Glucagon and plasma catecholamines during beta-receptor blockade in exercising man

Area of Science:

• Exercise Physiology
• Endocrinology
• Metabolic Regulation

Background:

• Hormonal responses during prolonged exercise are critical for maintaining glucose homeostasis.
• The roles of beta-adrenergic blockade and lipolytic blockade in modulating these responses require further investigation.

Purpose of the Study:

• To investigate the effects of beta-adrenergic blockade (propranolol) and lipolytic blockade (nicotinic acid) on hormonal and metabolic responses during exhaustive exercise at 60% maximal oxygen uptake.
• To determine the influence of these blockades on glucagon secretion and substrate utilization.

Main Methods:

• Seven healthy men performed incremental exercise tests to exhaustion under three conditions: propranolol (P), nicotinic acid (N), and control (C).
• Measurements included total work time, plasma epinephrine, norepinephrine, insulin, glucagon, glucose, non-esterified fatty acids (NEFA), and glycerol.
• Carbohydrate combustion was also assessed.

Main Results:

• Total work time was significantly shorter in the propranolol (P) and nicotinic acid (N) conditions compared to control (C).
• Epinephrine levels at exhaustion were highest in P, followed by N and C.
• Glucagon increased progressively in all conditions, reaching three times pre-exercise values at exhaustion, irrespective of beta-adrenergic blockade.
• Carbohydrate combustion was lower, and NEFA and glycerol concentrations were higher during C experiments.

Conclusions:

• Beta-adrenergic blockade does not diminish, nor does intensified alpha-receptor stimulation increase, the exercise-induced glucagon response.
• Neither adrenergic receptor stimulation nor NEFA and alanine concentrations are primary drivers of exercise-induced glucagon secretion.
• Reduced glucose availability is suggested to be a key factor enhancing glucagon and epinephrine secretion during prolonged exercise.