Changes in vagal phasic chronotropic responses with sympathetic stimulation in the dog

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Function
• Cardiac Electrophysiology

Background:

• The autonomic nervous system, comprising sympathetic and parasympathetic (vagal) components, critically regulates heart rate and cardiac function.
• Sympathetic stimulation is known to increase heart rate and contractility, while vagal stimulation slows the heart rate.
• Interactions between sympathetic and vagal influences are complex and crucial for maintaining cardiac homeostasis.

Purpose of the Study:

• To investigate the effects of sympathetic stimulation on the heart's chronotropic responses to brief periods of vagal stimulation in a canine model.
• To elucidate the impact of sympathetic tone on the sinoatrial nodal pacemaker cells' response to phasic vagal stimuli.
• To determine how sympathetic activity modulates the timing and magnitude of cardiac cycle adjustments influenced by vagal nerve stimulation.

Main Methods:

• Utilizing an open-chest anesthetized dog model.
• Applying controlled sympathetic stimulation.
• Administering brief, repetitive bursts of vagal stimulation at varying frequencies and timings within the cardiac cycle.
• Measuring cardiac cycle length and chronotropic responses to assess heart rate changes.

Main Results:

• Sympathetic stimulation shortens the overall cardiac cycle length.
• A paradoxical response was observed in sinoatrial nodal cells, where increased vagal stimulation frequency paradoxically decreased cardiac cycle length within a specific phase.
• Sympathetic stimulation attenuated the magnitude of the cardiac chronotropic response to vagal stimuli, irrespective of stimulus timing, but did not alter the interval from peak vagal effect to atrial depolarization.

Conclusions:

• Sympathetic stimulation significantly modifies the heart's response to vagal nerve activity, primarily by reducing the chronotropic effect of vagal bursts.
• The temporal relationship between sympathetic and vagal stimulation is critical; alterations in sympathetic tone can profoundly impact cardiac response to precisely timed vagal stimuli.
• Understanding these complex interactions is vital for comprehending cardiac autonomic regulation and potential therapeutic interventions.