|"There are at least four different levels of
regulation of carrier activity (Table 3): (i) the transport activity simply depends
on the availability and concentration of transport substrates and competing
ligands in relation to their affinity for the carrier protein, (ii) it may also be
influenced by parameters directly modulating kinetics of a carrier system such as
intra- and extracellular pH, availability of cotransport ions, ion gradients and
membrane potential, or by the medium osmolarity (iii) a further level of control
in the short-time range is regulation of the level of activity by interaction with
particular effectors which may be internal metabolites (e.g. cyclic AMP), intracellular
regulatory proteins or components of other transport systems (e.g.
the HPr component of phosphotransferase systems  (iv) as an adaptive
mechanism to changes in the growth conditions, many carrier systems are
effectively regulated on the level of expression."