Cardiac and coronary vasodilator effects of gentamicin (GM) were investigated in isolated, blood-perfused papillary muscle, sino-atrial (SA) node and atrioventricular (AV) node preparations of dogs. GM (0.3–100 μmol) was injected intraarterially. GM produced an increase in coronary blood flow in all preparations. In paced papillary muscle preparations, GM reduced the force of contraction. In spontaneously beating papillary muscle preparations, GM decreased the rate of automaticity and the force of contraction. In SA node preparations, GM decreased the sinus rate. In AV node preparations, GM injected into the posterior septal artery (which supplies the AV node) increased AV conduction time and in large doses, produced third-degree AV block. In the same preparations, GM in large doses injected into the anterior septal artery (which supplies the His-Purkinje-ventricular system) prolonged AV conduction time (i.e., intraventricular conduction time) and reduced the amplitude of ventricular bipolar electrograms. The order of potencies of GM on the above cardiovascular variables is as follows: Coronary blood flow≧ventricular muscle contraction>ventricular automaticity>SA nodal automaticity>AV nodal conduction>intraventricular conduction. This cardiovascular profile is different from those of organic calcium-antagonists, but rather similar to that of manganese ions, reflecting its own mechanism of action.
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