Intracellular Calcium Stores and Sodium- Calcium Exchanger in Cardiac Myocytes

Cytosolic Ca2+, [Ca]I , has a key role in intracellular signalling during excitation-contraction coupling (E-C coupling) in cardiac myocytes. The sarcoplasmic reticulum (SR) is a main intracellular Ca2+ store and the Na+-Ca2+ exchanger (NaCaX) is a major mechanism to extrude Ca2+ for balancing the Ca2+ influx via L-type Ca2+ channels during excitation. Furthermore, [Ca]I also affects the configuration of the action potential (AP). The present study, by combination of animal experiments and computer simulations, investigated the roles of [Ca]I, SR and NaCaX in cardiac myocytes, in Ca2+-induced Ca2+ release (CICR) and in modulation of APs. The following were studied: (I) the stretch-induced effects on rat atrium and the role of [Ca]I in modulation of AP; (II) the role of the SR in modulation in rat atrium by stretch; (III) the role of NaCaX; (IV) the role of [Ca]I in modulation of action potential duration (APD) in myocytes with short and long action potential duration. In isolated rat atrial preparations, the physiological or moderate stretch stimulus caused twophasic rise of developed contraction, rapid and slow phases, accompanied with slow increments of [Ca]I and prolongations of action potentials durations in continuous recordings. In sustained stretch, the APD and [Ca]I were all increased significantly when intra-atrial pressure increased from 1 to 3 mmHg. In computer simulations, employing a rat atrial model (RA model), it was found that stretch-activated channels and increased Tn C affinity for Ca2+ alone could not produce the changes in the experiments. Only after both mechanisms applied to model cells, the main experimental findings could be mimicked (I). The prolongation of APD induced by stretch in rat atrial preparations was reversed after depleting the Ca2+ content of the SR by application of the SR functional inhibitors, ryanodine, thapsigargin and caffeine (II). In the computer simulation using modified guinea pig ventricular model, the Ca2+ entry via the reversal of NaCaX was found to be accounting 25% of the total activator Ca2+ for triggering Ca2+ release from the SR during normal excitation. This contribution increases with elevated [Na]i (III). In a guinea pig ventricular model (GPV model) and a RA model were employed for investigating the regulation of APD by [Ca]Idependent membrane currents. Increased SR Ca2+ content produced an elevated [Ca]I in both model cells, leading to prolongation of APD in the RA model but shortening in the GPV model. Increased [Ca]I enhances the NaCaX current in the same scale in both models, but inhibits L-type Ca current much more in the GPV model than the RA model (IV). In conclusion, (I) Stretch-induced [Ca]I increase prolongs the rat atrial AP by enhancing the NaCaX inward current. Stretch-activated channels (SACs) and increased affinity of TnC for Ca2+ are main general factors responsible for the variety of changes of cardiac muscles induced by stretch. (II) The SR plays a crucial role in the modulation of myocytes by accumulating the additional Ca2+ influx via sarcolemma during stretch. (III) The NaCaX contributes a small part for activator Ca2+ for calcium release from the SR during normal cardiac E-C coupling. However, this contribution is [Na]idependent, and in some pathological conditions, it may be a potential factor for cardiac arrhythmogenesis. (IV) Different effects on the NaCaX and L-type channels induced by increased [Ca]I leads to the dispersion of the change of APD in myocytes with long and short AP during inotropic interventions that increase the [Ca]I.