Improvement of cardiac performance by carboxylic ionophore monensin in greyhound and mongrel dogs.

The effects of the cardiotonic ionophore monensin on coronary blood flow, cardiac output, left ventricular (LV) O2 consumption, LV contractility and arterial blood pressure were measured in greyhounds and compared with those in mongrel dogs under pentobarbital anaesthesia. Following intravenous injection of 100 micrograms/kg monensin, all parameters rose, the greyhounds showing a relatively greater response in peak values of coronary blood flow, LV contractility and cardiac output. The increase in the calculated LV mechanical work was proportionately greater than the corresponding increase in LV O2 consumption, hence the derived index, external mechanical efficiency, rose rapidly from an initial value of 0.165 to 0.285 (73%) in greyhounds; mongrel dogs showed a more modest rise from 0.094 to 0.120 (28%) during the same interval. The difference in the responses in the two breeds of dogs are attributed mainly to larger heart size and exercise training in greyhounds as compared with untrained mongrel dogs.