P-31-MAGNETIC RESONANCE SPECTROSCOPY OF THE RABBIT MASSETER MUSCLE

Dynamic biochemical changes in the masseter muscle were studied in 14 New Zealand adult male rabbits by P-31-nuclear magnetic resonance (NMR ) spectroscopy. NMR spectra were obtained during rest and electrical s timulation of the muscle in the anaesthetized animal at 33 recording s essions. Electrical stimulation was applied by a pair of copper wires placed separately with hypodermic needles into the muscle. NMR spectra were acquired with a 2 x 3 cm, double-turn, copper transmit/receive c oil. Sixteen spectra were averaged over 30 s to obtain averaged spectr a continuously during a 30-min recording. The spectra were processed a utomatically using a non-linear 'least-squares' fitting program on the spectrometer. A Lorentzian line shape was assumed for the peaks, and values of peak height, area and chemical shifts were generated. Each a veraged spectrum consisted of five peaks: inorganic phosphate (P-i), c reatine phosphate (P-Cr), and three peaks related to ATP. Data were an alysed as to absolute changes in P-i and P-Cr, in the ratio of P-i/P-C r, and the shift of P-i to P-Cr, to estimate pH. Several protocols wer e used in which ranges of frequency, intensity and duration of electri cal stimulation were tested. The protocol for detailed studies involve d stimulating the muscle twice at 5 Hz for 3 min separated by a 3-min rest period, then stimulating twice at 50 Hz for 3 min separated by a rest period. During contraction of the muscle, there was a significant increase in the P-i/P-Cr, ratio (p < 0.05) as compared to the resting level. The ratio reached a plateau over a 3-min contraction using 5-H z stimulation, then increased significantly more with the 50-Hz stimul ation but decayed during the 3 min. Sustained stimulation with 50 Hz f or 15-45 min evoked an initial sharp change in P-i/P-Cr, which then re ached a steady plateau that remained over the entire stimulation. Thes e findings indicate that the rabbit masseter muscle is relatively fati gue resistant in maintaining a steady-state equilibrium in the relatio n of P-i to P-Cr.