Ca2+ uptake by cardiac sarcoplasmic reticulum from patients with idiopathic dilated cardiomyopathy.

We measured Ca2+ uptake by sarcoplasmic reticulum prepared from left ventricular myocardium obtained from six nonfailing human hearts and nine excised hearts from patients with class IV idiopathic dilated cardiomyopathy. Ca2+ uptake had a Vmax of 593 +/- 82 nmol/mg-min, a K0.5 of 0.68 +/- 0.07 microM, and an nHill of 1.7 +/- 0.1 in the nonfailing hearts. The corresponding values in the excised failing hearts were 593 +/- 36 nmol/mg-min, 0.63 +/- 0.03 microM, and 1.6 +/- 0.1. The beta-receptor density in crude sarcolemma prepared from left ventricular myocardium was 110.0 +/- 15.3 fmol/mg in the unmatched donors and 52.1 +/- 4.5 fmol/mg in the excised failing hearts. These results suggest that abnormal Ca2+ handling in idiopathic dilated cardiomyopathy in humans is not the result of any intrinsic alteration of Ca2+ uptake by sarcoplasmic reticulum.