Background: Abnormal intracellular Ca2+ homeostasis occurs in chronic atrial fibrillation(AF). The intracellular Ca2+ concentration is regulated by ryanodine and inositol 1,4,5-trisphosphate (IP3) receptors. Changes occur in ryanodine receptors in atrial tissue from patients in chronic AF. Whether AF patients have alterations in atrial IP3 receptors was investigated.
Methods: IP3 receptor expression was analyzed in the right atrial myocardium from 13 mitral valvular disease (MVD) patients with AF (MVD/AF), 5MVD patients with normal sinus rhythm(MVD/NSR), and 8 control patients with NSR(tissue obtained during coronary artery bypass surgery). Hemodynamic and echocardiographic data were obtained preoperatively, and an immunohistochemical study was performed on the atrial tissue.
Results: The relative expression level of IP3 receptor protein was significantly greater in MVD/AF (0.75 +/- 0.26) than in MVD/NSR (0.42 +/- 0.13, p < 0.01), and both were significantly above the control value (0.14 +/- 0.08). The relative expression level of IP3 receptor mRNA was significantly greater in MVD/AF(0.028 +/- 0.008) than in control subjects (0.015 +/- 0.004, p < 0.01), but MVD/AF patients did not differ from MVD/NSR (0.020 +/- 0.006) patients. The relative expression levels of IP3 receptor protein and mRNA were higher in patients with left atrial dimension > or = 40 mm, pulmonary capillary wedge pressure > or = 10 mmHg, and right atrial pressure > or = 5 mmHg. IP3 receptors were overexpressed in the cytosol and at the nuclear envelope of atrial myocytes in MVD.
Conclusions: Since chronic mechanical overload of the atrial myocardium increases IP3 receptor expression, especially in patients with chronic AF, up-regulation of IP3 receptors may be important in modulating intracellular Ca2+ homeostasis and initiating and/or perpetuating AF.