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MEMBRANE PERMEABILITY, CALCIUM, AND OSMOTIC PRESSURE
In: Wurts, W. A. 1987. An evaluation of specific ionic and growth parameters affecting the feasibility of
commercially producing red drum (Sciaenops ocellatus). Doctoral dissertation. Texas A&M University,
College Station, TX. © 1988
William A. Wurts, Senior State Specialist for Aquaculture
Kentucky State University Cooperative Extension Program
www.ca.uky.edu/wkrec/Wurtspage.htm
DISCUSSION (excerpts)
[Commentary:
It seems plausible that the presence and/or absence of calcium ions on specific areas of ion pores/channels
results in a change of the protein’s shape. Calcium may affect the distribution of electrons on the protein and
alter the atomic/intra-molecular bond angles. This could modify the structural conformation of the
pore/channel proteins, creating differences in membrane permeability to specific ions.]
Cell membranes, which are vital to the maintenance of intra and extracellular ion concentrations, appear to
be important ion permeability barriers in fish. Generally, a cell from any vertebrate is bathed
in extracellular fluid (blood and interstitial fluids). The fluid found within the cell (intracellular fluid) has a
distinct ionic composition, different from extracellular fluid. By maintaining this ionic disparity, cells are
able to maintain internal fluid pressure and hence their shape. Additionally, a membrane potential (an
electrical voltage gradient) is generated by maintaining the difference in ionic composition as well as
differences in charged components across the cell membrane. This occurs as a result of the semi-permeable
nature (differential permeability to select ions) of cell membranes. The ionic gradient is thought to be
generated and maintained by energy dependent processes. Many of the regulated ions have important
functions for enzymatic reactions, the generation and conductance of electrical impulses and the maintenance
of cell integrity.
The two ions which are most conspicuously regulated are sodium and potassium. The cell membrane is 100
times more permeable to potassium than sodium. The concentration of sodium is highest
in extracellular fluid while potassium is highest in intracellular fluid. Cell membranes become less
permeable to molecules as they increase in size as well as charge or polarity. The cell membrane behaves as
if it had numerous pores traversing it. These hypothesized pores appear to have a diameter of 8.0 angstroms
and could account for the differences in membrane permeability to sodium and potassium (Solomon, 1960;
Guyton, 1971). More recent work has described and elucidated the presence of specific cellular pores or
channels from excitable tissues (Matsuda and Noma, 1984; Moczydlowski et al., 1985). It is postulated that
the membrane pore is lined or gated with positively charged prosthetic groups. Further, it is theorized that
the prosthetic groups are divalent cations; calcium, in particular. These ions are attached presumably to
binding sites along the surface of the pore (Frankenhaeuser and Hodgkin, 1957; Solomon, 1960; Guyton,
1971; Moczydlowski et al., 1985).
An element in its ionic form has an electrical charge whose field lines approximate a sphere, at the center of
which is located the atom. It is believed the charged fields of divalent calcium ions allow only certain
molecules to pass through the membrane pore. The pore itself restricts the size of particles passing through
it. Positively charged calcium ions lining the pore restrict the passage of other positively charged ions on the
basis of the magnitude of their charged sphere. Sodium has a greater sphere of positive charge than
potassium and should pass through the cell membrane (pore) less readily than potassium. This theory is
compatible with the presence of higher concentrations of sodium and lower concentrations of potassium
in extracellular fluids with the converse being true for intracellular fluids. Ostensibly, the pore (passive
mechanism) coupled with energy dependent ion pumps (active mechanism) sustain this gradient.
A pertinent model for studying ion fluxes across cell membranes is the nerve cell. The nerve cell generates,
conducts and transmits electrical impulses to the tissue that it innervates by creating a wave of membrane
depolarization. This is caused by a rapid but transient change in the permeability of the cell membrane to
both sodium and potassium ions. Depolarization is caused by a rapid influx of sodium ions into the nerve
cell which disrupts the charge gradient across the membrane. This depolarization starts at one point on the
cell membrane and spreads over the entire surface of the nerve, apparently by exciting (depolarizing) all
areas of the cell membrane directly contiguous to the initial point of depolarization. It has been hypothesized
(Frankenhaeuser and Hodgkin, 1957; Guyton, 1971) that a sudden yet temporary removal of the charged
calcium that lines the membrane pore allows sodium to more readily penetrate or move through the
membrane. This could possibly occur in response to the release of nerve transmitters, hormones or other
unknown chemical substances.
