Passage Whole Grains Have Been Shown to Reduce the Risk of of Cardiovascular

Passage
Whole grains have been shown to reduce the risk of cardiovascular disorders.  A team of scientists discovered that the compound 5-aminovaleric acid betaine (5-AVAB) was present at high levels in cardiomyocytes of mice given a diet rich in whole grains and decided to investigate its role in metabolism.  Preliminary evidence indicated that 5-AVAB did not alter the citric acid cycle in these cardiomyocytes, but that it did alter fatty acid metabolism.The plasma membrane transport protein organic cation/carnitine transporter 2 (OCTN2) transports extracellular L-carnitine into the cell against its concentration gradient using the energy provided by the simultaneous transport of Na⁺ into the cell.  Cellular L-carnitine is required for the transport of fatty acids from the cytosol into the mitochondria.  Based on the structures of 5-AVAB and L-carnitine (Figure 1) , scientists hypothesized that 5-AVAB may interfere with OCTN2-mediated transport of L-carnitine.
Figure 1  Structures of L-carnitine and 5-AVABTo test their hypothesis, scientists incubated mouse cardiomyocytes with various concentrations of 5-AVAB for 24 hours.  Cells were then provided with the fatty acid palmitate for energy and were deprived of all other energy sources.  The cardiomyocytes were also given 50 μM L-carnitine (endogenous extracellular concentration) or 2 mM L-carnitine, and the rate of oxygen consumption was monitored (Figure 2) .
Figure 2  Oxygen consumption rate of cardiomyocytes at various concentrations of 5-AVAB and L-carnitineIn a follow-up experiment, the scientists used siRNA to silence expression of OCTN2 and found that intracellular L-carnitine levels were similarly depleted in siRNA-treated cells as in cells treated with 100 μM 5-AVAB.
Adapted from Kärkkäinen O, Tuomainen T, Koistinen V, et al. Whole grain intake associated molecule 5-aminovaleric acid betaine decreases ß-oxidation of fatty acids in mouse cardiomyocytes. Sci Rep. 2018;8(1) :13036.
-The information in the passage and the data in Figure 2 best support which statement about cardiomyocytes when palmitate is their only energy source?

A) Cardiomyocytes cannot digest palmitate in the presence of 5-AVAB.
B) 5-AVAB inhibits synthesis of L-carnitine in a dose-dependent manner.
C) Mitochondria cannot import L-carnitine in the presence of 5-AVAB.
D) 5-AVAB reduces the rate of β-oxidation in a dose-dependent manner.

Correct Answer:

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