Comparison of mammalian cardiac ␣-and ␤-myosin heavy chain isoforms reveals 93% identity. To date, genetic methodologies have effected only minor switches in the mammalian cardiac myosin isoforms. Using cardiac-specific transgenesis, we have now obtained major myosin isoform shifts and/or replacements. Clusters of non-identical amino acids are found in functionally important regions, i.e. the surface loops 1 and 2, suggesting that these structures may regulate isoform-specific characteristics. Loop 1 alters filament sliding velocity, whereas Loop 2 modulates actin-activated ATPase rate in Dictyostelium myosin, but this remains untested in mammalian cardiac myosins. ␣ 3 ␤ isoform switches were engineered into mouse hearts via transgenesis. To assess the structural basis of isoform diversity, chimeric myosins in which the sequences of either Loop 1؉Loop 2 or Loop 2 of ␣-myosin were exchanged for those of ␤-myosin were expressed in vivo. 2-fold differences in filament sliding velocity and ATPase activity were found between the two isoforms. Filament sliding velocity of the Loop 1؉Loop 2 chimera and the ATPase activities of both loop chimeras were not significantly different compared with ␣-myosin. In mouse cardiac isoforms, myosin functionality does not depend on Loop 1 or Loop 2 sequences and must lie partially in other non-homologous residues.Myosin, the molecular motor of the heart, generates force and motion by coupling its ATPase activity to its cyclic interaction with actin. Myosin is a hexameric protein and is composed of two heavy chains (MHC) 1 and two essential and two regulatory myosin light chains. Structurally, MHC is composed of a number of discrete domains: a helical rod necessary for thick filament formation, and a globular head that contains the actin-binding site, catalytic, and motor domains (1).In the mammalian heart, two functionally distinct MHC isoforms, termed V 1 and V 3 , are present. V 1 is a homodimer of two ␣-MHC molecules, whereas V 3 is a ␤␤-homodimer. Expression of V 1 and V 3 is controlled both developmentally and hormonally. In the mouse, ␤-MHC expression in the ventricles predominates prenatally. However, via thyroid hormone regulation, ␤-MHC expression is silenced at birth, and ␣-MHC is transcribed (2). The functional differences between V 1 and V 3 myosin in terms of shortening velocity, force generation, and ATPase activity are profound. For example, rabbit V 1 myosin has a 2-3-fold faster actin filament sliding velocity than V 3 , but generates only half the average isometric force (3, 4). Likewise, both the Ca 2ϩ -stimulated and actin-activated ATPase activities of rabbit V 1 myosin are ϳ2-3 times greater than for V 3 myosin (3, 5). Similar differences in actin velocity and myofibrillar ATPase activity have been observed between mouse V 1

This study demonstrated that a call to the emergency medical services is underutilized by patients with symptoms of myocardial infarction, and documented the beneficial effects of an emergency medical services call by reducing pre-hospital delays and increasing early revascularization therapies.