Abstract 410: Cardiac Ryanodine Receptor (RyR) Channels Communicate among themselves and with Dyhidropyridine Receptor L-type Calcium Channels (DHPR)
In the heart, during the action potential, Ca2+ entry through DHPRs induces massive Ca2+ mobilization from the sarcoplasmic reticulum (SR). Global release arises from spatio-temporal summation of many localized elementary Ca2+ release events (Ca2+ sparks). Sparks, in turn, represent the coordinated activation/deactivation of a cluster of RyRs. Accordingly, RyR are found associated in arrays in the SR membrane. Here, we present evidence that RyR channels remain associated after reconstitution into planar lipid bilayers and synchronously gate (“coupled gating”). This coupled gating is subject of modulation by SR Ca2+ (possibly via feed-through regulation of neighboring channels), cytosolic Mg2+ as well as by various pharmacological agents. We found that the activity of coupled RyRs in bilayers appears to be much higher than in intact cells. We hypothesize that these differences may reflect the existence in cells (but not in bilayers) of Ca2+ entry-independent modulation of RyRs by DHPRs. We studied Ca2+ sparks in cardiac myocytes permeabilized with saponin or internally perfused via a patch pipette. Both, bathing and pipette solutions contained same low [Ca2+] (100 nM) which renders the DHPR ineffective as a Ca2+ channel. Still, DHPR blockers (e.g. nimodipine and FS-2) decreased (>50%), and DHPR agonists Bay-K8644 and FPL-64176 increased Ca2+ spark frequency. The DHPR modulators did not affect SR Ca2+ load (estimated from caffeine-induced Ca2+ transients) or SERCA activity (measured as Ca2+ loading rates of isolated SR microsomes) and did not change cardiac RyR channel gating (planar lipid bilayer studies). This suggests that Ca2+ sparks may be modulated by conformational changes in neighboring DHPRs. When aiming to understand cardiac calcium signaling in health as well as its alteration in disease, the focus is generally on the behavior of RyR (mainly single channels) or DHPR Ca2+ currents. Our results suggest that attention should be also given to the physiological role of RyR-RyR and DHPR-RyR communication processes during EC-coupling. They appear to be potential sites for pathological failure and possibly therapeutic modulation. This work was supported by Grants from NIH (to MF and LAB), AHA (to AZ, JAC and LAB) and by MDA (to JAC).