Membrane current and contraction in frog atrial fibres
- 1 December 1972
- journal article
- Published by Wiley in The Journal of Physiology
- Vol. 227 (1), 141-171
- https://doi.org/10.1113/jphysiol.1972.sp010024
Abstract
1. 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 4–7° 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. 8. Removal of Ca ions from the external medium resulted in an almost complete abolition of phasic contraction within 1–2 min and a gradual decrease of tonic contraction during the first 10 min. Application of a ‘Ca inhibitor’ to normal Ringer fluid caused a strong reduction of both ICa and phasic contraction without affecting tonic contractions. 9. It is concluded that phasic contractions are directly activated by the flow of ICa. Generation of tonic contractions may be attributed to a Ca transfer mechanism different from ICa or a release of Ca from intracellular stores.Keywords
This publication has 43 references indexed in Scilit:
- Two Inward Currents in Frog Atrial MuscleThe Journal of general physiology, 1971
- The dependence of the contractile force generated by frog auricular trabeculae upon the external calcium concentrationThe Journal of Physiology, 1971
- Effects of calcium on the contraction of the hypodynamic frog heartThe Journal of Physiology, 1970
- Treppe and total calcium content of the frog ventricleAmerican Journal of Physiology-Legacy Content, 1970
- Control of muscle contractionQuarterly Reviews of Biophysics, 1969
- Outward membrane currents activated in the plateau range of potentials in cardiac Purkinje fibresThe Journal of Physiology, 1969
- Existence and role of a slow inward current during the frog atrial action potentialPflügers Archiv - European Journal of Physiology, 1969
- The effect of the duration of the action potential on contraction in the mammalian heart musclePflügers Archiv - European Journal of Physiology, 1968
- The dependence of contraction and relaxation of muscle fibres from the crab Maia squinado on the internal concentration of free calcium ionsBiochimica et Biophysica Acta (BBA) - Specialized Section on Biophysical Subjects, 1964
- The ‘staircase’ phenomenon and the action of calcium on the heartThe Journal of Physiology, 1956