Percent change in baseline sarcomere length (SL), peak shortening, departure velocity, and return velocity in response to 15 minute perfusion with 10−8 M hRFRP-1 in the presence of 5 × 10−7 M bis-1, or 10−8 M hRFRP-1 only, and controls of 5 × 10−7 M bis-1 or media only (Table 6). There were no significant changes in baseline sarcomere length. The influence of 10−8 M hRFRP-1 in the presence of 5 × 10−7 M bis-1 was blocked and was not significantly different from control on peak shortening, departure time, and return velocity. Application of bis-1 in DMSO was not significantly different from control. Data were analyzed using 1-way ANOVA followed by a Dunnett's Multiple Comparison Test with p < 0.05 considered statistically significant (*; Table 6). Recordings were made from n = 6-23, 1-day and 2-day myocytes isolated from n = 1-3 hearts.