Effects of metoprolol on left ventricular function in rats with myocardial infarction

To study the effect of beta-receptor-blocking agents in an animal model of left ventricular (LV) dysfunction, we measured LV performance in vivo and in vitro in 69 rats with or without metoprolol (M) treatment 3 wk after left coronary arterial ligation or sham operation. Rats were divided into six groups including control (C) and M noninfarct (C-N and M-N), C and M small infarct (C-S and M-S), and C and M large infarct (C-L and M-L). LV function was measured as slope of change in systolic vs. diastolic pressure (pressure-function curve) during pressor response after administration of a bolus of phenylephrine (5 micrograms/kg i.v.). Reduction of LV function was noted in C-L compared with C-N and C-S (slope of pressure-function curve 3.3 +/- 0.3 vs. 11.0 +/- 1.9 and 11.9 +/- 2.3, respectively) and in M-L compared with M-N and M-S rats (slope of 5.5 +/- 1.4 vs. 11.3 +/- 2.0 and 12.1 +/- 1.4, respectively). There was no significant difference between C and M rats, although there was a trend toward partial correction of the pressure-function curves in M-L compared with C-L rats. In muscle bath preparations the uninfarcted LV posterior papillary muscle from shams and rats with small infarcts showed a dose-related increase in peak rate of tension development with isoproterenol stimulation, but this response was lacking in both C-L and M-L. Tissue assays showed no change in beta-receptor number.(ABSTRACT TRUNCATED AT 250 WORDS)