Sinus Baroceptor Reflex and Ouabain Interaction on Cardiac Contractility

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Abstract
Activation of sinus baroceptors significantly decreased right ventricular contractile force in the cat. The reflex decrease in cardiac activity is believed to be mediated by modifying the rate of tonic sympathetic impulses to the heart. In fact, 30% to 40% of the contractile force measured in these vagotomized preparations results from the tonic sympathetic impulses impelling cardiac muscle. Although the sinus reflex was effective in reducing cardiac contractile force in the fully digitalized preparation, the change was never as large as in the control state. Stimulation of the sympathetic nerves to the heart was not as effective in increasing contractile force after administration of the glycoside as before the drug. The competitive action of the glycoside and catacholamines is explained by proposing that both inotrophic agents affect cardiac force by altering the Ca availability for the excitation-contraction coupling process. If the tissue Ca relevant to contraction is fixed, a limited metabolic pool, then the upper limits of effective change in force under the influence of the 2 agents would be easily reached.