1. Confocal microscopy was used to investigate hormone‐induced subcellular Ca2+ release signals from the endoplasmic reticulum (ER) in a prototype non‐excitable cell line (HeLa cells). 2. Histamine application evoked two types of elementary Ca2+ signals: (i) Ca2+ blips arising from single ER Ca2+ release channels (amplitude, 30 nM; lateral spreading, 1.3 microns); (ii) Ca2+ puffs resulting from the concerted activation of several Ca2+ blips (amplitude, 170 nM; spreading, 4 microns). 3. Ca2+ waves in the HeLa cells arose from a variable number of initiation sites, but for individual cells, the number and subcellular location of the initiation sites were constant. The kinetics and amplitude of global Ca2+ signals were directly proportional to the number of initiation sites recruited. 4. Reduction of the feedback inherent in intracellular Ca2+ release caused saltatoric Ca2+ waves, revealing the two principal steps underlying wave propagation: diffusion and regeneration. Threshold stimulation evoked abortive Ca2+ waves, caused by the limited recruitment of Ca2+ puffs. 5. The hierarchy of Ca2+ signalling events, from fundamental levels (blips) to intermediate levels (puffs) to Ca2+ waves, is a prototype for Ca2+ signal transduction for non‐excitable cells, and is also analogous to the Ca2+ quarks, Ca2+ sparks and Ca2+ waves in cardiac muscle cells.

Abstract-Vascular calcification is associated with an increased risk of myocardial infarction; however, the mechanisms linking these 2 processes are unknown. Studies in macrophages have suggested that calcium phosphate crystals induce the release of proinflammatory cytokines; however, no studies have been performed on the effects of calcium phosphate crystals on vascular smooth muscle cell function. In the present study, we found that calcium phosphate crystals induced cell death in human aortic vascular smooth muscle cells with their potency depending on their size and composition. Calcium phosphate crystals of approximately 1 m or less in diameter caused rapid rises in intracellular calcium concentration, an effect that was inhibited by the lysosomal proton pump inhibitor, bafilomycin A1. Bafilomycin A1 also blocked vascular smooth muscle cell death suggesting that crystal dissolution in lysosomes leads to an increase in intracellular calcium levels and subsequent cell death. These studies give novel insights into the bioactivity of calcified deposits and suggest that small calcium phosphate crystals could destabilize atherosclerotic plaques by initiating inflammation and by causing vascular smooth muscle cell death. (Circ Res. 2008;103:e28-e34.)

We have used reverse transcriptase-polymerase chain reaction to investigate the expression of ryanodine receptors in several excitable and nonexcitable cell types. Consistent with previous reports, we detected ryanodine receptor expression in brain, heart, and skeletal muscle. In addition, we detected ryanodine receptor expression in various other excitable cells including PC12 and A7r5 cells. Several muscle cell lines (BC 3 H1, C2C12, L6, and Sol8) weakly expressed ryanodine receptor when undifferentiated but strongly expressed type 1 and type 3 ryanodine receptor isoforms when differentiated into a muscle phenotype. Only 2 (HeLa and LLC-PK1 cells) out of 11 nonexcitable cell types examined expressed ryanodine receptors. Expression of ryanodine receptors at the protein level in these cells was confirmed using [

Physiological role of atrial myocytesThe heart undergoes a cycle of events that cause blood to be propelled to the lungs and the body. These events can be divided into two stages: diastole and systole. During the diastolic stage, the atrial and ventricular myocytes are relaxed. Systole refers to the period of contraction and consequent ejection of blood from the ventricles to the pulmonary artery or aorta. The cardiac cycle is initiated by the sinoatrial nodea group of specialised non-contractile cardiac myocytes positioned in the wall of the right atrium. All of the cells in the heart have an intrinsic ability generate action potentials (electrical impulses). However, the sinoatrial node acts as the primary pacemaker because its cells naturally discharge at a high rate, and so override the other potential pacemaking sites. Hormonal modulation of the sinoatrial node is one of the principal mechanisms for altering the frequency of the heartbeat. From the sinoatrial node, the action potential spreads over the atria causing them to contract and push blood into the ventricles. As the wave of depolarisation sweeps across the heart, it reaches the atrioventricular node, which filters and relays the signal to the ventricles via specialised conduction tissue including the Purkinje fibres. The atrial chambers contract and relax before the ventricular systole, and their activation is evident as separate electrical activity in an electrocardiogram. The time course of contraction is marginally shorter in atria compared to ventricles. Both cell types reach peak contraction within a few tens of milliseconds (Luss et al., 1999).The ventricles contract more forcefully than the atrial chambers and are predominantly responsible for forcing blood out of the heart. However, atria can play a significant role in altering the amount of blood that loads into the ventricles before to systole. When a person is at rest, the contribution of atria to the filling of ventricles with blood is relatively low. The majority (~90%) of ventricular refilling occurs as a consequence of the venous pressure of blood returning to the heart. However, if the heart rate increases, such as during exercise or stress, then atrial contraction can account for ~20-30% of the volume of blood in the ventricles. This contribution to ventricular refilling is known as 'atrial kick' (Lo et al., 1999). Under some pathological conditions (such as mitral valve stenosis) and during ageing, the dependence on atrial kick can be critical (Nicod et al., 1986).A common form of cardiac dysrhythmia known as 'atrial fibrillation' occurs when electrical impulses do not solely arise from the sinoatrial node, but instead spontaneously occur with high frequency from sites around the atria (~350 discharges per minute, compared with the normal sinoatrial rhythm of 60-80 beats per minute). The rapid and irregular electrical discharges during atrial fibrillation cause the atria to quiver and thereby

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The pacemaker Ca2+ puff sites that initiate Ca2+ responses are temporally and spatially stable within cells. These Ca2+ release sites are distinguished from their neighbours by an intrinsically higher InsP3 sensitivity.

Inositol 1,4,5′-triphosphate receptor II (IP3RII) calcium channel expression is increased in both hypertrophic failing human myocardium and experimentally induced models of the disease. The ectopic calcium released from these receptors induces pro-hypertrophic gene expression and may promote arrhythmias. Here, we show that IP3RII expression was constitutively restrained by the muscle-specific miRNA, miR-133a. During the hypertrophic response to pressure overload or neurohormonal stimuli, miR-133a down-regulation permitted IP3RII levels to increase, instigating pro-hypertrophic calcium signaling and concomitant pathological remodeling. Using a combination of in vivo and in vitro approaches, we demonstrated that IP3-induced calcium release (IICR) initiated the hypertrophy-associated decrease in miR-133a. In this manner, hypertrophic stimuli that engage IICR set a feed-forward mechanism in motion whereby IICR decreased miR-133a expression, further augmenting IP3RII levels and therefore pro-hypertrophic calcium release. Consequently, IICR can be considered as both an initiating event and a driving force for pathological remodeling.

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