EFFECT OF PHENYTOIN ON SKELETAL-MUSCLE FROM QUARTER HORSES WITH HYPERKALEMIC PERIODIC PARALYSIS

The contractile activity, the threshold for calcium-induced calcium re lease in fractions of sarcoplasmic reticulum and the potassium concent ration were determined in preparations of semimembranosus muscle from normal quarter horses and quarter horses with hyperkalaemic periodic p aralysis before and after they were treated with phenytoin. Before the treatment there was no difference in caffeine contracture or electric ally elicited twitch response between the two groups. For one week aft er the treatment, the time to peak tension of caffeine contractures wa s significantly (P<0.005) reduced in the horses with hyperkalaemic per iodic paralysis but unchanged in the normal horses. The variance but n ot the mean values for the threshold for Ca2+-induced Ca2+ release fro m the sarcoplasmic reticulum was greater for the horses with hyperkala emic period paralysis before but not after the treatment with phenytoi n.