Porcine malignant hyperthermia: halothane effects on force generation in skeletal muscles.

Halothane-induced malignant hyperthermia (MH) is thought to result from a defect in the regulation of cytosolic calcium concentration in MH-susceptible (MHS) skeletal muscle. Such a defect might be expected to alter the time course of contractile responses. To test this hypothesis, isolated intact cell bundles from external intercostal and common digital extensor muscles of normal and MHS pigs were stimulated electrically to elicit twitch and tetanic tension in the presence and absence of halothane (2.5%). Time intervals measured for both twitches and tetani were (1) the latent period between the stimulus and tension increase, (2) the time to peak tension, and (3) the half-relaxation time. In contrast to previous reports, halothane had no effect on any measured time course parameter of twitches of either type of normal or MHS muscle, nor did the twitches of MHS and normal muscles differ in any parameter in the absence of halothane. However, the tetanic tension relaxation in both types of MHS muscle was markedly slowed by halothane, whereas in normal muscles there was little change. The slower rate of relaxation induced by halothane in MHS muscles suggests that halothane, either directly or indirectly, enhances the release or slows the removal of calcium in intact MHS muscles following maximal activation. This slowed tetanus relaxation could be of use in identification of MHS individuals.