Myocyte response to beta-adrenergic stimulation is preserved in the noninfarcted myocardium of globally dysfunctional rat hearts after myocardial infarction.

BACKGROUND

Cellular mechanisms underlying the diminished inotropic response of remodeled hearts after myocardial infarction (MI) remain unclear.

RESULTS

Left ventricular (LV) remodeling and function were assessed by 2D echocardiography and isolated perfused heart studies in 6-week post-MI and sham-operated rats. LV myocytes from sham and noninfarcted MI hearts were used for morphometric and functional studies. Beta-adrenergic receptor (beta-AR) agonist isoproterenol (ISO)-induced contractile response was measured in isolated hearts. The effects of ISO and forskolin on contractile function and calcium transients of isolated myocytes were recorded. ISO-induced cAMP generation was compared in sham and MI myocytes. beta-AR density was measured by radioligand binding. MI hearts were remodeled (LV diameter 8.5+/-0.3 versus 5.7+/-0.3 mm, P:<0.001) and showed global (% fractional shortening 19.1+/-2.5 versus 55.3+/-2.2, P:<0.01) and regional contractile dysfunction of noninfarcted myocardium (% systolic posterior wall thickening 37+/-4 versus 62+/-10, P:<0.01). Isolated heart function (LV developed pressure 58+/-2 versus 72+/-3 mm Hg, P:=0.004) and ISO concentration response were reduced in MI hearts. Myocytes from the noninfarcted LV were structurally remodeled (32% longer and 18% wider), but their contractile response and intracellular calcium kinetics to ISO and forskolin were not diminished. beta-AR receptor density (B(max) 24+/-1.5 versus 22.4+/-1.6 fmol/mg protein) and beta-AR agonist-stimulated cAMP were similar in both groups.

CONCLUSIONS

Isolated myocytes from the remodeled and dysfunctional myocardium are structurally modified but contract normally under basal conditions and in response to beta-AR stimulation. beta-AR density is preserved in remodeled myocytes. Nonmyocyte factors may be more important in the genesis of contractile dysfunction in the remodeled rat heart up to 6 weeks after MI.