As noted earlier, measured activities of
Na+,K+-ATPase were higher when the main
pH buffer of the incubation medium was
HCO 3 /CO 2. In such studies, the CO 2 con-
centration in the extracellular fluid would
be relatively high, reducing the concentra-
tion gradient of CO 2 between the incuba-
tion medium and intracellular fluid. Since
CO 2 diffuses out of cells down this concen-
tration gradient, this results in lower rates
of CO 2 loss from cells and would increase
the potential for facilitated diffusion of
CO 2 , increasing Na+,K+-ATPase activity.
Extracellular CO 2 levels will be lower
when alternative pH buffers, such as
HEPES, are used.
Sies et al. (1973), from measurements
of the change in pH of media draining
perfused rat liver, have shown that addi-
tion of acetate (0–5 mM) to the perfusate
(pH 7.4) caused a rapid change in proton
uptake into hepatocytes, and using data
supplied in this and an earlier publication
(Sies and Noack, 1972) it can be calculated
that the rates of proton uptake observed
would cause changes in Na+,K+-ATPase
activity similar to those reported above,
assuming the previously discussed linkage
between proton uptake and sodium
pumping. The data from Sies’ group do not
differentiate between rapid initial extru-
sion of protons caused by equilibration of
acetate across the plasma membrane and
the proposed steady-state efflux of protons
caused by acetate cycling. A similar
response was seen when the perfused liver
was exposed to differing levels of
CO 2 /HCO 3 .
The ability of acetate to act as a proton
ionophore will depend on the concentra-
tion of acetate in the extracellular fluid
relative to the cell’s ability to metabolize it.
Thus the balance of nutrients available will
determine metabolic efficiency, and it is to
be expected that the optimal balance of
nutrients will vary from tissue to tissue.
Scollan and Jessop (1995) showed that in
sheep given diets which would result in an
imbalance of ingested nutrients (and would
be expected to be used with a lower
efficiency), blood acetate was markedly
higher than in those fed on a more balanced
diet. Cronjé et al. (1991) have shown that
nutrient balance has a marked effect on
acetate clearance, and Leng (1990) has
discussed the influence of nutrient balance
on the efficiency of use of metabolizable
energy, pointing out the importance of the
protein:energy ratio on the overall
efficiency of energy use, as well as on the
level of feed consumption.Leakage
Biological membranes are composed of
lipid bilayers in which many different
proteins are embedded. Due to their
arrangement, they have a hydrophobic
inner layer which is presumed to be imper-
meable to charged or ionic species (or more
specifically to molecules with high charge
densities). As discussed above, this is not
strictly true and many detailed electro-
physiological studies have shown that the
permeabilities of cell membranes to
various ions, whilst low, are not zero (e.g.
Hume and Thomas, 1989). It is not clear to
what extent such permeabilities are due to
the presence of specific ion channels or to
non-specific leakage of ions across the lipid
bilayer – maybe at regions where the lipid
bilayer interacts with embedded proteins –
or indeed to the processes outlined above.
Studies from Hulbert’s group in Australia
have made detailed, comparative studies of
the leakiness of biological membranes.
They have studied the leakiness of both the
plasma membrane to Na+and the mito-
chondrial membrane to H+and have shown
that it varies markedly between cold-
blooded reptiles and mammals (e.g. Else
and Hulbert, 1987; Brand et al., 1991). This
may be due to much higher levels of unsat-
urated fatty acids in the mammal’s mem-
branes and differences in levels of thyroid
hormones (Hulbert, 1987). This group have
proposed that it is the higher rate of both
Na+flux across the plasma membrane and
H+ leak across the mitochondrial mem-
brane in warm-blooded animals that is
important for maintaining higher rates of
heat production, and hence the ability to
keep body temperature at constant levels.Aspects of Cellular Energetics 155