SARCOPLASMIC-RETICULUM IN CARDIAC LENGTH-DEPENDENT ACTIVATION IN RABBITS

After a step increase in length of rabbit right ventricular papillary muscles, active stress increased immediately followed by a further slo w increase in stress over 15-20 min. We studied the contribution of th e sarcoplasmic reticulum (SR) to the slow change in stress (SCS) after changing muscle length from 85 to 95% of length at which active force development was maximal. SCS amounted to 32.5 +/- 12.2% (mean +/- SD, n = 19) of the total increase in active stress. This was associated w ith a 13.2 +/- 8.7% increase in calcium content of the SR as estimated with rapid cooling contractures (P < 0.0001, n = 19). However, SCS wa s not dependent on SR calcium content. There was no significant attenu ation in SCS after SR calcium depletion with ryanodine (n = 6), SR Ca2 +-adenosinetriphosphatase inhibition with cyclopiazonic acid (n = 6), or combined treatment with ryanodine and cyclopiazonic acid (n = 3). W e conclude that, in the rabbit, SR calcium content increases slowly af ter a step increase in cardiac muscle length but the slow changes in a ctive stress are not dependent on the sarcoplasmic reticulum.