Action potential duration (APD) depends on both the preceding diastolic interval (DI) (restitution), and also on the activation history of the cardiomyocyte during the preceding minutes (short-term memory or hysteresis). Intracellular calcium cycling is an important determinant in arrhythmia susceptibility, yet hysteresis of calcium transient duration (CaTD) has not been extensively studied. We hypothesised that APD and CaTD restitution exhibit hysteresis.Methods
Hearts from five adult Sprague-Dawley rats were isolated, Langendorff-perfused and dual optical mapping was performed using the voltage-sensitive dye RH237 and the calcium indicator Rhod-2-AM. Stimuli of cycle length (CL) 300ms were delivered to the heart for 60 seconds and fluorescence emitted from the dyes was recorded. The CL was decreased from 300ms to 100ms in decrements of 20ms and the same measurements were taken (the descending limb), and then increased back up to 300ms in steps of 20ms (the ascending limb).Results
For voltage, a hysteresis loop was observed for each heart, where the APD for a given DI was greater when the CL was decreasing than when it was increasing (Fig. A). At DI 150ms, APD90 for Desc. limb 69.9 ± 5.6 vs. Asc. limb 52.4 ± 1.7ms (p=0.02). However, no hysteresis loop was observed for CaTD (Fig. B); at DI 100ms, CaTD90 for Desc. limb 116.8 ± 0.9 vs. Asc. limb 115.9 ± 2.0ms (p=NS). Whilst no basal-apical variation in epicardial APD was noted, a spatial gradient of voltage-calcium delay existed, increasing from apex to base (at CL=120ms: Apex 2.7 ± 0.9, Mid 4.7 ± 0.7, Base 5.8 ± 0.7ms. Apex vs. base p < 0.05).Conclusions
APD restitution demonstrates hysteresis, whereas CaTD restitution demonstrates no significant hysteresis. This suggests that APD restitution hysteresis does not depend on intracellular CaTD.