Porcine malignant hyperthermia: genotype and contractile threshold of immature muscles.

We investigated whether malignant hyperthermia (MH)-related contractile abnormalities, such as lowered contractile threshold, were expressed in MH-susceptible (MHS) immature muscles and myotubes. Muscles from neonatal piglets homozygous for Arg615 (normal) or for Cys615 (MHS) ryanodine receptor alleles, and heterozygotes were used. Intact cell bundles from piglet muscles generally were similar in contractile properties to adult muscles of the same genotype. Thresholds for K contractures in normal, heterozygous, and MHS piglet muscles (40 mmol/L, 25 mmol/L and 15 mmol/L K+, respectively) differed significantly. Cultured myotubes were subjected to a series of square pulses of varying strengths (-50 to +50 mV) and durations (25-300 ms) using whole cell patch-clamp techniques. Threshold for contraction differed significantly among the three genotypes, for example, with 300 msec pulses thresholds were -6.9 +/- 0.9, -12.4 +/- 1.6, and -22.6 +/- 2.6 mV for normal, heterozygous and MHS myotubes, respectively. Thus a significantly lower than normal threshold for contraction was expressed in MHS and heterozygous piglet muscles and myotubes. Further, these developmentally immature preparations are likely to express other differences characteristic of adult MHS muscles, and thus provide suitable preparations for clinically relevant studies of MH-related cellular abnormalities.