but not 1,25-(OH)2D3, given to +D lactating rats can cause hypercalcemia in the suckling pups. The hypercalcemic effect on the pups observed after vitamin D3 treatment of the mother is probably a result of transport of toxic amounts of primarily vitamin D3 into the milk and is not due to altered mineral composition of the milk.

Dynamic changes in circulating 1,25-dihydroxyvitamin D during reproduction in rats.

The concentrations of 1,25-dihydroxyvitamin D [1,25-(OH)2D], calcium, and phosphorus were measured in the serum of rats during pregnancy and at various stages of lactation. The concentration of 1,25-(OH)2D hormone increased almost two-fold during pregnancy and the latter part of lactation, but decreased to control levels or very low values immediately after birth and weaning, respectively. Furthermore, the concentration of 1,25-(OH)2D was inversely correlated with the concentration of calcium, suggesting that circulating 1,25-(OH)2D fluctuates in concert with calcium demands during the reproductive cycle. Parathyroidectomy in lactating rats caused a 70 percent inhibition of the normally observed 1,25-(OH)2D increase, indicating that parathyroid hormone, in response to changes in serum calcium, is a primary modulator of 1,25-(OH)2D during lactation.