A regional gradient of cardiac intrinsic rhythmicity depicted in embryoniccultured multiple hearts

We used optical methods to examine the spatial gradient of intrinsic rhythm icity in early-stage multiple-heart chick embryos. The latter were induced experimentally in whole-embryo culture. The embryos were cut microsurgicall y through the tissue of the anterior intestinal portal at the 5- to early 7 -somite developmental stage. Spontaneous electrical activity in 4 to 6 segm ented hearts, during the 7- to 10-somite stages of development, were monito red simultaneously by means of multiple-site optical recordings of membrane potential activity, using a voltage-sensitive merocyanine-rhodanine dye (N K2761). Each segment of the heart exhibited its own inherent rhythmicity. I n quadruple hearts, the order of the rhythmicity was often left-caudal segm ent>right-caudal segment>left-cephalic segment>right-cephalic segment; the heart rate in the left-caudal segment was often faster than that in the oth er segments. An atypical pattern of "bursting" rhythm was observed in the c ephalic segments suggesting that, in these segments, the development of rhy thmicity is relatively poor. These findings strongly emphasize the concept that, in the early phases of cardiogenesis, the'formation of a regional gra dient of pacemaker activity (i.e. a spatial gradient of intrinsic rhythmici ty) results in the functional self-organization of the pacemaking area.