Calcium is then restored preventing further sodium influx. When this occurs, a transient efflux of potassium
from the intracellular fluid to the outer surface of the cell membrane restores the original surface charge,
thus repolarizing the membrane. It is further recognized that once a point area of membrane is depolarized,
depolarization continues spontaneously. The wave of depolarization is self propagating (Guyton, 1971). It is
assumed that the mechanism of depolarization (the increase in sodium influx) is still the same. The wave of
increased sodium permeability would result from altering the binding affinity for calcium in the membrane
pore. This could occur as a result of the changed charge on the surface of depolarized membrane contiguous
to membrane that has not been depolarized. The change in charge of depolarized membrane could alter the
calcium binding affinity of the pores in stable membrane by affecting the charge of the calcium binding sites
(possibly pore proteins). Nerve transmitters and hormones may effect depolarization by the same
mechanism. Ion fluxes across cell membranes, as described above, appear to be transient and not directly
dependent on energy (ATP).
It has been recognized that the intensity of stimulus necessary to initiate sodium influx can be reduced by
lowering the concentration of calcium in the extracellular fluid (Frankenhaeuser and Hodgkin, 1957; Guyton,
1971). Presumably, this occurs in response to an altered binding affinity for calcium in the membrane
pores. These observations are consistent with the principles of cofactor-enzyme interactions as defined
by Michaelis-Menten enzyme kinetics (Lehninger, 1975). The affinity or rate of cofactor binding (calcium in
this example) is dependent upon the concentration or availability of that component in the reaction medium
(extracellular fluid bathing the nerve cell membrane). Low calcium in the extracellular fluid would result in
incomplete saturation of the calcium binding sites along the membrane pore, reducing the stimulus (force)
necessary to dislodge them in the process of depolarization. If calcium concentrations are sufficiently low in
the extracellular fluid, spontaneous sodium influx will occur (Frankenhaeuser and Hodgkin, 1957; Guyton,
1971). An understanding of this hypothetical mechanism simplifies the explanation of the results of this
dissertation and provides a parallel analogy for comparison with the effects of environmental calcium on red
drum performance.
The extracellular fluids (blood and interstitial fluids) of the red drum or any fish come into close contact with
the environment by way of the body surfaces and the gills. The extracellular fluids of teleosts have a unique
and relatively stable ionic composition. In general, most vertebrates are similar. Human extracellular fluid
has sodium, potassium and calcium at concentrations of 3264, 196 and 100 mg/1 respectively (Guyton,
1971). When compared, a wide variety of teleosts show similar ionic concentrations in
their extracellular fluids (Holmes and Donaldson, 1969).
The environment, whether fresh or saltwater, has unique yet different ionic characteristics from
the extracellular fluids of teleosts. Most marine waters (35 g/1 TDS) have much higher concentrations of
sodium, potassium and calcium at 10685, 396 and 410 mg/1 (Gross, 1977), respectively, than
the extracellular fluids of teleosts (e.g. red drum). Conversely, the concentrations of these ions in freshwater
(Boyd, 1979) are typically well below those found in the extracellular fluids of teleosts. Therefore, the
differences in ionic composition of teleost extracellular fluid and that of the environment generate an ionic
gradient across the membranes (semi-permeable) of teleost surface epithelia.
The surface epithelia of gills and body surfaces are protected from direct interaction with the environment by
mucous and intercellular junctions. Fish mucous appears to have calcium binding properties
(Chartier Baraduc, 1973). Mucous is a glycoprotein and could serve as a calcium chelating agent retarding
ion loss from epithelial cells as a charged surface coat or barrier. Intercellular junctions are specialized areas
of attachment between epithelial cells preventing the loss of ions and fluids from the interstitial area beneath
(Bloom and Fawcett, 1975). It has been postulated that cell membranes and intercellular junctions are
dependent on calcium for normal function (Loewenstein, 1966; Guyton, 1971; Bloom and Fawcett,
1975). This dependence may relate to the routes (perhaps pores or channels) of passive and active ion
movements (Guyton, 1971; Fromter and Diamond, 1972, Claude and Goodenough, 1973; Sardet et al.,
1979; Sardet, 1980; Nonnotte et al., 1982). While some ion loss or gain occurs as a result of leakiness, it is
assumed that energy dependent processes can compensate.
