Effects of high hydrostatic pressure on rat atrial muscle.

Muscle preparations from rat atria were hydraulically compressed in circulating Tris-buffered solution kept at 37 degrees C. Spontaneously beating preparations decreased their beating frequency with 37.3 +/- 13.5 beats/min (22.3% +/- 6.7%, P less than 0.001) and increased their force with 2.3 +/- 1.1 mN (48.6% +/- 17.5%, P less than 0.001) when they were compressed to 10 MPa (100 atm). Decompression gave values not significantly different from precompression control values. The increase in force could in part be explained by the hyperbaric bradycardia and negative force-frequency relation. The remaining force increase seemed to be an effect of the increase in hydrostatic pressure. Changes in action potential duration and Ca2+ availability for the contractile machinery are discussed as possible mechanisms. Electrically driven preparations increased their contraction force at compression if the stimulus strength was at least 175% of the threshold. At lower stimulus levels just above threshold and at higher frequencies, inconsistent results were obtained at pressure.