Action potential conduction velocity in myocardium is a function of gap junction (GJ) electrical resistance [1]. GJ resistance itself depends on the phosphorylation state of gap junction proteins, connexins. Consequently we investigated the role of serine threonine protein phosphatases (PPs) on gap junction conductance, in particular the Ca2+-dependent PP, calcineurin, or the Ca2+-independent PPs, PP1, PP2A.Male, Dunkin-Hartley guinea pigs were euthanised and the left atrium dissected from the heart. Strips were mounted in an oil-gap chamber for measurement of GJ resistance (Rj) [2] in control solution (Na=149.4 mM), or in low-Na solution (Na=29.4 mM) to raise the intracellular Ca2+ concentration. Calcineurin inhibitors, cyclosporine-A (CysA; 5 µM; n=6) or calcineurin auto-inhibitory peptide (CAIP; 50 µM, n=3) were used in control and low-Na solutions. Okadaic acid (OA) was used at 2 nM or 100 nM (n=4) to assess the role of PP2A or PP1, respectively, on Rj. The more selective PP1 inhibitor, tautomycin (TTM; 5 nM; n=7), was also used. The selective Ca2+/Calmodulin dependent protein kinase II (CaMKII) inhibitor, auto-inhibitory peptide (AIP; 10 µM; n=3), was used to assess the role of CaMKII. Values are mean ± SEM and were normalised to control (=100%) conditions. Differences between sets was tested by ANOVA; the null hypothesis was rejected at p<0.05.Low-Na solution significantly and reversibly increased Rj in all preparations. CysA had no significant effect under control conditions. However, CysA diminished the effects of low-Na solution on Rj (low-Na: 183±19.1% vs low-Na+CysA: 120±12.9%, p<0.05). CAIP also abolished the effect of low-Na on Rj (low-Na: 177±16.7% vs low-Na+CAIP: 93±12.6%, p<0.05). AIP had no effect on Rj in low-Na solution (low-Na: 174±4.2% vs low-Na+AIP: 183±4.1%). OA (2 nM) had no significant effect on Rj under control conditions (97±5.7%) but significantly reduced the effect of low-Na solution (low-Na: 180±13.7% vs low-Na+2 nM OA: 131±13.4%). The increase of Rj induced by low-Na was not affected upon addition of 100 nM OA (low-Na: 180±13.7% vs low-Na+OA: 188±18.8%) or TTM (low-Na: 142±13.4% vs low-Na+TTM: 153±10.7%).The phosphatases calcineurin and PP2A mediate the increase of Rj in the left atrium during elevated cytosolic Ca2+. However, PP1 has no role in regulating Rj. The particular connexin subtype that is targeted is currently unknown.