Properties of fibres, endplates and acetylcholine receptors in the diaphragm, masseter, laryngeal, abdominal and limb muscles in the goat.

PURPOSE

Although differences in fibre composition, fibre size or acetylcholine receptor (AChR) density between muscles have often been proposed to explain the unequal sensitivities of muscles to muscle relaxant drugs, it is not clear whether or how these parameters differ among muscles or are related to one another, In this study, several muscles were examined to determine the composition and cross-sectional area (CSA) of types I and II fibres, the surface area of their motor endplates (ESA), and their AChR density.

METHODS

Biopsies from the thyroarytenoideus, cricoarytenoideus dorsalis, masseter, diaphragm, transversus abdominis, rectus abdominis, gastrocnemius and soleus muscles of goats were processed by muscle histochemistry and morphometry and the ESA:CSA ratio was computed. The number and density of AChRs per endplate were estimated by 125I-alpha-bungarotoxin binding studies.

RESULTS

The mean type I fibre composition (range: 0-100%), fibre diameter (28-50 microns) and the ESA:CSA ratio (0.27-1.01) differed among muscles (P = 0.0001), but there were no significant differences (P > 0.05) in the mean endplate size (577-725 microns 2) AChR number (6.6-14.5 x 10(6)) or AChR density (8,900-22,300 microns-2) probably because of marked individual variations. Fibre size increased and the ESA:CSA ratio decreased in the order laryngeal, diaphragm, jaw, limb and abdominal muscles.

CONCLUSION

It is concluded that between muscles fibre size varies more than endplate size or AChR number.