Shaking up glycolysis: Sustained, high lactate flux during aerobic rattling

Abstract

Substantial ATP supply by glycolysis is thought to reflect cellular anoxia in vertebrate muscle. An alternative hypothesis is that the lactate generated during contraction reflects sustained glycolytic ATP supply under well-oxygenated conditions. We distinguished these hypotheses by comparing intracellular glycolysis during anoxia to lactate efflux from muscle during sustained, aerobic contractions. We examined the tailshaker muscle of the rattlesnake because of its uniform cell properties, exclusive blood circulation, and ability to sustain rattling for prolonged periods. Here we show that glycolysis is independent of the O(2) level and supplies one-third of the high ATP demands of sustained tailshaking. Fatigue is avoided by rapid H(+) and lactate efflux resulting from blood flow rates that are among the highest reported for vertebrate muscle. These results reject the hypothesis that glycolysis necessarily reflects cellular anoxia. Instead, they demonstrate that glycolysis can provide a high and sustainable supply of ATP along with oxidative phosphorylation without muscle fatigue.

Figures

Figure 1

PCr (A) and pH (B) levels starting at the onset of ischemia in resting muscle and through ischemic rattling. The contractile ATP demand is measured by Δ[PCr]i. After the onset of acidosis, the changes in [PCr] (Δ[PCr]e) and pH (ΔpHe) reflect glycolytic H+ production (Eq. 1; ref. 14). Values are means, n = 6. The dashed line in B is the resting muscle pH from high-resolution spectra.

Figure 2

PCr dynamics from rest to aerobic rattling (A, n = 6) and during recovery (B, n = 4). Δ[PCr]r is the change in [PCr] from rest to the rattling steady state. PCr recovery is characterized by the rate constant (kPCr = 1/τ) derived from a monoexponential fit. Values are means. The rattling period is shown by the solid horizontal line in A.

Figure 3

ATP balance during rattling is comprised of the ATP supply by glycolysis (striped box, n = 13) and oxidative phosphorylation (open box, n = 5) vs. the contractile ATP demand (filled box, n = 13). Values are means ± SEM.

Figure 4

Comparison of the ATP supply by glycolysis and oxidative phosphorylation as determined by MR spectroscopy (open bars) and blood measurements (closed bars). Values are means ± SEM. Number of determinations as in Fig. 3 and Table 1.

Figure 5

The percentage of glycolysis to total ATP supply in human muscle and rattlesnake tailshaker muscle during sustained contractions. The bars represent the following muscles: open bar, wrist flexor muscles (9); yellow bar, tibialis anterior (9); blue bar, quadriceps (34); and red bar, tailshaker muscles. The dashed horizontal line represents the 8% of ATP supplied when pyruvate is derived exclusively from glycogen, is the sole substrate for respiration, and is completely oxidized.