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Phospholemman (PLM, FXYD1), abundantly expressed in the heart, is the primary cardiac sarcolemmal substrate for PKA and PKC. Evidence supports the hypothesis that PLM is part of the cardiac Na-K pump complex and provides the link between kinase activity and pump modulation. PLM has also been proposed to modulate Na/Ca exchanger activity and may be involved in cell volume regulation. This study characterized the phenotype of the PLM knockout (KO) mouse heart to further our understanding of PLM function in the heart. PLM KO mice were bred on a congenic C57/BL6 background. In vivo conductance catheter measurements exhibited a mildly depressed cardiac contractile function in PLM KO mice, which was exacerbated when hearts were isolated and Langendorff perfused. There were no significant differences in action potential morphology in paced Langendorff-perfused hearts. Depressed contractile function was associated with a mild cardiac hypertrophy in PLM KO mice. Biochemical analysis of crude ventricular homogenates showed a significant increase in Na-K-ATPase activity in PLM KO hearts compared with wild-type controls. SDS-PAGE and Western blot analysis of ventricular homogenates revealed small, nonsignificant changes in Na-K-ATPase subunit expression, with two-dimensional gel (isoelectric focusing, SDS-PAGE) analysis revealing minimal changes in ventricular protein expression, indicating that deletion of PLM was the primary reason for the observed PLM KO phenotype. These studies demonstrate that PLM plays an important role in the contractile function of the normoxic mouse heart. Data are consistent with the hypothesis that PLM modulates Na-K-ATPase activity, indirectly affecting intracellular Ca and hence contractile function. FXYD1; contractile function; intracellular sodium regulation

Objective: The basal activity of p38 MAPK has recently been shown to impair myocardial contractility. This kinase is activated by ischemia and short-term hibernation. We hypothesized that p38 MAPK activation may contribute to the contractile deficit that characterizes low-flow ischemia. Methods: In Langendorff-perfused isolated C57BL/6 mouse hearts, perfusion pressure was reduced from 85 to 15 or 30 mm Hg for 120 min to induce ischemic left ventricular dysfunction. The effect of the p38 MAPK inhibitor SB203580 (1 μM/l) on contractile function and p38 MAPK activation was assessed. Results: Reduction in perfusion pressure to 15 or 30 mm Hg was accompanied by stable reductions in coronary flow (83±2% and 66±2%, respectively) and developed pressure (84±2% and 61±3%), with minimal infarction (15.6±0.69% and 10.6±0.98% of LV myocardium, respectively), but marked activation of p38 MAPK (reflected in pHSP27 1092±326% basal and 996±301% basal, respectively). The p38 MAPK inhibitor SB203580, present during the last 60 min of reduced pressure perfusion, prevented p38 MAPK activation (pHSP27 281±92% basal, p=0.01 and 186±72% basal, p=0.01) but, despite the presence of a contractile reserve, had no effect on developed pressure. Similarly, early treatment with SB203580 started 5 min after the onset of reduced flow also failed to attenuate contractile dysfunction. Conclusion: The p38 MAPK activation that accompanies short-term hibernation does not appear to contribute to the contractile deficit. [Copyright &y& Elsevier]