A method has been developed for culturing cardiac myocytes in a collagen matrix to produce a coherently contracting 3-dimensional model heart tissue that allows direct measurement of isometric contractile force. Embryonic chick cardiomyocytes were mixed with collagen solution and allowed to gel between two Velcro-coated glass tubes. During culture, the cardiomyocytes formed spontaneously beating cardiac myocyte-populated matrices (CMPMs) anchored at opposite ends to the Velcro-covered tubes through which they could be attached to a force measuring system. Immunohistochemistry and electron microscopy revealed a highly organized tissue-like structure of alpha-actin and alpha-tropomyosin-positive cardiac myocytes exhibiting typical cross-striation, sarcomeric myofilaments, intercalated discs, desmosomes, and tight junctions. Force measurements of paced or unpaced CMPMs were performed in organ baths after 6-11 days of cultivation and were stable for up to 24 h. Force increased with frequency between 0.8 and 2.0 Hz (positive "staircase"), increasing rest length (Starling mechanism), and increasing extracellular calcium. The utility of this system as a test bed for genetic manipulation was demonstrated by infecting the CMPMs with a recombinant beta-galactosidase-carrying adenovirus. Transduction efficiency increased from about 5% (MOI 0.1) to about 50% (MOI 100). CMPMs display more physiological characteristics of intact heart tissue than monolayer cultures. This approach, simpler and faster than generation of transgenic animals, should allow functional consequences of genetic or pharmacological manipulation of cardiomyocytes in vitro to be studied under highly controlled conditions.

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Observational clinical and ex vivo studies have established a strong association between atrial fibrillation and inflammation1. However, whether inflammation is the cause or the consequence of atrial fibrillation and which specific inflammatory mediators may increase the atria's susceptibility to fibrillation remain elusive. Here we provide experimental and clinical evidence for the mechanistic involvement of myeloperoxidase (MPO), a heme enzyme abundantly expressed by neutrophils, in the pathophysiology of atrial fibrillation. MPO-deficient mice pretreated with angiotensin II (AngII) to provoke leukocyte activation showed lower atrial tissue abundance of the MPO product 3-chlorotyrosine, reduced activity of matrix metalloproteinases and blunted atrial fibrosis as compared to wild-type mice. Upon right atrial electrophysiological stimulation, MPO-deficient mice were protected from atrial fibrillation, which was reversed when MPO was restored. Humans with atrial fibrillation had higher plasma concentrations of MPO and a larger MPO burden in right atrial tissue as compared to individuals devoid of atrial fibrillation. In the atria, MPO colocalized with markedly increased formation of 3-chlorotyrosine. Our data demonstrate that MPO is a crucial prerequisite for structural remodeling of the myocardium, leading to an increased vulnerability to atrial fibrillation.