3. CELLULAR ASPECTS OF ACTIVE TRANSPORT
187
tion of an action potential could be a release of Ca bound in the membrane (95, 187; see also Volume II, Chapter 10).
4. Passive Flux of Sodium and Potassium in Nonconducting
Cells
Calcium ions play also an important role in determining the passive
permeability characteristics of nonconducting cells. In Ca-free medium
the passive permeability to cations increases considerably in erythrocytes
from the turtle (188), the snapping turtle Chelydra serpentina
(189),
fishes (190), and man (191, 192). Acanthamoeba in Ca-free medium
loses about 8.5% of its total Κ within 5-10 minutes (193), Κ influx being
1.84 μ/mioles cm.
-2 sec.
-1
, while the efflux is 4.74 ^moles cm.
-2 sec.
-1
.
In the presence of 10 mM Ca, the influx decreases by 19% while the
efflux decreases by 32%.
These results seem to indicate that divalent cations, and more particularly Ca, are bound at the cell surface to the molecular architecture
responsible for the passive permeability characteristics of the membrane.
B. ACTIVE TRANSPORT
Without a careful analysis of the experimental conditions, it is rather
difficult to demonstrate a direct effect of a substance on the mechanism
of active transport of an ion. The flux of an ion across a living membrane
is indeed the result of various mechanisms, and even if we are dealing
with an active transport, passive permeability may be a limiting step.
To illustrate this point let us return to the frog skin. As shown in
Section III, Na is actively transported from the epithelial side to the
dermal side, the mechanism of active transport being located at the inner
boundary of the epithelial cells. The outer boundary is specifically permeable to Na ions (104). There is good experimental evidence that the
intracellular Na concentration sets the rate of the active transport mechanism. On the other hand, the intracellular Na concentration is related,
among other things, to the Na passive permeability at the outer border
of the cells. Thus if we modify the passive permeability to Na, we could
influence the active transport of Na through a modification of intracellular Na concentration, without affecting directly the mechanism responsible for active transport (100, 102, 108, 194). This example shows
how difficult it is to interpret the action of Ca and Mg on the active
transport of ions.
Mullins (195) has shown that if the Ca concentration of a Ringer
solution is doubled, the absorption of P0 4 by the isolated frog sciatic
decreases by 33%. According to Huf et al. (195a) Ca-free Ringer is
without any effect on the active transport of Na across the frog skin.
187
tion of an action potential could be a release of Ca bound in the membrane (95, 187; see also Volume II, Chapter 10).
4. Passive Flux of Sodium and Potassium in Nonconducting
Cells
Calcium ions play also an important role in determining the passive
permeability characteristics of nonconducting cells. In Ca-free medium
the passive permeability to cations increases considerably in erythrocytes
from the turtle (188), the snapping turtle Chelydra serpentina
(189),
fishes (190), and man (191, 192). Acanthamoeba in Ca-free medium
loses about 8.5% of its total Κ within 5-10 minutes (193), Κ influx being
1.84 μ/mioles cm.
-2 sec.
-1
, while the efflux is 4.74 ^moles cm.
-2 sec.
-1
.
In the presence of 10 mM Ca, the influx decreases by 19% while the
efflux decreases by 32%.
These results seem to indicate that divalent cations, and more particularly Ca, are bound at the cell surface to the molecular architecture
responsible for the passive permeability characteristics of the membrane.
B. ACTIVE TRANSPORT
Without a careful analysis of the experimental conditions, it is rather
difficult to demonstrate a direct effect of a substance on the mechanism
of active transport of an ion. The flux of an ion across a living membrane
is indeed the result of various mechanisms, and even if we are dealing
with an active transport, passive permeability may be a limiting step.
To illustrate this point let us return to the frog skin. As shown in
Section III, Na is actively transported from the epithelial side to the
dermal side, the mechanism of active transport being located at the inner
boundary of the epithelial cells. The outer boundary is specifically permeable to Na ions (104). There is good experimental evidence that the
intracellular Na concentration sets the rate of the active transport mechanism. On the other hand, the intracellular Na concentration is related,
among other things, to the Na passive permeability at the outer border
of the cells. Thus if we modify the passive permeability to Na, we could
influence the active transport of Na through a modification of intracellular Na concentration, without affecting directly the mechanism responsible for active transport (100, 102, 108, 194). This example shows
how difficult it is to interpret the action of Ca and Mg on the active
transport of ions.
Mullins (195) has shown that if the Ca concentration of a Ringer
solution is doubled, the absorption of P0 4 by the isolated frog sciatic
decreases by 33%. According to Huf et al. (195a) Ca-free Ringer is
without any effect on the active transport of Na across the frog skin.