The interpretation of abnormal ³¹P magnetic resonance saturation transfer measurements of P_i/ATP exchange in insulin-resistant skeletal muscle

Abstract

TO THE EDITOR: To study relationships between muscle mitochondrial function, fatty acid oxidation, intramyocellular lipid (IMCL), and insulin resistance (IR), Laurent et al. (21a) measured P i /ATP exchange by 31 P magnetic resonance saturation (MRS) transfer (ST) in resting rat muscle and found that it decreased transiently by high dietary fat (HF) intake. Following precedent, they identified P i /ATP exchange with mitochondrial ATP synthesis, noting that "actual rates obtained . . . in rats at rest are in excellent agreement with recent human data," and took the decrease as evidence of mitochondrial dysfunction (21a). I would like to make two points about absolute rates and pathophysiological logic. Absolute RatesGiven the allometry of basal metabolic rate (34), to which it makes a large contribution (33), resting muscle ATP turnover should presumably scale with weight (Ϫ0.25) , making it approximately four times greater in rat (ϳ7 in mouse) than in human. Unlike P i /ATP exchange (21a), published measurements by several other methods seem roughly consistent with this (Fig. 1A). Moreover, although "there is no gold standard against which the ATP synthesis rate measured by 31 P saturation transfer can easily be validated" (21a), the estimates it gives are notably high (Fig. 1A). For P i /ATP exchange to validly measure mitochondrial ATP synthesis (i.e., net flux through F 1 F 0 -ATPase) requires this to be far from equilibrium [as measurements in vitro support, at least at high turnover (21)] and glycolytic P i /ATP exchange (catalyzed by glyceraldehyde-3-phosphate dehydrogenase and 3-phosphoglycerate kinase) to be small (7, 15, 21a, 32). No direct comparisons have been made, but suprabasal P i /ATP exchange rates in stimulated rat muscle were noted (7) to be approximately consistent with O 2 consumption (12) and phosphocreatine (PCr) kinetic (35) data and with poststimulation PCr resynthesis rates (38). However, upon reexamination (Fig. 1B) these comparisons do not rule out appreciable glycolytic P i /ATP exchange. If skeletal muscle resembles heart, where this discrepancy is broadly independent of ATP turnover (32) (in effect, a constant error), the 20 -40% P i /ATP exchange defect in HF (21a) and other IR states (4,28,29,36) presumably underestimates that in ATP synthesis. However, the similar percent defects in P i /ATP exchange and tricarboxylic cycle (TCAC) flux measured by 13 C MRS in IR (4,28,29) suggest that the discrepancy is turnover dependent (a proportional error). This is not resolved by the observations that triiodothyronine (14, 22) and 2,4-dinitrophenol (14) decrease, whereas uncoupling protein-3 knockout increases (8), the ratio of P i /ATP exchange to TCAC flux [a qualitative index of coupling (15, 16)] or that 2,4-dinitrophenol does not affect directly measured glycolytic exchange (14). Flux Vs. Capacity, Supply Vs. DemandThe distinction between flux and capacity is why "maximal enzyme activities do not necessarily reflect actual flux through metabolic pathways" (21a, 29). On the conventio...

Comparison of measuring energy metabolism by different 31P‐magnetic resonance spectroscopy techniques in resting, ischemic, and exercising muscle

“…Finally, mitochondrial ATP formation is not strictly irreversible (34). As a consequence, the apparent forward flux (fATP) exceeds the net production many times (26). Nevertheless, our data show that this parameter seems to be linearly related to an alternate measure of resting ATP turnover (PCr decrease).…”

“…Saturation transfer measurements revealed more than 20-fold higher ATP production rates than estimation based on ischemia-induced PCr decrease. A meta-analysis of previous publications (26) reported also much higher values for saturation transfer than for other measurements of resting ATP turnover. To our knowledge, a direct comparison of saturation transfer with the other methods has not been reported before in one single group of humans.…”

Comparison of in vivo postexercise phosphocreatine recovery and resting ATP synthesis flux for the assessment of skeletal muscle mitochondrial function

“…Most investigated is the feedback control loop involving changes in the extramitochondrial concentrations of the ATP hydrolysis products ADP (23,25,66) and, to a lesser extent, P i (66). To interpret ATP synthesis flux data in terms of mitochondrial function, it is essential to take the error signals; i.e., concentrations of ADP and P i , into account (26). A lower ATP synthesis flux in combination with low error signals simply represents a lower ATP demand.…”

“…The lower ATP synthesis rates in resting muscle of insulin-resistant subjects (38,39) could actually reflect a normal regulatory response to a lower energy demand caused by impaired insulin signaling rather than an impairment of intrinsic mitochondrial function (1,26,51,65). Moreover, V ATP obtained from 31 P ST measurements is composed of both mitochondrial ATP synthase flux and glycolytic exchange flux, with the latter contributing by as much as 80% at rest (3,4,6,26,27). Therefore, decreased resting V ATP does not necessarily reflect a mitochondrial defect.…”

Skeletal muscle ATP kinetics are impaired in frail mice

“…P i →ATP flux) represent the sum of all ATP synthesis reactions involving P i through both glycolysis and mitochondrial oxidative phosphorylation (OxPhos), but this is dominated by glyoclytically mediated Pi-ATP exchange (Kemp 2008;Balaban and Koretsky 2011;From and Ugurbil 2011;Kemp and Brindle 2012). An approximately 55% decline in P i →ATP flux in IL10 tm/tm mice was observed and this lower unidirectional P i →ATP flux was not due to substrate driving the reaction since P i was higher, not lower, in IL10 tm/tm mice.…”