High-energy phosphates and tension production in rabbit tibialis anterior/extensor digitorum longus muscles

Ryschon, T. W., J. C. Jarvis, S. Salmons, and R. S. Balaban.High-energy phosphates and tension production in rabbit tibialisanterior/extensor digitorum longus muscles. J. Appl. Physiol. 82(3): 1024-1029, 1997.The effects ofrepetitive muscle contraction on energy state and tension productionwere studied in rabbit tibialis anterior/extensor digitorum longusmuscles that had been subjected to 90 days of continuous indirectelectrical stimulation at 10 Hz. Anesthetized chronically stimulatedand control rabbits were challenged with 15 min of stimulation at 4 and15 tetani/min.P_i-to-phosphocreatine (PCr) ratio(P_i/PCr) was measured in vivo before, during, andafter acute stimulation by³¹P-magnetic resonancespectroscopy, and tension was recorded at the same time. AlthoughP_i/PCr was low at rest, it wassignificantly higher in chronically stimulated muscle than in controlmuscle (0.20 ± 0.02 vs. 0.05 ± 0.01, P < 0.05). Stimulation of control muscle for 15 min at both 4 and 15 tetani/min induced a significant rise in P_i/PCr, whereas the sameconditions in chronically stimulated muscle did not produce anysignificant departure from initial levels. The tension produced bycontrol muscle fell to 93 ± 3% of its initial value duringstimulation at 4 tetani/min and to 61 ± 7% at 15 tetani/min,respectively. In chronically stimulated muscle, on the other hand,tension was potentiated above its initial level at both stimulationrates (135 ± 15 and 138 ± 11%, respectively) and remainedsignificantly elevated throughout each trial. The ability ofchronically stimulated muscle to sustain high levels of activity withminimal perturbations in P_i/PCr ordecrement in tension is attributable to cellular adaptations thatinclude a well-documented increase in oxidative capacity.

     

Normalized metabolic stress for 31P-MR spectroscopy studies of human skeletal muscle: MVC vs. muscle volume

Fowler, M. D., T. W. Ryschon, R. E. Wysong, C. A. Combs, andR. S. Balaban. Normalized metabolic stress for³¹P-MR spectroscopy studies ofhuman skeletal muscle: MVC vs. muscle volume. J. Appl.Physiol. 83(3): 875-883, 1997.A criticalrequirement of submaximal exercise tests is the comparability ofworkload and associated metabolic stress between subjects. In thisstudy, ³¹P-magnetic resonancespectroscopy was used to estimate metabolic strain in the soleus muscleduring dynamic, submaximal plantar flexion in which target torque was10 and 15% of a maximal voluntary contraction (MVC). In 10 healthy,normally active adults, (PCr + P_i)/PCr, where PCr isphosphocreatine, was highly correlated with power output normalized tothe volume of muscle in the plantar flexor compartment(r = 0.89, P < 0.001). The same variable was also correlated, although less strongly(r = 0.78, P < 0.001), with power normalized toplantar flexor cross-sectional area. These findings suggest thatcomparable levels of metabolic strain can be obtained in subjects ofdifferent size when the power output, or stress, for dynamic plantarflexion is selected as a function of plantar flexor muscle volume. Incontrast, selecting power output as a function of MVC resulted in apositive linear relationship between (PCr + P_i)/PCr and thetorque produced, indicating that metabolic strain was increasing ratherthan achieving constancy as a function of MVC. These findings providenew insight into the design of dynamic muscle contraction protocolsaimed at detecting metabolic differences between subjects of differentbody size but having similar blood flow capacity and mitochondrialvolume per unit of muscle.

     

Association between Vitamin D receptor polymorphisms and the rate of bone loss in elderly women-importance of adjusting for dietary and lifestyle factors

The association between the restriction length polymorphisms of the Vitamin D receptor (VDR) gene and the bone mineral density (BMD) or the rate of bone loss is still under debate. In a longitudinal study of untreated postmenopausal elderly women, we evaluated the relationship between the VDR gene polymorphisms (BsmI, TaqI, ApaI, and FokI) and the rate of bone loss over a 3-year period. We also examined the effect of adjustments for dietary and lifestyle factors on these associations. Before adjustments, the rate of femoral neck bone loss was - 3.76 +/- 1.58% in women with BB genotype and 0.45 +/- 0.65% in women with bb genotype, which was not significantly different. Upon adjustment for dietary and lifestyle factors, statistically significant (P = 0.03) bone loss was observed at femoral neck in women with BB genotype (- 3.66 +/- 2.44%) compared to that of bb genotype (2.39 +/- 1.32%). Similar results were observed with TaqI genotypes. The rates of bone loss at other skeletal sites were not different between VDR genotypes defined by BsmI and TaqI. VDR gene polymorphisms defined by ApaI and FokI were not related to the rate of bone loss.     

Efficiency of human skeletal muscle in vivo: comparison of isometric, concentric, and eccentric muscle action

Ryschon, T. W., Fowler, R. E. Wysong, A.-R. Anthony, and R. S. Balaban. Efficiency of human skeletal muscle in vivo: comparison of isometric, concentric, and eccentric muscle action. J. Appl. Physiol. 83(3): 867-874, 1997.The purpose of this study was to estimate the efficiency of ATPutilization for concentric, eccentric, and isometric muscle action inthe human tibialis anterior and extensor digitorum longus in vivo. Adynamometer was used to quantitate muscle work, or tension, whilesimultaneous ³¹P-nuclear magneticresonance data were collected to monitor ATP, phosphocreatine,inorganic phosphate, and pH. The relative efficiency of the actions wasestimated in two ways: steady-state effects on high-energyphosphates and a direct comparison of ATP synthesis rates with work. Inthe steady state, the cytosolic free energy dropped to the lowest valuewith concentric activity, followed by eccentric and isometric actionfor comparative muscle tensions. Estimates of ATP synthesis ratesrevealed a mechanochemical efficiency [i.e., ATP productionrate/work (both in J/s)] of 15.0 ± 1.3% in concentric and34.7 ± 6.1% in eccentric activity. The estimated maximum ATPproduction rate was highest in concentric action, suggesting anactivation of energy metabolism under these conditions. By using directmeasures of metabolic strain and ATP turnover, these data demonstrate adecreasing metabolic efficiency in human muscle action from isometric,to eccentric, to concentric action.