Calcium-calmodulin gating of a pH-insensitive isoform of connexin43 gap junctions.

Intracellular protons and calcium ions are two major chemical factors that regulate connexin43 (Cx43) gap junction communication and the synergism or antagonism between pH and Ca(2+) has been questioned for decades. To assess the ability of Ca(2+) ions to modulate Cx43 junctional conductance (g(j)) in the absence of pH-sensitivity, patch clamp experiments were performed on Neuroblastoma-2a (N2a) cells or neonatal mouse ventricular myocytes (NMVMs) expressing either full-length Cx43 or the Cx43-M257 (Cx43K258stop) mutant protein, a carboxyl-terminus (CT) truncated version of Cx43 lacking pH-sensitivity. The addition of 1†μM ionomycin to normal calcium saline reduced Cx43 or Cx43-M257 g(j) to zero within 15 min of perfusion. This response was prevented by Ca(2+)-free saline or addition of 100†nM calmodulin (CaM) inhibitory peptide to the internal pipette solution. Internal addition of a connexin50 cytoplasmic loop calmodulin-binding domain (CaMBD) mimetic peptide (200†nM) prevented the Ca(2+)/ionomycin-induced decrease in Cx43 g(j), while 100†μM Gap19 peptide had minimal effect. The investigation of the transjunctional voltage (V(j)) gating properties of NMVM Cx43-M257 gap junctions confirmed the loss of the fast inactivation of Cx43-M257 g(j), but also noted the abolishment of the previously reported facilitated recovery of g(j) from inactivating potentials. We conclude that the distal CT domain of Cx43 contributes to the V(j)-dependent fast inactivation and facilitated recovery of Cx43 gap junctions, but the Ca(2+)/CaM-dependent gating mechanism remains intact in its absence. Sequence-specific connexin CaMBD mimetic peptides act by binding Ca(2+)/CaM non-specifically and the Cx43 mimetic Gap19 peptide has negligible effect on this chemical gating mechanism.