THE EFFECT OF K-PIPIENS( ON THE RECOVERY OF THE TWITCH AND TETANIC FORCE FOLLOWING FATIGUE IN THE SARTORIUS MUSCLE OF THE FROG, RANA)

The goal of this study was to investigate how an increase in the extra cellular K+ (K-0(+)) concentration immediately after fatigue affects t he recovery of the resting potential, the twitch and tetanic contracti on of frog sartorius muscle to further understand the role of K+ in th e mechanism of fatigue. Resting potentials were measured with conventi onal microelectrodes. Twitch and tetanic contractions were elicited by field stimulation. All muscles were fatigued with tetanic contraction s at a rate of one contraction per second for 3 min while being expose d to 3 mmole l(-1) K-0(+). During fatigue development the resting pote ntial decreased by 16 mV (control group and pH(0), 7.2, extracellular pH), while the decrease in the twitch force was 32.8%, compared to 79. 3% for the tetanic force, and 84.6% for the maximum rate of force deve lopment of the tetanus. Fatigued muscles were also unable to maintain a plateau phase during a tetanus: force declined by 14.8% during this phase. During the recovery period under control conditions (3 mmole l( -1) K-0(+)), all four parameters returned to their pre-fatigue values, the recovery of the plateau phase was the fastest (10 min), while tha t of the twitch force was the slowest (80 min). When K-0(+) was increa sed to 7.5 or 9.5 mmole l(-1) immediately after fatigue, the recovery rate of the tetanic force and plateau phase was reduced. The maximum r ate of force development of the tetanus, however, recovered at a faste r rate than control muscles. The recovery of the twitch force was also increased above that of control when K-0(+) was increased to 9.0 mmol e l(-1) (a concentration which maximally potentiates the twitch force of unfatigued muscle). Frog sartorius muscles were also tested at pH(0 ) 6.4, a pH(0) which inhibits force recovery. At that pH(0) the effect s of K-0(+) were similar to those observed at pH(0) 7.2. It is conclud ed that the role of K+ in muscle fatigue is more complex and may not i nvolve just a contribution to the decrease in force during fatigue dev elopment, but may also contribute to an increase in force development under some conditions.