The efflux of potassium sodium chloride calcium and sulphate ions and of sorbitol and glycerol during the cardiac cycle in frogs ventricle

摘要

1. The exchanges of potassium and various other substances have been measured in beating frog's ventricles, using both superfused and distended preparations. In both preparations the high fluid flow rates used (1 ml./sec) cleared the ventricular cavity with a half-time (T½) of about 130 msec.2. Histological sections show that the modal strand radius in the relaxed or contracted distended ventricle is 17·5 μ, and in the relaxed and contracted superfused ventricle is 17·5 and 27·5 μ respectively.3. In quiescent ventricles the resting potassium influx and efflux are approximately equal at about 16 p-mole/cm².sec. This figure is computed from Niedergerke's (1963b) estimate of a cell size of 3·5 μ taken from electron-micrographs. If the older figure of 9·2 μ from single isolated cells is used (Skramlik, 1921) then the fluxes are about 44 p-mole/cm².sec. To allow for some cell damage in these preparations a further increase in flux of about 30% may be necessary.4. Contraction leads to a diminution of both potassium influx and efflux. Measurements made at 100 msec intervals throughout the cardiac cycle have demonstrated (a) that this decreased K efflux occurs at the same time as the mechanical twitch, and (b) that the size of the decrease is dependent on the external calcium concentration. Other experiments show that a similar decrease can be obtained by inducing a contracture at a constant membrane potential. It is concluded that the decreased K efflux during contraction is due to mechanical distortion of the tissue. This leads to a further slowing of the K diffusion and allows considerable reabsorption of K to occur into the cells.5. Efflux analysis suggests that normal K diffusion in the extracellular space may be about 1/10 of that in free solution. If this is correct the true membrane fluxes may be × 5 those measured.6. Phasic efflux measurements of Na, Ca, K, Cl, SO4, sorbitol and erythritol show that a peak of efflux occurs just after the point of maximum rate of contraction of the ventricle. The peak efflux of K is least but all the other substances show similar patterns. In calcium-free solutions these phasic changes are absent. It is concluded that these effects are mechanical.7. Net K and Na changes were measured in ventricles poisoned by ouabain. The computed net changes for quiescent ventricles were a gain of 2·8 p-mole/cm².sec of Na and a loss of 5·3 p-mole/cm².sec of K. On stimulation a further increase in Na uptake of 8 p-mole/cm² occurred with no further loss of potassium. These results are computed for a cell diameter of 3·5 μ, for the larger diameter of 9·2 μ appropriate values of Na and K are 7·4 and 13·4 p-mole/cm².sec respectively for quiescent ventricles and an extra Na uptake of 21 p-mole/cm² per action potential. These results: (a) show that no large degree of single-file interaction occurs on the K movements, (b) are in agreement with the hypothesis that the membrane K fluxes are underestimated and (c) show that sufficient Na enters the cells per action potential to discharge a capacity of about 4 μF/cm².8. A general conclusion reached in these experiments is that ion movements during the long cardiac action potential cannot easily be measured because of mechanical artifacts.