Effect of low pH on single skeletal muscle myosin mechanics and kinetics.

Acidosis (low pH) is the oldest putative agent of muscular fatigue, but the molecular mechanism underlying its depressive effect on muscular performance remains unresolved. Therefore, the effect of low pH on the molecular mechanics and kinetics of chicken skeletal muscle myosin was studied using in vitro motility (IVM) and single molecule laser trap assays. Decreasing pH from 7.4 to 6.4 at saturating ATP slowed actin filament velocity (V(actin)) in the IVM by 36%. Single molecule experiments, at 1 microM ATP, decreased the average unitary step size of myosin (d) from 10 +/- 2 nm (pH 7.4) to 2 +/- 1 nm (pH 6.4). Individual binding events at low pH were consistent with the presence of a population of both productive (average d = 10 nm) and nonproductive (average d = 0 nm) actomyosin interactions. Raising the ATP concentration from 1 microM to 1 mM at pH 6.4 restored d (9 +/- 3 nm), suggesting that the lifetime of the nonproductive interactions is solely dependent on the [ATP]. V(actin), however, was not restored by raising the [ATP] (1-10 mM) in the IVM assay, suggesting that low pH also prolongs actin strong binding (t(on)). Measurement of t(on) as a function of the [ATP] in the single molecule assay suggested that acidosis prolongs t(on) by slowing the rate of ADP release. Thus, in a detachment limited model of motility (i.e., V(actin) approximately d/t(on)), a slowed rate of ADP release and the presence of nonproductive actomyosin interactions could account for the acidosis-induced decrease in V(actin), suggesting a molecular explanation for this component of muscular fatigue.

The depressive effect of Pi on the force-pCa relationship in skinned single muscle fibers is temperature dependent.

Increases in P(i) combined with decreases in myoplasmic Ca(2+) are believed to cause a significant portion of the decrease in muscular force during fatigue. To investigate this further, we determined the effect of 30 mM P(i) on the force-Ca(2+) relationship of chemically skinned single muscle fibers at near-physiological temperature (30 degrees C). Fibers isolated from rat soleus (slow) and gastrocnemius (fast) muscle were subjected to a series of solutions with an increasing free Ca(2+) concentration in the presence and absence of 30 mM P(i) at both low (15 degrees C) and high (30 degrees C) temperature. In slow fibers, 30 mM P(i) significantly increased the Ca(2+) required to elicit measurable force, referred to as the activation threshold at both low and high temperatures; however, the effect was twofold greater at the higher temperature. In fast fibers, the activation threshold was unaffected by elevating P(i) at 15 degrees C but was significantly increased at 30 degrees C. At both low and high temperatures, 30 mM P(i) increased the Ca(2+) required to elicit half-maximal force (pCa(50)) in both slow and fast fibers, with the effect of P(i) twofold greater at the higher temperature. These data suggest that during fatigue, reductions in the myoplasmic Ca(2+) and increases in P(i) act synergistically to reduce muscular force. Consequently, the combined changes in these ions likely account for a greater portion of fatigue than previously predicted based on studies at lower temperatures or high temperatures at saturating Ca(2+) levels.

Fiber type and temperature dependence of inorganic phosphate: implications for fatigue.

Elevated levels of P(i) are thought to cause a substantial proportion of the loss in muscular force and power output during fatigue from intense contractile activity. However, support for this hypothesis is based, in part, on data from skinned single fibers obtained at low temperatures (< or =15 degrees C). The effect of high (30 mM) P(i) concentration on the contractile function of chemically skinned single fibers was examined at both low (15 degrees C) and high (30 degrees C) temperatures using fibers isolated from rat soleus (type I fibers) and gastrocnemius (type II fibers) muscles. Elevating P(i) from 0 to 30 mM at saturating free Ca(2+) levels depressed maximum isometric force (P(o)) by 54% at 15 degrees C and by 19% at 30 degrees C (P < 0.05; significant interaction) in type I fibers. Similarly, the P(o) of type II fibers was significantly more sensitive to high levels of P(i) at the lower (50% decrease) vs. higher temperature (5% decrease). The maximal shortening velocity of both type I and type II fibers was not significantly affected by elevated P(i) at either temperature. However, peak fiber power was depressed by 49% at 15 degrees C but by only 16% at 30 degrees C in type I fibers. Similarly, in type II fibers, peak power was depressed by 40 and 18% at 15 and 30 degrees C, respectively. These data suggest that near physiological temperatures and at saturating levels of intracellular Ca(2+), elevated levels of P(i) contribute less to fatigue than might be inferred from data obtained at lower temperatures.

Preexercise feeding in untrained adolescent boys does not affect responses to endurance exercise or performance.

The effects of preexercise feeding on responses to endurance exercise and performance were investigated. Untrained adolescent boys (N = 13, age 14.9 +/- 0.5 years) completed three endurance test sessions separated by a minimum of 72 hr. Each session consisted of 75 min of cycling at 60% of VO2 max followed by a high-intensity performance test. Dietary conditions were a candy bar (C1: 280 kcal, 36 g CHO), fat-free fig bars (C2: 200 kcal, 44 g CHO), and a nonnutritive sweetened drink (C3: placebo), ingested 10 min prior to exercise. Respiratory gases, heart rate, blood glucose, and lactate concentrations were measured throughout the test. ANOVA results revealed significant time effects for all variables; however, no differences were seen among the conditions. Performance times, 311.9 +/- 38.5 s in C1, 316.2 +/- 37.3 s in C2, and 328.1 +/- 46.4 s in C3, were not significantly different among conditions. Thus, preexercise feeding did not affect responses to endurance exercise or performance in adolescent boys.