Re: Why does smooth muscle not need a supply of creatine phosphate?

Hi Lloyd!  Thanks for your question!  

The reaction that you're familiar with is the following:

ADP + Creatine Phosphate -->(catalyzed by Creatine Phosphokinase) --> ATP + 
Creatine

At rest, our muscles maintain the minimum amounts of ATP in order to 
conserve energy.  During exercise or moments of activity, however, the 
muscles in use will produce large amounts of ATP via the above reaction at 
a constant rate.  creatine phosphokinase, the catalyst of this reaction, is 
so fast and efficient, that at any given moment during this reaction, there 
is almost ten times as much creatine phosphate present and waiting to be 
used than ATP generated!

Creatine phosphate, a high-energy molecule that contains a phosphoryl 
group, is found mainly in skeletal muscles because these muscles need large 
amounts of ATP for strenuous activity, unpredictable bursts of energy, 
motion....   In order to maintain the supply of ATP as energy is being 
expended due to contractions of the muscles used, creatine phosphokinase 
pulls the phosphate group off of creatine phosphate and gives it to the ADP 
(adenosine DIphosphate) molecule, turning it into ATP (adenosine 
TRIphosphate).

In contrast, smooth muscle, which contracts much more regularly and on a 
continual basis, requires ATP at a much slower rate and therefore utilizes 
a different reaction.  For example:  While there is a great need for 
skeletal muscles to help you run quickly and suddenly away from a surprise 
Tyrannosaurus Rex attack, there is really no reason for the smooth muscles 
in your gastrointestinal tract to immediately and suddenly process a potato 
chip.  The reactions that provide ATP to smooth muscle are different than 
the above reaction for skeletal muscle and do not involve creatine 
phosphate.   Smooth muscle contraction is based on a different series of 
reactions that I will simplify briefly:   Calcium binds to a molecule 
called calmodulin.  This calcium-calmodulin complex then binds to Myosin 
Light Chain Kinase and activates it, causing it to phosphorylate an 
inactive Myosin Light Chain molecule.  This ultimately leads to the 
contraction of the smooth muscle.  Likewise, smooth muscle relaxation 
occurs when the phosphate is removed from the activated Myosin Light Chain.

On an interesting note (courtesy of www.academics.smcvt.edu), some people 
take creatine phosphate as a dietary supplement with the hopes that it will 
enhance their athletic performance.  Their theory is that by increasing the 
amounts of creatine phosphate in the muscles than what would normally be 
there physiologically, the ATP supply can be replenished for even longer, 
delaying fatigue.  Sounds like a clever idea --- however, researchers are 
finding that the human body can only store a specific amount of creatine 
phosphate --- any extra onboard, and the body eliminates it!  

Hope this information helps, Lloyd!

Sincerely, 
G. Monreal

For further information, consult the following excellent websites:
	www.academics.smcvt.edu, www.uic.edu, www.bmb.leeds.ac.uk