Modulation of junctional permeability.

Changes in intercellular coupling can be accomplished by continuous synthesis and destruction of intercellular channels and through a modulation of unit channel permeability. The increase in free [Ca2+]i caused by activation of sodium-calcium exchange or by metabolic inhibition leads to cell decoupling. In embryonic cells the conductance of the gap junction is strongly dependent on pHi. The exact role of Ca2+ and H+ in the physiological modulation of junctional conductance remains unknown. The cyclic AMP (cAMP)-calcium hypothesis is presented. According to this view, cAMP modulates the junctional permeability through specific kinases. A feedback mechanism between calcium and cAMP might be relevant in the physiological control of junctional conductance.