Human stem cell-derived cardiomyocytes hold promise for heart repair, disease modeling, drug screening, and for studies of developmental biology. All of these applications can be improved by assessing the contractility of cardiomyocytes at the single cell level. We have developed an in vitro platform for assessing the contractile performance of stem cell-derived cardiomyocytes that is compatible with other common endpoints such as microscopy and molecular biology. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were seeded onto elastomeric micropost arrays in order to characterize the contractile force, velocity, and power produced by these cells. We assessed contractile function by tracking the deflection of microposts beneath an individual hiPSC-CM with optical microscopy. Immunofluorescent staining of these cells was employed to assess their spread area, nucleation, and sarcomeric structure on the microposts. Following seeding of hiPSC-CMs onto microposts coated with fibronectin, laminin, and collagen IV, we found that hiPSC-CMs on laminin coatings demonstrated higher attachment, spread area, and contractile velocity than those seeded on fibronectin or collagen IV coatings. Under optimized conditions, hiPSC-CMs spread to an area of approximately 420 μm2, generated systolic forces of approximately 15 nN/cell, showed contraction and relaxation rates of 1.74 μm/s and 1.46 μm/s, respectively, and had a peak contraction power of 29 fW. Thus, elastomeric micropost arrays can be used to study the contractile strength and kinetics of hiPSC-CMs. This system should facilitate studies of hiPSC-CM maturation, disease modeling, and drug screens as well as fundamental studies of human cardiac contraction.

The zebrafish (Danio rerio) has been intensively used in cardiovascular biology mainly for study of heart development. This is a promising costeffective model system to study structure-function due to the ease of genetic manipulations. However, the basic contractile physiology of zebrafish heart is still incompletely understood. Moreover, the applicability of the fish heart as a model system for study mammalian heart contractility needs to be established. The aim of our work was to establish the zebrafish as a model system to approach structure-function cardiac myocyte biology. Accordingly, we performed experiments with a focus on the Frank-Starling mechanisms at two levels: cellular (intact cells) and myofilament level (permeabilized cells). In single cell experiments we were able to attach a single enzymatically isolated zebrafish ventricular myocytes to myotak coated carbon probes and measure force, intracellular calcium, cell length, and sarcomere length in electrically paced cells over a range (20%) of cell lengths. In skinned cell experiments, we used a single myofibril technique to measure activation/ relaxation kinetics and force-Ca 2þ relations at short and long sarcomere lengths (2.2 and 2.4 um). Zebrafish intact myocytes displayed robust length induced increase in twitch force in the absence of calcium transient alteration. In skinned zebrafish muscle we found robust myofilament length dependent activation as well as characteristic activation/relaxation dynamics. We conclude that the zebrafish heart is an appropriate model to study cardiac structure-function relationships.