Molecules or ions diffuse from areas of high concentration to areas of low concentration until they are
equally distributed. This is true for each specific component of any solution. It is of interest for this
discussion that sodium, potassium and calcium concentrations in sea water are higher than those of
fish extracellular fluid while those of fresh water are lower. In sea water, the tendency is for sodium,
potassium and calcium to diffuse into fish extracellular fluid. In fresh water, the reverse would occur. It is
assumed that what diffusion does occur, does so through ion pores in epithelial cell membranes and
intercellular junctions (gills and body surfaces). Permeability barriers (ion pores and mucous) and energy
dependent ion pumps prevent the ionic composition of fish extracellular fluid from equilibrating with that of
the environment.
The results of the ion experiments in this dissertation can be related to the ion pore theory as previously
described. In low calcium aquatic environments, the ion pores of the surface epithelia would
be submaximally saturated with calcium. This would lower the force or kinetic energy necessary to strip
calcium from the pore. If environmental calcium were sufficiently low, a rapid and spontaneous flux of
sodium (possibly potassium as well) could occur moving from the fluid of highest concentration to the fluid
of lowest concentration. Diffusion would be rapid enough that active (energy dependent) uptake or
elimination of ions could not compensate. Death would occur as a result of altered circulatory volume and/or
disrupted ion metabolism. This is consistent with the results observed in this study as well as the
observations of other researchers (as discussed in the introduction).
----------
As explained in the introduction and the first pages of discussion, there is considerable evidence to indicate
that the ionic composition of a teleost's environment directly affects either gill ionic exchange mechanisms
and/or their permeability to certain ions and water. The results of this dissertation, particularly the ion
studies, appear to substantiate these findings. The concentration of environmental calcium appears to
directly affect survival of red drum in both saltwater and freshwater environments.
The diffusional gradients for sodium and calcium across the surface epithelia of fish placed in low calcium
(less than 100 mg/1) sea water (35 g/1 TDS) are in opposite directions. Sodium ions diffuse from sea water
into the extracellular fluid while calcium would be driven towards sea water. The sharp sodium gradient
causes sodium to influx with high energy. Low calcium concentrations in the environment would tend to
dislodge calcium from its ion pore binding sites. As calcium begins to flux, the influx of sodium would
become rapid pushing free calcium ions toward the extracellular fluid. Apparently, the force
of influxing sodium is so great that a minimum concentration of 176 mg/1 calcium (lowest calcium level
with survival, Table 7) in sea water is necessary to begin saturating ion pore binding sites, thus retarding the
influx of sodium and perhaps, that of potassium. Sodium influx and the concomitant efflux of water are too
great in saltwater environments containing 120 mg/1 calcium or less (treatments with 100% mortality, Table
7). The resultant loss of fluid volume and the increased sodium (and potassium) content of
the extracellular fluid may cause circulatory shock and cardiac failure (Guyton, 1971), resulting in death.
In fresh water, the diffusional gradients for sodium, potassium and calcium are in the same direction across
the surface epithelia. This unidirectional ion flow, through the ion pores, is the most reasonable explanation
that the euryhaline red drum can tolerate much lower calcium concentrations in fresh water than in salt water,
9 mg/1 as opposed to 176 mg/1, respectively. Since calcium is diffusing in the same direction as sodium
and potassium at a relatively constant concentration, it would keep ion pore binding sites in a state of
comparative saturation, thus retarding ion effluxes. However, when the environmental concentration falls
below a minimum level 1.7 mg/1 (in this study), the kinetic energy driving calcium and monovalent ions
through the pore would tend to continuously desaturate calcium binding sites allowing sodium and potassium
(to a lesser extent, calcium) to diffuse into the environment at a rate greater than active uptake mechanisms
could replace them. There would be a simultaneous water influx. The net effect would be the reduction in
concentration of these ions in the extracellular fluid. When ionic concentrations reach a critical low level,
cardiac spasms (low ionic tetany) result (Guyton, 1971), causing death.
If one uses the concentrations of sodium, potassium and calcium found in saltwater,
vertebrate extracellular fluid, and the minimum survival treatments of this study and converts them as
discussed by Guyton (1971), one can estimate the osmotic pressure (kinetic energy) driving each ionic
species through the ion pore (Table 23). It is evident from this table that sodium exerts the greatest force or
pressure across cell membranes (gill epithelia). The ion pore must counteract a sodium pressure in sea water
which is much greater than that in fresh water or low salinity water. It would seem that the binding affinity
for calcium can be altered in accordance with specific environmental ionic concentrations and total dissolved
solids.
