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The load clamp analysis of mammalian heart contraction in the presence of noradrenaline.

In experiments on isolated guinea pig papillary muscles superfused by Tyrode's solution the effects of abrupt alterations in load on length transients were investigated in controls and the presence of noradrenaline (2 mg/L) at 25 +/- 2 degrees C. In controls short load clamps (50 ms, 0.6 g) had no effect on the isotonic contraction if they were imposed in the fast shortening phase but accelerated relaxation when applied at the later phases of the contractile response. The timing of this stretch intervention determined the effect of the load clamps on length transients. The load clamp analysis revealed two components after the addition of noradrenaline even if splitting was absent on the contraction curve. The amplitude of the first component increased by 81 +/- 22% (n = 9) when the stimulation frequency was raised from 0.1 to 1.0 Hz; its time to peak tension did not change and was equal to 169 +/- 6ms(n = 12). The second component did not change between 0.1 and 0.5 Hz but significantly decreased to 19 +/- 10% (n = 4) at stimulation frequency 1.0 Hz; its time to peak tension correlated with action potential duration (r = 0.98) and decreased from 518 +/- 12 ms at 0.1 Hz to 314 +/- 20 ms at 1.0 Hz (n = 12). It was concluded that the first component is induced by calcium release from sarcoplasmic reticulum and the second one is determined by the action potential duration.