Abstract

The kinetics underlying plasma epinephrine concentrations were studied. Six athletes (T) and six sedentary males (C) were given intravenous infusions of 3H-labeled epinephrine, after which arterial blood was drawn. They rested sitting and bicycled continuously to exhaustion (60 min at 125 W, 60 min at 160 W, 40 min at 200 W, and 240 W to the end). Work time was 154 +/- 13 (SE) (T) and 75 +/- 6 (C) min. At rest, epinephrine clearance was identical, but plasma concentration and, accordingly, secretion were higher in T than C subjects. Epinephrine clearance was closely related to relative work load, decreasing from 15 above the basal level at 30 of VO2max to 22 below at 76 of VO2max. Epinephrine concentrations increased much more with work intensity than could be accounted for by changes in clearance and were, at exhaustion, higher in T subjects despite similar glucose, heart rate, and hematocrit values. At a given load, epinephrine clearance rapidly became constant, whereas concentration increased continuously. Forearm extraction of epinephrine invalidated use of blood from a cubital vein or a hand vein arterializedby hot water in turnover measurements. During exercise, changes in epinephrine concentrations reflect changes in secretion rather than in clearance. Training may increase adrenal medullary secretory capacity. Verf.-Referat