Excitation-contraction Coupling in a Barnacle Muscle Fiber As Examined with Voltage Clamp Technique

Relations between the membrane potential and the tension associated with changes in membrane potential were analyzed in barnacle giant muscle fibers by using voltage clamp techniques. With a step change in membrane potential the tension reaches its final level with a time course which is expressed by the difference of two exponential functions. The time constants tau(1) (0.2-0.4 sec at 23 degrees C) and tau(2) (0.07-0.12 sec at 23 degrees C) are independent of the new membrane potential at least for a relatively small membrane potential change while the final level of tension is a function of the potential. Decreasing the temperature increases both tau(1) and tau(2) (Q(10) = -2 to -3) and the increase of the tonicity of the external medium increases tau(1) but not tau(2). The final level of tension is related by an S-shaped curve to the membrane potential. The slope of the final tension-membrane potential curve increases with increasing external Ca concentration and is reduced when a small amount of transition metal ions is added to the medium. This suggests that the influx of Ca ions through the membrane is an important factor in the development of tension.