SUMMARY1. To explore how maximum velocity of shortening (Vmax) of fibres varies within one muscle and how Vmax varies with body size, we measured Vmax of muscle fibres from soleus muscle of a large animal, the horse.2. Vmax was determined by the slack test on skinned single muscle fibres at 15 'C during maximal activation (pCa = 5 2). The fibre type was subsequently determined by a combination of single-cell histochemistry and gel electrophoresis of the myosin light chains.3. Vmax values for the type I, IIA and IIB muscle fibres were 0 33+0 04 muscle lengths/s (ML/s) (±S.E.M., n = 6), 1-33+008 ML/s (n = 7) and 3-20+0-26 ML/s (n = 6), respectively. It is likely that the large range in Vmax is due to differences observed in the myosin heavy chains and light chains associated with the three fibre types.4. Comparison of Vmax over a 1200-fold range (450 kg horse vs. 0-38 kg rat) of body mass (Mb) suggests that slow fibres scale more dramatically (Mb-018) than do fast glycolytic fibres . This difference may enable the slow fibres to work at high efficiencies in the large animal while the fast fibres can still generate a large mechanical power when necessary.

Results suggest that in horses with black walnut-induced laminitis, there is an early decrease in LMBF followed by reperfusion prior to onset of clinical signs. Treatment with glyceryl trinitrate after development of clinical signs of laminitis did not have a significant effect on LMBF.

Seventeen horses with a total of 18 ocular or periorbital squamous cell carcinomas were irradiated with Strontium-90 surface therapy, or Radon-222, Iodine-125, or Iridium-192 interstitial implants. Tumors were surgically cytoreduced prior to irradiation in all but two horses. Follow-up datawas obtained for 2 years in each horse. Two-year non-recurrence rates (no grossly visible evidence of tumor) were 87.5% for wSr beta therapy, and 60% (70% when corrected for non-tumor-related deaths) for interstitial implant therapy. Data regarding frequency of occurrence, location of lesions, age of onset, and non-recurrence rates were similar to previous literature reports utilizing variable or shorter follow-up periods. The effects of cytoreductive surgery of the tumors was not studied.

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Until now, there has been no reliable method for histochemical determination of fiber types of single skinned muscle fibers. The major problem arises from the fact that most histochemical techniques use cross-sections of a large group of fibers and compare a given fiber with those surrounding it. This is not possible with a single skinned fiber which has been separated from a bundle to be used for mechanical analysis. A further problem is that the skinning procedure itself may alter the staining pattern. We have developed a procedure by which multiple cross-sections of single skinned fibers can be exposed to various histochemical reactions and the staining patterns compared on the same slide to those of frozen muscle and skinned bundles. By this procedure, three fiber types were distinguished by both Ca2+-ATPase and SDH reactions. The fiber typings determined from both enzyme systems correlated well with each other. Although we were able to differentiate only between slow and fast fibers by SDS-PAGE, these results corroborated the histochemical classification. This procedure will clearly be useful in skinned single muscle fiber mechanics experiments performed to determine functional differences among fiber types.