influence the rate of Na+entry into the cell,
and hence Na+,K+-ATPase activity. Nearly
all measurements of ion transport have
been made on isolated cells or pieces of
tissue removed from the animal and
incubated in vitro. The incubation medium
used is usually devoid of hormones and
contains standardized (and often high)
levels of substrate. The differences
observed indicate that some intrinsic
property of cells has been altered and that
this persists for the period of time between
tissue removal and the measurements
being made.
Consideration of physiological pro-
cesses at the level of individual cells
enables the understanding of energy use. It
can be appreciated that many energetic
costs are consequences of particular meta-
bolic or physical processes, and thus the
opportunity to manipulate them is limited.
High rates of metabolic activity equate with
increased basal metabolic requirements of
individual cells. For example, high rates of
CO 2 production will increase the rate of H+
efflux from cells as part of facilitated
diffusion of CO 2 , in turn increasing energy
use by Na+,K+-ATPase. The ability of cells
to divide or to grow is dependent on
hormonal stimuli that increase pHias part
of the sequence of events they trigger. This
increases basal energy use by increasing
the rate of H+ efflux, again increasing
Na+,K+-ATPase activity.
Ensuring that the balance of nutrients
is optimal will ensure that the potential for
wasteful cycling of acetate across the cell
membrane is minimized. Acetate cycling
will be greater when the concentration of
acetate in blood is high relative to the cell’s
ability to metabolize it. The availability of
other nutrients can influence this (Illius
and Jessop, 1996).
Therefore, the ability of weak acids
such as acetic and carbonic acids to act as
proton ionophores is worthy of further
investigation as a new concept which
might offer the possibility of a unifying
theory to account for previously contradic-
tory observations. It may explain the
changes in efficiency of use of metaboliz-
able energy with differing diets, the causes
of which have not yet been elucidated
satisfactorily. Understanding the causes of
reduced metabolic efficiency and the
associated heat increment would be of
tremendous advantage, for example in
developing improved feeding strategies
when combating heat stress. The possi-
bility of metabolic energy dissipation as a
result of a physical transmembrane
influence of metabolites could yield a very
profound new insight into the energy
metabolism of many species beyond
ruminants. Such information will be
required in order to assess the causes of
differences in energy use between animals
of differing genotype. Taylor et al.(1987)
reported that maintenance needs of cattle
varied with breed. Such differences cannot
be accounted for by changes in the propor-
tions of metabolically active tissues (Taylor
et al., 1991; Webster, 1993), thus indicating
that differences in metabolic efficiency
exist. Identification of nutrients and
hormones that have a direct influence on
metabolic efficiency will be necessary to
understand the causes of such variation.158 N.S. Jessop
References
Adeola, O., Young, L.G., McBride, B.W. and Ball, R.O. (1989) In vitroNa+,K+-ATPase dependent
respiration and protein synthesis in skeletal muscle of pigs fed at three dietary protein levels.
British Journal of Nutrition61, 453–465.
AFRC (1993) Energy and Protein Requirements of Ruminants. An advisory manual prepared by the
AFRC Technical Committee on Responses to Nutrients. CAB International, Wallingford, UK.
Baldwin, R.L., Smith, N.E., Taylor, J. and Sharp, M. (1980) Manipulating metabolic parameters to
improve growth rate and milk secretion. Journal of Animal Science51, 1416–1428.
Boron, W.F. (1983) Transport of H+and of ionic weak acids and bases.Journal of Membrane Biology
72, 1–16.