Halothane sensitivity of young pigs in vivo and in vitro

Piglets less than 8 wk of age that are known by genotype to be malignant hyperthermia-susceptible (MHS) do not usually develop characteristic hyperthermia and limb muscle rigidity in response to a brief halothane exposure (5 min of 3%). To determine whether a malignant hyperthermia (MH) episode could nevertheless be provoked by a more rigorous challenge, both genetically MHS (Pietrain) and normal (Yorkshire) 5-wk-old piglets were exposed to a combined halothane-succinylcholine challenge. Only two of eight MHS piglets developed limb rigidity; however, all MHS piglets (and no normal piglets) developed clinical signs of MH episode initiation during the 30-min challenge. Temperatures rose from 37.4 to 38.6 degrees C in MHS piglets while falling slightly in normal piglets. In MHS piglets, venous pH fell from 7.46 +/- 0.02 to 6.88 +/- 0.07, PVCO2 rose from 36 +/- 2 to 126 +/- 17 mmHg, and plasma concentration of K+ rose from 4.0 +/- 0.1 to 7.1 +/- 0.6 mM, whereas all values remained stable in normal piglets. Muscles removed from the same piglets before the halothane-succinylcholine challenge were exposed to halothane in vitro. The muscles from genetically MHS piglets responded to halothane with characteristic depression of tetanic tension and prolonged tetanus relaxation time but did not develop halothane-induced contractures. We conclude that, in the absence of either halothane-induced limb rigidity or in vitro contractures, these young animals were still susceptible to potentially fatal MH episodes on exposure to appropriate triggering agents. The MH defect is apparently partially masked in piglets and expressed fully only in older pigs.