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 Ca²⁺ entry into the mitochondria. Accordingly, we established a system to determine the role of the mitochondrial Ca²⁺ 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. Ru₃₆₀ and spermine were used to modify mitochondrial Ca²⁺ uniporter activity. Changes in mitochondrial Ca²⁺ content and energetic phosphate metabolite levels were determined. Key Results The addition of Ru₃₆₀, a selective inhibitor of the mitochondrial Ca²⁺ uniporter, induced progressively and sustained negative inotropic effects that were dose-dependent with an EC₅₀ of 7 μM. Treatment with spermine, a uniporter agonist, showed a positive inotropic effect that was blocked by Ru₃₆₀. Inotropic stimulation with isoprenaline elevated oxygen consumption (2.7-fold), Ca²⁺-dependent activation of pyruvate dehydrogenase (5-fold) and mitochondrial Ca²⁺ content (2.5-fold). However, in Ru₃₆₀-treated hearts, this parameter was attenuated. In addition, β-adrenoceptor stimulation in the presence of Ru₃₆₀ did not affect intracellular Ca²⁺ handling, PKA or Ca²⁺/calmodulin-dependent PK signalling. Conclusions and Implications Inhibition of the mitochondrial Ca²⁺ 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 Ca²⁺ uniporter activity.