Abstract

Myocardial action potential conduction velocity is a function of gap junction (GJ) electrical conductance (Gj). The phosphorylation state of GJ proteins, in particular Cx43 determines Gj and the role of protein phosphatases (PPs) remains unclear. This study aims to investigate the role of serine-threonine PPs in regulating Gj, in particular the Ca2+-dependent PP, calcineurin, and the Ca2+-independent PPs, PP1 and PP2A.

Left atrial strips were dissected from male, Dunkin-Hartley guinea pigs to estimate Gj by measuring the frequency-dependent intracellular conductance1 in control solution (Na=149.4 mM), or low-Na solution (Na=29.4 mM) to raise [Ca2+]i. Inhibitors to calcinuerin, (cyclosporine-A; 5 µM; calcineurin auto-inhibitory peptide; CAIP, 50 µM); PPI (tautomycin, 5 nM) or PP2A (okadaic acid, 2 nM; fostreicin 100 nM) were used. Western blots were conducted on inhibitor perfused tissue. Values are mean±SEM, normalised to control. Differences were tested by ANOVA; null hypotheses rejected at p<0.05.

Low-Na significantly and reversibly decreased Gj, and in control conditions individual inhibitors had no effect. Cyclosporine-A and CAIP diminished the effects of low-Na (Gj: low-Na, 55±5.8% low-Na+ cyclosporine-A (n=6): 85±9.7%; low-Na+CAIP (n=3): 104±10.2% control). Fostreicin also reduced the low-Na effect (low-Na: 54±2.5%; low-Na+fostreicin (n=6): 77±6.1% control) but not tautomycin (low-Na: 65±4.9%; low-Na+TTM (n=7): 66±3.9%). Phosphorylation of Cx43 during low-Na was diminished by CysA, CAIP and FST.

The phosphatases calcineurin and PP2A mediate the decrease of Gj in the left atrium during elevated [Ca2+]i. Connexin43 is one of the targets for calcineurin and PP2A.