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Fig. 3 mapre2 loss of function leads to decreased Na+ current. A, Voltage clamp protocol used to measure sodium current (INa; top) and typical INa traces ( bottom) of a freshly isolated wild-type (WT) and knockout (KO) myocyte. As detailed in the Supplemental Material, we collected ventricles from 4 to 5 adult fish that were pooled for each cell isolation. n indicates the number of cells measured from 3 isolations. B, Average current-voltage (I-V) relationships ( left) and dot plots of INa density at −20 mV ( right) in WT and KO myocytes showing a decrease of INa density in KO myocytes. I-V relationships were compared with the 2-way repeated measures ANOVA (P=0.036) followed by pairwise comparison using the Student-Newman-Keuls test. The INa densities at −20 mV were compared using the Student t test. The numbers indicate the P values <0.05. C, The time course of current inactivation at −20 mV was fitted by a double-exponential equation: I/Imax=Af×exp(−t/τf)+As×exp(−t/τs), where Af and As are the fractions of the fast and slow inactivation components, and τf and τs are the time constants of the fast and slow inactivating components, respectively. Data did not differ significantly (Student t test). Neither the time constants nor the relative amplitudes were different between WT and KO myocytes. D, Voltage dependency of activation ( left) and V1/2 and k of the Boltzmann fits of every cell measured ( right). Solid lines are Boltzmann fits to the average data. Data did not differ significantly (Student t test). E, Voltage clamp protocol used to measure voltage dependency of INa inactivation ( top) and typical INa inactivating traces ( bottom). F, Voltage dependency of inactivation and V1/2 and k of the Boltzmann fits of all cells ( right). Solid lines are Boltzmann fits to the average data. Data did not differ significantly (Student t test).