Mitochondrial Sensitivity To Submaximal ADP Concentrations Following Bed Rest: A Double Exponential Model Analysis

Abstract

We recently demonstrated that a 10-day exposure to inactivity/microgravity (bed rest, BR) determined impairments of peak oxygen uptake (V̇O2), peak cardiac output, peripheral and microvascular function, whereas maximal ADP-stimulated mitochondrial respiration ex vivo in isolated permeabilized fibers and muscle V̇O2 off-kinetics in vivo were unaffected. PURPOSE: To evaluate mitochondrial sensitivity to submaximal [ADP]. METHODS: Isolated permeabilized vastus lateralis fibers were analyzed by high-resolution respirometry, before (PRE) and after (POST) a 10-day BR in 10 young males. 11 submaximal titrations of ADP (from 12.5 to 10,000 μM) were utilized to assess complex I + II-linked ADP sensitivity which was analyzed by both a Michaelis-Menten (MM) kinetics equation, with the calculation of the apparent Km by standard methods, and by applying a double-exponential function, with the calculation of [ADP] corresponding to 50% of the maximal mitochondrial respiration ([ADP] at 50% JO2max). RESULTS: The double-exponential function provided a better fitting of the data according to AIC model selection. The sum of squared residuals was substantially lower with the double-exponential vs. MM fitting (411 ± 419 and 1209 ± 495 [pmol/s/mg]2, respectively). R2 was higher with the double-exponential vs. the MM fitting (0.97 ± 0.04 and 0.90 ± 0.05, respectively). Analysis of residuals showed that the fitting by MM markedly overestimated JO2 from 1000 to 2000 μM, and underestimated JO2 from 6000 to 10,000 μM. The apparent Km for MM was lower than the [ADP] at 50% JO2max, both in PRE (843 ± 706 and 1270 ± 1015 μM, respectively) and in POST (375 ± 173 and 409 ± 398 μM, respectively). Only for the [ADP] at 50% JO2max the difference between POST vs. PRE was statistically significant (P = 0.046). CONCLUSIONS: The double-exponential function provided a better fit of experimental data. It might also give insights into cellular mechanisms regulating mitochondrial respiration, possibly related to the transport of nucleotides and phosphate across external and internal mitochondrial membranes, ATP synthase activity and respiratory chain activity. The enhanced mitochondrial sensitivity to submaximal [ADP] after BR could represent an attempt to maintain metabolic homeostasis in the presence of impairments upstream of mitochondria.