Ellis E. Golub

The effect of age and 1,25-dihydroxyvitamin D3 treatment on the intestinal calcium-binding protein of suckling rats

Abstract The intestinal level of the vitamin D-dependent duodenal calcium-binding protein was assayed by an equilibrated column technique in rat embryos, neonates, and pups. Calcium-binding protein was undetectable in unborn, newborn, and 1- to 2-day-old rats i.e., the level was lower than in severely vitamin D-deficient animals. Calcium-binding protein was detected after the animals were 5-days old and thereafter rose monotonically as a function of body weight. Treatment with 1,25-dihydroxyvitamin D 3 failed to raise the calcium-binding protein levels of newborn or 1-day-old rats, but doubled the level in 11- or 12-day-old pups. Plasma calcium was raised in all treated animals. The failure to detect calcium-binding protein in vitamin D-replete suckling animals provides evidence for a dissociation between calcium absorption and calcium binding protein.

25-Hydroxycholecalciferol: high affinity substrate for hepatic cytochrome P-450.

Summary Addition of 25-hydroxycholecalciferol (25-OH-D 3 ) to rat liver microsomes revealed a type I spectral change with a spectral dissociation constant (K S ) of approximately 80nM; this value remained unchanged in phenobarbital-treated rats, although the maximum binding value was tripled. The inhibition constant, K i , of 25-OH-D 3 for aminopyrine N-demethylation was 59nM, in good agreement with the K S value. Stopped-flow studieds showed a two-fold increase in microsomal NADPH cytochrome P-450 reductase activity in the presence of 25-OH-D 3 . These findings suggest that the P-450 system plays a role in the blotransformation of 25-OH-D 3 .

Regulation of intestinal calcium-binding protein in rats: Role of parathyroid hormone

SummaryIntestinal calcium-binding protein (CaBP) levels of rats fed a high (1.5%) Ca diet were the same whether the animals were parathyroidectomized (PTX), sham-operated controls pair-fed with the PTX animals, or sham-operated controls fed ad libitum. Consequently, a given base level of CaBP seems to be parathyroid hormone independent and not closely related to feed intake. On the other hand, whereas the ad libitum fed controls more than doubled their intestinal CaBP in response to a 2-day low-calcium (0.02%) regimen, neither the parathyroidectomized animals nor the pair-fed sham-operated controls were able to do so. Since the latter two groups consumed less feed and therefore less vitamin D than the ad libitum fed animals, the inability to increase CaBP in response to a low-calcium diet may have been caused by a restricted vitamin D intake rather than by the absence of parathyroid hormone.

Calcium binding protein and regulation of calcium transport

SummaryWe have discussed the calcium-dependent feedback regulation of calcium absorption. This appears to involve two major mechanisms:a) The calcium-binding protein-related saturable calcium transport in the duodenum. This increases when calcium intake is lowered. b) The ileal calcium flux from lumen to blood which decreases when calcium intake is high. This is accompanied, by an unmodified flux from blood to lumen so that net ileal flux into the lumen increases with high calcium intake.

The Effect of Vitamin D on Renal Calcium Clearance

1. C: Vu 0.07 + 0.03 Si (0.5%Ca,0.5%P, no vito D) 2. CD: Vu -0.41 + 0.02 Si (0.5%Ca,0.5%P, 2200u.vit.D/kg) 3. R: Vu 10.1 0.09 Si (1. 5%Ca,0.14%P, no vito D) 4. RD: Vu 1.3 + 0.03 Si (1. 5%Ca, 0 .14%p, 2200u.vit.D/kg) 5. PTX: Vu 0.36 + 0.02 Si (0.5%Ca,0.5%P, 2200u.vit.D/kg parathyroidectomized animals) 6. N: Vu 0.30 + 0.02 Si (0.5%Ca,0.5%P, 2200u.vit.D/kg) Vu urinary calcium excretion, Groups C, CD, R, and RD remg/day ceived 18% protein in semiSi net calcium absorption, synthetic diets; groups PTX mg/day and N received 28% protein.

25-Hydroxycholecalciferol: High Affinity Substrate for Hepatic Cytochrome P-450

In vitro studies with hepatic microsomes showed that 25-hydroxy-vitamin D3 (25-OH-D3) is bound tightly by the microsomal cytochrome P-450 system, with the spectral dissociation constant of 84 nM the lowest reported to-date for a natural or xenobiotic compound. Vitamin D2 and dihydrotachysterol also were bound, but their dissociation constants were 200–300 times higher. Aminopyrine demethyl-ation was competitively inhibited in the presence of 25-OH-D3 and NADPH cytochrome. P-450 reductase activity was doubled by 25-OH-D3 addition. Taken together these findings suggest that liver microsomes are involved in the transformation and degradation of 25-OH-D3 and other vitamin D congeners. However, the enzyme systems are not vitamin D-dependent. Moreover, even though phenobarbital treatment led to a doubling of enzyme activity and, in animals undergoing vitamin D depletion, to a 40% faster time-dependent drop of 25-OH-D3 plasma levels, as compared to untreated controls, the two groups had the same levels of intestinal calcium-binding protein (CaBP). If CaBP is one expression of the active metabolite, 1,25-dihydroxy vitamin D3 (1,25-(OH) 2-D3), then alterations of a substrate (25-OH-D3)-product (1,25-(OH)2-D3) relationship at a single control point (liver), brought about by phenobarbital treatment, did not lead to detectable changes in hormone expression. The experiments underscore the stability of the calcium regulating system and point to the existence of multiple control loops.