To the Editor: Williams et al. (March 28 issue)1 demonstrate that the rise in the plasma potassium level that occurs during exercise is associated with increased plasma catecholamine levels. This elevation of the plasma potassium level is enhanced by β-adrenergic blockade and reduced by α-adrenergic blockade. We recently evaluated the effect of clonidine on the hormonal response to exercise in healthy volunteers and also observed its effect on plasma potassium levels during and after exercise. Clonidine in the doses used has its main effect on central postsynaptic α-adrenergic receptors, resulting in reduced sympathetic outflow.2 It reduces circulating levels of.
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