A 31P-NMR study of tissue respiration in working dog muscle during reduced O2 delivery conditions.

To investigate the role of tissue oxygenation as one of the control factors regulating tissue respiration, 31P-nuclear magnetic resonance spectroscopy (31P-NMR) was used to estimate muscle metabolites in isolated working muscle during varied tissue oxygenation conditions. O2 delivery (muscle blood flow x arterial O2 content) was varied to isolated in situ working dog gastrocnemius (n = 6) by decreases in arterial PO2 (hypoxemia; H) and by decreases in muscle blood flow (ischemia; I). O2 uptake (VO2) was measured at rest and during work at two or three stimulation intensities (isometric twitch contractions at 3, 5, and occasionally 7 Hz) during three separate conditions: normal O2 delivery (C) and reduced O2 delivery during H and I, with blood flow controlled by pump perfusion. Biochemical metabolites were measured during the last 2 min of each 3-min work period by use of 31P-NMR, and arterial and venous blood samples were drawn and muscle blood flow measured during the last 30 s of each work period. Muscle [ATP] did not fall below resting values at any work intensity, even during O2-limited highly fatiguing work, and was never different among the three conditions. Muscle O2 delivery and VO2 were significantly less (P < 0.05) at the highest work intensities for both I and H than for C but were not different between H and I. As VO2 increased with stimulation intensity, a larger change in any of the proposed regulators of tissue respiration (ADP, P(i), ATP/ADP.P(i), and phosphocreatine) was required during H and I than during C to elicit a given VO2, but requirements were similar for H and I.(ABSTRACT TRUNCATED AT 250 WORDS)     

Oxidation/reduction state of cytochrome oxidase during repetitive contractions

There is disagreement regarding whether inadequate O2 determines maximal O2 uptake (VO2max) and lactic acid output (L) during muscular activity. Direct assessment of mitochondrial cytochrome oxidase (cytochrome a-a3) oxidation/reduction (O/R) state should provide an unequivocal answer for this issue. A new near-infrared spectrophotometric method was used to measure the O/R state of cytochrome a-a3 of dog gastrocnemius-plantaris muscle in situ during repetitive isotonic twitch and tetanic contractions. Three contraction frequencies were used for each contraction type in alternating sequence to provide a wide range of VO2 up to VO2max. VO2 and L were measured after 3 and 9 min of a 10-min contraction period, and 15 min were allowed for recovery between contraction periods. VO2 increased with contraction frequency. L was variably increased with contraction frequency at 3 min and uptake usually occurred at 9 min, except at the highest tetanic frequency. The O/R span of cytochrome a-a3 was determined by respiring the animals with 100% N2 to determine the most reduced state. This was followed by respiration with 100% O2, which gave the most oxidized state transiently during recovery. Within this span in muscles at rest, cytochrome a-a3 was 50-80% oxidized. During contractions of both types at all frequencies, cytochrome a-a3 always became more oxidized by an additional 10-20%. These findings should put to rest any arguments that inadequate O2 is a determinant of VO2max or L under the conditions of these experiments: repetitive contractions with free flow in self-perfused muscles and normoxia.     

Ouabain-insensitive salt and water movements in duck red cells. II. Norepinephrine stimulation of sodium plus potassium cotransport

Catecholamines induce net salt and water movements in duck red cells incubated in isotonic solutions. The rate of this response is approximately three times greater than a comparable effect observed in 400 mosmol hypertonic solutions in the absence of hormone (W.F. Schmidt and T. J. McManus. 1977 a.J. Gen. Physiol. 70:59-79. Otherwise, these two systems share a great many similarities. In both cases, net water and salt movements have a marked dependence on external cation concentrations, are sensitive to furosemide and insensitive to ouabain, and allow the substitution of rubidium for external potassium. In the presence of ouabain, but the absence of external potassium (or rubidium), a furosemide-sensitive net extrusion of sodium against a large electrochemical gradient can be demonstrated. When norepinephrine-treated cells are incubated with ouabain and sufficient external sodium, the furosemide-sensitive, unidirectional influxes of both sodium and rubidium are half- maximally saturated at similar rubidium concentrations; with saturating external rubidium, the same fluxes are half-maximal at comparable levels of external sodium. In the absence of sodium, a catecholamine-stimulated, furosemide-sensitive influx of rubidium persists. In the absence of rubidium, a similar but smaller component of sodium influx can be seen. We interpret these results in terms of a cotransport model for sodium plus potassium which is activated by hypertonicity or norepinephrine. When either ion is absent from the incubation medium, the system promotes an exchange-diffusion type of movement of the co-ion into the cells. In the absence of external potassium, net movement of potassium out of the cell leads to a coupled extrusion of sodium against its electrochemical gradient.     

Sprint performance is related to muscle fascicle length in male 100-m sprinters.

The purpose of this study was to investigate the relationship between muscle fascicle length and sprint running performance in 37 male 100-m sprinters. The sample was divided into two performance groups by the personal-best 100-m time: 10.00-10.90 s (S10; n = 22) and 11.00-11.70 s (S11; n = 15). Muscle thickness and fascicle pennation angle of the vastus lateralis and gastrocnemius medialis and lateralis muscles were measured by B-mode ultrasonography, and fascicle length was estimated. Standing height, body weight, and leg length were similar between groups. Muscle thickness was similar between groups for vastus lateralis and gastrocnemius medialis, but S10 had a significantly greater gastrocnemius lateralis muscle thickness. S10 also had a greater muscle thickness in the upper portion of the thigh, which, given similar limb lengths, demonstrates an altered "muscle shape." Pennation angle was always less in S10 than in S11. In all muscles, S10 had significantly greater fascicle length than did S11, which significantly correlated with 100-m best performance (r values from -0.40 to -0.57). It is concluded that longer fascicle length is associated with greater sprinting performance.     

