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  • 1
    Electronic Resource
    Electronic Resource
    Evidence for vitamin D-independent active calcium absorption in newborn piglets (1993)
    Springer
    Calcified tissue international 52 (1993), S. 305-309 
    Details
    ISSN: 1432-0827
    Keywords: Calcitriol ; Ca2+ absorption ; Vitamin D receptor ; Piglets ; Pseudo vitamin D deficiency ; Rickets
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Physics
    Notes: Summary The role of 1,25-dihydroxycholecalciferol (calcitriol) for intestinal calcium (Ca2+) absorption was studied in newborn (〈1 week old) and weaned piglets (〉6 weeks old). In both groups, normal piglets and piglets suffering from inherited pseudo vitamin D-deficiency rickets, type I (PVDRI) were used. In this inherited disorder, renal production of calcitriol is absent. Plasma samples were assayed for calcitriol and total Ca, and dissociation constants (Kd) and maximum binding capacities (Bmax) of intestinal calcitriol receptors were determined under equilibrium conditions at 4°C. Unidirectional Ca2+-flux rates were measured across stripped duodenal mucosae in Ussing chambers in the absence of electrochemical gradients. The plasma calcitriol concentrations of neonatal (26.5±7.1 pg/ml, n=11; $$\bar x$$ ± SEM) and weaned PVDRI piglets (18.8±5.7 pg/ml, n=8)were unphysiologically low and differed significantly from control animals (83.6±14.8 pg/ml, n=8, and 86.9±9.6 pg/ml, n=11, respectively). However, newborn PVDRI piglets had normal plasma Ca levels at least during the first days of life. They became hypocalcemic and developed clinical symptoms of rickets during the following weeks. In newborn PVDRI and control piglets, Bmax was significantly lower (84±28 fmol/mg protein and 127±55 fmol/mg protein, n=9, respectively) than in weaned piglets (741±82 fmol/mg protein, n=9, and 778±121 fmol/mg protein, n=8, respectively). Significant net Ca2+-fluxes were found in both newborn PVDRI and control piglets (88.8±25.1 nmol · cm-2 · h-1, n=6, and 86.5±10.5 nmol · cm−2 · h−1,n=9, respectively). However, active net Ca2+ absorption was completely absent in weaned PVDRI piglets. These results indicate the presence of vitamin D-independent mechanisms for active intestinal Ca2+ absorption during early postnatal life in pigs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00296656
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  • 2
    Electronic Resource
    Electronic Resource
    Mechanisms of phosphate uptake into brush-border membrane vesicles from goat jejunum (1996)
    Springer
    Journal of comparative physiology 166 (1996), S. 230-240 
    Details
    ISSN: 1432-136X
    Keywords: Phosphate transport ; Jejunum ; Calcitriol ; Goats
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract This study concerns the uptake of inorganic phosphate into brush-border membrane vesicles prepared from jejunal tissues of either control or Ca-and/or P-depleted goats. The brush-border membrane vesicles showed a time-dependent accumulation of inorganic phosphate with a typical overshoot phenomenon in the presence of an inwardly directed Na+ gradient. The Na+-dependent inorganic phosphate uptake was completely inhibited by application of 5 mmol·l-1 sodium arsenate. Half-maximal stimulation of inorganic phosphate uptake into brush-border membrane vesicles was found with Na+ concentrations in the order of 5 mmol·l-1. Inorganic phosphate accumulation was not affected by a K+ diffusion potential (inside negative), suggesting an electroneutral transport process. Stoichiometry suggested an interaction of two or more Na ions with one inorganic phosphate ion at pH 7.4. Na+-dependent inorganic phosphate uptake into jejunal brush-border membrane vesicles from normal goats as a function of inorganic phosphate concentration showed typical Michaelis-Menten kinetic with V max=0.42±0.08 nmol·mg-1 protein per 15 s-1 and K m=0.03±0.01 mmol·l-1 (n=4, x ±SEM). Long-term P depletion had no effect on these kinetic parameters. Increased plasma calcitriol concentrations in Ca-depleted goats, however, were associated with significant increases of V max by 35–80%, irrespective of the level of P intake. In the presence of an inwardly directed Na+ gradient inorganic phosphate uptake was significantly stimulated by almost 60% when the external pH was decreased to 5.4 (pHout/pHin=5.4/7.4). The proton gradient had no effect on inorganic phosphate uptake in absence of Na+. In summary, in goats Na+ and calcitriol-dependent mechanisms are involved in inorganic phosphate transport into jejunal brush-border membrane vesicles which can be stimulated by protons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00263987
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  • 3
    Electronic Resource
    Electronic Resource
    Comparative studies on Na-dependent Pi transport in ovine, caprine and porcine renal cortex (2000)
    Springer
    Journal of comparative physiology 170 (2000), S. 387-393 
    Details
    ISSN: 1432-136X
    Keywords: Key words Renal Pi transport ; P depletion ; Ca depletion ; Renal cortex brush border membranes ; AbbreviationsAP alkaline phosphatase ; BBM brush border membranes ; BBMV brush-border membrane vesicles ; DM dry matter ; Pi inorganic phosphate ; PTH parathyroid hormone ; RT-PCR reverse transcriptase-polymerase chain reaction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract In contrast to monogastric species, renal excretion of inorganic phosphate (Pi) in ruminants is low and this could be attributed to an almost complete tubular Pi reabsorption. However, the functional and regulatory basis for this phenomenon has not yet been clarified. Therefore, it was the aim of the present study to characterize the kinetic parameters of the tubular Pi reabsorption system as affected by P or Ca depletion using preparations of renal cortex brush border membrane vesicles (BBMV) from goats and sheep and to compare the data with respective parameters of porcine preparations. Na-dependent Pi uptake into renal cortex BBMV as a function of Pi concentration showed typical Michaelis-Menten kinetic and respective Scatchard plot analysis of the specific Pi uptake revealed linearity indicating the predominant presence of a single type of Pi transporters in the preparations. Under control conditions Vmax values of Na-dependent Pi uptake into BBMV were highest in goats and sheep and lowest in pigs (1.98, 1.39 and 0.95 nmol · mg−1 protein · 10 s−1, respectively). Km values were not different between goats and sheep under all feeding conditions and ranged from between 0.34 mmol · l−1 and 0.55 mmol · l−1 which was three- to five-times higher than that found in pigs (0.11 mmol · l−1). Oligonucleotides derived from rat kidney cortex type IIa Na/Pi cDNA were used for reverse transcriptase-polymerase chain reaction (RT-PCR) in goat, sheep and pig kidney cortex. The products isolated were 768 bp for sheep and pigs and 765 bp for goats, with the respective amino acids sequences, representing a segment of approximately 40% in length of the entire transporter, exhibiting an at least 92% sequence homology between different species. From the results, involvement of type IIa Na/Pi cotransport in tubular Pi reabsorption in small ruminants can be postulated. However, it should not be considered that a potential role of other Pi transport systems be completely be excluded. Interestingly, neither P nor Ca depletion caused significant effects on Na-dependent Pi transport capacities and affinities in goats and sheep. From this, parathyroid-hormone independent regulatory pathways of tubular Pi reabsorption can be assumed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003600000115
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