INFLUENCE OF MUSCLE LENGTH ON WORK FROM TRABECULAR MUSCLE OF FROG ATRIUM AND VENTRICLE

The work capacity of segments of atrial and ventricular muscle from th e frog Rana pipiens was measured as a function of muscle length using the work loop technique. Both the work done during shortening and the work required to re-lengthen the muscle after shortening increased wit h muscle length. Net work increased with length up to a maximum, beyon d which work declined. The optimum sarcomere length for work output wa s 2.5-2.6 mu m for both atrial and ventricular muscle. Isometric force increased with muscle length to lengths well beyond the optimum for w ork output. Thus, the decline in work at long lengths is not simply a consequence of a reduction in the capacity of heart muscle to generate force. It is proposed that it is the non-linear increase in work requ ired to re-lengthen muscle with increasing muscle length which limits net work output and leads to a maximum in the relationship between net work and muscle length. Extension of the results from muscle strips t o intact hearts suggests that the work required to fill the ventricle exceeds that available from atrial muscle at all but rather short vent ricular muscle lengths.