Cardiorenal hemodynamics and sodium excretion in rats with myocardial infarction

The relation between left ventricular function and renal excretion of sodium (Na+) was studied in rats with myocardial infarction (MI) and varying degrees of left ventricular dysfunction. Three groups of rats were defined: 1) control sustained no infarct, 2) small to moderate infarcts involved 10-40% of the left ventricular circumference, and 3) large infarcts involved greater than 40%. In conscious rats, Na+ excretion was measured after administration of saline load by gavage. Four hours after the load, rats with large MI excreted less than one half the amount of Na+ excreted by control rats, whereas rats with small to moderate MI excreted an intermediate amount. In a second group of anesthetized rats, Na+ excretion, renal hemodynamics, and ventricular performance were determined before and after acute intravenous volume expansion with a balanced salt solution. Rats with small to moderate MI demonstrated minimal impairment in ventricular pumping ability but excreted less Na+ after volume expansion than did control rats. However, rats with large MI demonstrated marked impairment in left ventricular performance and exhibited the least natriuretic response to volume loading. Glomerular filtration rate and renal plasma flow failed to increase with volume expansion in both groups of rats with MI. Thus Na+ excretion in response to acute volume loading was diminished in rats with large MI and markedly impaired cardiac performance but was also reduced in rats with small to moderate MI and minimal changes in ventricular pumping capacity.