If you've ever had a severe asthma attack or gone into premature labor, there is a good chance you were given the drug terbutaline. Terbutaline can relax your involuntary smooth muscle when it's causing problems: in constricted
If you've ever had a severe asthma attack or gone into premature labor, there is a good chance you were given the drug terbutaline. Terbutaline can relax your involuntary smooth muscle when it's causing problems: in constricted airways during an asthma attack, or in the uterus during contractions. But if you've taken terbutaline, you've probably also noticed another effect: it can induce a pounding, racing heartbeat. How can one drug produce such opposite effects - relaxing smooth muscle in some parts of your body, while making your cardiac muscle work harder?
The answer is that terbutaline switches on a common information-processing module, called a signaling pathway, which gets used over and over in different cells to perform very different jobs. This information-processing module can be plugged into different cell types, where it will transmit signals from the environment outside the cell to the inside where the information is processed and acted upon. Because our cells use a common set of information-processing modules to carry out so many different jobs, it's easy for drugs that act on these modules to produce a wide range of side-effects.
Anatomy of an Information Processing Module
Let's take a look at one of these information-processing modules to see how terbutaline produces it's varied effects. Terbutaline closely resembles a hormone produced by your body, epinephrine (also known as adrenaline). Like epinephrine, terbutaline switches on a signaling pathway when it binds to a protein on the surface of a cell, a protein called a beta-adrenergic receptor. (Aficionados will know that there are several types of these receptors, and they have somewhat different roles in different tissues; terbutaline acts primarily on beta-2 receptors.) Once terbutaline binds the receptor, we get a cascade of chemical reactions like this:
1. The receptor activates a G-protein.
2. The G-protein turns on an enzyme called adenylyl cyclase.
3. Adenylyl cyclase produces a molecule called cAMP.
4. cAMP activates a protein called Protein Kinase A, or PKA.
5. PKA initiates many different downstream processes that produce a change in the cell (stronger muscle contractions in your heart, relaxation in your smooth muscle cells, and other effects in your gut and liver).
Like components in your computer, this pathway can be plug