Control of intercellular communication by voltage dependence of gap junctional conductance

The junctional conductance between coupled amphibian blastomeres exhibits a high degree of voltage dependence, as previously described in voltage clamp studies (Spray, D.C., A.L. Harris, and M.V.L. Bennett (1981) J. Gen. Physiol. 77: 77-95; Harris, A.L., D.C. Spray, and M.V.L. Bennett (1981) J. Gen. Physiol. 77: 95-117). The present study examines the properties which this voltage dependence confers on electrotonic coupling between cells. The effects of applied pulses and ramps of current are studied experimentally and are modeled by calculation. During sufficiently large current pulses applied to one cell of a pair, the cells uncouple and then recouple after termination of the pulses. Ramps of current applied to one of the cells can give voltage-current (V-I) relations with a region of hysteresis within which the cells are stably coupled or stably uncoupled depending on previous history. Intrinsically generated currents are able to cause bistability of coupling in the absence of externally applied current. Calculations from the parameters of junctional conductance defined under voltage clamp fully account for these findings and illustrate how junctional and nonjunctional conductances affect the V-I relations in the region of bistability. Recordings from several cells within a small group show that boundaries of intercellular communication can be altered by applied current, a finding that also can be accounted for by voltage dependence of junctional conductance. The "Appendix" examines quantitatively the criteria required for bistability of coupling and the relevance of bistability for intercellular signaling. The plasticity of coupling which the voltage dependence of junctional conductance confers on cells offers an intriguing mechanism by which patterns of intercellular communication could be determined and changed in developing tissues.