Percent change in baseline sarcomere length (SL), peak shortening, departure velocity, and return velocity in response to 15 minutes of perfusion with 10−8 M rRFRP-1, 10−8 M hRFRP-1, and control (media, only) in isolated adult rat cardiac myocytes (Table 3). There were no significant changes in baseline sarcomere length. The influence of 10−8 M rRFRP-1 and 10−8 M hRFRP-1 on departure velocity and in return velocity were comparable and significantly different from control values; however, 10−8 M rRFRP-1 did not produce the significant decrease in peak shortening observed with 10−8 M hRFRP-1 and was not significantly different from the media only control response. Data were analyzed using 1-way ANOVA followed by a Dunnett's Multiple Comparison Test with p < 0.05 considered statistically significant (*; Table 3). Recordings were made from n = 12-14, 1-day and 2-day myocytes isolated from n = 2-4 hearts.