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Chapter 31: Nitrogen: Amino Acid–Derived Biomolecules

A 58-year-old man visits your office and complains about impotence. Upon questioning you learn that he is capable of erections, but that they do not last as long and that they are of lesser strength than previously. Which of the following neurotransmitters is primarily responsible for dilation of the penile artery during erections?

A. acetylcholine

B. epinephrine

C. GABA

D. nitric oxide

E. norepinephrine

Answer D: Nitric oxide has important neurotransmitter-like functions. Its short half-life, due to spontaneous decay, limits its range and action. Nitric oxide acts on smooth muscle cells in a generally inhibitory manner; it relaxes the GI muscles and sphincters and dilates blood vessels. Vascular relaxation allows more blood to flow into the penis resulting in increased tumescence. In blood vessels, nitric oxide is derived from endothelial cells and was originally identified as endothelium-derived relaxing factor (EDRF).

Measurement of the rate of creatinine clearance is used as a key determinant of renal function. In which of the following tissues is creatinine generated?

A. adipose

B. kidney

C. liver

D. lung

E. skeletal muscle

Answer E: In cardiac and skeletal muscle, high-energy phosphate is stored through the transfer of a phosphate from ATP to creatine generating creatine phosphate. Creatine is synthesized in liver from guanidoacetate. Guanidoacetate is derived from the amino acid arginine in the kidneys. In muscle cells, creatinine is a nonenzymatic metabolite of creatine phosphate. When measured in the serum, levels of creatinine are remarkably constant from day to day and are proportional to muscle mass. In renal dysfunction, the clearance of creatinine will be impaired and its levels will therefore rise in the serum. Although creatine is synthesized in the liver from guanidoacetate, which is produced in the kidney, it is not used by these 2 tissues. Once synthesized, creatine is transported to cardiac and skeletal muscle where it is phosphorylated and stored for future energy needs.

The action of many vasodilators within the vasculature involves the influx of calcium ions. A defect in which of the following proteins would most likely result in reduced effectiveness of these types of drugs?

A. adenylate cyclase

B. calmodulin

C. eNOS (NOS-3)

D. cGMP phosphodiesterase

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