Antagonistic effects of 24R,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 on L-type Ca2+ channels and Na+/Ca2+ exchange in enterocytes from Atlantic cod (Gadus morhua)

Influence of the aquatic environment and 1α,25(OH)2 vitamin D3 on calcium influx in the intestine of adult zebrafish

We investigated the effects of environment calcium challenge and 1α,25(OH)2 vitamin D3 (1,25-D3) on 45Ca2+ influx in the intestine of zebrafish (ZF). In vitro45Ca2+ influx was analyzed using intestines from fed and fasted fish. ZF were held in water containing Ca2+ (0.02, 0.7, 2.0 mM) to analyze the ex vivo45Ca2+ influx in the intestine and for histology. Intestines from fish held in water with Ca2+ were incubated ex vivo to characterize ion channels, receptors, ATPases and ion exchangers that orchestrate 45Ca2+ influx. For in vitro studies, intestines were incubated with antagonists/agonist or inhibitors to study the mechanism of 1,25-D3 on 45Ca2+ influx. Fasted ZF reached a plateau for 45Ca2+ influx at 30 min. In vivo fish at high Ca2+ stimulated ex vivo45Ca2+ influx and increased the height of intestinal villi in low calcium. In the normal calcium, 45Ca2+ influx was maintained by the reverse-mode Na+/Ca2+ (NCX) activation, Na+/K+-ATPase pump and sarco/endoplasmic reticulum calcium ATPase (SERCA) pump. However, Ca2+ hyperosmolarity is supported by L-type voltage-dependent calcium channels (L-VDCC), transient receptor potential vanilloid subfamily 1 (TRPV1) and Na+/K+-ATPase activity. The calcium challenge causes morphological alteration and changes the ion type-channels involved in the intestine to maintain hyperosmolarity. 1,25-D3 stimulates Ca2+ influx in normal osmolarity coordinated by L-VDCC activation and SERCA inhibition to keeps high intracellular calcium in intestine. Our data showed that the adult ZF regulates the calcium challenge (per se osmolarity), independently of the hormonal regulation to maintain the calcium balance through the intestine to support ionic adaptation.

Alternative perspective on intestinal calcium absorption: proposed complementary actions of Ca(v)1.3 and TRPV6

Transcellular models of dietary Ca(2+) absorption by the intestine assign essential roles to TRPV6 and calbindin-D(9K) . However, studies with gene-knockout mice challenge this view. Something fundamental is missing. The L-type channel Ca(v) 1.3 is located in the apical membrane from the duodenum to the ileum. In perfused rat jejunum in vivo and in Caco-2 cells, Ca(v) 1.3 mediates sodium glucose transporter 1 (SGLT1)-dependent and prolactin-induced active, transcellular Ca(2+) absorption, respectively. TRPV6 is activated by hyperpolarization and is vitamin D dependent; in contrast, Ca(v) 1.3 is activated by depolarization and is independent of calbindin-D(9K) and vitamin D. This review considers evidence supporting the idea that Ca(v) 1.3 and TRPV6 have complementary roles in the regulation of intestinal Ca(2+) absorption as depolarization and repolarization of the apical membrane occur during and between digestive periods, respectively, and as chyme moves from one intestinal segment to another and food transit times increase. Reassessment of current arguments for paracellular flow reveals that key phenomena have alternative explanations within the integrated Ca(v) 1.3/TRPV6 view of transcellular Ca(2+) absorption.

Effects of dietary vitamin D3 administration on innate immune parameters of seabream (Sparus aurata L.)

The present study assesses the in vivo effect of vitamin D(3) or cholecalciferol on some innate immune parameters of the gilthead seabream (Sparus aurata L.). Cholecalciferol was orally administered to seabream specimens in a commercial pellet food supplemented with 0 (control); 3750; 18,750 or 37,500 U kg(-1) and fish were sampled after 1, 2 and 4 weeks of treatment. Serum and head- kidney leucocytes were obtained and humoral (peroxidase and complement activity) and cellular (leucocyte peroxidase content, phagocytic, respiratory burst and natural cytotoxic activities) innate immune parameters were measured. Diet supplementation with 37,500 U kg(-1) cholecalciferol for 2 or 4 weeks resulted in a significant increase in phagocytic ability or serum peroxidase content, respectively, whereas the 3750 and 18,750 U kg(-1) supplemented diets led to significant increases in the phagocytic capacity of leucocytes at week 2 compared with the values found in control fish. Natural cytotoxic activity was increased in leucocytes from fish fed for 1 week with 3750 U kg(-1) cholecalciferol. No significant differences were observed in complement activity or in respiratory burst activity in the assayed conditions. These results suggested that dietary vitamin D(3) administration has an effect on the innate immune parameters of gilthead seabream. The immunostimulant effect was greater on the cellular innate immune parameters assayed, suggesting that similar receptors to those present in mammals are involved in the action of this vitamin in the fish immune system.

