63之38第7题

lydiar_020

PASSAGE 38
* It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, however, we have discovered that the production and release in brain neurons of the neuro-
5) transmitter serotonin
neurotransmitters are compounds that neurons use to transmit signals to other cells) depend directly on the food that the body processes.
* Our first studies sought to determine whether the increase in serotonin observed in rats given a large injec-
10)tion of the amino acid tryptophan might also occur after rats ate meals that change tryptophan levels in the blood. We found that, immediately after the rats began to eat, parallel elevations occurred in blood tryptophan, brain tryptophan, and brain serotonin levels. These find-
15) ings suggested that the production and release of sero- tonin in brain neurons were normally coupled with blood-tryptophan increases. In later studies we found that injecting insulin into a rat's bloodstream also caused parallel elevations in blood and brain tryptophan levels
20) and in serotonin levels. We then decided to see whether the secretion of the animal's own insulin similarly affected serotonin production. We gave the rats a carbohydrate- containing meal that we knew would elicit insulin secre- tion. As we had hypothesized, the blood tryptophan
25) level and the concentrations of tryptophan serotonin in the brain increased after the meal.
* Surprisingly, however, when we added a largeamount of protein to the meal, brain tryptophan and serotonin levels fell. Since protein contains tryptophan,
30) why should it depress brain tryptophan levels? The answer lies in the mechanism that provides blood tryp- tophan to the brain cells. This same mechanism also provides the brain cells with other amino acids found in protein, such as tyrosine and leucine. The consumption
35) of protein increases blood concentration of the other amino acids much more, proportionately, than it does that of tryptophan. The more protein in the meal, the lower is the ratio of the resulting blood-tryptophan concentration to the concentration of competing amino
40) acids, and the more slowly is tryptophan provided to the brain. Thus the more protein in a meal, the less serotonin subsequently produced and released.  

7. According to the passage, an injection of insulin wasmost similar in its effect on rats to an injection of
(A) tyrosine
(B) leucine
(C) blood
(D) tryptophan
(E) protein

答案是D，我不太明白，好像选E更对，原文好像不是假设injection of insulin后，tryptophan会升高，但实际相反。所以我不理解，injection of insulin和an injection of tryptophan怎么可能是一样的呢？

skayouyou

1.large injection of tryptophan-->serotonin increase(line9)

2.insulin定位，line19--whether insulin secretion similarly affect serotonin-->similar to tryptophan(line15可看出)--->line 25 As we hypothesized.......
serotonin果然increase了

所以，injection of tryptophan 和insulin有similar effect: 都能使 serotonin increase

所以选D,similar to tryptophan