Calcium and phosphate homeostasis

Ionic calcium (Ca2+) is crucial for many cellular
functions including neurotransmitter release and
nerve impulse propagation; contraction of skeletal,
cardiac and smooth muscle; blood clotting; exocrine
and endocrine secretory processes; and cell proliferation.
Ca2+ also acts as a ‘second messenger’, connecting
some stimuli (certain hormones, growth factors
and neurotransmitters) with physiological responses.
Bones and teeth contain about 99% of the body’s calcium;
the other 1% is distributed in both intra- and
extracellular fl uids.
The blood level of Ca2+ is very closely regulated:
even small changes in Ca2+ concentration can be
fatal. Acute hypocalcaemia in the human causes essentially
no other signifi cant effects besides tetany,
because tetany kills the patient before other effects
can develop. In tetany, the low concentration of Ca2+
in the extracellular fl uid causes the nervous system
to become progressively more excitable because of
increased neuronal membrane permeability. Eventually,
peripheral nerve fi bres become so excitable
that they begin to discharge spontaneously, initiating
nerve impulses that pass to the skeletal muscles, where
they elicit the muscular spasms of tetany. In hypercalcaemia,
the nervous system is depressed, and refl ex
actions of the central nervous system become sluggish.
There is also constipation and lack of appetite,
probably because of depressed contractability of the
muscular walls of the gastrointestinal tract. Above a
certain high level of blood Ca2+, calcium phosphate is
likely to precipitate throughout the blood and the soft
tissues. Deposition of calcium in the kidney or heart
causes death due to renal failure or cardiac arrest.
Phosphorus is a component of hydroxyapatite in
the skeleton, of phospholipids in cell membranes,
and of the nucleic acids, DNA and RNA. In cells,
phosphorus participates in energy metabolism and
acid–base regulation. Many signalling molecules depend
on the phosphorylation of enzymes to elicit an
hormonal response. Approximately 85% of the body’s
phosphorus is in the skeleton, 14% is associated with
soft tissue such as muscle, and 1% is found in the
blood and body fl uids. Within the physiological range
of pH values, inorganic phosphate (Pi) is present in
two different ionic species, H2PO4
– (monovalent) and
HPO4
2– (divalent). The relative concentration of each
is dependent on the ambient pH. Thus variation in the
pH value may have marked effects on the transport of
phosphate by altering the concentration ratio of these
phosphate species.
1α,25-Dihydroxyvitamin D restores low plasma
concentrations of Ca2+ and Pi to normal by action at
the three major targets, namely intestine, bone and
kidney. The hormone (1) stimulates the intestinal absorption
of Ca2+ and Pi by independent mechanisms,
(2) stimulates the transport of Ca2+ (accompanied
by Pi) from the bone fl uid compartment to the extracellular
fl uid compartment, and (3) facilitates the
renal reabsorption of Ca2+. These three mechanisms
provide calcium for bone mineralization and prevent
hypocalcaemic tetany.
1α,25-Dihydroxyvitamin D3 regulates the synthesis
of two classes of calcium-binding proteins (calbindins)
found in mammalian intestine and kidney. An
intestinal 9-kDa protein (calbindin-D9k) binds two
calcium ions per molecule, and a renal 28-kDa protein
(calbindin-D28k) binds fi ve to six calcium ions per
molecule (Lowe et al., 1992). Gross & Kumar (1990)
reviewed extensive evidence that the calbindins are
involved in transcellular calcium transport.
8.6.2 Intestinal calcium absorption
Calcium is present in foods and dietary supplements
as relatively insoluble salts. Because calcium
is absorbed only in its ionized form, it must fi rst be
released from the salts. Solubilization of most calcium
salts takes place in the acidic medium of the stomach
but, on reaching the alkaline environment of the small
intestine, some of the Ca2+ may complex with minerals
or other specifi c dietary constituents, thereby limiting
calcium bioavailability.
The mammalian intestine has developed special
vitamin D-dependent mechanisms to ensure the
absorption of appropriate amounts of calcium in the
face of changing needs and varying dietary calcium
intakes. Present knowledge of these mechanisms
is rather limited, although it appears that multiple
mechanisms are involved. In view of the amount of
controversy and continued research into this subject,
any model of vitamin D action must be tentative.
Calcium absorption takes place by the translocation
of luminal Ca2+ through the enterocytes (transcellular
route) and between adjacent enterocytes via the tight
Vitamin D 209
junctions (paracellular route). Transcellular movement
is a saturable, energy-dependent process that
is subject to regulation by vitamin D and is confi ned
almost entirely to the duodenum and upper jejunum.
