Considerable debate surrounds the identity of the precise cellular site(s) of inertia that limit the contribution of mitochondrial ATP resynthesis towards a step increase in workload at the onset of muscular contraction. By detailing the relationship between canine gracilis muscle energy metabolism and contractile function during constant‐flow ischaemia, in the absence (control) and presence of pyruvate dehydrogenase complex activation by dichloroacetate, the present study examined whether there is a period at the onset of contraction when acetyl‐coenzyme A (acetyl‐CoA) availability limits mitochondrial ATP resynthesis, i.e. whether a limitation in mitochondrial acetyl group provision exists. Secondly, assuming it does exist, we also aimed to identify the mechanism by which dichloroacetate overcomes this ‘acetyl group deficit’. No increase in pyruvate dehydrogenase complex activation or acetyl group availability occurred during the first 20 s of contraction in the control condition, with strong trends for both acetyl‐CoA and acetylcarnitine to actually decline (indicating the existence of an acetyl group deficit). Dichloroacetate increased resting pyruvate dehydrogenase complex activation, acetyl‐CoA and acetylcarnitine by ≈20‐fold (P < 0.01), ≈3‐fold (P < 0.01) and ≈4‐fold (P < 0.01), respectively, and overcame the acetyl group deficit at the onset of contraction. As a consequence, the reliance upon non‐oxidative ATP resynthesis was reduced by ≈40 % (P < 0.01) and tension development was increased by ≈20 % (P < 0.05) following 5 min of contraction. The present study has demonstrated, for the first time, the existence of an acetyl group deficit at the onset of contraction and has confirmed the metabolic and functional benefits to be gained from overcoming this inertia.

Background and purpose: Inhibition of hepatic glycogen phosphorylase is a potential treatment for glycaemic control in type 2 diabetes. Selective inhibition of the liver phosphorylase isoform could minimize adverse effects in other tissues. We investigated the potential selectivity of two indole site phosphorylase inhibitors, GPi688 and GPi819. Experimental approach: The activity of glycogen phosphorylase was modulated using the allosteric effectors glucose or caffeine to promote the less active T state, and AMP to promote the more active R state. In vitro potency of indole site inhibitors against liver and muscle glycogen phosphorylase a was examined at different effector concentrations using purified recombinant enzymes. The potency of GPi819 was compared with its in vivo efficacy at raising glycogen concentrations in liver and muscle of Zucker (fa/fa) rats. Key results: In vitro potency of indole site inhibitors depended upon the activity state of phosphorylase a. Both inhibitors showed selectivity for liver phosphorylase a when the isoform specific activities were equal. After 5 days dosing of GPi819 (37.5 mmol kg À1 ), where free compound levels in plasma and tissue were at steady state, glycogen elevation was 1.5-fold greater in soleus muscle than in liver (Po0.05). Conclusions and implications:The in vivo selectivity of GPi819 did not match that seen in vitro when the specific activities of phosphorylase a isoforms are equal. This suggests T state promoters may be important physiological regulators in skeletal muscle. The greater efficacy of indole site inhibitors in skeletal muscle has implications for the overall safety profile of such drugs.

-We examined the effects of increasing acetylcarnitine and acetyl-CoA availability at rest, independent of pyruvate dehydrogenase complex (PDC) activation, on energy production and tension development during the rest-to-work transition in canine skeletal muscle. We aimed to elucidate whether the lag in PDC-derived acetyl-CoA delivery toward the TCA cycle at the onset of exercise can be overcome by increasing acetyl group availability independently of PDC activation or is intimately dependent on PDCderived acetyl-CoA. Gracilis muscle pretreated with saline or sodium acetate (360 mg/kg body mass) (both n ϭ 6) was sampled repeatedly during 5 min of ischemic contraction. Acetate increased acetylcarnitine and acetyl-CoA availability (both P Ͻ 0.01) above control at rest and throughout contraction (P Ͻ 0.05), independently of differences in resting PDC activation between treatments. Acetate reduced oxygen-independent ATP resynthesis ϳ40% (P Ͻ 0.05) during the first minute of contraction. No difference in oxygen-independent ATP resynthesis existed between treatments from 1 to 3 min of contraction; however, energy production via this route increased ϳ25% (P Ͻ 0.05) above control in the acetate-treated group during the final 2 min of contraction. Tension development was ϳ20% greater after 5-min contraction after acetate treatment than in control (P Ͻ 0.05). In conclusion, at the immediate onset of contraction, when PDC was largely inactive, increasing cellular acetyl group availability overcame inertia in mitochondrial ATP regeneration. However, after the first minute, when PDC was near maximally activated in both groups, it appears that PDC-derived acetyl-CoA, rather than increased cellular acetyl group availability per se, dictated mitochondrial ATP resynthesis.acetylcarnitine; oxygen deficit; oxidative phosphorylation; phosphocreatine; sodium acetate THE READJUSTMENT of oxidative (mitochondrial) ATP production to meet the increase in muscular energy demand during the transition from rest to exercise, or the step increase from one workload to another, is delayed and follows an approximately exponential time course (for review, see Ref. 37). During this period of latency, the transient shortfall in mitochondrial ATP production, classically termed the "oxygen deficit" (21, 30), is supplemented by ATP resynthesis from non-oxygen-dependent routes [i.e., ATP and phosphocreatine (PCr) breakdown and glycolysis]. Although ATP production from oxygen-independent routes enables rapid rates of ATP turnover to be achieved, it has only a finite capacity and also results in the accumulation of metabolic byproducts that are deleterious to muscular contractile function (hydrogen ions, lactate ions, and inorganic phosphate; see Ref. 8). Indeed, without the progressive increase in mitochondrial ATP production at the onset of contraction, the onset of muscular fatigue would be markedly accelerated. Classically, the lag in oxidative ATP production and resulting oxygen deficit have been attributed to a lag in muscle blood flow and thereby m...

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