Daniel J. Krause

Long-term caloric restriction abrogates the age-related decline in skeletal muscle aerobic function

The purpose of this study was to determine the effect of long‐term caloric restriction (CR) on the age‐associated decline of skeletal muscle aerobic function. Skeletal muscle maximal aerobic performance (VO2max) was assessed in ad libitum (AL) and CR rats aged 8–10 months and 35 months using a pump‐perfused hindlimb model to match oxygen delivery to muscle mass between groups. Whereas there was a 46% decline in muscle mass‐specific VO2max between 8–10 mo (524±13 µmol•min−1•100 g−1; mean±se) and 35 mo (281±54 µmol min−1•100 g−1) in AL rats, not only did CR rats begin at the same point in 8–10 mo old rats (490±42 µmol•min−1•100 g−1), we found no decline in 35 mo old CR animals (484±49 µmol•min−1•100 g−1). Interestingly, although most markers of oxidative capacity began at a lower point in young adult CR animals, CR rats exhibited a higher in situ activity of complex IV at VO2max. This activity allows the young adult CR animals to exhibit normal aerobic capacity despite the lower oxidative enzyme activities. In stark contrast to the 19–41% decline in activities of citrate synthase, complexes I–III, and complex IV in homogenates prepared from the plantaris muscle and mixed region of gastrocnemius muscle with aging in AL rats, no age‐related decline was found in CR animals. Thus, our results showed that CR preserves aerobic function in aged skeletal muscles by facilitating a higher in situ function of complex IV and by preventing the age‐related decline in mitochondrial oxidative capacity.

Exercise training in late middle‐aged male Fischer 344 × Brown Norway F1‐hybrid rats improves skeletal muscle aerobic function

The Fischer 344 × Brown Norway F1‐hybrid (F344BN) rat has become an increasingly popular and useful strain for studying age‐related declines in skeletal muscle function because this strain lives long enough to experience significant declines in muscle mass. Since exercise is often considered a mechanism to combat age‐related declines in muscle function, determining the utility of this strain of rat for studying the effects of exercise on the ageing process is necessary. The purpose of this study was to evaluate the plasticity of skeletal muscle aerobic function in late middle‐aged male rats following 7 weeks of treadmill exercise training. Training consisted of 60 min per day, 5 days per week with velocity gradually increasing over the training period according to the capabilities of individual rats. The final 3 weeks involved 2 min high‐intensity intervals to increase the training stimulus. We used in situ skeletal muscle aerobic metabolic responses and in vitro assessment of muscle mitochondrial oxidative capacity to describe the adaptations of aerobic function from the training. Training increased running endurance from 11.3 ± 0.6 to 15.5 ± 0.8 min, an improvement of ∼60%. Similarly, distal hindlimb muscles from trained rats exhibited a higher maximal oxygen consumption in situ (23.2 ± 1.3 versus 19.7 ± 0.8 μmol min−1 for trained versus sedentary rats, respectively) and greater citrate synthase and complex IV enzyme activities in gastrocnemius (29 and 19%, respectively) and plantaris muscles (24 and 28%, respectively) compared with age‐matched sedentary control animals. Our results demonstrate that skeletal muscles from late middle‐aged rats adapt to treadmill exercise by improving skeletal muscle aerobic function and mitochondrial enzyme activities. This rat strain seems suitable for further investigations using exercise as an intervention to combat ageing‐related declines of skeletal muscle aerobic function.

Nitric oxide synthase inhibition reduces O2 cost of force development and spares high-energy phosphates following contractions in pump-perfused rat hindlimb muscles

The purpose of the present experiments was to test the hypotheses that: (i) nitric oxide synthase (NOS) inhibition reduces the O2 cost of force development across a range of contractile demands; and (ii) this reduced O2 cost of force development would be reflected in a sparing of intramuscular higher energy phosphates. Rat distal hindlimb muscles were pump perfused in situ and electrically stimulated (200 ms trains with pulses at 100 Hz, each pulse 0.05 ms duration) for 1 min each at 15, 30 and 60 tetani min−1 and for 2 min at 90 tetani min−1 in three groups: 0.01 mm adenosine; 1 mm d‐NAME and 0.01 mm adenosine (d‐NAME); and 1 mm l‐NAME and 0.01 mm adenosine (l‐NAME). The gastrocnemius–plantaris–soleus muscle group was freeze clamped post‐contractions for metabolite analyses. Force was 19% higher and oxygen uptake was 20% lower with l‐NAME versus adenosine, and there was a 35% reduction in /time‐integrated tension versus adenosine and 24% versusd‐NAME that was independent of contraction frequency. l‐NAME treatment produced a 33% sparing of muscle phosphocreatine (PCr), and intramuscular lactate was no different between groups. In contrast, d‐NAME reduced force by 30%, by 29% and the O2 cost of force development by 15% compared with adenosine, but had no effect on the degree of intramuscular ATP and PCr depletion. These results show that NOS inhibition improved the metabolic efficiency of force development, either by improving the ATP yield for a given O2 consumption or by reducing the ATP cost of force development. In addition, these effects were independent of contraction frequency.

Nitric oxide synthase inhibition reduces the O2 cost of force development in rat hindlimb muscles pump perfused at matched convective O2 delivery

Nitric oxide (NO) is a physiological mediator of skeletal muscle function. Specifically, NO affects cellular respiration and muscle contractility; however, the reduced blood flow and convective O2 delivery that result from impaired vasodilatation when NO synthase (NOS) is inhibited in vivo have obscured past interpretations of the effects of NO. Therefore, we studied the effect of NOS inhibition in an in situ pump‐perfused rat hindlimb to test the hypothesis that NOS inhibition would improve contractile and aerobic metabolic performance. Pump perfusion permitted matching of convective O2 delivery (516 ± 16 μmol O2 min−1 (100 g)−1; mean ±s.e.m.) between groups, allowing us to investigate the effects of NOS inhibition independent of this variable. Three groups were studied. The perfusate of one group was treated with both adenosine (0.01 mm) and the NOS inhibitor, Nω‐nitro‐l‐arginine methyl ester (l‐NAME; 1 mm). Adenosine is a vasodilator that can act through both NO‐dependent and ‐independent pathways; the NO‐independent vasodilatory action of adenosine allowed us to match the perfusion rate and convective O2 delivery in this l‐NAME group to those of the other groups. In the second group the perfusate was treated with adenosine only (Ado). In the third group the perfusate received no treatment and served as a control (Con). Oxygen consumption ( ) was on average 26 and 14% lower during the contraction bout in l‐NAME and Ado, respectively, versus Con. In Ado, lactate efflux was similar to Con and force was reduced in proportion to versus Con, whereas l‐NAME was associated with a 32% lower lactate efflux and similar force to Con. Therefore, the lower :force development ratio in the l‐NAME group demonstrates that the O2 cost of force development is reduced by NOS inhibition independent of convective O2 delivery.