Fig. 5.
Myosuppressin caused dose-dependent increases in contraction duration in whole heart preparations, with similar effects on a range of parameters related to duration. (A) Duration of contractions, measured at half-amplitude, increased with increasing myosuppressin concentration. (B) Recordings of a single contraction in control saline (black) and in myosuppressin (red) illustrate the difference in time course and amplitude of heart beats. The recording in control saline has been scaled to the amplitude of the contraction in myosuppressin (blue), showing that the biggest change is in the rate and duration of the rising phase of the contraction. (C) Rise time, measured as the time from the onset of contraction to the maximum contraction, increased with increasing myosuppressin concentration. (D) Fall time, measured as the time from the peak of contraction to the return to baseline tension, increased to a lesser extent. (E) Burst duration, measured on the anterolateral nerves, likewise increased with increasing myosuppressin concentration. A,C—E are all shown on the same vertical scale to facilitate comparisons between graphs. (F) Number of spikes per burst similarly increased with increasing myosuppressin concentration; spike frequency (not shown) thus remained approximately constant. Threshold for contraction duration, rise time and fall time was between 10−8 and 10−9 mol l−1; threshold for burst duration and number of spikes per burst was between 10−7 and 10−8 mol l−1. *Significantly different from zero, one-sample, two-tailed t-test, P<0.05. N=6, 6, 11, 14 for 10−9, 10−8, 10−7 and 10−6 mol l−1 myosuppressin, respectively. Error bars represent standard errors.