Raised intracellular calcium decreased gap junction conductance from 3.80 to 2.00 mS/cm and altered Cx43 phosphorylation, effects that were reversed by calcineurin inhibitors.
Raised intracellular calcium reduces cardiac gap junction conductance and conduction velocity via a calcineurin-dependent pathway that alters Cx43 phosphorylation, suggesting a mechanism for arrhythmias in conditions like ischemia or hypertrophy.
Absolute Event Rate: 2% vs 3.8%
p-value: p=<0.0001
Cardiac arrhythmias are associated with raised intracellular Ca2+ and slowed action potential conduction caused by reduced gap junction (GJ) electrical conductance (Gj). Ventricular GJs are composed of connexin proteins (Cx43), with Gj determined by Cx43 phosphorylation status. Connexin phosphorylation is an interplay between protein kinases and phosphatases but the precise pathways are unknown. We aimed to identify key Ca2+-dependent phosphorylation sites on Cx43 that regulate cardiac gap junction conductance and action potential conduction velocity. We investigated the role of the Ca2+-dependent phosphatase, calcineurin. Intracellular Ca2+ was raised in guinea-pig myocardium by a low-Na solution or increased stimulation. Conduction velocity and Gj were measured in multicellular strips. Phosphorylation of Cx43 serine residues (S365 and S368) and of the intermediary regulator I1 at threonine35 was measured by Western blot. Measurements were made in the presence and absence of inhibitors to calcineurin, I1 or protein phosphatase-1 and phosphatase-2. Raised Ca2 +i decreased Gj, reduced Cx43 phosphorylation at S365 and increased it at S368; these changes were reversed by calcineurin inhibitors. Cx43-S368 phosphorylation was reversed by the protein kinase C inhibitor chelerythrine. Raised Ca2+i also decreased I1 phosphorylation, also prevented by calcineurin inhibitors, to increase activity of the Ca2+-independent phosphatase, PPI. The PP1 inhibitor, tautomycin, prevented Cx43-365 dephosphorylation, Cx43-S368 phosphorylation and Gj reduction in raised Ca2+i. PP2A had no role. Conduction velocity was reduced by raised Ca2+i and reversed by calcineurin inhibitors. Reduced action potential conduction and Gj in raised Ca2+ are regulated by calcineurin-dependent Cx43-S365 phosphorylation, leading to Cx43-S368 dephosphorylation. The calcineurin action is indirect, via I1 dephosphorylation and subsequent activation of PP1.
Jabr et al. (Tue,) conducted a other in Cardiac arrhythmias (preclinical model). Low-Na solution and calcineurin inhibitors (CysA, CAIP) vs. Control Tyrode's solution was evaluated on Gap junction conductance (Gj) (p=<0.0001). Raised intracellular calcium decreased gap junction conductance from 3.80 to 2.00 mS/cm and altered Cx43 phosphorylation, effects that were reversed by calcineurin inhibitors.
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