Cardiovascular, ventilatory and catecholamine responses to maximal dynamic exercise in partially curarized man.

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Abstract
1. In ten young men the ventilatory, cardiovascular, catecholamine and metabolic responses to maximal dynamic leg exercise on a stationary bicycle were followed during partial neuromuscular blockade with tubocurarine. Maximal exercise was performed when the drug effect was at its maximum as well as during the subsequent reduction in the effect allowing a gradually increasing work intensity. The results were compared with those obtained during submaximal and maximal exercise performed without tubocurarine. Partial neuromuscular blockade decreased hand-grip strength to 41 .+-. 1.1% (S.E. of mean) and the maximal work load to 27 .+-. 2.4% of control values. Voluntary effort was maximal and the rate of perceived exertion was high at all levels of exercise with tubocurarine indicating a maintained intense central nervous motor command. 2. During maximal action of the drug oxygen uptake was 1.67 .+-. 0.11 l/min while only 0.91 .+-. 0.13 l/min (P < 0.01) at the same work intensity without neuromuscular blockade. This difference may reflect a dominant reliance on fast-twitch muscle fibres when work was performed under the influence of tubocurarine. 3. Compared at a given oxygen uptake ventilation was higher during work with tubocurarine than during control exercise (e.g. 55 .+-. 4.2 and 40 .+-. 2.21 l/min, respectively (P < 0.01), at a mean oxygen uptake of 1.9 l/min), while heart rate did not differ significantly (146 .+-. 4.4 and 139 .+-. 3.0 beats/min). With decreasing drug effect both variables increased towards the maximum values of 138 .+-. 4.5 l/min and 183 .+-. 3.9 beats/min, respectively, achieved in control experiments at an oxygen uptake of 3.8 .+-. 0.2 l/min. Like heart rate the mean arterial blood pressure increased with increasing work load and was similar at a given oxygen uptake with and without tubocurarine. 4. During maximal exercise at peak tubocurarine effect plasma adrenaline and noradrenaline concentrations were smaller than during control maximum, 1.6 .+-. 0.27 versus 3.4 .+-. 0.55 nmol/l (P < 0.01) and 7.5 .+-. 1.3 versus 12.6 .+-. 1.8 nmol/l (P < 0.05), respectively. However, comparisons at identical oxygen uptake rates revealed that catecholamine responses were markedly enhanced during tubocurarine treatment. Also, blood lactate concentrations were smaller at peak tubocurarine action than during control maximum, 1.9 .+-. 0.42 mmol/l and 6.1 .+-. 0.49 mmol/l (P < 0.01). In contrast the concentration of free fatty acids was higher in tubocurarine experiments: 0.5 .+-. 0.07 against 0.3 .+-. 0.03 mmol/l (P < 0.01) in agreement with the view that lactate inhibits the release of free fatty acids. 5. The higher ventilations and plasma catecholamine values at given oxygen uptake rates in tubocurarine compared to control experiments suggest that activity in motor centres in the central nervous system (central command) has a role in regulation of these variables. On the other hand ventilation, heart rate, arterial blood pressure and, less so, catecholamine concentrations were much smaller than during control maximal exercise, and increased as absolute work load increased during fading away of the neuromuscular blockade in curare experiments, arguing for the view that factors originating in the working muscles and depending on their metabolism dominate the regulation of these variables during voluntary dynamic exercise.