Opposing responses in sympathetic nerve activity induced by central injections of ouabain

Cardiac glycosides induce changes in activity of peripheral sympathetic nerves. Arrhythmic doses have been suggested to act in the central nervous system to produce these changes, but direct central effects of digitalis on sympathetic discharge have not been documented. Thus, the central action of ouabain on sympathetic outflow was examined in baroreceptor and chemoreceptor denervated cats. Ouabain was injected into 128 vasoconstrictor or cardioaccelerator sites in the medulla or hypothalamus. Electrical stimulation of 24% of these sites evoked arrhythmias and stimulation consistently caused marked increases in heart rate, blood pressure and nerve activity, but ouabain had several effects, inducing either no change or increases or decreases in spontaneous activity of vasoconstrictor and cardioaccelerator nerves. Effects of ouabain were also observed on signal-averaged potentials evoked in these nerves by electrical stimulation of the medullary or hypothalamic sites of injection. The amplitude of the evoked potentials was either increased, decreased or left unchanged. In general, spontaneous and evoked activities were inhibited by ouabain more frequently than they were enhanced. The pattern of nervous responses to ouabain did not relate to the dose of drug or to the anatomical site of injection. Medullary and hypothalamic injections of ouabain often produced large changes in blood pressure, heart rate and nerve activity, but these effects were not accompanied by alterations in cardiac rhythm. Thus, central microinjections of ouabain produced heterogeneous patterns of effects on activity of peripheral sympathetic nerves, and these microinjections were not sufficient to evoke cardiac arrhythmias in cats with sectioned cranial nerves IX and X.