Lower Ca2+ enhances the K+-induced force depression in normal and HyperKPP mouse muscles

Area of Science:

• Muscle Physiology
• Ion Channel Function
• Neuromuscular Disorders

Background:

• Hyperkalemic periodic paralysis (HyperKPP) causes episodic muscle weakness.
• The mechanism by which calcium (Ca2+) ingestion alleviates HyperKPP symptoms is unknown.
• Lowering extracellular Ca2+ ([Ca2+]e) normally has minimal effect on muscle force but can affect voltage-gated channels when membranes are depolarized.

Purpose of the Study:

• To test if lowering [Ca2+]e depresses force in normal muscles under depolarizing conditions.
• To investigate if HyperKPP muscles exhibit greater sensitivity to low Ca2+-induced force depression due to pre-existing depolarization.

Main Methods:

• Compared force development and membrane excitability in wild-type and HyperKPP muscles.
• Manipulated extracellular Ca2+ and K+ concentrations.
• Assessed the effects of salbutamol and Mg2+ substitution.

Main Results:

• Lowering [Ca2+]e significantly enhanced K+-induced force and excitability depression in wild-type muscles only under elevated K+.
• In HyperKPP muscles, lowering [Ca2+]e exacerbated K+-induced effects even at normal K+ levels, and increased contractures.
• Reduced salbutamol efficacy and inconsistent Mg2+ reversal were observed with lowered [Ca2+]e.

Conclusions:

• HyperKPP muscles show greater Ca2+ sensitivity, particularly in depolarized fibers.
• A higher proportion of depolarized muscle fibers in HyperKPP contributes to this sensitivity.
• These findings elucidate a key mechanism underlying HyperKPP symptom exacerbation and potential treatment interactions.