THE EFFECT OF ISOFLURANE AND TEMPERATURE ON THE ACTIONS OF MUSCLE-RELAXANTS IN RAT IN-VITRO

Hypothermia and isoflurane alone increase the potencies of steroidal m uscle relaxants (MRs). We studied the combined influence of isoflurane and hypothermia on MR potency. Phrenic nerve-hemidiaphragm preparatio ns of rats were mounted in modified Krebs' solution and aerated with 5 % CO2-95% O-2 gas mixture at 37 degrees C and 4% CO2 at 27 degrees C t o maintain the CO2 content constant. Phrenic nerves were stimulated wi th 0.1 Hz supramaximal impulses and elicited tension of the diaphragm was recorded. Isoflurane 1% was added after stabilization of twitch te nsion and MR was added 60 min later. Twitch tension was reduced by 20% +/- 2.5% at 37 degrees C and 3.5% +/- 0.7% at 27 degrees C from contr ol with only isoflurane. The IC50 (inhibitory concentration, 50%) valu es of the MRs decreased significantly (P (P < 0.05) with isoflurane at both temperatures. The ratios of the IC50 values without and with isof lurane of the benzylisoquinolinium MRs were significantly more at both temperatures (P < 0.05) indicating the enhancement of potentiation of their action by isoflurane over steroidal MRs. When the soluble conce ntration of isoflurane at 27 degrees C was kept similar to that of at 37 degrees C, the ratios of all the MRs were reduced significantly fro m the ratios at 37 degrees C, indicating a reduction of potentiation. When the partial pressure of isoflurane was kept constant at 37 degree s C and 27 degrees C, the potentiating action of the MRs by isoflurane was similar. But when the partial pressure was deceased to keep the c oncentration of isoflurane constant, the potentiation was reduced. The difference suggests that the partial pressure is a determinant of iso flurane's neuromuscular effect which is attenuated by hypothermia.