of sodium ions, and hence serves to main-
tain intracellular sodium ion concentration
within certain limits (Smith and
Rozengurt, 1978). It has been reported that
the activity of the sodium pump varies
between different tissues and between
different physiological states.
The research group of Milligan and
McBride have made extensive measure-
ments of ion transport costs, mainly that of
the sodium pump. It has been shown that
the activity of this pump utilizes a substan-
tial proportion of the cell’s ATP production
and that this cost varies with physiological
state, hormonal status, environmental tem-
perature and diet. Gregg and Milligan
(1982b) measured O 2 consumption and the
percentage of this inhibited by ouabain in
muscle from calves of different ages and
breeds. In the youngest animals (2–3 weeks
old), total O 2 consumption was higher than
in older animals (7 months old) and breed
was found to have little effect. Sodium
pump activity accounted for 40% of total O 2
consumption and did not vary with age or
breed. McBride and Milligan (1984, 1985a)
made similar measurements in duodenal
mucosa taken from cows at different stages
of lactation or from sheep on different
planes of nutrition. In both cases, total O 2
consumption varied relatively little between
treatments, but Na+,K+-ATPase activity
increased from 35% in non-lactating or end
of lactation cows to 55% during the peak of
mid-lactation. With sheep, sodium pump
activity accounted for 28, 50 and 61% of
total O 2 consumption at feeding levels of
zero, maintenance and twice maintenance,
respectively. In studies with liver, McBride
and Milligan (1985b) again found no
difference in total O 2 consumption between
samples taken from sheep that were starved
or fed and non-lactating, at peak lactation or
at a late stage of lactation. However, Na+,K+-
ATPase activity was lowest (at 18% of total)
for starved animals and highest (at 45% of
total) for animals at peak lactation.
Other reported measurements of
sodium pump activity vary widely,
accounting for between 60 and 10% of total
cellular energy production. One cause of
this variation is the nature of the incuba-
tion medium used (Milligan and Summers,
1986; Jessop, 1988), with higher activities
observed when HCO 3 /CO 2 -buffered, simple
salt solutions were used and lower
activities when more complex, HEPES-
buffered cell culture media were used.
There has been one reported study in
which Na+,K+-ATPase activity has been
measured in vivo (Swaminathan et al.,
1989). In this study, guinea-pigs were
injected intraperitoneally with ouabain
which caused a 40% reduction in whole-
body metabolic rate.
The studies reviewed above have all
been observational in nature, often termed
empirical. They illustrate that variation in
such processes occurs, but they do not
provide any understanding of the causes of
such variation. For example, changes in
Na+,K+-ATPase activity imply that the rate of
sodium ion entry into cells differs accord-
ingly. Why might this be so? One major
problem might be that of in vitromeasure-
ment, in that tissues or cells have been
removed from the environment in vivo.
Hormonal levels will be different, as might
the concentration of substrates and other
ions. Care has to be taken in order to ensure
that the plasma membranes of the cell sus-
pensions have not been damaged by the
isolation procedure, as any changes in mem-
brane permeability will give rise to large
changes in sodium pump activity. In many
cases, cell viability is assessed by means of
dye exclusion. What causes sodium to enter
cells? Figure 7.1 shows the major causes of
Na+entry into a cell across its plasma mem-
brane. As each Na+ion enters, so it must be
removed by the sodium pump in order to
maintain the intracellular Na+ concentra-
tion. Since one mole of ATP is hydrolysed
for every 3 moles of Na+pumped, every Na+
entering the cell costs one-third of an ATP.
Consider a cell in its environment. It
contains within its plasma membrane,
nucleic acids (DNA and RNA) and many
proteins. Proteins and nucleic acids are
negatively charged when hydrated and
these molecules, because of their size,
cannot pass across the cell membrane.
Hence they are often referred to as the
cell’s fixed negative charge. The negativeAspects of Cellular Energetics 151