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Difficulty: MediumEndocrine and Metabolic Mechanisms

A 62-year-old man comes to the clinic for a routine follow-up. He has a 10-year history of type 2 diabetes mellitus and a history of heart failure with preserved ejection fraction (ejection fraction of 55%55\%). His current medications include metformin, lisinopril, and amlodipine. He feels well and reports no shortness of breath, orthopnea, or lower extremity edema. His blood pressure is 128/78 mm Hg128/78 \text{ mm Hg}, and his pulse is 72/min72/\text{min}. Physical examination reveals clear lungs bilaterally and no jugular venous distention. Laboratory evaluation shows a hemoglobin A1c of 8.6%8.6\%. The physician considers adding pioglitazone to the patient's regimen to improve glycemic control but decides against it due to the patient's cardiac history. Which of the following best describes the cellular mechanism by which the avoided medication promotes fluid retention?

  1. A
    Decreased degradation of bradykinin leading to increased capillary permeability
  2. B
    Direct negative inotropic effect on ventricular cardiomyocytes
  3. Upregulation of epithelial sodium channel transcription in the renal collecting tubulesAnswer
  4. D
    Competitive inhibition of sodium-glucose cotransporter 2 resulting in compensatory aldosterone release
  5. E
    Arteriolar smooth muscle relaxation leading to preferential precapillary vasodilation

Answer

The avoided medication promotes fluid retention via the upregulation of epithelial sodium channel transcription in the renal collecting tubules.
Pioglitazone is a thiazolidinedione (TZD) that exerts its pharmacological effects by binding to and activating the peroxisome proliferator-activated receptor-gamma (PPAR-\gamma), an intracellular nuclear transcription factor. While its primary therapeutic effect is improving insulin sensitivity in adipose tissue and skeletal muscle, PPAR-\gamma receptors are also abundantly expressed in the principal cells of the renal collecting tubules. Activation of these receptors in the kidney increases the transcription and expression of the epithelial sodium channel (ENaC) and the basolateral Na+/K+-ATPase. This leads to significantly increased sodium reabsorption and subsequent water retention. The resulting intravascular volume expansion increases cardiac preload, which can precipitate or exacerbate congestive heart failure. For this reason, TZDs are strictly contraindicated in patients with symptomatic heart failure.

Step-by-Step Solution

1
Identify the medication class being considered and rejected.
The physician considered pioglitazone, a thiazolidinedione (TZD), but rejected it due to the patient's history of heart failure.
Recognizing the drug and its primary contraindication is required to understand which metabolic mechanism is being tested.
2
Recall the cellular target of thiazolidinediones.
TZDs bind to and activate the peroxisome proliferator-activated receptor-gamma (PPAR-\gamma), a nuclear transcription factor.
The question asks for the cellular mechanism leading to the specific side effect of fluid retention.
3
Determine how PPAR-\gamma activation leads to fluid retention.
PPAR-\gamma receptors in the renal collecting ducts stimulate the transcription of the epithelial sodium channel (ENaC), increasing renal sodium and water reabsorption.
Connecting the molecular target to the organ-specific physiological response explains the mechanism of the contraindication.

Key Concept

Mechanism of thiazolidinedione-induced fluid retention and heart failure exacerbation
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Topics

Endocrine and Metabolic Mechanisms

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