The purpose of the present investigation was to study the relation between energy metabolism and contractile function in the isovolumic guinea pig heart. 31P nuclear magnetic resonance spectroscopy was used to measure changes in the intracellular levels of creatine phosphate, ATP, inorganic phosphate, and pH during 2.43 min total global ischemia and 2.43 min reperfusion, with a time resolution of 9.7 s. From these data, cytosolic changes in the phosphorylation potential, [ATP]-to-[ADP] ratio, free-energy change of ATP hydrolysis, and concentration of free ADP were estimated. The simultaneous monitoring of functional and biochemical parameters allowed them to be directly correlated with respect to time and with respect to each other. No significant changes in ATP were detected at any time, but changes in all other biochemical data were highly correlated with changes in contractile function. Kinetic analysis, using a nonlinear least-squares fit of the experimental points, revealed that the changes in most parameters fitted monoexponential functions. Each parameter was ranked according to its half time, which revealed that the phosphorylation potential was the only metabolic parameter to change at a rate faster than loss of contractile function during ischemia, and all metabolic changes, with the exception of pH, led the recovery of contractile function during reperfusion, the most rapid change occurring in the free ADP concentration. It is concluded that the cytosolic phosphorylation potential controls the contractile function of the heart and that cytosolic free ADP is important in the control of mitochondrial oxidative phosphorylation.

This study examined the time course of adaptions in insulin sensitivity (IS) in adolescent boys after acute high-intensity interval exercise (HIIE) and moderate-intensity exercise (MIE). Eight boys (15.1±0.4 y) completed three 3-day experimental trials in a randomised order: 1) 8×1 min cycling at 90% peak power with 75 s recovery (HIIE); 2) cycling at 90% of gas exchange threshold for a duration to match work during HIIE (MIE); and 3) rest (CON). Plasma [glucose] and [insulin] were measured before (PRE-Ex), 24 and 48 h post (24 h-POST, 48 h-POST) in a fasted state, and 40 min (POST-Ex) and 24 h (24 h-POST) post in response to an oral glucose tolerance test (OGTT). IS was estimated using the Cederholm (OGTT) and HOMA (fasted) indices. There was no change to HOMA at 24 h or 48 h-POST (all 0.05). IS from the OGTT was higher POST-EX for HIIE compared to CON (17.4%,=0.010, ES=1.06), and a non-significant increase in IS after MIE compared to CON (9.0%, =0.14, ES=0.59). At 24 h-POST, IS was higher following both HIIE and MIE compared to CON (HIIE:=0.019, 13.2%, ES=0.88; MIE: 9.7%, =0.024, ES=0.65). In conclusion, improvements to IS after a single bout of HIIE and MIE persist up to 24 h after exercise when assessed by OGTT.

This study had two objectives: 1) to examine whether the validity of the supramaximal verification test for maximal oxygen uptake (̇O2max) differs in children and adolescents when stratified for sex, body mass and cardiorespiratory fitness (CRF); and 2) to assess sensitivity and specificity of primary and secondary objective criteria from the incremental test to verify ̇O 2max. Methods: 128 children and adolescents (76 males, 52 females; 9.3-17.4 y) performed a ramp-incremental test to exhaustion on a cycle ergometer followed by a supramaximal test to verify ̇O 2max. Results: Supramaximal tests verified ̇O 2max in 88% of participants. Group incremental test peak ̇O 2 was greater than the supramaximal test (2.27 ± 0.65 L•min -1 and 2.17 ± 0.63 L•min -1 ; P<0.001), although were correlated (r =0.94; P<0.001). No differences were found in ̇O 2 plateau attainment or supramaximal test verification between sexes, body mass or CRF statuses (all P>0.18).Supramaximal test time to exhaustion predicted supramaximal test ̇O 2max verification (P=0.040). Primary and secondary objective criteria had insufficient sensitivity (7.1-24.1%) and specificity (50-100%) to verify ̇O 2max. Conclusion: The utility of supramaximal testing to verify ̇O 2max is not affected by sex, body mass or CRF status.Supramaximal testing should replace secondary objective criteria to verify ̇O 2max.