Prolonged potentials in gastrointestinal muscles induced by calcium chelation

Area of Science:

• Physiology
• Muscle Electrophysiology

Background:

• Smooth muscle exhibits spontaneous electrical activity, including spikes and slow waves.
• Calcium ions (Ca) are crucial for muscle contraction and electrical signaling.

Purpose of the Study:

• To investigate the role of bound calcium in regulating smooth muscle electrical activity.
• To characterize the ionic mechanisms underlying spontaneous electrical events in smooth muscle.

Main Methods:

• Treatment of skate, toad, frog stomach, and cat intestinal muscles with calcium-free solutions containing EGTA or EDTA.
• Electrophysiological recordings to monitor membrane potential and electrical activity.
• Ionic substitution experiments (Na replacement) and pharmacological blockade (TTX, Mn, Co, La, verapamil, D600).

Main Results:

• Calcium chelation reversibly abolished spontaneous spikes and slow waves.
• Prolonged rhythmic potentials (several seconds duration) appeared after calcium removal, characterized by rapid depolarization and repolarization.
• These EGTA-induced waves were dependent on sodium (Na) ions and insensitive to TTX.
• The waves were abolished by divalent cations (Mn, Co, La) and channel blockers (verapamil, D600), suggesting involvement of calcium-permeable channels.

Conclusions:

• Removal of bound calcium by chelators leads to a non-specific reduction in membrane resistance.
• Sodium ions (Na) can rhythmically enter smooth muscle cells through channels typically utilized by calcium (Ca).
• This study reveals a novel pathway for sodium entry mediated by the absence of calcium, impacting smooth muscle excitability.