PLN and myocardial infarction: Although Iso resulted in the shortening of APD80 and CaTD80 from baseline values in both sham and MI hearts, slightly greater relative changes in CaTD80 were observed in MI (Fig. 4), which may suggest enhanced responsiveness of Ca2+-handling proteins modulated by β-adrenergic activation (e.g., phospholamban phosphorylation on sarcoendoplasmic reticulum calcium ATPase activity).