The mechanical characteristics of the central segment of isolated cat papillary muscle were determined with recently developed equipment. Two small sharpened stainless steel pins, inserted transversely through the muscle, were used to mark the ends of a segment not damaged by attachments. Installation of the pins did not affect the performance of the muscle. The distance between the pins was measured and controlled to produce isometric and afterloaded isotonic contractions of the segment of the muscle between the pins. Data from such contractions were compared with traditional whole muscle measurements made on the same preparation. The isometric length-tension curve of the central segment was significantly higher than that of the whole muscle, and there was no plateau of developed force at long lengths in five of six muscles studied. In the resting state, the segment was more compliant than the whole muscle for physiologic lengths and much stiffer for longer lengths. Segment velocity and shortening were significantly higher than whole muscle velocity and shortening at comparable loads.
A modified test of postextrasystolic potentiation achieved with a brief episode of rapid pacing followed by a 6-second pause (RPP maneuver) was used to evoke maximal force in isolated intact ferret right ventricular papillary muscles. Maximal RPP tensions were examined under length-clamped conditions and compared with the steady-state forces obtained when further increases in [Ca2+]o, did not further increase force and to the tensions recorded at the point of saturation of force when similarly length-clamped muscles were subjected to caffeine-induced tetanization. The results show that the calculated maximal twitch tension achieved with RPP is comparable to the 25-35 g/mm2 observed in intact single skeletal muscle fibers. The study also shows that the beat-to-beat decay of the potentiated contraction is exponential. While the amount of the constant fractional beat-to-beat decay is a function of [Ca2+]o, it is not influenced by length. During the decay of potentiation, the ratio of the potentiation of any beat divided by that of the previous beat is a constant, called (X). With certain assumptions, it is shown that (X) is a measure of the fraction of activator calcium taken up by the sarcoplasmic reticulum in each beat and, in the steady state, the fraction of activator calcium that comes from the sarcoplasmic reticulum. The (X) amounted to 33%, 50%, and 65% when [Ca2+]o was 1.25, 2.50, and 5.0 mM, respectively. Thus, at 1.25 mM [Ca2+]o, some two thirds of the total calcium required to activate the myofilaments comes from the extracellular compartment during excitation and only one third is contributed via release from the sarcoplasmic reticulum. In the region of optimal myofilament overlap, RPP force-length curves are remarkably shallow and almost indistinguishable from the sarcomere length-tension relation observed in skinned single cardiac cells. Tetanus plateau tensions are significantly smaller than RPP forces at any length, and the slope of the tetanus force-length curves is greater than that obtained with RPP. Thus, and by exclusion, we also suggest that caffeine may exert significant downstream inhibitory effects.
Articles you may be interested inA long-range-corrected density functional that performs well for both ground-state properties and timedependent density functional theory excitation energies, including charge-transfer excited states Development of a sum-over-states density functional theory for both electric and magnetic static response properties Not only for the the heart, but also for papillary muscle a contraction variable K can be defined (in analogy with the deformation variable A ) in a phenomenological approach that makes use of the finding that two of the three constants (c 2 and c 3 ) in the isochronics of such a muscle can be considered functions of the third one (K). This fact provides four constants (k i' k2' k~, and k 4 ) of an energy density function. A tentative bridge to the heart (for clinical application) is laid; four analogous constants of the same order of magnitude can be measured there.
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