Force-velocity shifts with repetitive isometric and isotonic contractions of canine gastrocnemius in situ.

The force-velocity (F-V) relationships of canine gastrocnemius-plantaris muscles at optimal muscle length in situ were studied before and after 10 min of repetitive isometric or isotonic tetanic contractions induced by electrical stimulation of the sciatic nerve (200-ms trains, 50 impulses/s, 1 contraction/s). F-V relationships and maximal velocity of shortening (Vmax) were determined by curve fitting with the Hill equation. Mean Vmax before fatigue was 3.8 +/- 0.2 (SE) average fiber lengths/s; mean maximal isometric tension (Po) was 508 +/- 15 g/g. With a significant decrease of force development during isometric contractions (-27 +/- 4%, P < 0.01, n = 5), Vmax was unchanged. However, with repetitive isotonic contractions at a low load (P/Po = 0.25, n = 5), a significant decrease in Vmax was observed (-21 +/- 2%, P < 0.01), whereas Po was unchanged. Isotonic contractions at an intermediate load (P/Po = 0.5, n = 4) resulted in significant decreases in both Vmax (-26 +/- 6%, P < 0.05) and Po (-12 +/- 2%, P < 0.01). These results show that repeated contractions of canine skeletal muscle produce specific changes in the F-V relationship that are dependent on the type of contractions being performed and indicate that decreases in other contractile properties, such as velocity development and shortening, can occur independently of changes in isometric tension.     

Restriction-map variation associated with the G6PD polymorphism in natural populations of Drosophila melanogaster

Restriction-map variation was studied in 126 copies of the G6pd region in X chromosome lines of Drosophila melanogaster from North America, Europe, and Africa. Special attention was focused on the distribution of variation relative to the geographically variable polymorphism for two electrophoretic variants. Nucleotide heterozygosity as determined by eight six-cutter restriction enzymes for the 13-kb region is estimated, on the basis of the worldwide sample, to be 0.065%, which is the lowest value reported for any comparable region in the D. melanogaster genome. Significant linkage disequilibrium between electrophoretic alleles and restriction-site variation is observed for several sites. In contrast to published studies of other genetic regions, there are large insertions that reach significant frequencies and are found across considerable geographic distances. There is a clustering of this variation inside the first large intervening sequence of the G6PD gene.      

Fatigue of mammalian skeletal muscle in situ during repetitive contractions

When the gastrocnemius-plantaris muscle group of the dog is stimulated to contract repetitively for 30 min at frequencies high enough to generate VO2 levels at or near VO2 max, VO2 and mechanical performance decline with time. This decline with time is fatigue, and it occurs during twitch and tetanic contractions that are isometric or isotonic. There is oxidation of the mitochondrial electron transport system, and net lactic acid output is transient, ending after 20 min of contractions. Energy and substrate stores and intracellular pH are only moderately changed and do not appear to be well correlated with the development of fatigue. Blood flow through the muscle is well correlated with the development of fatigue and decreases as fatigue develops in a manner that keeps the blood arteriovenous O2 difference nearly constant. Changing the blood flow alters the rate of development of fatigue as an inverse relation, and this response does not appear to be related to changes in the availability of O2 in the mitochondria. Nerve-muscle transmission of excitation does not seem to be involved in the development of fatigue. Excitation-contraction coupling is well accepted to be at least part of the genesis of the development of fatigue. Metabolic limitations and control may affect excitation-contraction coupling by one or more changes in the internal environment. Blood flow affects this system by an unknown mechanism. The role of blood flow in fatigue deserves further consideration.     

Fasting protects against the side effects of irinotecan but preserves its anti-tumor effect in Apc15lox mutant mice

Irinotecan is a widely used topoisomerase-I-inhibitor with a very narrow therapeutic window because of its severe toxicity. In the current study we have examined the effects of fasting prior to irinotecan treatment on toxicity and anti-tumor activity. FabplCre;Apc^15lox/+ mice, which spontaneously develop intestinal tumors, of 27 weeks of age were randomized into 3-day fasted and ad libitum fed groups, followed by treatment with a flat-fixed high dose of irinotecan or vehicle. Side-effects were recorded until 11 days after the start of the experiment. Tumor size, and markers for cell-cycle activity, proliferation, angiogenesis, and senescence were measured. Fasted mice were protected against the side-effects of irinotecan treatment. Ad libitum fed mice developed visible signs of discomfort including weight loss, lower activity, ruffled coat, hunched-back posture, diarrhea, and leukopenia. Irinotecan reduced tumor size in fasted and ad libitum fed groups similarly compared to untreated controls (2.4 ± 0.67 mm and 2.4 ± 0.82 mm versus 3.0 ± 1.05 mm and 2.8 ± 1.08 mm respectively, P < 0.001). Immunohistochemical analysis showed reduced proliferation, a reduced number of vascular endothelial cells, and increased levels of senescence in tumors of both irinotecan treated groups. In conclusion, 3 days of fasting protects against the toxic side-effects of irinotecan in a clinically relevant mouse model of spontaneously developing colorectal cancer without affecting its anti-tumor activity. These results support fasting as a powerful way to improve treatment of colorectal carcinoma patients.