Table 23
A comparison of osmotic pressure (mmHg) and the direction of diffusion (flux) for specific ions in fresh
and saline waters relative toextracellular fluid (ECF) at 28ºC
Specific
ions
Osmotic
pressure sea
water: ECF
Ion
flux
Osmotic
pressure1 fresh
water: ECF
Ion
flux
Osmotic
pressure2 saline
water: ECF
Ion
flux
Sodium
6049
influx
2523
efflux
1417
efflux
Potassium
95
influx
93
efflux
67
efflux
Calcium
145
influx
-
-
19
efflux
Low calcium
353a
103b
423c
influx
influx
efflux
384a
474b
-
efflux
efflux
-
-
-
-
-
-
-
1Sodium and potassium concentrations are those of untreated well water (Table 1).
2Saline water in this example is sea water (35 g/l) diluted to a strength of 5 g/l (Table 2).
3a) Minimum calcium concentration (176 mg/l) for seawater survival (Tables 7 and 8).
b) Highest seawater calcium concentration (120 mg/l) at which 100% mortality occurred (Table 7).
c) Lowest seawater calcium concentration (1 mg/l) at which 100% mortality occurred (Table 8).
4a) Mean minimum calcium concentration (19 mg/l) for acceptable freshwater survival (Tables 11 and 12).
b) Calcium concentration (1.3 mg/l) at which freshwater survival was significantly affected (Tables 15
and 16).
Concentrations (mg/l except for pH) of major ionic constituents in sea water [natural or formulated
with synthetic sea salts (35 g/l TDS)], dilute sea water (5 g/l TDS) and vertebrate extracellular fluid
(ECF) - - adapted from Wurts and Stickney, 1989, Aquaculture, 76: 21-35.
Ions
Sea water1
(35 g/l TDS)
Dilute sea water
(5 g/l TDS)
ECF2
(9 g/l TDS)
Sodium
10685
1526
3265
Potassium
396
57
195
Calcium
410
59
100
Magnesium
1287
184
36
Chloride
19215
2745
3652
Bicarbonate
142
20
1708
Sulfate
2511
359
48
pH
7.8-8.4
7.8-8.4
7.4
1Gross (1977).
2Guyton (1971).
Oduleye (1976) noted that brown trout pre-adapted to high calcium environments displayed increased
salinity tolerance. The rate of water influx into isolated gills of the thick lipped mullet could be
progressively reduced with adaptation to fresh water (Gallis et al. , 1979). Low environmental calcium
concentrations have been observed to stimulate prolactin production in sticklebacks (Wendelaar Bonga,
1978) and tilapia (Wendelaar Bonga et al., 1983). Olesen (1985) demonstrated that serotonin could effect a
calcium related permeability increase in the microvessels of frog brain. Fleming et al. (1974) indicated that
low calcium sea water stimulated RNA metabolism in the gills of F. kansae. It is possible that pore binding
site affinity for calcium can be altered in response to osmotic or ionic stress, humoral factors (e.g. hormones)
or messengers to and from individual cell nucleii. Again, it is important to emphasize the transient nature of
the calcium phenomena detected in the results of this dissertation. It is unlikely that compensatory
mechanisms can be sufficiently activated over short periods of time (96 hours).
The results of the freshwater pond culture studies indicated that small red drum (4-6 g) performed poorly
(slow growth and high mortality). Larger red drum fingerlings (35-45 g) displayed rapid growth and good
survival. There are two reasonable explanations for these observations. Apparently, fish of greater size
underwent less osmotic stress and were not food limited.
It is generally recognized that larger animals have both a lower surface area to volume ratio and a lower
metabolic rate/unit weight. The gills of fish represent more than 60% of the exposed body surface (Ogawa,
1975). Large fish would have a lower surface area to volume ratio than small fish with respect to their gills
as well as body surfaces. In small fish, more extracellular fluid is brought into close contact with the
environment by way of body surfaces. Therefore, small fish lose relatively more ions to their environment
(fresh water) as a result of diffusion across leaky permeability barriers. This places a greater demand on
energy dependent mechanisms of ion homeostasis. In addition to osmotic stress, fish are stressed as a result
of handling during stocking. Stress can effect the release of catecholamines, corticosteroids and possibly
pituitary hormones, all of which can affect hydromineral balance (Chan et al., 1968; Guyton, 1971; Johnson,
1973; Sage, 1973; Pic et al., 1974; Shuttleworth, 1978; Panget al., 1980b; Pang and Yee, 1980; Tomasso et
al., 1980; Robertson, 1984). Hence, small fish are subjected to greater osmotic stress due to size (surface
area) and higher metabolic rates (greater energy demands). Stress can increase susceptibility to disease,
retard growth and cause death (Stickney, 1979).
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