Effect Of Extracellular Ca2+ On Intracellular Ca2+ Dynamics In Intact Hearts Of Wildtype And Calsequestrin 2 Ko Mice

Abstract

Free [Ca2+] in the lumen of the sarcoplasmic reticulum (SR) is a critical factor controlling Ca2+-induced Ca2+-release (CICR). Ca2+-binding protein calsequestrin 2 (Casq2) located in SR lumen is important component in the regulation of CICR. One of the possible roles of Casq2 could be to prevent the depletion of the luminal Ca2+ stores during Ca2+ release. In order to modify the SR Ca2+ content we changed extracellular Ca2+ concentration in hearts from wildtype and Casq2 KO mice. The dynamics of intra-SR Ca2+ depletion, myoplasmic free Ca2+ and time course of the action potentials (APs) were measured from the epicardial layer of murine hearts using Pulsed Local Field Fluorescence Microscopy. Lowering extracellular Ca2+ resulted in smaller amplitude of Ca2+ transients, acceleration of the restitution of CICR and diminishing Ca2+ alternans. The ablation of Casq2 led to noticeable changes in the dynamics of CICR especially at low extracellular Ca2+. The prolongation of the release can be explained by the modification of the properties of the ryanodine receptors (RyR2) in the absence of Casq2. The restitution of CICR, which was already accelerated by low extracellular Ca2+ was even faster in hearts of KO mice. In addition, the decline in cytosolic Ca2+ level observed in response to low extracellular Ca2+ was more pronounced in KOs. APs (conducted in the presence of blebbistatin) display a prolongation of the phase 2 of cardiac APs at 37°C as extracellular Ca2+ was decreased. Interestingly, an opposite effect was observed at room temperature (21°C). In conclusion, the results obtained on transgenic mice lacking Casq2 suggest that this protein can be engaged in controlling amplitude of CICR not only as a Ca2+ buffer but also a modulator of RyR2.