Effects of Fatiguing Stimulation on Intracellular Na+ and K+ in Frog Skeletal Muscle
Format of Original
American Physiological Society
Journal of Applied Physiology
Original Item ID
The purpose of this study was to describe the alterations in the intracellular concentrations of sodium ([Na+]i) and potassium ([K+]i) and the membrane potential (Em) as a result of fatiguing stimulation of the frog semitendinosus muscle and to relate these changes to the alterations in the sarcolemma action potential and force-generating ability of the muscle. [Na+]i and [K+]i were measured by using ion-selective microelectrodes. Before stimulation (100-ms trains at 150 Hz, 1 stimulus/s for 5 min), [Na+]i, [K+]i, and Em were 16 ± 1 mM, 142 ± 5 mM, and -83 ± 1 mV, respectively. As a result of stimulation, [Na+]i rose to 49 ± 6 mM and recovered to 16 ± 2 mM with a time constant (τ) of 70 s·[K+]i fell to 97 ± 8 mM as a result of stimulation, then recovered to 148 ± 5 mM with τ= 56 s. Em depolarized to -74 ± 3 mV then recovered to -83 ± 2 mV with τ= 53 s. The Na+/K+ permeability ratio of the resting membrane fell 3%, whereas at the peak of the action potential the permeability ratio fell 38%. A previous study using the same muscle and stimulation protocol showed force to recover with a fast initial phase (~2 min) and a much slower second phase (~50 min). The recovery of [Na+]i, [K+]i, and Em was similar to the fast phase of force recovery; thus the altered Na+ and K+ concentration gradient across the sarcolemma and t-tubular membrane may contribute to this component of fatigue. The possible fatigue mechanisms induced by the altered ionic gradients include 1) complete block of the action potential propagation; 2) depolarization-induced inactivation of t-tubular charge movement; and 3) a reduced magnitude of the t-tubular charge due to the lower action potential spike potential.
Balog, Edward Michael and Fitts, Robert H., "Effects of Fatiguing Stimulation on Intracellular Na+ and K+ in Frog Skeletal Muscle" (1996). Biological Sciences Faculty Research and Publications. 455.