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Zorluk: OrtaEndocrine and Metabolic Mechanisms

A 45-year-old man comes to the clinic for a routine follow-up examination of type 2 diabetes mellitus. His current regimen includes metformin and a newly added oral antihyperglycemic medication, which was started four weeks ago. The patient reports that his home fasting blood glucose readings have improved significantly. However, he has noticed that his shoes feel tighter and he has gained 2.5 kg (5.5 lb) since his last visit. He has a history of mild heart failure with preserved ejection fraction. Physical examination reveals 2+ bilateral pitting edema of the lower extremities. Laboratory studies show a fasting plasma glucose of 112 mg/dL. The physician realizes the new medication is responsible for the patient's fluid retention, which is exacerbating his underlying cardiac condition, and plans to discontinue it immediately. The discontinued drug most likely exerts its primary therapeutic effect by initially binding to which of the following cellular targets?

  1. Intracellular receptor that heterodimerizes with the retinoid X receptorCevap
  2. B
    ATP-sensitive potassium channel on the pancreatic beta-cell membrane
  3. C
    Membrane-bound tyrosine kinase receptor leading to autophosphorylation
  4. D
    Sodium-glucose cotransporter in the proximal renal tubule
  5. E
    G protein-coupled receptor that increases intracellular cyclic AMP

Cevap

The correct target is an intracellular receptor that heterodimerizes with the retinoid X receptor, which correctly describes the mechanism of action of thiazolidinediones.
The clinical presentation of improved glycemic control accompanied by new-onset weight gain and peripheral edema in a patient with a history of heart failure strongly points to the recent initiation of a thiazolidinedione (TZD), such as pioglitazone or rosiglitazone. TZDs are agonists of the peroxisome proliferator-activated receptor gamma (PPAR-gamma). PPAR-gamma is an intracellular (nuclear) receptor. Upon binding a TZD, PPAR-gamma forms a heterodimer with the retinoid X receptor (RXR). This complex binds to specific DNA response elements to alter the transcription of various genes, ultimately increasing adiponectin levels, promoting adipocyte differentiation, and increasing insulin sensitivity (e.g., via GLUT4 upregulation). A major side effect of TZDs is fluid retention due to increased sodium reabsorption in the renal collecting ducts via upregulation of epithelial sodium channels (ENaC). This fluid retention can precipitate or exacerbate heart failure, making TZDs strictly contraindicated in these patients.

Adım Adım Çözüm

1
Analyze the patient's new symptoms following the initiation of an oral antidiabetic drug.
The patient developed significant weight gain and bilateral pitting edema of the lower extremities.
Identifying the specific side effect profile is essential to determining the drug class responsible.
2
Correlate the side effects and the patient's medical history with antidiabetic medication classes.
Thiazolidinediones (e.g., pioglitazone) are known to cause fluid retention and weight gain, and they carry a black box warning for causing or exacerbating heart failure.
This confirms the newly added medication was a thiazolidinedione, explaining the physician's immediate decision to discontinue it.
3
Identify the cellular mechanism of action for thiazolidinediones.
They bind to peroxisome proliferator-activated receptor gamma (PPAR-gamma), an intracellular nuclear receptor.
The question specifically asks for the cellular target of the discontinued medication.
4
Determine the downstream molecular pathway of PPAR-gamma activation.
Upon binding its ligand, PPAR-gamma heterodimerizes with the retinoid X receptor (RXR) to alter the transcription of insulin-responsive genes.
This matches the correct option describing the receptor's specific behavior.

Anahtar Kavram

Thiazolidinediones (TZDs) improve insulin sensitivity by binding to PPAR-gamma, an intracellular nuclear receptor that heterodimerizes with the retinoid X receptor (RXR) to regulate the transcription of genes involved in glucose and lipid metabolism. Due to their tendency to increase renal sodium reabsorption leading to fluid retention, TZDs are contraindicated in patients with symptomatic heart failure.
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Endocrine and Metabolic Mechanisms

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