Tension components and tension fall of the bullfrog atrial muscle during depolarization.

1) Development and fall of tension during depolarization in the bullfrog atrial muscle were studied using a double gap voltage clamp arrangement. 2) Phasic and tonic tension developments and the following decay were noticed in the tension produced by depolarizing pulse of different durations and amplitudes. The phasic component appeared in response to a short-lasting pulse (0.1-0.2 sec), and reached a peak tension at around 73 mV depolarization, being depressed for stronger pulses. La (1 mM) and Mn ions (5 mM) eliminated the component. 3) Tonic contraction was generated by long-lasting depolarization. The development of tension was not affected by these ions, but increased in strength and duration of depolarizing pulse up to 170 mV and 1.0 sec. 4) When a depolarizing pulse was sustained, the tonic tension decayed gradually after the peak. The tension decay was composed of three different phases, initial retardation, relatively rapid relaxation and extremely slow final relaxation. 5) The rapid relaxation phase appeared exponential with time when an excess tension above provisional final tension (sustained tension) was plotted, but the time constant increased with strength of depolarization. 6) Logarithm of the final tension appeared proportional to the membrane depolarization in between 20 to 100 mV. Under the simultaneous presence of La (1 mM) and caffeine (10 mM) the tension decay during depolarization was abolished resulting in a marked elevation of the final tension. 7) The final tension under normal conditions was considered to be equilibrium tension between the tonic tension and relaxing components, and thus, at least three components, the phasic and tonic tensions and a relaxing factor, appeared to control muscle contraction during depola-