For Langendorff perfusion of the whole heart and for superfusion of isolated right ventricular preparations, we used a standard Tyrode solution composed of (in mM): 135 NaCl, 5.4 KCl, , , , 10 glucose, 10 HEPES (pH 7.4 adjusted with NaOH). The bolus dye injections were prepared from stock solutions containing 25 mM di-4-ANBDQBS in ethanol, or 10 mM di-4-ANEPPS in DMSO. The dyes were further dissolved in Tyrode to final concentrations of 50 μM and 25 μM for di-4-ANBDQBS and di-4-ANEPPS, respectively. To facilitate dye loading, 0.01% Pluronic F-127 was added. To eliminate motion artifact, we used 20 μM blebbistatin. The dye di-4-ANBDQBS was provided by Dr. L. Loew (University of Connecticut, USA); di-4-ANEPPS came from Invitrogen, USA; blebbistatin came from Calbiochem, UK; and all other chemicals were from Sigma-Aldrich.