Re: How many ATP are needed for the heart to beat?

Hello Linda,

That is a very interesting and complicated question, which I have no chance 
of giving a full answer to but I have had a search through the literature 
for information to give you an idea of how ATP is used in the heart.
There are 3 main enzymes that use ATP during the contraction of heart 
muscle.  These are: the sarcolemal Na+ K+ ATPase; myosin; and the 
sarco/endoplasmic reticulum Ca2+ ATPase (SERCA). 
The Na+ K+ ATPase is the primary transport system that maintains the ionic 
imbalance between the cell exterior and interior.  Each pump cycle for the 
breakdown of one molecule of ATP extrudes 3 Na+ ions out of the cell and 
transports 2K+ ions into the cell, thus building up [K+] and reducing [Na+] 
This sets up and maintains the ionic gradients that generate the currents 
responsible for the action potential.  The rate of hydrolysis of ATP by the 
Na/K ATPase enzyme can be measured in the test tube and it has been found 
that one milligram of this protein will breakdown 966-1400nmoles of ATP per 
minute at 37oC (Tao et al., 1995 Journal of Cardiovascular Pharmacology Vol 
25 pp859-863).
Myosin is the major protein in the contractile apparatus of the muscle, 
which binds to actin.  As myosin breaks down a molecule of ATP it undergoes 
a conformational change so that its lever arm moves which generates force 
and moves actin filaments along relative to itself, which is the molecular 
event involved in muscle shortening.  One milligram of human cardiac 
ventricular myosin breaks down ATP at a rate of 124 nmols per minute at 
37oC (Schier et al., 1982  Journal of Clinical Investigation. Vol 69 
pp816-825.
The SERCA protein utilises ATP to remove Ca2+ from inside the cell 
following the end of the action potential to enable the muscle to relax.  
For the breakdown of one molecule of ATP the pump removes two Ca2+ ions 
from the cell.  The ATP hydrolysis rate for 1milligram of SERCA is 800nmols 
of ATP per minute at 35oC (Wolosker et al., 1997 Biochemical Journal Vol 
321 pp545-550).

Unfortunately to extrapolate this information to ATP turnover in the whole 
heart is not possible.  In the intact heart these proteins will not be 
working at a constant rate and not all the protein in the heart will be 
working at the same time.  Also the quantity of each of these proteins in 
the whole heart is not known.
The ATP concentration in the heart is around 0.7mmoles per gram wet weight. 
 But ATP turnover in the heart is very fast and ATP can be generated very 
quickly as it is needed.

I hope that I have managed to give you some more insight into the 
utilisation of ATP in the heart and I am sorry for not being able to fully 
answer your question.

Dave Burton