The sodium of animal cell membranes converts the chemical energy obtained from the hydrolysis
of adenosine 5' -triphosphate into a movement of the cations Na + and K + against an
electrochemical gradient. The gradient is used subse quently as an energy source to drive the
uptake of metabolic substrates in polar epithelial cells and to use it for purposes of
communications in excitable cells. The biological importance of the sodium pump is evident from
the fact that be tween 20-70% of the cell's metabolic energy is consumed for the pumping pro
cess. Moreover the sodium pump is an important biological system involved in regulatory
processes like the maintenance of the cells' and organism's water me tabolism. It is therefore
understandable that special cellular demands are han dled better by special isoforms of the
sodium pump that the expression of the sodium pump and their isoforms is regulated by hormones
as is the activity of the sodium pump via hormone-regulated protein kinases. Additionally the
sodium pump itself seems to be a receptor for a putative new group of hormones the endogenous
digitalis-like substances which still have to be defined in most cases in their structure.
This group of substances has its chemically well known coun terpart in steroids from plant and
toad origin which are generally known as car diac glycosides. They are in medical use since at
least 200 years in medicine in the treatment of heart diseases.