Molecular characterization of an epithelial Ca2+ channel-like gene from crayfish Procambarus clarkii

This study describes the cloning, sequencing and functional characterization of an epithelial Ca(2+) channel (ECaC)-like gene isolated from antennal gland (kidney) of the freshwater crayfish Procambarus clarkii. The full-length cDNA consisted of 2687 bp with an open reading frame of 2169 bp encoding a protein of 722 amino acids with a predicted molecular mass of 81.7 kDa. Crayfish ECaC had 76-78% identity at the mRNA level (80-82% amino acid identity) with published fish sequences and 56-62% identity at the mRNA level (52-60% amino acid identity) with mammalian ECaCs. Secondary structure of the crayfish ECaC closely resembled that of cloned ECaCs. Postmolt ECaC expression was exclusively restricted to epithelia associated with Ca(2+) influx and was virtually undetectable in non-epithelial tissues (eggs, muscle). Compared with expression levels in hepatopancreas, expression in gill was 10-fold greater and expression was highest in antennal gland (15-fold greater than in hepatopancreas). Compared with baseline expression levels in intermolt stage, expression of ECaC in antennal gland increased 7.4- and 23.8-fold, respectively, in pre- and postmolt stages of the molting cycle. This increase was localized primarily in the labyrinth and nephridial canal, regions of the antennal gland associated with renal Ca(2+) reabsorption. The ECaC in crayfish appears to be expressed in epithelia associated with unidirectional Ca(2+) influx and relative expression is correlated with rate of Ca(2+) influx.

Effects of Supplemental Dietary Phytase and 25-Hydroxycholecalciferol on the Digestive and Reproductive Organ Characteristics of Commercial Layers Inoculated Before or at the Onset of Lay with the F-Strain of Mycoplasma gallisepticum

In 3 trials, the effects of dietary supplementation with phytase (PHY) and 25-hydroxycholecalciferol (25-D3) on the digestive and reproductive organ characteristics of commercial layers that were inoculated prelay (12 wk of age) or at the onset of lay (22 wk of age) with F-strain Mycoplasma gallisepticum (FMG) were assessed at 58 wk of age. Experimental layer diets that included a basal control diet or a control diet supplemented with 0.025% PHY and 25-D3 were fed from 20 through 58 wk of age. As a percentage of total oviduct weight, magnum weight was lower in birds that were inoculated (sham or FMG) at lay onset compared with those that were inoculated prelay, and in FMG-inoculated birds, relative duodenum length was greater in those inoculated at 12 compared with 22 wk. Also, as percentages of organ weight or length, infundibulum length and isthmus weight were increased, whereas duodenum length was decreased by dietary supplementation with PHY and 25-D3. The overall timing (12 vs. 22 wk) of inoculation can affect the reproductive organ characteristics of layers, whereas, more specifically, the timing of an FMG inoculation may affect their digestive organ structure. Furthermore, independent of inoculation timing and type, the reproductive organ and digestive systems of laying hens may be influenced by dietary supplementation with PHY and 25-D3.

Environmental salinity regulates the in vitro production of [3H]-1,25-dihydroxyvitamin D3 and [3H]-24,25 dihydroxyvitamin D3 in rainbow trout (Oncorhynchus mykiss)

Previous studies have shown that specific binding of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) to enterocyte basolateral membranes (BLM), as well as circulating concentrations, is affected in response to changes in environmental salinity. It is not known if the production of 1,25(OH)2D3 and 24,25(OH)2D3 is affected by environmental salinity. The aim of the present study was to measure the in vitro production of [3H]-1,25(OH)2D3 and [3H]-24,25(OH)2D3 in fresh water (FW) and after 1, 2, 3, and 7 days after transfer to seawater (SW). Pooled sub-cellular fractions (mitochondria and microsomes) from liver or kidney was incubated with [3H]-25(OH)D3 and the produced metabolites were separated using HPLC. Hepatic production of [3H]-1,25(OH)2D3 was decreased after 24h in SW. This was followed by an up-regulation after 48h and a second, slower decrease in production rate which leveled out after 7 days in SW. The production rate in SW was lower than the original rate in FW-adapted fish. For hepatic [3H]-24,25(OH)2D3 production the pattern was reversed. Renal production of [3H]-24,25(OH)2D3 increased significantly during the period of SW acclimation. These results suggest that environmental salinity regulates the production rate of the two antagonizing calcium regulatory hormones; 1,25(OH)2D3 and 24,25(OH)2D3. This gives further evidence to the hypothesis that there is a physiological regulation and a differentiated importance of 1,25(OH)2D3 and 24,25(OH)2D3 in relation to environmental calcium concentrations.

