Membrane potential, contractile activation and relaxation rates in voltage clamped short muscle fibres of the frog.

Abstract

SUMMARY1. Voltage clamped short (a 15 mm) muscle fibres of the frog can develop maximum tension of 4-3 kg/cm2.2. The time course of contractile responses to prolonged depolarization is markedly dependent on the fibre membrane potential. With sufficiently long pulses the responses present a plateau and a spontaneous relaxation phase.3. At room temperature (20-22 TC), and at membrane potentials of -10 mV the plateau duration is about 2 see and the spontaneous relaxation rate is 0 50 sec-'. At membrane potentials of -35 mV the plateau duration is 4x6 see and the spontaneous relaxation rate is 0x28 sec-'.4. When the fibres are depolarized at room temperature with relatively short pulses ( < 2 see), the contractile responses are cut short at the end of the pulse, and the fibres relax at a rate (11 sec-') which is independent on the pulse amplitude and duration.5. The relaxation rate after a short pulse can be affected by membrane potential only in the region near the contractile threshold where further release of contractile activator is expected to occur.6. The time course of contractile responses to prolonged depolarization may be shortened by conditioning depolarization.7. The system responsible for the release of contractile activator may be tentatively described by three states, resting, activated, and inactivated, in analogy with the model proposed by Chandler, Rakowski & Schneider (1976) to describe the possible configurations of the potential dependent charge movement in muscle.

Effects of external calcium reduction on biphasic potassium contractures and action of divalent cations on the calcium reduction in frog single twitch muscle fibers.

“…3c and 4c) may be explained from the viewpoint of the biphasic K contractures; such different effects may be due to the difference between the actions of these divalent cations on the activation mechanism of the initial component and those of the secondary component. CAPUTO (1972CAPUTO ( , 1981 and CAPUTO and FERNANDEZ DE BOLANOS (1979) have proposed a model to interpret the peak tension and the time course of K contractures in terms of an activation and an inactivation mechanism (Caputo model). LUTTGAU and SPIECKER (1979) and COTA and STEFANI (1981, 1982 explained the effect of external Cat + reduction and the effect of Mgt + on the Cat reduction according to this model.…”

Asymmetrical charge movement in slow‐ and fast‐twitch mammalian muscle fibres in normal and paraplegic rats.

“…There are no descriptions of charge movement or mechanical activation in the different twitch fibre types. However, it has been found that the voltage dependence of tension is similar in slow tonic fibres and twitch fibres (Gilly & Hui, 1980a;Caputo & de Bolanos, 1979; Hodgkin & Horowicz, 1960;Nasledov, 1969). The voltage dependence of charge movement is also similar in slow tonic and twitch fibres, but the maximum charge movement in the tonic fibres is one quarter to one third of that in the twitch fibres (Gilly & Hui, 1980b).…”

Involvement of Mg2+ in terminating Ca2+ release in cultured rat skeletal muscle

“…It is now functionally well established that the Ca 2÷ release channel responsible for this process is the ryanodine receptor (RyR) [2,3]. Earlier studies on depolarization-induced Ca 2+ release (or contraction) showed that Ca 2÷ release is turned off by membrane repolarization [4][5][6][7][8], and based on this evidence the rate of Ca 2÷ release was estimated. Since Ca 2+ release flux started to decay within a millisecond of repolarization, there seemed to be a direct mechanical coupling between the voltage sensor and the release channel [5,9].…”