Inositol 1,4,5-Trisphosphate Directs Ca²⁺Flow between Mitochondria and the Endoplasmic/Sarcoplasmic Reticulum: A Role in Regulating Cardiac Autonomic Ca²⁺Spiking

Abstract

The signaling role of the Ca(2+) releaser inositol 1,4, 5-trisphosphate (IP(3)) has been associated with diverse cell functions. Yet, the physiological significance of IP(3) in tissues that feature a ryanodine-sensitive sarcoplasmic reticulum has remained elusive. IP(3) generated by photolysis of caged IP(3) or by purinergic activation of phospholipase Cgamma slowed down or abolished autonomic Ca(2+) spiking in neonatal rat cardiomyocytes. Microinjection of heparin, blocking dominant-negative fusion protein, or anti-phospholipase Cgamma antibody prevented the IP(3)-mediated purinergic effect. IP(3) triggered a ryanodine- and caffeine-insensitive Ca(2+) release restricted to the perinuclear region. In cells loaded with Rhod2 or expressing a mitochondria-targeted cameleon and TMRM to monitor mitochondrial Ca(2+) and potential, IP(3) induced transient Ca(2+) loading and depolarization of the organelles. These mitochondrial changes were associated with Ca(2+) depletion of the sarcoplasmic reticulum and preceded the arrest of cellular Ca(2+) spiking. Thus, IP(3) acting within a restricted cellular region regulates the dynamic of calcium flow between mitochondria and the endoplasmic/sarcoplasmic reticulum. We have thus uncovered a novel role for IP(3) in excitable cells, the regulation of cardiac autonomic activity.

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

“…Purkinje cells from infarcted hearts show proarrhythmic propagating Ca 2+ waves that are reduced in frequency and amplitude by 2-APB. Also consistent with the notion of IP 3 Rs regulating automaticity of Purkinje fibers, reduction of IP 3 production using a phospholipase C inhibitor was demonstrated to inhibit their autonomous Ca 2+ spiking [102].…”

“…Indeed, neonatal animals have a relatively high expression of IP 3 Rs (compared to adult cells), and the channels are demonstrably functional. For example, addition of IP 3 to permeabilized cells, or uncaging of photo-releasable IP 3 , has directly demonstrated the presence of active IP 3 Rs in situ within neonatal myocytes [102,103]. Furthermore, neonatal cardiac myocytes respond to several agonists (e.g.…”

“…Furthermore, neonatal cardiac myocytes respond to several agonists (e.g. endothelin-1, angiotensin II, phenylephrine, ATP, pros-taglandins, IGF-1) with IP 3 production and Ca 2+ mobilization [102,104]. These same agonists also promote hypertrophic growth of neonatal cardiac myocytes [104,105].…”

“…These same agonists also promote hypertrophic growth of neonatal cardiac myocytes [104,105]. Studies using immunostaining, Western blotting and analysis of mRNA have suggested that IP 3 R types 1, 2 and 3 are expressed in neonatal cells [102,103,106].…”

“…Almost all studies that examined either the distribution of IP 3 Rs, or the spatial pattern of IP 3 -evoked Ca 2+ signals, have reported that the channels are largely expressed in close proximity to the nucleus in neonatal cells [102,103,[106][107][108]. This is unlike RyRs, which have a more widespread expression pattern in neonatal cells [102]. The perinuclear release of IP 3 -sensitive Ca 2+ stores can impact specifically on nucleoplasmic Ca 2+ levels [103] and thereby promote hypertrophic gene transcription [109].…”