Unaltered norepinephrine-heart rate relationship in exercise with exogenous heat

Area of Science:

• Exercise Physiology
• Autonomic Nervous System Function
• Environmental Stress Response

Background:

• Sympathetic nervous system activity is commonly assessed using plasma norepinephrine (NE) and epinephrine (E) concentrations.
• Heat stress is known to increase heart rate and sympathetic nervous system activity during exercise.
• The relationship between heart rate and NE concentration may be altered by superimposed heat stress.

Purpose of the Study:

• To investigate whether heat stress alters the relationship between heart rate and plasma norepinephrine concentration during graded exercise.
• To determine if heat stress affects plasma epinephrine concentration during exercise.

Main Methods:

• Six healthy young men performed graded exercise (50-200 W) under two conditions: normal temperature and heat stress (38°C skin temperature).
• Plasma NE and E concentrations were measured throughout the exercise protocol.
• Heart rate and workload were continuously monitored.

Main Results:

• Plasma NE concentration increased proportionally with heart rate during exercise, even with superimposed heat stress, maintaining the established relationship.
• Heat stress did not significantly alter plasma E concentration compared to exercise alone.
• The findings suggest that heat stress primarily impacts sympathetic neural activity (NE) rather than adrenal medulla activity (E) during exercise.

Conclusions:

• The close relationship between heart rate and norepinephrine concentration is maintained during exercise with heat stress.
• Heat stress during exercise increases sympathetic neural activity without a corresponding increase in adrenal epinephrine release.
• This suggests a specific modulation of the autonomic nervous system response to combined exercise and heat stress.