Plasticity of craniomandibular muscle function: 31P magnetic resonance spectroscopy of the rabbit masseter muscle.

The masseter muscle was studied during postnatal development of the rabbit from the juvenile to adult stage in which the oral function was altered during maturation by modifying the diet to soft food. The muscle was assessed using phosphate magnetic resonance (31P NMR) spectroscopy with a single-turn copper surface coil to study potential changes in phosphate metabolism. The 31P NMR spectra consisted of five peaks related to unbound forms of inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to the adenosine triphosphate (ATP). The masseter was assessed in one group of five rabbits at 8 weeks postnatally (juvenile) and after 4 months of this experimental group masticating on soft food. They were compared with a control group of five rabbits raised on a normal hard diet. The Pi/PCr ratio increased in the adult masseter much higher during twitching, tetany, and periodic contraction than in the juvenile regardless as to whether the adult animal had been raised from the juvenile period on soft or hard diet. There were relatively few differences between the experimental adult animals raised on a soft diet and the normal adult animals despite the soft diet animals demonstrating a significantly lower weight and smaller muscle mass. These findings suggest that chronic underuse of the masseter muscle by decreasing the masticatory loads has a minimal effect on the phosphate metabolism of the maturing masseter.