In the perfused chick intestine, the stimulation of
calcium transport by 1α,25(OH)2D3 is suppressed by
24R,25(OH)2D3 (Nemere, 1999). Paracellular movement
is passive, independent of vitamin D status, and
exists all along the small intestine (Pansu et al., 1983).
Two models which describe the mechanism of
transcellular absorption are the calbindin-based diffusional-
active transport model and the vesicular
transport model (Wasserman & Fullmer, 1995).
The calbindin-based diffusional-active transport
model
This transcellular pathway is a complex process involving
three steps: (1) entry by movement of Ca2+
from lumen through the brush-border membrane
of the enterocyte, (2) intracellular diffusion, and (3)
extrusion from the cell across the basolateral membrane.
The major action of vitamin D in regulating
this process is on the steps involved in Ca2+ movement
beyond brush-border entry (Roche et al., 1986; Schedl
et al., 1994). The concentration of cytosolic Ca2+ within
the enterocyte is closely controlled at about 10–7 M,
and this is ultimately a function of the rate of entry
and the rate of exit of Ca2+ across the cell boundaries.
Intracellular organelles, including mitochondria, microsomes
and lysosomes, play a major role in controlling
cytosolic Ca2+ by storing and releasing the cation
as appropriate. The overall rate of transcellular movement
is determined by the intracellular diffusion,
which is the rate-limiting step (Bronner, 1990).
One of the most striking effects of vitamin D is
the induction of calbindin-D9k, which is distributed
throughout the cytoplasm of the enterocyte. Absence
of intestinal calbindin-D9k may be considered a
molecular index of vitamin D defi ciency (Bronner,
1991).
Entry
This step involves the movement of luminal Ca2+
across the brush-border membrane of the enterocyte
into the cytosol. The downhill electrochemical gradient
permits the entry of luminal Ca2+ without the
input of metabolic energy.
Fullmer (1992) reviewed data from several laboratories
that suggested a ‘liponomic regulation’ of intestinal
calcium transport by 1α,25(OH)2D3. In this model,
1α,25(OH)2D3 alters the phospholipid structure of the
brush-border membrane, causing an increase in membrane
fl uidity, which, in turn, leads to a specifi c increase
in the permeability of the membrane to Ca2+. Among
these reports, 1α,25(OH)2D3 enhanced the synthesis of
phosphatidylcholine and also increased the incorporation
of unsaturated fatty acids into phosphatidylcholine
in chick duodenal enterocytes (Matsumoto et al.,
1981). Incorporation of methyl cis-vaccinic acid (a
fatty acid known to increase membrane fl uidity) into
brush-border membrane vesicles from vitamin D-defi
cient chickens caused an increase in rate of vesicular
calcium uptake, but there was no such effect in vesicles
from 1α,25(OH)2D3-treated chickens. Conversely,
methyl trans-vaccinic acid (a fatty acid known to decrease
membrane fl uidity) caused a decrease in rate of
calcium uptake in vesicles from 1α,25(OH)2D3-treated
chickens, but no change in vesicles from vitamin Ddefi
cient chickens (Fontaine et al., 1981). Brasitus et
al. (1986) demonstrated that the fl uidity of the brushborder
membrane from vitamin D-deprived rats was
lower than that of vitamin D-replete control animals.
Treatment with 1α,25(OH)2D3 restored fl uidity to control
levels within 1–2 hours. The changes in membrane
fl uidity were associated with appropriate changes in
lipid composition, and preceded detectable increases
in calcium absorption (demonstrable only during the
5th hour). The lack of temporal correspondence indicates
that the early changes in membrane composition
attributable to 1α,25(OH)2D3 are probably not a major
factor in calcium absorption. The same conclusion was
reached by Schedl et al. (1994), who found no signifi -
cant effect of vitamin D on saturable or nonsaturable
uptake of calcium.
The observation that Ca2+ uptake at the brush border
has a saturable component suggests an interaction
with a low-affi nity binding site, which might be associated
with a calcium channel or a carrier of some sort.
The results of in vitro and in vivo studies using calcium
channel blocking drugs suggest that voltage-activated
calcium channels, such as those found in excitable
tissues (nerve, muscle) are unlikely to be present in
intestinal brush borders (Favus & Tembe, 1992). This
does not exclude the possibility that the brush-border
membrane may contain calcium channels with properties
distinct from those found in nerve and muscle,
although no such channels have as yet been identifi ed
in this membrane.