The primary electrolyte responsible for maintaining resting membrane potential in cardiac cells is:

Appeared in: BTSC 3 August 2025

Explanation

Potassium (K⁺) is the principal ion that determines the resting membrane potential (Phase 4) of cardiac myocytes.
During rest, the cardiac cell membrane is significantly more permeable to K⁺ than to other ions.
The outward leak of positively charged K⁺ ions down its steep concentration gradient is the primary factor that creates the negative charge (around -90 mV) inside the cell.
This outward potassium current, known as I_K1, keeps the membrane potential stable near the potassium equilibrium potential until the next depolarization event.

Option A: Sodium (Na⁺) is responsible for the rapid depolarization (upstroke) of the action potential, which is Phase 0, not the resting potential.
Option C: Calcium (Ca²⁺) influx is responsible for the plateau phase (Phase 2) of the action potential and is essential for triggering myocardial contraction.
Option D: Magnesium (Mg²⁺) is an important cofactor for many enzymes, including the Na⁺/K⁺-ATPase pump, which helps maintain the ion gradients over time. However, it does not directly establish the resting membrane potential.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain The role of electrolytes in establishing the cardiac resting membrane potential as background academic context rather than a clinical decision trigger.
Abnormal serum potassium levels (hypokalemia or hyperkalemia) can dangerously alter the resting membrane potential, leading to life-threatening cardiac arrhythmias.
Nurses must diligently monitor potassium levels in patients with cardiac conditions, kidney disease, or those taking medications like diuretics or ACE inhibitors.
What if? If a patient develops severe hyperkalemia (high potassium), the resting membrane potential becomes less negative (e.g., moves from -90mV to -70mV). This initially increases excitability but can then lead to inactivation of sodium channels, causing slowed conduction, bradycardia, and potentially cardiac arrest.

How to Approach the Question

First, identify the key concept in the question: the electrolyte responsible for the 'resting membrane potential' in 'cardiac cells'.
Recall the phases of the cardiac action potential (Phases 0-4). The 'resting' state corresponds to Phase 4.
Systematically review the primary role of each electrolyte option in the cardiac cycle.
Sodium (Na⁺) is for depolarization (Phase 0).
Calcium (Ca²⁺) is for the plateau and contraction (Phase 2).
Potassium (K⁺) is for repolarization (Phase 3) and maintaining the resting potential (Phase 4).

Concept Tested & Keywords

Concept Tested: The role of electrolytes in establishing the cardiac resting membrane potential.
Stem keywords: primary electrolyte, resting membrane potential, cardiac cells
Lead-in keywords: is
Negative lead-in flag: false

QCdSMojwpbEwqroa-uGgjo

E6 Pharmacology Katzung 16e pp. 355-357, 356-358

Practise the full BTSC 3 August 2025

Attempt every question from this paper in a timed mock, then review the full solution for each one.