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Chapter 7: Biological Membranes and Membrane Transport

Digoxin is a member of the cardiac glycoside family of drugs. This drug can be used to treat patients with atrial fibrillation since it slows heart rate. Cardiac glycosides function by inhibiting which class of membrane transporter?

A. antiporters

B. facilitative diffusers

C. symporters

D. uniporters

Answer A: The cardiac glycosides function by inhibiting the action of Na+/K+-ATPases. These transporters transport Na+ into the cell while simultaneously transporting K+ out. Thus, they function as classical antiporters.

Aquaporins are a class of transporters that are involved in the transport of water across membranes. The aquaporins belong to which type of transporter family?

A. active

B. β-barrel channels (porins)

C. α-channels

D. facilitated

E. passive

Answer C: The aquaporin proteins are made up of 6 transmembrane α-helices arranged in a right-handed bundle. The a-type channels are homo- or hetero-oligomeric structures that in the latter case consist of several different proteins. The a-type class of channel protein has between 2 and 22 transmembrane α-helical domains, thus the derivation of the name of this type of channel.

You are treating a patient who complains of having an intense or uncontrollable thirst with a strong craving for ice water. The patient produces large amounts of urine (8-10 L per day). Additional symptoms include fatigue, headache, irritability, and muscle pains. Given the symptoms you suspect nephrogenic diabetes insipidus. If this diagnosis is correct, which of the following membrane transporter types would be defective in your patient?

A. aquaporin

B. F-type ATPase

C. Na+/K+-ATPase

D. P-type ATPase

E. SLC40A1 (ferroportin)

Answer A: Nephrogenic diabetes insipidus (NDI) is a disorder in which a defect in the tubules in the kidneys causes a person to pass a large amount of urine. This disorder is caused by an improper response of the kidney to antidiuretic hormone (ADH, also called vasopressin), leading to a decrease in the ability of the kidney to concentrate the urine by removing free water. The primary cause of NDI is a mutation in the ADH receptor in the kidney. However, NDI can also result from mutations in the aquaporin-2 gene.

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