Transmembrane potential changes caused by shocks in guinea pig papillary muscle

Area of Science:

• Cardiovascular Physiology
• Electrophysiology
• Cellular Electrophysiology

Background:

• Transmembrane potential (Vm) changes are crucial for cardiac function.
• Extracellular electrical stimulation is used to study cardiac electrophysiology.
• Understanding Vm dynamics under different stimulation conditions is essential.

Purpose of the Study:

• To investigate the changes in transmembrane potential (delta Vm) induced by extracellular field stimulation in guinea pig papillary muscles.
• To determine the relationship between shock strength and delta Vm.
• To explore the role of sodium channels in action potential generation during field stimulation.

Main Methods:

• Recorded Vm in guinea pig papillary muscles using a double-barrel microelectrode.
• Applied 10-ms extracellular electrical shocks of varying strengths and polarities during action potential plateau and diastole.
• Administered tetrodotoxin (TTX) or TTX plus Ca(2+)-free perfusion to assess sodium channel involvement.

Main Results:

• The relationship between delta Vm and shock potential gradient was non-linear.
• Hyperpolarizing shocks during diastole induced complex Vm changes, including an immediate depolarization upstroke post-shock.
• Field stimulation, but not point stimulation, could induce action potentials when sodium channels were inactivated by TTX.

Conclusions:

• The Vm response to extracellular field stimulation is complex and non-linear.
• Field stimulation can overcome sodium channel inactivation to initiate action potentials.
• Sodium channels are activated twice by hyperpolarizing shocks during diastole.