Membrane depolarization as a cause of tension development in mammalian ventricular muscle

294

112

1. Experiments were performed to determine whether intercalated disks represent a high or a low resistance to the diffusion of K ions. 2. Bundles of sheep ventricular fibres, 0·6‐0·8 mm thick and 7‐11 mm long, were pulled through a hole in a partition. One half was charged by radiopotassium (K*); the other half was washed by inactive Tyrode solution. 3. A steady state with respect to the distribution of K* along the bundle was reached 6 hr after the exposure to radioactivity. 4. On the ‘washed’ side [K*] fell along a relatively smooth curve and did not tend to the value zero towards the cut end of the bundle. The average space constant for the decrease of [K*] was 1·55 mm, corresponding to a distance of about 12 × the length of a cell. This indicates that the disk resistance is low and that the fibres are healed over at their cut ends. 5. Rhythmical contraction had no detectable effect on the space constant. This may mean either that shortening and thickening of a cell does not effectively mix its content, or that stirring goes on even in resting cells. 6. On the assumption of mixing between disks the average resistance of 1 cm2 of disk to the movement of K ions is 3 Ω. If there is no mixing this value would be even lower. 7. The assumption that K* distribution within the ‘washed’ half of the bundle is a consequence of cell‐to‐cell rather than of extracellular movement is supported by the results of several control experiments. 8. It is concluded that the propagation of the cardiac action potential is possible by local circuit currents.

Section: Discussionmentioning

confidence: 64%

The diffusion of radiopotassium across intercalated disks of mammalian cardiac muscle

Weidmann¹,

Hodgkin²

1966

294

112

“…Further prolongation of the pulse duration, however, evoked a second contractile response which relaxed only after repolarization (cf. Kavaler, 1959;Morad & Trautwein, 1968;Reuter & Scholz, 1968). Wood et al (1969) have shown that in sheep trabeculae the second contractile response may strongly influence the twitch tension during the following depolarization.…”

Section: Resultsmentioning

confidence: 99%

“…Most of the earlier studies, however, had to deal with the difficulty that during excitation the membrane potential is not constant. Therefore, Kavaler's (1959) experiments were particularly interesting, as he was able to maintain the membrane potential, measured in ventricular trabeculae from sheep and calf hearts, for several seconds at approximately the plateau level of a normal action potential by applying constant current from an external source. In this condition, he found a biphasic contractile response consisting of an initial twitch with incomplete relaxation and a sustained contracture which relaxed only after repolarization.…”

Section: Introductionmentioning

confidence: 99%

The relation between membrane potential, membrane currents and activation of contraction in ventricular myocardial fibres

Beeler¹,

Réuter²

1970

286

105

4. The ability of a fibre bundle to contract again after a preceding twitch is greatly dependent on the membrane potential between two equal depolarizations. In Tyrode solutions with 1*8 and 7-2 mM-CaCl2 half restoration of this ability occurred at -45 + 3 mV ( + s.E. of mean) and -23 + 4 mV, respectively. 5. In sodium-free bathing solutions, steady-state tension was attained upon the first depolarization provided ICa was activated.

“…The initial twitch seems to be basic to the normal activity during an action potential. The occurrence of a tonic contraction could account for the observation made in some cardiac structures that lengthening of the action potential duration by cathodal pulses leads to a corresponding prolongation of maintenance of tension (Kavaler, 1959;Morad & Trautwein, 1968;Wood, Heppner & Weidmann, 1969).…”

mentioning

confidence: 97%

Membrane current and contraction in frog atrial fibres

Einwächter¹,

Haas²,

Kern³

1972

SUMMARY1. Membrane current and mechanical activity were recorded from short segments of frog atrial muscle strips using a double sucrose gap voltage clamp arrangement. Experiments were performed at 4-7O C. Two types of contraction were observed dependent upon the duration of the clamp.2. Short-lasting depolarizations caused a flow of Ca inward current, ICa, and development of a phasic contraction. Time to peak tension approximated 400 msec. Both ICa and contraction, as functions of membrane potential, had a threshold of about -40 mV and were maximal at inside positive potentials in normal Ringer fluid. Peak tension decreased at strong depolarizations. 3. The minimum time of depolarization required for initiation of a phasic contraction was 40-70 msec. The time necessary for full activation of contraction was 200-300 msec and comparable to the period of time covered by the flow of ICa-4. There was no marked change in peak tension upon repetitive depolarization to the same membrane potential.5. Restoration of (phasic) contractility after a preceding contraction was strongly dependent on the level of membrane potential between conditioning and test pulse. Restoration was half complete at potentials around -45 mV.6. Long-lasting depolarizations generated tonic (sustained) contractions superimposed on the phasic (transient) ones. Threshold potential for initiation of tonic contractions was usually positive to the threshold of phasic contractions. The time taken to attain the final level of tension ranged between 0 7 and 3 sec. Plateau tension, as a function of membrane potential, increased with increasing depolarization and reached a flat maximum at about + 50 mV in normal Ringer fluid. 7. At membrane potentials near zero level, plateau tension developed by the tonic mechanism was about twice peak tension due to phasic contraction.