The development of beat-rate synchronization of rat myocyte pairs in cell culture.

When two spontaneously beating neonatal rat heart cells in tissue culture were allowed to grow together they synchronized their originally independent beats to a common rhythm, as measured with an opto-electronic technique. Both single isolated cells and cell pairs exhibited a highly irregular beating pattern. Beating irregularity was strongly and positively correlated with mean interbeat interval. Synchronization of beating occurred in 50% of the pairs studied within one beating interval. In the remaining cell pairs, the first synchronized beat was followed by a 4-65 s period of partial synchronization. The time difference between contraction moments of two cells in a pair respective to each other (latency) changed upon synchronization from a random value to a fixed value. In a few cases the latency decreased during 20 to 30 s after the first synchronized beat before a steady-state value was reached. The mean interbeat interval (IBI) of the synchronized cell pairs was governed by the mean IBI of the originally faster beating cells. In 83% of the cases the mean IBI of the cell pairs was between that of the originally isolated beating cells. We conclude from the experiments described that physical coupling (i.e. gap junction formation) is virtually complete before beating synchronization occurs.

The establishment of regular beating in populations of pacemaker heart cells. A study with tissue-cultured rat heart cells.

Single isolated neonatal rat heart cells beat slowly (mean beating interval duration in the range of seconds) and irregularly (coefficient of variation greater than 40%). It is shown that slowness and irregularity of beating are intrinsic properties of the cells and are not caused by dissociation damage or lack of conditioning factors in the culture medium. When cell contacts are established either by letting the cultures grow for given amounts of time or by plating cells at increasing densities both interval duration and irregularity decrease. The beating regularity of small groups of interconnected cells (3 to 35 cells) and larger groups (200 to 15000 cells) is comparable. There is no clear cut proportionality between number of interconnected cells and beating regularity. Confluent monolayers beat fast (mean interval duration ranging between 200 and 400 ms and regular (coefficient of variation less than 5%). The hypothesis is discussed that this clock-like behavior of monolayers of heart cells is caused by the interaction of several pacemaker centers which are by themselves less regular and beat more slowly.