The ionic nature of slow inward current and its relation to contraction
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The amplitude of the slow inward current decreases on lowering the extracellular calcium concentration. In zero calcium slow inward current is not observed.
The slow inward current is not affected by lowering the sodium concentration in the bathing solution to 10% of the normal concentration.
The membrane conductance is increased during the slow inward current by a factor of about 2 and not fully inactivated during maintained depolarization.
The threshold potential for both slow inward current and contraction is at −35 mV.
At potentials more positive than +80 mV (at 3.6 mM extracellular calcium) and more positive than +40 mV (at 0.9 mM calcium) the amplitude of the contraction declines. Also slow inward current reverses polarity at these potentials and calcium concentrations.
The amplitude of the steady state contraction rises with the amplitude of clamp depolarization in an S-shaped manner from threshold to the maximum of contraction at zero millivolt. Plotted in normalized fashion (maximum amplitude of contraction = 1), the curve relating amplitude of contraction to clamp potential is shifted by 6.6 mV in negative direction when the extracellular calcium concentration is reduced by a factor of 4.
Reduction of extracellular sodium results in large increase of amplitude of contraction. The plot of normalized amplitude of contraction vs. clamp potential is shifted towards negative membrane potentials and both, the reversal potential of slow inward current and the maximum amplitude of contraction, occur between +10 mV and +30 mV.
The slow inward current is interpreted as a calcium current. The amount of calcium uptake is not less than 10−7M/l fibre volume and beat, but can be up to onehundred times larger. The calcium current is supposed to fill intracellular stores and to release calcium from such stores. For the release effect the conductance change, rather than the inflowing calcium seems to be responsible. Direct activation of myofibrils by the inflowing calcium is not likely to occur but cannot be excluded.
Key wordsSlow Inward Current Calcium Current Contraction
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