Functional characterisation and genomic analysis of an epithelial calcium channel (ECaC) from pufferfish, Fugu rubripes

An orthologue to the mammalian epithelial calcium channels, ECaC1 (TRPV5) and ECaC2 (TRPV6), was cloned from gill of pufferfish (Fugu rubripes) and characterised, demonstrating that this gene predates the evolution of land-living vertebrates. The F. rubripes ECaC (FrECaC) protein displays all structural features typical for mammalian ECaCs including three ankyrin repeats, six transmembrane domains, and a putative pore region between TM V and TM VI. Functional expression of FrECaC in Madin-Darby canine kidney (MDCK) cells confirmed that the channel mediates Ca(2+) influx. FrECaC was also permeable to Zn(2+) and, to a small extent, to the Fe(2+) ion. Thus, in addition to a role in Ca(2+) uptake FrECaC might serve as a pathway for zinc and iron acquisition. FrECaC mRNA was highly abundant in the gill, but sparsely present in the intestine. Calcium absorption via FrECaC in pufferfish may be subject to the regulation of 1.25(OH)(2)D(3), estrogen and progesterone as consensus cis regulatory elements for the respective steroid hormone receptors were found in the upstream regulatory region of the FrECaC gene. FrECaC gene organisation is very conserved when compared with mammalian ECaCs. Only one ECaC gene seems to exist in the F. rubripes genome, and the corresponding protein clusters together with ECaC2 from mammals upon phylogenetic analysis. Thus, the two mammalian ECaC genes may originate from a single ancestral ECaC2 gene in vertebrates appearing early in evolution.

Direct, rapid effects of 25-hydroxyvitamin D3 on isolated intestinal cells

Scattered reports in the literature have suggested that the metabolite 25-hydroxyvitamin D(3) [25(OH)D(3)] has biological activity. In the present work, perfusion of isolated duodenal loops of normal chickens with 100 nM 25(OH)D(3) resulted in enhanced transport of (45)Ca within 2 min relative to the vehicle controls. We then tested the effect of a range of 25(OH)D(3) concentrations on (45)Ca handling by isolated intestinal cells in time course studies. Following a basal uptake period, cell suspensions from 7-week old chicks were treated either with 25, 100, or 300 nM 25(OH)D(3), or the vehicle ethanol (0.01%, final concentration). Both 25 and 100 nM 25(OH)D(3) resulted in a significant (P < 0.05) reduction in (45)Ca levels, relative to controls, between 1-10 min after treatment, while 300 nM 25(OH)D(3) resulted in a significant increase in (45)Ca levels, relative to controls, after 10 min of incubation. The effect of 100 nM 25(OH)D(3) (a physiological level) on cell calcium was abolished by the presence of 6.5 nM 24,25-dihydroxyvitamin D(3). In cell preparations from 14- or 28-week old birds 100nM 25(OH)D(3) had no effect, relative to vehicle controls. Incubation of cells with 2 microM BAY K8644, a calcium channel activator, stimulated (45)Ca uptake within 3 min relative to vehicle controls (P < 0.05), while addition of either 20 microM forskolin or 100 nM phorbol ester (stimulators of the PKA and PKC pathways, respectively) resulted in enhanced radionuclide levels after 10 min of incubation (P < 0.05, relative to corresponding controls). Finally, cells were treated with 100 nM 25(OH)D(3) or vehicle and samples taken at various times for analyses of protein kinase C and A activities. No effect of 25(OH)D(3) on protein kinase C activity was observed, while protein kinase A activity was stimulated to nearly 200% of controls at 1 min after 25(OH)D(3) addition (P < 0.05, relative to corresponding controls) and began declining at 3 min, returning to control levels 5 min after additions. We conclude that 25(OH)D(3) has a direct effect on calcium handling in enterocytes of young animals that may in part be mediated by the protein kinase A signal transduction pathway.

Environmental salinity regulates receptor expression, cellular effects, and circulating levels of two antagonizing hormones, 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3, in rainbow trout

In freshwater-adapted rainbow trout, intestinal cells (enterocytes) possess receptors for 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] in the basolateral membrane, and respond to treatment with 1,25(OH)(2)D(3) with increased intracellular calcium concentrations. No receptors are found for the antagonizing hormone 24,25-dihydroxyvitamin D(3) [24,25(OH)(2)D(3)] at the enterocyte basolateral membrane, and it has no effect on enterocyte calcium homeostasis. After acclimation to seawater, however, the enterocyte membrane receptors for 1,25(OH)(2)D(3) are down-regulated and specific binding for 24,25(OH)(2)D(3) appears, which is further up-regulated with time spent in seawater. This shift in receptor expression is concurrent with an increased sensitivity of the enterocytes to 24,25(OH)(2)D(3) and a decreased sensitivity to 1,25(OH)(2)D(3). This results in a partial inhibition of intracellular calcium uptake, which would be beneficial when inhabiting a calcium-rich environment like seawater.