SYMMETRICAL SEGREGATION OF POTASSIUM CHANNELS AT CYTOKINESIS

To determine how voltage-gated ion channels segregate between sibling cells at cytokinesis, we used a whole-cell patch clamp to measure the electrophysiological phenotypes of siblings within 45 min of division. Recently born siblings in an immortalized line of embryonic retinal c ells were identified as pairs of spherical cells adhering to one anoth er. All siblings were electrically coupled when cells were simultaneou sly voltage clamped, whereas nonsiblings were not coupled. Twelve pair s of siblings were electrically isolated by mechanical separation so t hat their phenotypes could be measured independently. Cells expressed two principal membrane conductances, delayed rectifier-like (I(K)) and inward rectifier (I(K(IR)) potassium currents. Despite qualitative an d quantitative variability in I(K) and I(K(IR)) expression within the population, each cell of a given pair expressed similar steady-state c urrent densities between -110 and +50 mV. We estimated I(K(IR)) slope conductance by blocking the current specifically with 5 mM Cs and calc ulated I(K(IR)) ratios in siblings and nonsiblings. Three pairs of sib lings expressed I(K(IR)) ratios of approximately 1.2, while ratios in three pairs of adhered nonsiblings varied between 1.6 and 5.4. When cu rrents were sampled continuously through cytokinesis by using the perf orated-patch recording mode, current amplitude showed no net change wi thin 30 min of division. Because channel number did not appear to chan ge in siblings during this interval, parental channels were inherited by each daughter in proportion to the area of membrane received. Heter ogeneity therefore arises after siblings reenter interphase and is not due to the asymmetrical segregation of channels at cytokinesis.