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.