CK inhibition accelerates transcytosolic energy signaling during rapid workload steps in isolated rabbit hearts

The effect of graded creatine kinase (CK) inhibition on the response time of mitochondrial O2 consumption to dynamic workload jumps (tmito) was studied in isolated rabbit hearts. Tyrode-perfused hearts (n = 7/group) were exposed to 15 min of 0, 0.1, 0.2, or 0.4 mM iodoacetamide (IA) (CK activity = 100, 14, 6, and 3%, respectively). Pretreatment tmito was similar across groups at 6.5 +/- 0.5 s (mean +/- SE). The increase observed over time in control hearts (33 +/- 8%) was progressively reversed to 16 +/- 6, -20 +/- 6 (P < 0.01 vs. control), and -46 +/- 6 (P < 0.01 vs. control) % in the 0.1, 0.2 and 0.4 mM IA groups, respectively. The faster response times occurred without reductions in mitochondrial oxidative capacity (assessed in vitro) or myocardial O2 consumption of the whole heart during workload steps. Isovolumic contractile function assessed as rate-pressure product (RPP) and contractile reserve (increase in RPP during heart rate steps) were significantly reduced by IA. We conclude that CK in the myofibrils and/or cytosol does not speed up transfer of the energy-related signal to the mitochondria but rather acts as an energetic buffer, effectively slowing the stimulus between myofibrils/ion pumps and oxidative phosphorylation. This argues against the existence of an obligatory creatine phosphate energy shuttle, because CK is effectively bypassed.