Ca²⁺ homeostasis in skeletal muscle is regulated by a variety of intracellular second-messenger systems. Alterations in some second-messenger systems have been found to be associated with MH [2, 3]; the cyclic AMP (cAMP) system also seems to be affected in MH. In skeletal-muscle cells from MH-susceptible (MHS) patients and animals, higher cAMP levels were measured compared to normal (MHN) subjects [4–7].

The mode of action of methylxanthines such as caffeine and theophylline is concentration dependent [8]. While they act by adenosine-receptor antagonism at lower concentrations, at higher concentrations they are also unspecific phosphodiesterase (PDE) inhibitors. PDE inhibitors produce a receptor-independent increase in Ca²⁺ release from the SR via the RyR in cardiac muscle by decreasing the rate of cAMP degradation. This effect could also be demonstrated in skeletal muscle as an increase in isometric contraction after application of PDE inhibitors [9, 10].

Swine were fasted overnight with free access to water. General anesthesia was induced by administration of ketamine 10mg·kg⁻¹ intramuscularly (Ketavet, Pharmacia & Upjohn, Erlangen, Germany). After insertion of a venous cannula into an ear vein, anesthesia was deepened with propofol 10mg·kg⁻¹ (Diprivan 2%, Astra-Zeneca, Plankstadt, Germany) and fentanyl 10μg·kg⁻¹ (Fentanyl-Janssen, Janssen-Cilag, Neuss, Germany) intravenously (i.v.). After tracheotomy and intubation, the lungs were mechanically ventilated with an air/oxygen mixture (fraction of inspired oxygen (FiO₂) 0.4). Anesthesia was maintained with propofol 10mg·kg⁻¹·h⁻¹ and fentanyl 50μg·kg⁻¹·h⁻¹. Neuromuscular blocking drugs were not administered. A multilumen central-venous line was inserted into the right internal-jugular vein. One lumen was used for withdrawal of blood samples and measurement of central-venous pressure (CVP), the second for administration of theophylline and fluid infusion (5–10mL·kg⁻¹·h⁻¹ Ringer's solution), and the third for administration of anesthetics. Cannulas were inserted into both femoral arteries: one was used for withdrawal of blood samples, the other for continuous measurement of arterial pressure and body core temperature (PiCCO, Pulsion Medical Systems, Munich, Germany). Normothermia was maintained by forced-air warming, and rectal and intravascular body temperatures were measured continuously.

A blood-gas analyzer (ABL625, Radiometer, Copenhagen, Denmark) was used for monitoring arterial and venous oxygen saturation, oxygen partial pressure (pO₂), carbon dioxide partial pressure (pCO₂), pH, and potassium and lactate levels. Mechanical ventilation was adjusted to maintain venous pCO₂ at 46 ± 4mmHg; the body core temperature was adjusted to 37.5 ± 0.2°C before the experiment was started. Once a steady state was achieved for at least 30 minutes, baseline values were recorded for all variables.

Theophylline (Bronchoparat, Klinge Pharma, Munich, Germany) 1.0mg·kg⁻¹ was administered as an i.v. bolus. Subsequent doses were given every 10 minutes to reach cumulative doses of 3.5, 8.5, 18.5, 33.5, 53.5, and 93.5mg·kg⁻¹. The clinical occurrence of MH was defined by the development of two of three conditions: central-venous pCO₂ ≥75mmHg, central-venous pH ≤ 7.20, and an increase of intravascular body core temperature by ≥2.0°C as measured using the PiCCO system.

During the experiments, hemodynamic variables (heart rate (HR), mean arterial pressure (MAP), CVP, cardiac output), end-tidal CO₂ concentration (etCO₂), rectal and intravascular body temperature (°C), blood-gas concentrations (O₂ saturation (SaO₂), pCO₂, pH), and lactate levels were measured every 5 minutes. Every 10 minutes, blood samples were taken for gas-chromatographic measurement of theophylline concentrations. After all experiments were completed, the pigs were killed using 10% magnesium chloride solution i.v.

The pCO₂ and venous pH did not change after administration of theophylline (Figures 1(a) and 1(b)). The baseline body temperatures of the MHS (37.5°C (36.6–38.6°C)) and MHN swine (37.4°C (35.8–37.8°C)) as well as lactate levels of 1.8mmol·L⁻¹ (0.4–3.4mmol·L⁻¹) in MHS and 1.8mmol·L⁻¹ (0.8–6.5mmol·L⁻¹) in MHN animals remained unchanged. The HR of all swine increased at cumulative doses of 93.5mg·kg⁻¹ theophylline (Figure 1(c)), while the MAP decreased. In MHS swine MAP decreased from 84.5mmHg (60–104mmHg) to 65.5mmHg (48–67mmHg) after administration of 93.5mg·kg⁻¹ theophylline. In MHN swine the decrease was more marked, from 84mmHg (67–98mmHg) to 53.5mmHg (50–68mmHg).

There are several case reports describing patients who developed MH-like symptoms without receiving the classic trigger substances such as succinylcholine or volatile anesthetics [1]. However, MH symptoms are largely unspecific. The clinical presentation of theophylline intoxication can be very similar to MH, for example, with tachycardia, hyperthermia, dehydration, convulsions, and coma as well as skeletal-muscle cramps and rhabdomyolysis. In individual cases, hypermetabolic theophylline overdoses have been treated successfully with dantrolene [14]. Therefore, a possible trigger mechanism of theophylline for MH was suggested [15].

The MH trigger potential of methylxanthines has been tested in several animal studies. Aminophylline induced MH-like symptoms in anesthetized rabbits, but a genetic basis of MH susceptibility in these animals was not examined in this study [16]. In MHS swine, i.v. and oral administration of caffeine potentiated the development of MH crises induced by halothane and succinylcholine [17]. It was therefore suggested that caffeine may reduce the threshold of MH trigger agents. Furthermore, the detection of specific in vitro effects of theophylline in porcine skeletal-muscle preparations supported the hypothesis of an MH trigger potential for methylxanthines [11].

In this study, theophylline was not a trigger of MH in susceptible swine. No significant differences between MHS and MHN swine or symptoms of hypermetabolism could be found during cumulative administration of theophylline. The therapeutic blood level of theophylline in humans is estimated at between 8.0 and 20.0mg·mL⁻¹. After cumulative administration of 93.5mg theophylline, the swine in our study died in hemodynamic decompensation. The mean blood level measured at this time was 122.4mg·mL⁻¹(99.3–154.4mg·mL⁻¹) or 0.68mmol·L⁻¹ (0.55–0.86mmol·L⁻¹) theophylline. A direct comparison between in vivo blood concentrations and concentrations in IVCTs is impossible. However, in vitro contracture development in isolated porcine MHS muscle specimens started at concentrations of 3.0mmol·L⁻¹ theophylline. Therefore, it could be speculated that the in vivo theophylline concentrations in this study were high enough to kill the animals, but too low to trigger MH.