Mitochondrial calcium content in isolated perfused heart: effects of inotropic stimulation.

We tested the hypothesis that in the intact heart, mitochondrial metabolism is activated by mitochondrial Ca2+ uptake during increased work. We measured left ventricular pressure (LVP), pyruvate dehydrogenase (PDH) activity, and mitochondrial and A band elemental content by electron probe microanalysis (EPMA) in Langendorff-perfused hamster hearts under control conditions, after isoproterenol (10(-6) M) stimulation, and after increasing perfusion pressure from 60 to 100 mmHg. Hearts were rapidly frozen, then EPMA was performed on cryosections cut from the surface of the frozen hearts; PDH activity was measured from the same area. Isoproterenol and elevated perfusion pressure increased LVP by 185 +/- 21 and 58 +/- 14%, respectively, versus controls. PDH activity increased from 10.4 +/- 1.5 (mean +/- SE) nmol.min-1. mg protein-1 (controls) to 21.6 +/- 3.5 (isoproterenol) and 18.5 +/- 3.2 nmol.min-1.mg protein-1 (increased perfusion pressure). There was no significant change in mitochondrial Ca1 in response to isoproterenol [1.2 +/- 0.1 (mean +/- SE) mmol/kg dry wt] or increased perfusion pressure (1.1 +/- 0.1) versus controls (1.0 +/- 0.1). These results suggest that, in the intact heart, mechanisms other than mitochondrial Ca2+ uptake may contribute to PDH activation and increased cardiac work.