Plasma catecholamines and heart rate at the beginning of muscular exercise in man

The relationship between the time course of heart rate and venous blood norepinephrine (NE) and epinephrine (E) concentrations was studied in 7 sedentary young men before and during 3 bicycle exercises of 5 min each (respectively 23±2.8%, 45±2.6% and 65±2.4% {Mathematical expression}, mean ±SE). During the low level exercise the change in heart rate is monoexponential (τ =5.7±1.2s) and no increment above the resting level of NE (ΔNE) or of E (ΔE) occurs. At the medium and highest intensity of exercise: a) the change in heart rate is biexponential, τ for the fast and the slow component averaging about 3 and 80 s respectively; b) ΔNE (but not ΔE) increases continuously with time of exercise; c) at the 5th min of exercise heart rate increments are related to ΔNE; d) between 20s and 5 min, at corresponding sampling times, the heart rate of the slow component is linearly related to ΔNE. At exercise levels higher than 33% {Mathematical expression} the increase in heart rate described by the slow component of the biexponential kinetic could be due to an augmented sympathetic activity revealed by increased NE blood levels.