EFFECT OF MATURATION ON P-31 MAGNETIC-RESONANCE SPECTROSCOPY OF THE RABBIT MASSETER MUSCLE

This work studies the dynamic metabolic changes of the rabbit masseter muscle during post-natal development. The composition and proportion of oxidative and glycolytic muscle fibers alter during maturation. The masseter muscle, as most muscles of the craniofacial region, exhibits unusual development in composition of isoforms of myosin. The effect of this unusual composition on the dynamic metabolic properties of the masseter muscle have not been assessed. The metabolism of the rabbit masseter muscle was studied by means of P-31-nuclear magnetic resonanc e (NMR) spectroscopy. Contraction was elicited by electrical stimulati on of the muscle in the anesthetized animal. Five animals were studied at 8 weeks and 24 weeks so that both the juvenile and adult stages co uld be evaluated. The dynamic biochemical changes in the masseter musc le were studied by the analysis of NMX spectra. A single-turn surface coil (copper) was used, and the original signal was treated with Fouri er transforms to obtain P-31 spectra. The low signal-to-noise ratio re quired averaging 16 acquisitions (acquisition time = 400 msec, repetit ion rate = 1.8 sec) in 30 sec and then obtaining continuous spectra fo r 27 min. Each averaged spectrum demonstrated five peaks: inorganic ph osphate (Pi), creatine phosphate (PCr), and three peaks related to ade nosine triphosphate (Am). The protocol involved recording an initial t hree-minute rest period, stimulating the muscle at 5 Hz for 3 min twic e, separated by three-minute rest periods, and stimulating the muscle at 50 Hz twice for 3 min separated by rest periods. The Pi/PCr ratio i ncreased significantly in the adult masseter during both 5-Hz stimulat ions, evoking twitching, and the first 50-Hz stimulation, evoking teta ny (repeated ANOVA, P < 0.05). The resting pH (6.96 +/- 0.13) was sign ificantly lowered during both twitching (6.85 +/- 0.10; P < 0.0038) an d tetany (6.55 +/- 0.13; P < 0.0001), but only in the adult masseter m uscle. These findings suggest that the adult masseter muscle possesses more glycolytic fibers as it modifies its metabolism during postnatal development.