malignant hyperthermia ( mh ) is an autosomally inherited , potentially lethal myopathy with a heterogeneous etiology that is usually triggered by volatile anesthetics and depolarizing muscle relaxants .
it is widely accepted that susceptibility to mh is caused by abnormal ca metabolism within the skeletal - muscle fiber .
the site of the defect in mh appears to lie in the ca release mechanism of the sarcoplasmic reticulum ( sr ) in skeletal muscle , namely , in the complex of the dihydropyridine ( dhpr ) and ryanodine receptors ( ryr ) .  
ca 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 [ 47 ] .
the mode of action of methylxanthines such as caffeine and theophylline is concentration dependent . 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 ca 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 ] .  
theophylline ( 1,3-dimethylxanthine ) was shown to induce contracture development in porcine skeletal muscle in vitro .
furthermore , clear differences could be demonstrated between mhn and mhs muscle : mhs muscle developed significantly greater contractures compared to mhn specimens after bolus administration of 3  mmoll or 5  mmoll theophylline .
these results were comparable with in vitro contracture tests ( ivcts ) with caffeine and could be explained by the chemical analogies of both substances .
the purpose of the current study was to investigate the in vivo effects of theophylline in mhs and mhn swine .
after approval by the animal research committee of the university hospital hamburg - eppendorf , 6 mhn german landrace pigs weighing 3948  kg , aged 35 months , and 6 mhs pietrain pigs weighing 2633  kg , aged 35 months , from a special breeding farm ( research station thalhausen , technical university munich , germany ) were investigated .
prior to the study , in all animals genomic dna was isolated from blood preserved in ethylenediaminetetraacetic acid to check for the presence of the arg615-cys point mutation on chromosome 6 , indicating mh susceptibility .
general anesthesia was induced by administration of ketamine 10  mgkg intramuscularly ( ketavet , pharmacia & upjohn , erlangen , germany ) .
after insertion of a venous cannula into an ear vein , anesthesia was deepened with propofol 10  mgkg ( diprivan 2% , astra - zeneca , plankstadt , germany ) and fentanyl 10  gkg ( 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 ) .
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 ( 510  mlkgh 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  4  mmhg ; the body core temperature was adjusted to 37.5  0.2c 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 ) 1.0  mgkg was administered as an i.v .
subsequent doses were given every 10 minutes to reach cumulative doses of 3.5 , 8.5 , 18.5 , 33.5 , 53.5 , and 93.5  mgkg .
the clinical occurrence of mh was defined by the development of two of three conditions : central - venous pco2 75  mmhg , central - venous ph  7.20 , and an increase of intravascular body core temperature by 2.0c 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 .
statistical evaluation was performed using a computer - based program ( statview 4.57 , abacus concepts , berkeley , ca ) .
intergroup variations were calculated with the mann - whitney test ; intragroup differences were calculated with analysis of variance for repeated measures .
theophylline induced comparable clinical alterations in the anesthetized mhs and mhn swine , especially in regard to hemodynamic data . however , significant differences could not be found between mhs and mhn swine .
furthermore , clinical signs of muscular alterations , for example , general muscle rigidity , were not observed in any swine .
after cumulative administration of 93.5  mgkg , all animals died with tachycardia and hypotension followed by ventricular fibrillation .
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 ( 37.5c ( 36.638.6c ) ) and mhn swine ( 37.4c ( 35.837.8c ) ) as well as lactate levels of 1.8  mmoll ( 0.43.4  mmoll ) in mhs and 1.8  mmoll ( 0.86.5  mmoll ) in mhn animals remained unchanged .
the hr of all swine increased at cumulative doses of 93.5  mgkg theophylline ( figure 1(c ) ) , while the map decreased .
in mhs swine map decreased from 84.5  mmhg ( 60104  mmhg ) to 65.5  mmhg ( 4867  mmhg ) after administration of 93.5  mgkg theophylline . in mhn swine
the decrease was more marked , from 84  mmhg ( 6798  mmhg ) to 53.5  mmhg ( 5068  mmhg ) .  
there were no differences in blood theophylline concentrations between mhs and mhn swine . a mean blood concentration of 13.9  mgml ( 12.316.2  mgml ) theophylline was found after 30 minutes and a cumulative dose of 8.5  mgkg ( figure 1(d ) ) .
ten minutes after the last dose ( 93.5  mgkg ) the mean blood concentration was 122.4  mgml ( 99.3154.4  mgml ) .
the cytoplasmic ca concentration is regulated by a variety of intracellular second - messenger systems .
alterations in second - messenger systems , for example , the serotonin system or inositol polyphosphates , have been found to be associated with mh [ 2 , 3 ] .
higher cytoplasmic camp content and adenyl - cyclase activity have been found in mhs compared to mhn patients
. additionally , serum camp concentrations during and after physical exercise increased more and for longer time periods in mhs compared to mhn patients
. one mode of action of methylxanthines is the unspecific inhibition of pdes , which leads to enhancement of cytoplasmic camp .
furthermore , a mh - specific effect of the methylxanthine caffeine is well known and is used as a gold standard in the ivcts for the diagnosis of mh [ 10 , 13 ] .  
there are several case reports describing patients who developed mh - like symptoms without receiving the classic trigger substances such as succinylcholine or volatile anesthetics .
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 .
aminophylline induced mh - like symptoms in anesthetized rabbits , but a genetic basis of mh susceptibility in these animals was not examined in this study . in mhs swine , i.v . and oral administration of caffeine potentiated the development of mh crises induced by halothane and succinylcholine .
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 .
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 20.0  mgml .
after cumulative administration of 93.5  mg theophylline , the swine in our study died in hemodynamic decompensation .
the mean blood level measured at this time was 122.4  mgml(99.3154.4  mgml ) or 0.68  mmoll ( 0.550.86  mmoll ) theophylline .
however , in vitro contracture development in isolated porcine mhs muscle specimens started at concentrations of 3.0  mmoll 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 .
stress is a well - known trigger of mh in swine , whereas the role of stress in human mh is still unclear [ 9 , 18 ] .
considering the results of this study , theophylline might be a cofactor in the induction of mh in humans , but does not appear to be a primary trigger of mh .
 
 background . 
 theophylline was shown to induce contracture development in porcine malignant hyperthermia ( mh ) susceptible ( mhs ) skeletal muscles in vitro . 
 the purpose of the current study was to investigate the in vivo effects of theophylline in mhs and mh normal ( mhn ) swine . 
 methods . 
 mh - trigger - free general anesthesia was performed in mhs and mhn swine . 
 theophylline was administered intravenously in cumulative doses up to 93.5  mgkg1 . 
 the clinical occurrence of mh was defined by changes of central - venous pco2 , central - venous ph , and body core temperature . results . 
 theophylline induced comparable clinical alterations in the anesthetized mhs and mhn swine , especially in regard to hemodynamic data . 
 no pig developed hypermetabolism and/or mh according to defined criteria . 
 all animals died with tachycardia followed by ventricular fibrillation . 
 conclusions . 
 the cumulative theophylline doses used in this study were much higher than doses used therapeutically in humans , as demonstrated by measured blood concentrations . 
 theophylline is thus not a trigger of mh in genetically determined swine . 
1. Introduction
2. Methods
3. Results
4. Discussion