doi:10.1155/2012/371415 Research Article Length and PKA Dependence of Force Generation and Loaded Shortening in Porcine Cardiac Myocytes

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In healthy hearts, ventricular ejection is determined by three myofibrillar properties; force, force development rate, and rate of loaded shortening (i.e., power). The sarcomere length and PKA dependence of these mechanical properties were measured in porcine cardiac myocytes. Permeabilized myocytes were prepared from left ventricular free walls and myocyte preparations were calcium activated to yield ∼50 % maximal force after which isometric force was measured at varied sarcomere lengths. Porcine myocyte preparations exhibited two populations of length-tension relationships, one being shallower than the other. Moreover, myocytes with shallow length-tension relationships displayed steeper relationships following PKA. Sarcomere length-Ktr relationships also were measured andKtr remained nearly constant over∼2.30 μm to∼1.90 μm and then increased at lengths below 1.90 μm. Loaded-shortening and peak-normalized power output was similar at ∼2.30 μm and ∼1.90 μm even during activations with the same [Ca2+], implicating amyofibrillar mechanism that sustainsmyocyte power at lower preloads. PKA increasedmyocyte power and yielded greater shortening-induced cooperative deactivation in myocytes, which likely provides a myofibrillar mecha-nism to assist ventricular relaxation. Overall, the bimodal distribution of myocyte length-tension relationships and the PKA-mediated changes in myocyte length-tension and power are likely important modulators of Frank-Starling relationships i