Effect of dissociating cytosolic calcium and metabolic rate on intracellularPO2 kinetics in single frog myocytes.

The purpose of this investigation was to utilize 2,3-butanedione monoxime (BDM; an inhibitor of contractile activation) to dissociate cytosolic[Ca²⁺] ([Ca²⁺]_c) from the putative respiratory regulators that arise from muscle contraction-induced ATP utilization in order to determine the relative contribution of[Ca²⁺]_c on intracellularP_O2 (P_iO2) kinetics during the transition from rest to contractions in single skeletal myocytes isolated fromXenopus laevislumbrical muscle. Myocytes were subjected to electrically induced isometric tetanic contractions (0.25 Hz; 2-min bouts) while peak tension and either[Ca²⁺]_c (n= 7; ratiometric fluorescence microscopy) orP_iO2 (n= 7; phosphorescence microscopy) was measured continuously. Cells were studied under both control and 3 mmBDM conditions in randomized order. Initial (control, 100± 0%; BDM, 72.6± 4.6%), midpoint (control, 86.7± 1.8%; BDM, 61.6± 4.1%) and end (control, 85.0± 2.8%; BDM, 57.5± 5.0%) peak tensions (normalized to initial control values) were significantly reducedwith BDM compared with control (n= 14). Despite the reduced peak tension, peak[Ca²⁺]_c was not alteredbetween control and BDM trials. Thus, the peak tension-to-peak[Ca²⁺]_c ratio was reduced with BDM compared with control. The absolute fall inP_iO2 with contractions, which is proportional to the rise in, was significantly reduced with BDM (13.2± 1.3 mmHg) compared with control (22.0± 2.0 mmHg). However,P_iO2 onset kinetics (i.e. mean response time (MRT)) was not altered between BDM (66.8± 8.0 s) and control (64.9± 6.3 s) trials. Therefore, the initial rate of change (defined as the fall inP_iO2/MRT) was significantly slower in BDM fibres compared with control. These data demonstrate in these isolated single skeletal muscle fibres that unchanged peak[Ca²⁺]_c in the face of reduced metabolic feedback from the contractile sites evoked with BDM did not alterP_iO2 onset kinetics in isolated single frog myocytes, suggesting that metabolic signals arising from the contractile sites play a more substantial role than[Ca²⁺]_c in the signalling pathway to oxidative phosphorylation during the transition from rest to repeated tetanic contractions. [ABSTRACT FROM AUTHOR]