3.6: Faulty membranes can cause diseases.

The single most common fatal inherited disease in the United States is cystic fibrosis, a disease that results from an improperly functioning membrane. At any given time, about 30,000 people in the United States have cystic fibrosis.

Cystic fibrosis occurs when an individual inherits from both parents incorrect genetic instructions for producing one type of transmembrane protein that allows chloride ions to get into and out of cells. This transport protein occurs primarily in the membranes of cells in the lungs and digestive tract.

These genetic instructions can be defective in more than a thousand different ways, but the result is the same: malfunction of chloride passageways in a cell’s membrane that causes gradual accumulation of chloride ions within cells. In nearly all cases of cystic fibrosis, two primary effects occur: an improper salt balance in the cells and a buildup of thick, sticky mucus—particularly in the lungs. Normal mucus helps to protect the lungs by trapping dust and bacteria. This mucus is then moved out of the lungs (helped along by coughing). The mucus produced by someone with cystic fibrosis, however, is too thick and sticky to be moved out of the lungs, so it collects there, where it impairs lung function and increases the risk of bacterial infection. Because of the improper cellular salt balance, one way to test for cystic fibrosis is to measure the concentration of salt in the sweat—abnormally high concentrations indicate that the person may have the disease

Although many high-tech treatments have been promised for the sufferers of cystic fibrosis and a great deal of research is being done on this disease, one of the most common treatments is decidedly low-tech. Parents help their children with cystic fibrosis clear the mucus out of their lungs by holding them on a steep slant, almost upside down, and vigorously patting or thumping their chest and back. This shakes loose the mucus in their lungs and moves it to a place where they can cough it up (FIGURE 3-13). With careful treatment, the life expectancy of someone with cystic fibrosis can be 35–40 years or longer.

Faulty membranes also play a role in many other diseases, including heart disease (familial hypercholesterolemia), diabetes, and hormonal disorders (such as Graves disease). In most of these cases, the membrane component that is improperly functioning is commonly a receptor protein or a transport protein.

It shouldn’t be surprising, then, to discover that much pharmaceutical research focuses on altering cell membrane functioning, One group of drugs that alter membrane function—called “beta-blockers”—is extremely effective at reducing anxiety. This effect was discovered almost accidentally. The drugs were actually developed as a treatment for high blood pressure—the only use for which they are approved by the FDA. But as a result of several decades of clinical research studies documenting the effectiveness of beta-blockers in treating anxiety, many doctors now prescribe beta-blockers (in a legal practice called “off-label” prescribing) to control the symptoms of anxiety.

Here’s how beta-blockers work. Many cells in your body, particularly the cells of the heart, have receptor proteins on their plasma membranes that can bind to adrenaline, which helps your body cope with stressful situations. These receptor proteins are called “beta-adrenergic receptors,” or beta-receptors. In stressful situations, your adrenal glands pump out adrenaline (FIGURE 3-14). On reaching cells in your heart (among other locations in your body) and binding to beta-receptors, the adrenaline promptly causes your heart to beat faster and more forcefully, increasing your blood pressure in the process. This reaction is fine in a short-term fight-or-flight situation, but it is not healthy over the long run because the increased pressure can damage blood vessels. Depending on its severity, this reaction can also be problematic if you are giving a presentation or taking a test, or if you’re in any other anxiety-producing situation.

When you take a beta-blocker pill, the pill dissolves and the chemicals travel throughout your body until they encounter the beta-receptors. They bind to the receptors, hold on, and block the adrenaline from doing its job. This outcome slows your heart rate, causes a reduction in blood pressure, and can bring great relief to those suffering from the sweating and trembling associated with anxiety.