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Understanding the evolution of phenotypic plasticities and the connections among the environment, genotype, and phenotype requires detailed understanding of the proximate mechanisms regulating morphological differences between phenotypes. Spea multiplicata tadpoles can develop into two different phenotypes, i.e. carnivores and omnivores, which differ in many morphological and behavioral traits. One of the major differences is enlargement of the jaw and tail musculature in carnivores relative to those of omnivores. We investigated pattern of muscle enlargement by measuring differences in myofiber number and cross-sectional area between the phenotypes during early and mid-development. The data show that both hyperplasia and hypertrophy underlie the carnivores' enlargement of both the orbitohyoideus jaw muscle (OH) and the tail muscle (TL). Carnivores had more OH and TL myofibers than did omnivores at all ages, but the rate of myofiber addition differed, by approximately 9 and 17 myofibers per day respectively. Carnivores also had larger OH and TL myofibers than did omnivores, at many of the ages studied, and the rate of myofiber cross-sectional area increase (log-transformed myofiber cross-sectional area plotted against age in days) was significantly greater for carnivores than for omnivores in the internal, but not the peripheral, regions for both the OH and TL muscle., (2009 Wiley-Liss, Inc.)

Measurements of the intracellular diffusion coefficients (Di) of ATP and creatine phosphate (PCr) in stable, isolated preparations of skeletal muscle were made by means of pulsed field gradient (PFG) 31P NMR. Experiments used a PFG NMR probe specifically designed for small, superfused biological samples. This provided a magnetic field gradient in the z axis of up to 195 G/cm with minimal eddy currents. DiATP and DiPCr in white (fast, glycolytic) skeletal muscle from goldfish (Carassius auratus) were determined to be 2.48 +/- 0.33 and 3.49 +/- 0.33 x 10(-6) cm2/s, respectively, at 25 degrees C and a diffusion time of approximately 19 ms. For comparison with Di values, diffusion coefficients of ATP and PCr also were measured in solutions of ionic composition similar to that of fish muscle cytosol. The in vitro diffusion coefficients of ATP and PCr were 3.54 +/- 0.11 and 5.28 +/- 0.08 x 10(-6) cm2/s, respectively, at 25 degrees C.

Mice were treated for 7-12 wk with the creatine analogue beta-guanidinopropionic acid (beta-GPA). Treatment reduced total creatine to approximately 5% of control values in soleus (SOL) and extensor digitorum longus (EDL) muscles. In both muscles from treated mice, phosphorylated beta-GPA accumulated and resting [ATP] decreased by approximately 50%. Relative to controls, cytochrome oxidase and citrate synthase activities increased significantly in EDL from treated mice, but not in SOL; creatine kinase activity decreased significantly in SOL, but not in EDL. Measurements of poststimulation energy metabolism show that the energy cost to maintain tension in SOL and EDL from treated mice was approximately 50% of that in control muscle. Relative to controls, first-order rate constants of poststimulation O2 demand were 2- and 3.6-fold greater in SOL and EDL, respectively, from treated mice. Increased economy of SOL and EDL from treated mice is consistent with previously reported changes in myosin isoenzymes. Increases in rate constants of O2 utilization in creatine-depleted muscle are inconsistent with the hypothesis that cytoplasmic or mitochondrial creatine kinase is rate limiting for cellular respiration.

Phosphorus NMR spectroscopy and HPLC analyses were made on isolated rat and mouse muscles selected for different volume fractions of the major known fiber types. We tested the hypothesis that muscle cell types at rest have intrinsically different contents of PCr, ATP and Pi. The Pi content was low and the PCr and ATP contents were high in muscles with large contents of type 2b and 2a fibers, and vice versa in muscles with large volume fraction of types 1 and 2x fibers. From the profile of these metabolites we could distinguish only two classes of fibers in the murine muscles and predict well the composition of cat muscles. For the first class, types 2a and 2b fibers, the intracellular concentrations were: ATP 8 mM; total Cr 39 mM; PCr 32 mM; Pi 0.8 mM; ADP 8 microM. For the second class, type 1 and 2x fibers, these quantities are: ATP 5 mM; TCr 23 mM; PCr 16 mM; Pi 6 mM; ADP 11 microM. Thus our results establish a new and apparently general criterion upon which to distinguish skeletal muscle cells, one based on the resting content of bioenergetically important metabolites.

