Fig. 2. Effects of long steady stretches on the heartbeat rhythm. (Ai—iv) In each record: top trace, membrane potential recording from the heart muscle showing bursts of excitatory junctional potentials (EJPs); middle trace, displacement of the unfixed stage, showing a 10 s stretch; bottom trace, a plot of instantaneous frequency of the EJP burst. The amplitude of displacement is shown on top of each record. Asterisks indicate slow muscle depolarization without EJPs. The records were obtained from a single preparation. The width of the opened heart during the 1.1 mm stretch (Aiv) was 2.9 mm, which is 38% larger than the calculated diastlic dimension (2.1 mm). The 1.1 mm stretch was approximately 2.5x larger than the amplitude of the circumferential dimension change in a given heart. (B) Steady-state burst frequency during the stretch (duration, 10-30 s) was plotted against amount of extension of the opened heart. Values for the burst frequency were normalized to the free-run frequency, and the change in width of the opened heart was normalized to the estimated circumferential dimension change in a beating heart tube (see Materials and methods). The horizontal bar in the graph shows a range of the changes in opened heart width (±0.5) that is equivalent to the circumferential dimension change in a beating heart. The control position is set at the median value between the maximum and minimum circumferential dimensions. Regression lines were calculated from data with the heart width less than the diastolic dimension (normalized heart width change <0.5, y=0.02x+0.99, r²=0.03) and from data with the heart width larger than the diastolic dimension (normalized heart width change >0.5, y=-0.29x+1.08, r²=0.53). Data points were obtained from 11 preparations.

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