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At least 235 records · Page 13Linked to original sources
PubMed1998

Oxygen isotopic abundances in calcium- aluminum-rich inclusions from ordinary chondrites: implications for nebular heterogeneity.

The oxygen isotopic compositions of two calcium-aluminum-rich inclusions (CAIs) from the unequilibrated ordinary chondrite meteorites Quinyambie and Semarkona are enriched in 16O by an amount similar to that in CAIs from carbonaceous chondrites. This may indicate that most CAIs formed in a restricted region of the solar nebula and were then unevenly distributed throughout the various chondrite accretion regions. The Semarkona CAI is isotopically homogeneous and contains highly 16O-enriched melilite, supporting the hypothesis that all CAI minerals were originally 16O-rich, but that in most carbonaceous chondrite inclusions some minerals exchanged oxygen isotopes with an external reservoir following crystallization.

Aluminum↗
PubMedDate not supplied

[Interaction of alkaline earth and transition metals with Na+/Ca2+-plasma membrane exchanger in secretory cells of the gastric gland].

Investigation of Na(+)-dependent Ca2+ uptake into the secretory cells of isolated gastric glands from guinea pig with the use of calcium isotope (45Ca2+) has been performed. The presence of Na+/Ca(2+)-exchanger in the cells membrane was established. Ca2+ uptake into the cells through Na+/Ca(2+)-exchanger was competitively inhibited by the number of alkaline earthy and transient metals" cations. Potency of inhibition increases in such an order (Ki, mM): Ba2+ (117.7) < Sr2+ (53.4) < < Mn2+ (15.2) < < Co2+ (12.8) < Cd2+ (8.6). By one-factor dispersion analysis it was shown that potency of inhibition depends on ionic radii and hydration enthalpy of metals" cations (hx2 = 93.93-94.15%) and also on stability constants of their complexes with oxygen-containing bioligands (acetic, aspartic and glutamic acid) (hx2 = 82.32-82.47%). Dependence of inhibitory constants from ionic radii is most adequately described by the parabolic equation, such dependence from hydration enthalpy and stability constants with oxygen-containing bioligands--by exponential or multiplicative equations. The conclusion has been made that velocity of Ca2+ transport through Na+/Ca(2+)-exchanger and potency of its inhibition by metals" cations is determined by the interaction between energy of their interaction with cation-binding sites of transport system and energy of hydration. Energetics of such interactions mainly depends on the steric conformity between the metal cation and cation-binding sites of the exchanger.

Animals↗
PubMed1975

Calcium absorption as a function of calcium intake.

The relationships between the amount of calcium absorbed and the quantity ingested was evaluated in 180 adult humans. Absorption was measured from the concentration ratio of concurrently administered oral and intravenous calcium isotopes. Intake ranged from 0.163 to 7.48 Gm. Ca per day. In 14 subjects, intakes were artificially elevated for purposes of this study. All others were studied at their usual intake levels. Absorption (Ca Abs) was found to follow a curvillnear relationship with intake (Ca-D), and was characterized by the following equation: Ca Abs equals 0.1541 - Ca-D plus 0.3127[exp(-1.0539 - Ca-D)] - Ca-D. The exponential term of this equation provided the major component of total absorption at intakes below 0.8 Gm. per day, but fell to negligible values when intake reached 2 to 3 Gm. per day, above which absorption was characterized by a simple linear function of intake. We found that there was no detectable upper limit to absorption capacity, which, at the 7.48 Gm. intake level, averaged more than 1.0 Gm per day. The observed mathematical description is consistent with the generally recognized inverse relationship between absorption efficiency and intake. At the same time it indicates that a component of absorption is independent of control mechanisms and is related solely to intake. A more general form of the foregoing equation, suggesting provision for other physiological variables such as growth hormone and cortisol, is proposed and discussed.

Acromegaly↗
PubMed1994

Absorbability of the calcium in a high-calcium mineral water.

The availability of the calcium contained in a high-calcium mineral water (Sangemini), popular in Italy, was compared in 18 healthy women with the availability of milk calcium ingested at the same calcium load, using 45Ca as the tracer in a randomized cross-over design. At an ingested calcium load of 2.5 mmol, absorption fraction averaged 0.433 for milk and 0.475 for Sangemini water. The mean quotient of the two (Sangemini/milk) was 1.129 (+/- 0.056, SEM, P < 0.05). The calcium of Sangemini water is thus highly bioavailable, and at least as bioavailable as milk calcium.

Absorption↗