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In mammals, including humans, nitric oxide (NO) is an important cellular signaling molecule involved in many physiological processes. Research into its function led to the 1998 Nobel Prize for discovering its role in cardiovascular function. One specific role of nitric oxide in cardiac function is the dilation of blood vessels, a process called vasodilation. Vasodilation of the arteries lowers blood pressure and decreases the force that the heart muscle needs to exert to pump blood.
The cell signaling mechanism begins when NO diffuses into the smooth muscle cells of the blood vessel and activates Guanylate Cyclase which, in turn, stimulates the Guanylate Cyclase to generate cyclic GMP (guanosine monophosphate) from Guanosine Triphosphate (GTP). Protein Kinase G phosphorylates several proteins that regulate calcium concentrations and, in this case, causes alterations in thin and thick muscle filaments that result in smooth muscle relaxation. This signaling mechanism is illustrated below:
Signal transduction pathways are composed of a complex series of biochemical reactions but they can be described in three essential steps.
Which statement best identifies the role and sequence of the primary molecules involved in nitric oxide cell signaling?
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A
The signal transduction pathway begins with nitric oxide (NO) acting as the signal, cyclic GMP as the second messenger that results in amplification of the signal via a phosphorylation cascade. The cellular response is smooth muscle relaxation.
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B
The signal transduction pathway begins with nitric oxide (NO) acting as the signal and Protein Kinase G as the second messenger, triggering a phosphorylation cascade. The cellular response is smooth muscle contraction.
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C
The signal transduction pathway begins with nitric oxide (NO) acting as the signal and Guanylyl Cyclase as the second messenger. This results in amplification of the signal via a phosphorylation cascade. The cellular response is smooth muscle contraction.
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D
The signal transduction pathway begins with the activation of Guanylyl Cyclase acting as the signal and Protein Kinase G as the second messenger, triggering a phosphorylation cascade. The cellular response is smooth muscle contraction.
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Solution

The correct option is A The signal transduction pathway begins with nitric oxide (NO) acting as the signal, cyclic GMP as the second messenger that results in amplification of the signal via a phosphorylation cascade. The cellular response is smooth muscle relaxation.
The signal transduction for the NO-Smooth muscle relaxation pathway is activated by NO, where cGMP acts as a second messenger, leads to relaxation of the smooth muscles
So, the correct answer is 'The signal transduction pathway begins with nitric oxide (NO) acting as the signal, cyclic GMP as the second messenger that results in amplification of the signal via a phosphorylation cascade. The cellular response is smooth muscle relaxation'

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Q. In mammals, including humans, nitric oxide (NO) is an important cellular signaling molecule involved in many physiological processes. Research into its function led to the 1998 Nobel Prize for discovering its role in cardiovascular function. One specific role of nitric oxide in cardiac function is the dilation of blood vessels, a process called vasodilation. Vasodilation of the arteries lowers blood pressure and decreases the force that the heart muscle needs to exert to pump blood.
The cell signaling mechanism begins when NO diffuses into the smooth muscle cells of the blood vessel and activates guanylyl cyclase. The complete signaling mechanism is illustrated in Figure 1 above:
Fig. 1: Signaling cascade of nitric oxide involving cyclic GMP (guanosine monophosphate), guanosine triphosphate (GTP), Protein Kinase G, calcium ions (Ca2+), and PDE (phosphodiesterase).
Nitroglycerin became famous in 1867 when Alfred Nobel patented an explosive mixture of nitroglycerin and diatomaceous earth as dynamite. Since then it has been widely used as a medication to treat the medical condition angina, which is chest pain caused by a lack of oxygen in the heart muscle and a common symptom of heart disease. Nitroglycerine is converted to nitric oxide by mitochondrial aldehyde dehydrogenase (Chen et al. 2007), which then functions as the signal in the NO transduction pathway. Unfortunately, patients quickly develop a tolerance to nitroglycerin and this prevents continuous administration.
Which of the following is NOT a plausible explanation for nitroglycerin tolerance in smooth muscle cells?
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Q. In mammals, including humans, nitric oxide (NO) is an important cellular signaling molecule involved in many physiological processes. Research into its function led to the 1998 Nobel Prize for discovering its role in cardiovascular function. One specific role of nitric oxide in cardiac function is the dilation of blood vessels, a process called vasodilation. Vasodilation of the arteries lowers blood pressure and decreases the force that the heart muscle needs to exert to pump blood.
The cell signaling mechanism begins when NO diffuses into the smooth muscle cells of the blood vessel and activates guanylate cyclase which, in turn, stimulates the guanylate cyclase to generate cyclic GMP (guanosine monophosphate) from guanosine triphosphate (GTP). Protein Kinase G phosphorylates several proteins that regulate calcium concentrations and, in this case, causes alterations in thin and thick muscle filaments that results in smooth muscle relaxation. This signaling mechanism is illustrated below:
Research published by Gustafsson, Persson, and Moncada (1990) found that nitric oxide production in the lungs decreases immediately under acute hypoxia (low oxygen), which causes the pulmonary blood vessels to constrict. This, in turn, limits oxygen delivery to the lungs, heart, red blood cells and mitochondria. A study published in the journal Nature (Beall et al. 2001) examined chronically hypoxic, high altitude populations of Tibetans and Bolivian Aymara, two countries almost halfway across the world from one other, and found that exhaled levels of Nitric oxide were unusually high relative to a low-altitude reference sample from the United States.
What is a likely conclusion that can be made with these findings?
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