Qβ and Qγ components of intramembranous charge movement in frog cut twitch fibers

• Published in: The Journal of general physiology Vol. 98; no. 3; pp. 429 - 464
• Main Authors: CHIU SHUEN HUI, CHANDLER, W. K
• Format: Journal Article
• Language: English
• Published: New York, NY Rockefeller University Press 01.09.1991; The Rockefeller University Press
• Subjects: Animals; Anura; Biological and medical sciences; Electric Stimulation; electrophysiology; Fundamental and applied biological sciences. Psychology; membrane currents; Membrane Potentials; Models, Biological; Muscles - physiology; Ranidae - physiology; Saponins - metabolism; skeletal muscle; Striated muscle. Tendons; Time Factors; Vertebrates: osteoarticular system, musculoskeletal system; Depolarization; Vertebrata; Frog; Amphibia; Electrophysiology; Salientia; Muscle; Muscular fiber; Membrane potential; Charge movement
• ISSN: 0022-1295; 1540-7748
• DOI: 10.1085/jgp.98.3.429

Intramembranous charge movement was measured in frog cut twitch fibers mounted in a double Vaseline-gap chamber with a TEA.Cl solution at 13-14 degrees C in the central pool. When a fiber was depolarized from a holding potential of -90 mV to a potential near -60 mV, the current from intramembranous charge movement was outward in direction and had an early, rapid component and a late, more slowly developing component, referred to as I beta and I gamma, respectively (1979. J. Physiol. [Lond.]. 289:83-97). When the pulse to -60 mV was preceded by a 100-600-ms pulse to -40 mV, early I beta and late I gamma components were also observed, but in the inward direction. The shape of the Q gamma vs. voltage curve can be estimated with this two-pulse protocol. The first pulse to voltage V allows the amounts of Q beta and Q gamma charge in the active state to change from their respective resting levels, Q beta (-90) and Q gamma (-90), to new steady levels, Q beta (V) and Q gamma (V). A second 100-120-ms pulse, usually to -60 mV, allows the amount of Q beta charge in the active state to change from Q beta (V) to Q beta (-60) but is not sufficiently long for the amount of Q gamma charge to change completely from Q gamma (V) to Q gamma (-60). The difference between the amount of Q gamma charge at the end of the second pulse and Q gamma (-60) is estimated from the OFF charge that is observed on repolarization to -90 mV. The OFF charge vs. voltage data were fitted, with gap corrections, with a Boltzmann distribution function plus a constant. The mean values of V (the potential at which, in the steady state, charge is distributed equally between the resting and active states) and k (the voltage dependence factor) were -59.2 mV (SEM, 1.1 mV) and 1.2 mV (SEM, 0.6 mV), respectively. The one-pulse charge vs. voltage data from the same fibers were fitted with a sum of two Boltzmann functions (1990. J. Gen. Physiol. 96:257-297). The mean values of V and k for the steeply voltage-dependent Boltzmann function, which is likely to be associated with the Q gamma component of charge, were -55.3 mV (SEM, 1.3 mV) and 3.3 mV (SEM, 0.6 mV), respectively, similar to the corresponding values obtained with the two-pulse protocol.