Force production in voltage-clamped human atrial muscle

Human atrial muscle preparations obtained during open heart surgery were mounted in a sucrose gap. Force and membrane currents were recorded during voltage clamp. After a 20-s rest, 10 clamps from a holding potential of -40 to 0 mV at 1.0 Hz were given. This was followed by a test clamp (called 1) of a varied duration and amplitude and two more test clamps (called 2 and 3) as during the priming period. Peak force of contraction 1 (F1) was independent of clamp duration from 2s to about 100 ms but declined at shorter durations. Peak force of contraction 2 (F2) and 3 (F3) increased with the duration and became potentiated. Increasing the clamp amplitude raised F1 to an optimum value at about +10 mV and there was a decline at higher voltages. Both F2 and F3 increased at higher amplitudes. A conventional bell-shaped current-voltage relation for the second inward current was obtained during clamp 1 with maximum inward current around -10 mV. In control experiments on isolated human myocytes peak current was recorded at somewhat more positive potentials. The relation between F3 and F2 was linear both when duration and amplitude of clamp 1 was varied. The slope of the line, interpreted as a measure of recirculation of activator calcium, was 0.4. It is concluded that force during voltage clamp in human atrial muscle is similarly related to membrane voltage as previously reported for guinea pig and ferret preparations.