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Figure 6 Immunostaining for inositol 1,4,5 trisphosphate receptor (IP3 receptor) protein in the atrial myocardium. In a control patient (A), immunoreactivity for IP3R protein was found within the cytosol and at the nuclear envelope of myocytes. In patients with mitral valvular disease with either normal sinus rhythm (B) or atrial fibrillation (C), the expression was increased both in the cytosol and at the nuclear envelope.