THE EFFECT OF GLIBENCLAMIDE ON FROG SKELETAL-MUSCLE - EVIDENCE FOR K+(ATP) CHANNEL ACTIVATION DURING FATIGUE

1. The purpose of this study was to determine whether ATP-sensitive K (K(ATP)+) channels are activated and contribute to the decrease in fo rce during fatigue development in the sartorius muscle of the frog, Ra na pipiens. Tetanic force (elicited by field stimulation), action pote ntial and membrane conductance (using conventional microelectrodes), w ere measured in the presence and absence of glibenclamide, a K(ATP)+ c hannel antagonist. Experiments were performed in bicarbonate-buffered solutions at pH 7.2. 2. In unfatigued muscle 100 mumol l-1 glibenclami de had no effect on the resting potential, the overshoot, the half-dep olarization time or the maximum rate of depolarization of action poten tials, while the mean half-repolarization time increased by 19 +/- 4% (+/- S.E.M.) and the maximum rate of repolarization decreased by 17 +/ - 5%. 3. Fatigue was elicited using 100 ms tetanic contractions every 1 s for 3 min. In the absence of glibenclamide the mean half-repolariz ation time increased from 0.57 +/- 0.05 to 0.89 +/- 0.05 ms during fat igue. The mean half-repolarization times after fatigue, when muscle fi bres were exposed to 100 mumol l-1 glibenclamide either 60 min prior t o fatigue or 60 s before the end of fatigue, were 1.16 +/- 0.08 and 1. 17 +/- 0.07 ms respectively. Application of 100 mumol l-1 glibenclamid e after 5 min of recovery did not increase the half-repolarization tim e, but decreased the rate of recovery compared to control values. 4. I n unfatigued muscles, 100 mumol l-1 glibenclamide did not affect the t etanic contraction. In the absence of glibenclamide, the mean tetanic force after fatigue was 11.0 +/- 0.9% of prefatigue values. Applicatio n of 100 mumol l-1 glibenclamide 60 min before fatigue increased the r ate of fatigue development as the mean tetanic force was 4.8 +/- 0.8% after 3 min of stimulation. The addition of 100 mumol l-1 glibenclamid e 60 s before the end of fatigue had no effect on tetanic force during this time compared to control. 5. In the absence of glibenclamide, mu scles recovered 90.1 +/- 1.6% of their tetanic force after 100 min. Ad dition of 100 mumol l-1 glibenclamide 60 min prior to fatigue signific antly reduced the capacity of muscles to recover their tetanic force: after 100 min of recovery tetanic force was only 47.3 +/- 9.4% of the pre-fatigue value. Application of 100 mumol l-1 glibenclamide 60 s pri or to the end of fatigue had a much smaller effect on the recovery as 79.4 +/- 6.2% of the tetanic force was recovered in 100 min. Addition of glibenclamide after 5 min of recovery had no effect. 6. The results from this study support the proposal that K(ATP)+ channels are activa ted during fatigue and they contribute to the repolarization phase of the action potential. Although no evidence was found that activation o f K(ATP)+ channels during fatigue contributes to the force decrease du ring fatigue development, the impairment of force recovery following f atigue in the presence of glibenclamide supports the notion that K(ATP )+ channels play an important protective role.