The relationships between muscle glycogenolysis, glycolysis, and H+ concentration were examined in eight subjects performing three 30-s bouts of maximal isokinetic cycling at 100 rpm. Bouts were separated by 4 min of rest, and muscle biopsies were obtained before and after bouts 2 and 3. Total work decreased from 20.5 +/- 0.7 kJ in bout 1 to 16.1 +/- 0.7 and 13.2 +/- 0.6 kJ in bouts 2 and 3. Glycogenolysis was 47.2 and 15.1 mmol glucosyl U/kg dry muscle during bouts 2 and 3, respectively. Lower accumulations of pathway intermediates in bout 3 confirmed a reduced glycolytic flux. In bout 3, the work done represented 82% of the work in bout 2, whereas glycogenolysis was only 32% of that in bout 2. Decreases in ATP and phosphocreatine contents were similar in the two bouts. Muscle [H+] increased from 195 +/- 12 to 274 +/- 19 nmol/l during bout 2, recovered to 226 +/- 8 nmol/l before bout 3, and increased to 315 +/- 24 nmol/l during bout 3. Muscle [H+] could not be predicted from lactate content, suggesting that ion fluxes are important in [H+] regulation in this exercise model. Low glycogenolysis in bout 3 may be due to an inhibitory effect of increased [H+] on glycogen phosphorylase activity. Alternately, reduced Ca2+ activation of fast-twitch fibers (including a possible H+ effect) may contribute to the low overall glycogenolysis. Total work in bout 3 is maintained by a greater reliance on slow-twitch fibers and oxidative metabolism.

Two experiments were conducted to study the effects of a subacute ruminal acidosis (SARA) model on diet choice in dairy cows. In the first experiment, 25% of the ad libitum dry matter intake (DMI) of the total mixed ration (TMR) was replaced with wheat-barley pellets (WBP, 50% ground wheat, 50% ground barley). Rumen pH was measured continuously via in-dwelling probes in 4 mid to late lactation cows. This diet change reduced rumen pH by 0.14 +/- 0.02 pH units (mean +/- SE) and increased time below pH 6.0, from 319 +/- 36 min(-1) to 641 +/- 36 min(-1). Hence, the nutritional model successfully induced SARA. The second experiment determined if inducing SARA increases the feed preference for long alfalfa hay compared with alfalfa pellets. The 2 wk of inducing SARA were separated by 1 control wk. Four cows on either SARA and control diets were given a choice of 2 feeds, 2 times per d, for 30 min. The preference ratios (PR = Amount of Hay consumed/Amount of Hay + Pellets consumed) for alfalfa hay during two SARA weeks was greater (0.85 +/- 0.03) compared with the control week (0.60 +/- 0.03). In SARA weeks, average rumen pH was 0.23 +/- 0.03 units lower, and time below pH 6.0 and 5.6 was higher compared to control. These results suggest that when given a choice of feeds, dairy cows alter their diet selection to attempt to attenuate SARA.

To assess the importance of factors influencing the resolution of exercise-associated acidosis, measurements of acid-base variables were made in nine healthy subjects after 30 s of maximal exercise on an isokinetic cycle ergometer. Quadriceps muscle biopsies (n = 6) were taken at rest, immediately after exercise, and at 3.5 and 9.5 min of recovery; arterial and femoral venous blood were sampled (n = 3) over the same time. Intracellular and plasma inorganic strong ions were measured by neutron activation and ion-selective electrodes, respectively; lactate concentration ([La-]) was measured enzymatically, and plasma PCO2 and pH were measured by electrodes. Immediately after exercise, intracellular [La-] increased to 47 meq/l, almost fully accounting for a reduction in intracellular strong ion difference ([SID]) from 154 to 106 meq/l. At the same time, femoral venous PCO2 increased to 100 Torr and plasma [La-] to 9.7 meq/l; however, plasma [SID] did not change because of a concomitant increase in inorganic [SID] secondary to increases in [K+], [Na+], and [Ca2+]. During recovery, muscle [La-] fell to 26 meq/l by 9.5 min; [SID] remained low (101 and 114 meq/l at 3.5 and 9.5 min, respectively) due almost equally to the elevated [La-] (30 and 26 meq/l) and reductions in [K+] (from 142 meq/l at rest to 123 and 128 meq/l). Femoral venous PCO2 rose to 106 Torr at 0.5 min postexercise and fell to resting values at 9.5 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Pyruvate dehydrogenase activity (PDHa) and acetyl group accumulation were examined in human skeletal muscle at rest and during exercise after different diets. Five males cycled at 75% of maximal O2 uptake (VO2 max) to exhaustion after consuming a low-carbohydrate diet (LCD) for 3 days and again 1-2 wk later for the same duration after consuming a high-carbohydrate diet (HCD) for 3 days. Resting PDHa was lower after a LCD (0.20 +/- 0.04 vs. 0.69 +/- 0.05 mmol.min-1.kg wet wt-1; P < 0.05) and coincided with a greater intramuscular acetyl-CoA-to-CoASH ratio, acetyl-CoA content, and acetylcarnitine content. PDHa increased during exercise in both conditions but at a lower rate in the LCD condition compared with the HCD condition (1.46 +/- 0.25 vs. 2.65 +/- 0.23 mmol.min-1.kg wet wt-1 at 16 min and 1.88 +/- 0.20 vs. 3.11 +/- 0.14 at the end of exercise; P < 0.05). During exercise muscle acetyl-CoA and acetylcarnitine content and the acetyl-CoA-to-CoASH ratio decreased in the LCD condition but increased in the HCD condition. Under resting conditions PDHa was influenced by the availability of fat or carbohydrate fuels acting through changes in the acetyl-CoA-to-CoASH ratio. However, during exercise the activation of PDHa occurred independent of changes in the acetyl-CoA-to-CoASH ratio, suggesting that other factors are more important.