MYOCARDIAL STRETCH ALTERS TWITCH CHARACTERISTICS AND CA2+ LOADING OF SARCOPLASMIC-RETICULUM IN RAT VENTRICULAR MUSCLE

Objective: The aim was to determine the influence of diastolic muscle length on force development and timing parameters of cardiac muscle twitch contraction and to determine whether a length dependency exists for the calcium loading capacity of the sarcoplasmic reticulum. Methods: Right ventricular papillary muscles and trabeculae were isolated from hearts of female Wistar rats weighing 220-280 g. Papillary muscles were stretched to diastolic lengths of 90, 95, and 100% L(max) and paced at 1.0 Hz. Individual twitch profiles were characterised by their peak force and the maximum rate (dF/dt) of the positive and negative force changes. Intrinsic timing was identified through waveform analysis that divided the twitch profile into time domains for the ascending limb (T0-T1; T1-T2) and the descending limb (T2-T3; T3-T4). Each domain was compared at three muscle lengths. The sarcoplasmic reticular calcium content at short (1.88-mu-m) and long (2.11-mu-m) sarcomere lengths was characterised by rapid cooling contractures after 1 s and 60 s of diastolic rest. Results: Peak developed force and the maximum rate of positive and negative force development decreased as diastolic muscle length was reduced from L(max) to 90% L(max). The intrinsic timing for the segment that reflects the relaxation phase of the twitch (T1-T4) was shortened as muscle length was reduced. The time domain that reflects the combined effects of calcium release and the early phase of contraction (T0-T1) was insensitive to diastolic muscle length. The fractional release of sarcoplasmic reticular calcium at different muscle lengths was approximately 32-35% of the total sarcoplasmic reticulum calcium pool. Conclusions: The data on the intrinsic timing of the twitch characteristics coupled with rapid cooling contracture analysis suggests a fractional calcium release that is approximately 32-35% of the total sarcoplasmic reticular capacity at either long or short muscle lengths. However, the loading capacity of the sarcoplasmic reticulum is greater when the muscle operates at a shorter diastolic length. This can be interpreted as meaning that diastolic muscle length differentially influences sarcoplasmic reticular calcium storage and release processes.