Cardiac responses to β ‐adrenoceptor stimulation is partly dependent on mitochondrial calcium uniporter activity

Background and Purpose Despite the importance of mitochondrial Ca 2+ to metabolic regulation and cell physiology, little is known about the mechanisms that regulate Ca 2+ entry into the mitochondria. Accordingly, we established a system to determine the role of the mitochondrial Ca 2+ uniporter in an isolated heart model, at baseline and during increased workload following β ‐adrenoceptor stimulation. Experimental Approach Cardiac contractility, oxygen consumption and intracellular Ca 2+ transients were measured in ex vivo perfused murine hearts. R u 360 and spermine were used to modify mitochondrial Ca 2+ uniporter activity. Changes in mitochondrial Ca 2+ content and energetic phosphate metabolite levels were determined. Key Results The addition of R u 360 , a selective inhibitor of the mitochondrial Ca 2+ uniporter, induced progressively and sustained negative inotropic effects that were dose‐dependent with an EC 50 of 7 μM. Treatment with spermine, a uniporter agonist, showed a positive inotropic effect that was blocked by R u 360 . Inotropic stimulation with isoprenaline elevated oxygen consumption (2.7‐fold), Ca 2+ ‐dependent activation of pyruvate dehydrogenase (5‐fold) and mitochondrial Ca 2+ content (2.5‐fold). However, in R u 360 ‐treated hearts, this parameter was attenuated. In addition, β ‐adrenoceptor stimulation in the presence of R u 360 did not affect intracellular Ca 2+ handling, PK A or Ca 2+ /calmodulin‐dependent PK signalling. Conclusions and Implications Inhibition of the mitochondrial Ca 2+ uniporter decreases β ‐adrenoceptor response, uncoupling between workload and production of energetic metabolites. Our results support the hypothesis that the coupling of workload and energy supply is partly dependent on mitochondrial Ca 2+ uniporter activity.