We tested the proposition that muscle cell types have different contents of phosphocreatine (PCr), ATP, and Pi by 31P NMR spectroscopy and HPLC analyses of adult rat and mouse muscles containing various volume fractions of different fiber types. There was a 2-fold difference in the PCr content between muscles with a high volume fraction of fiber types 1 and 2x versus those with fast-twitch (types 2a and 2b) fiber types. Pi content was low, and PCr and ATP contents were high in muscles with large contents of type 2b and 2a fibers; the reverse was true in muscles with a large volume fraction of type 1 and 2x fibers. There is a large range in the Pi/PCr ratios in normal resting muscles, from less than 0.05 in type 2 to 0.51 in type 1 fibers, depending upon the distribution of their component fiber types. In all muscles, the peak area resulting from the beta phosphate of ATP constituted approximately 13% of the sum of all peak areas observable in the 31P spectrum. Fiber types 2a and 2b were not distinguishable, and the content of type 2x fibers was similar to type 1 fibers. From the profile of these metabolites, we could distinguish only two classes of fibers. For type 2a and 2b fibers, the intracellular concentrations were 8 mM ATP, 39 mM total creatine, 32 mM PCr, 0.8 mM Pi, and 8 microM ADP. For type 1 and 2x fibers, these quantities were 5 mM ATP, 23 mM total creatine, 16 mM PCr, 6 mM Pi, and 11 microM ADP. Thus our results establish an additional criterion upon which to distinguish skeletal muscle cells, one based on the resting content of bioenergetically important metabolites. These results also provide the basis for estimating skeletal muscle fiber-type composition from noninvasive NMR spectroscopic data.

A creatine analogue, beta-guanidinopropionic acid (beta-GPA), was administered in the food (2% wt/wt) and the water (0.5% wt/vol) of male CD-1 mice. Uptake of the phosphorylated analogue and depletion of phosphocreatine in hindlimb muscle was monitored by 31P nuclear magnetic resonance and was found to be complete within 7 wk. After this time, the isomyosin composition of soleus, extensor digitorum longus (EDL), and ventricle was analyzed by pyrophosphate gel electrophoresis. The analogue was found to induce significant alterations in the type of myosin expressed in soleus and EDL. Normal soleus contains both intermediate (IM) and slow (SM) myosins, and treatment reduced the relative content of IM by approximately 50%. In EDL, treatment decreased fast isomyosin FM3 by 60% compared with controls. Sodium dodecyl sulfate-gel electrophoresis also showed a decrease of parvalbumin in EDL by approximately 50%. Treatment had no significant effect on the isomyosin composition of heart ventricle. Levels of physical activity and concentrations of serum glucose and thyroxine of treated mice were not significantly different from controls. These results indicate a role for intracellular energetics in mediating adaptive changes in the phenotype of muscle in mature animals.

The effects of acute and long-term changes in temperature upon catalytic and calcium regulatory function of red (slow oxidative) and white (fast glycolytic) muscle from striped bass (Morone saxatilis) were determined. Acclimation to 5 degrees C or 25 degrees C had no significant effect on catalytic function (ATPase activity) or regulatory sensitivity (Ca++-activation) of myofibrils from either muscle type. Substantial differences between red and white muscle were found in the intrinsic thermal sensitivity of maximally-activated Mg++-Ca++ myofibrillar ATPase. Arrhenius plots of myofibrillar ATPase from white muscle show one significant breakpoint at 29 degrees C, with activation energies (Ea) of 2.3 and 23.4 kcal mole-1 at temperatures above and below this transition, respectively. Arrhenius plots of myofibrillar ATPase from red muscle show two transitions occurring at 22 and 9 degrees C, with Ea of 7.6 kcal mole-1 above 22 degrees C and 18.3 kcal mole-1 between 9 and 22 degrees C. Activation energies for myofibrils from red muscle increase substantially to approximately 107.3 kcal mole-1 below the 9 degrees C breakpoint. Differences in the intrinsic thermal sensitivity of red and white muscle catalytic function are apparently due to interaction of actomyosins and calcium regulatory proteins which are specific to each muscle type. The results suggest that capacity for sustained swimming in striped bass, which is powered exclusively by red muscle, will be severely impaired at cold temperature unless compensations occur above the level of contractile proteins.