Which of the following symptoms is characteristic of Cushing syndrome?

Glycolysis is the fundamental metabolic pathway that catabolizes glucose into pyruvate to generate adenosine triphosphate (ATP). The pathway is tightly regulated at specific irreversible steps to maintain metabolic homeostasis and match cellular energy demands. The primary control point is governed by an allosteric enzyme that responds to the cell's energy charge. Proper regulation prevents the futile cycling of intermediates and ensures efficient glucose utilization.

Rationale:

A. Phosphoglucose isomerase facilitates the reversible conversion of glucose-6-phosphate to fructose-6-phosphate. Because this reaction operates near equilibrium and is easily reversible, it does not serve as a control point. It is not considered a regulatory enzyme in the glycolytic pathway. Its activity is governed strictly by the substrate concentration available in the cytosol.

B. Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, which is the rate-limiting step. This reaction is highly exergonic and irreversible under physiological conditions. PFK-1 is allosterically inhibited by ATP and citrate, while being activated by AMP and fructose-2,6-bisphosphate. This ensures glycolysis proceeds only when the cell requires additional chemical energy.

C. Hexokinase catalyzes the initial entry of glucose into the pathway by forming glucose-6-phosphate. While it is an irreversible step and subject to product inhibition, it is not the primary rate-limiting step for the entire pathway. Glucose-6-phosphate can enter other pathways like the pentose phosphate pathway. Therefore, it does not commit the molecule exclusively to anaerobic or aerobic catabolism.

D. Aldolase is responsible for the cleavage of fructose-1,6-bisphosphate into two three-carbon molecules, DHAP and glyceraldehyde-3-phosphate. This reaction is reversible and occurs downstream of the major regulatory commitments. Aldolase activity is not subject to the complex allosteric regulation seen in rate-limiting enzymes. It simply processes the flux provided by the earlier committed steps.

E. Glucokinase is an isoenzyme of hexokinase found primarily in the liver and pancreatic beta cells. It has a high Michaelis constant (Km) for glucose, allowing it to function as a glucose sensor. While important for managing postprandial glucose levels, it is not the universal rate-limiting step. The commitment to the glycolytic flow is primarily managed at the phosphofructokinase level.

Parasympathetic innervation of the heart is primarily mediated through the vagus nerve, which releases acetylcholine (ACh) onto the sinoatrial (SA) node. This chemical signal binds to muscarinic M2 receptors, triggering G-protein mediated changes in membrane potential. The resulting negative chronotropic effect slows the heart rate to maintain resting homeostasis and cardiac output efficiency during periods of low activity.

Rationale:

A. Decreasing permeability to potassium would lead to a buildup of positive charge inside the cell, causing depolarization rather than slowing the heart. This would make the cell more excitable and increase the heart rate. Acetylcholine acts to stabilize the membrane, not to make it more prone to reaching the threshold potential quickly.

B. While sodium channels are involved in the initial "funny" current of the pacemaker potential, closing them is not the primary mechanism of vagal hyperpolarization. The main inhibitory effect of acetylcholine relies on the movement of potassium ions out of the cell. Sodium channel modulation is a secondary effect compared to the direct potassium conductance increase.

C. Opening calcium channels would actually increase the rate of depolarization and strengthen muscular contraction. Acetylcholine actually inhibits the L-type calcium current in the nodal tissue to help slow the rate of firing. This choice incorrectly describes the ion flow and the resulting effect on the cardiac cycle timing.

D. Closing sodium channels would not lead to hypopolarization (becoming less negative). Furthermore, the vagus nerve's primary inhibitory action is not centered on simple sodium channel closure. The heart's response to acetylcholine is characterized by a significant membrane shift toward a more negative, stable state, which is the opposite of hypopolarization or depolarization.

E. Acetylcholine increases permeability to potassium in the sinus node by opening specialized GIRK (G-protein coupled inwardly rectifying potassium) channels. As potassium exits the cell, the membrane potential becomes more negative, a state called hyperpolarization. This moves the resting potential further from the threshold, effectively slowing the rate of pacemaker firing and heart rate.