Local activation of frog muscle fibres with linearly rising currents

Abstract

SUMMARY1. Single fast muscle fibres isolated from the semitendinosus muscles of the frog were locally activated by applying linearly rising current pulses to a pipette whose tip (diameter, 10-40,u) was in contact with the fibre surface.2. In normal or choline-Ringer solution with 1P8 mM-Ca, the threshold depolarization for producing a just perceptible local contraction was nearly constant over a wide range of the duration of linearly rising currents (0-5-20 sec or more).3. If [Ca]o was reduced to 0-25-0 1 mm, the threshold depolarization increased steeply with the increase in the duration of linearly rising currents, indicating a marked increase in the rate of an accommodation process in excitation-contraction coupling mechanism.

Activation of the contractile mechanism in the anterior byssal retractor muscle of Mytilus edulis.

“…2A) implies that the direct action of ACh on the internal structures, which is not mediated by membrane depolarization, may require some amount of bound Ca at or near the fibre membrane. Then, if it is assumed that the bound Ca is gradually removed during maintained depolarization as has been supposed for frog skeletal muscle fibres (Sugi, 1968) and squid axons (Frankenhaeuser & Hodgkin, 1957), the ineffectiveness of ACh in producing contracture tension in preparations depolarized with 200-300 mM-K (Fig. 6A) might result from the removal of the bound Ca necessary for the ACh-Ca release coupling, but not from the exhaustion of the stored Ca by high [K]0.…”

Inward spread of activation in frog muscle fibres investigated by means of high‐speed microcinematography

“…Since local membrane depolarizations of more than 100 mV tended to cause an irreversible reduction of membrane resistance (Strickholm, 1962;Sugi & Ochi, 1967b;Sugi, 1968), care was taken not to apply current pulses producing membrane depolarizations of more than 50-80 mV so that the surface membrane covered by the pipette remained intact during the experiments.…”

“…A limited area of surface membrane of the fibre was depolarized by applying rectanglular current pulses to a glass pipette (tip external diameter, 15-90 jsm) whose tip was in contact with the fibre surface, to make the interior of the pipette go negative with respect to the surrounding fluid (Huxley & Taylor, 1958;Strickholm, 1962;Sugi & Ochi, 1967a, b;Sugi, 1968). The contact resistance of the pipette, i.e.…”

“…With pipettes of 20-40 #tm diameter, the contraction was first initiated at the depolarized fibre surface (Sugi & Ochi, 1967b), and spread inwards with a velocity of 0-6-1 5 cm/sec. The threshold depolarization for producing a just perceptible contraction was 20-40 mV with current pulses of 100-500 msec duration (Sugi & Ochi, 1967b;Sugi, 1968). The velocity and extent of shortening and also the distance of inward spread of contraction were continuously graded according to the magnitude and duration of depolarization.…”

“…These findings may be taken to indicate that, at least in fibres which showed the phasic response, the T tubules have some kind of regenerating mechanism which contributes to the spread of depolarization along the T system in addition to the passive electrotonic spread. Then, the phasic nature of local contraction may well be accounted for by assuming that a brief, transient potential change analogous to the action potential at the surface membrane is set up in the T tubules by a moderate depolarization to cause a rapid phasic contraction; the rate of spontaneous relaxation of the phasic contraction is too rapid, and the magnitude of depolarization is too small to be explained by inactivation or accommodation process of excitation-contraction coupling (Hodgkin & Horowicz, 1960;Heistracher & Hunt, 1969;Sugi, 1968).…”