Re: How does caffeine act as a smooth muscle contractant?

Interesting question,

Actually, caffeine RELAX various smooth muscles (not contract), and the 
tremors some people experience from drinking caffeinated drinks are due to 
the central nervous system (CNS) stimulating effects of caffeine.

Caffeine is actually part of a group of compounds called methylxanthines, 
including: caffeine, xanthine, theophylline and theobromine.  The 
methylxanthines share in common a variety of effects, including: smooth 
muscle relaxation (especially bronchial smooth muscle); stimulate the CNS; 
stimulate cardiac muscle; and act on the kidney as a diuretic.

Some common food items and the methylxanthines they contain:
Tea: Mainly caffeine, and small amounts of theophylline and theobromine.
     From the leaves of Thea sinesis

Cocoa and chocolate: Mainly theobromine and some caffeine.
     From the seeds of Theobroma cacao

Coffee:  Manily caffeine.
     From the fruit of Coffea arabica and related species

Cola-flavored drinks: Mainly caffeine.
     From the nuts of Cola acuminata and the addition of caffeine itself

The cellular basis is not exactly known, but there are some proposed 
mechanisms that include:
1.  inhibition of phosphodiesterases (increasing cAMP)
2.  direct effects on intracellular calcium concentration
3.  indirect effects on intracellular calcium concentration
4.  uncoupling of intracellular calcium increases with muscle contractile 
elements.
5.  antagonism of adenosine receptors

Antagonism of adenosine receptors appear to be the most favored mechanism 
currently.

Reference:
The information provided above is taken directly from W.E. Serafin's 
Chapter 28 "Drugs used in the treatment of asthma", in Goodman and Gilman's 
Pharmacological Basis of Therapeutics (9th Edition, 1996). J.G. Hardman and 
L.E. Limbird (eds-in-chief), page 672-673.

Hope this helps!
....mark fung, md/phd