Inositol trisphosphate-dependent Ca stores and mitochondria modulate slow wave activity arising from the smooth muscle cells of the guinea pig prostate gland

Commonality and heterogeneity of pacemaker mechanisms in the male reproductive organs

During emission, the first phase of ejaculation, smooth muscle in organs of the male reproductive tract (MRT) vigorously contract upon sympathetic nerve excitation to expel semen consisting of sperm and seminal plasma. During inter-ejaculation phases, the epididymis, seminal vesicles and prostate undergo spontaneous phasic contractions (SPCs), this transporting and maintaining the quality of sperm and seminal plasma. Recent studies have revealed platelet-derived growth factor receptor α-expressing (PDGFRα+) subepithelial interstitial cells in seminal vesicles subserve the role of pacemaker cells that electrically drive SPCs in this organ. PDGFRα+ smooth muscle cells in the epididymis also appear to function as pacemaker cells implicating PDGFRα as a potential signature molecule in MRT pacemaking. The dominant mechanism driving pacemaking in these organs is the cytosolic Ca2+ oscillator. This operates through entrainment of the release-refill cycle of Ca2+ stores, the released Ca2+ ions opening Ca2+-activated chloride channels, including in some cases ANO1 (TMEM16A), with the resultant pacemaker potential activating L-type voltage-dependent Ca2+ channels in the smooth muscle causing contraction (viz. SPCs). A second pacemaker mechanism, namely the membrane oscillator also has a role in specific cases. Further investigations into the commonality and heterogeneity of MRT pacemakers will open an avenue for understanding the pathogenesis of male infertility associated with deterioration of seminal plasma.

Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

Propagation of Pacemaker Activity and Peristaltic Contractions in the Mouse Renal Pelvis Rely on Ca2+-activated Cl- Channels and T-Type Ca2+ Channels

The process of urine removal from the kidney occurs via the renal pelvis (RP). The RP demarcates the beginning of the upper urinary tract and is endowed with smooth muscle cells. Along the RP, organized contraction of smooth muscle cells generates the force required to move urine boluses toward the ureters and bladder. This process is mediated by specialized pacemaker cells that are highly expressed in the proximal RP that generate spontaneous rhythmic electrical activity to drive smooth muscle depolarization. The mechanisms by which peristaltic contractions propagate from the proximal to distal RP are not fully understood. In this study, we utilized a transgenic mouse that expresses the genetically encoded Ca²⁺ indicator, GCaMP3, under a myosin heavy chain promotor to visualize spreading peristaltic contractions in high spatial detail. Using this approach, we discovered variable effects of L-type Ca²⁺ channel antagonists on contraction parameters. Inhibition of T-type Ca²⁺ channels reduced the frequency and propagation distance of contractions. Similarly, antagonizing Ca²⁺-activated Cl^- channels or altering the transmembrane Cl^- gradient decreased contractile frequency and significantly inhibited peristaltic propagation. These data suggest that voltage-gated Ca²⁺ channels are important determinants of contraction initiation and maintain the fidelity of peristalsis as the spreading contraction moves further toward the ureter. Recruitment of Ca²⁺-activated Cl^- channels, likely Anoctamin-1, and T-type Ca²⁺ channels are required for efficiently conducting the depolarizing current throughout the length of the RP. These mechanisms are necessary for the efficient removal of urine from the kidney.

Uterine contractions in rodent models and humans

Aberrant uterine contractions can lead to preterm birth and other labour complications and are a significant cause of maternal morbidity and mortality. To investigate the mechanisms underlying dysfunctional uterine contractions, researchers have used experimentally tractable small animal models. However, biological differences between humans and rodents change how researchers select their animal model and interpret their results. Here, we provide a general review of studies of uterine excitation and contractions in mice, rats, guinea pigs, and humans, in an effort to introduce new researchers to the field and help in the design and interpretation of experiments in rodent models.

Generation and Regulation of Spontaneous Contractions in the Prostate

Spontaneous myogenic contractions have been shown to be significantly upregulated in prostate tissue collected from men with Benign Prostatic Hyperplasia (BPH), an extremely common disorder of the ageing male. Although originally thought likely to be involved in 'housekeeping' functions, mixing prostatic secretions to prevent stagnation, these spontaneous myogenic contractions provide a novel opportunity to understand and treat BPH. This treatment potential differs from previous models, which focused exclusively on attenuating nerve-mediated neurogenic contractions. Previous studies in the rodent prostate have provided an insight into the mechanisms underlying the regulation of myogenic contractions. 'Prostatic Interstitial Cells' (PICs) within the prostate appear to generate pacemaker potentials, which arise from the summation of number of spontaneous transient depolarisations triggered by the spontaneous release of Ca²⁺ from internal stores and the opening of Ca²⁺-activated Cl^- channels. Pacemaker potentials then conduct into neighbouring smooth muscle cells to generate spontaneous slow waves. These slow waves trigger the firing of 'spike-like' action potentials, Ca²⁺ entry and contraction, which are not attenuated by blockers of neurotransmission. However, these spontaneous prostatic contractions can be modulated by the autonomic nervous system. Here, we discuss the mechanisms underlying rodent and human prostate myogenic contractions and the actions of existing and novel pharmacotherapies for the treatment of BPH. Understanding the generation of human prostatic smooth muscle tone will confirm the mechanism of action of existing drugs, inform the identification and effectiveness of new pharmacotherapies, as well as predict patient outcomes.

Role of prostatic interstitial cells in prostate motility

The prostate is a gland whose secretions contribute to the seminal fluids ejaculated upon activation of autonomic sympathetic nerves. In elder males, the prostate undergoes an increase in stroma mass and myogenic tone, leading to benign prostatic hyperplasia that occludes the proximal urethra and the presentation of various lower urinary tract symptoms that decrease their quality of life. This review summarises the role of prostatic interstitial cells (PICs) in the generation of the spontaneous tone in the prostate. It presents current knowledge of the role of Ca²⁺ plays in PIC pacemaking, as well as the mechanisms by which this spontaneous activity triggers slow wave generation and stromal contraction. PICs display a small T-type Ca²⁺ current (I_CaT) and a large L-type Ca²⁺ current (I_CaL). In contrast to other interstitial cells in the urinary and gastrointestinal tracts, spontaneous Ca²⁺ signalling in PICs is uniquely dependent on Ca²⁺ influx through I_CaL channels. A model of prostatic pacemaking is presented describing how I_CaL can be triggered by an initial membrane depolarization evoked upon the selective opening of Ca²⁺-activated Cl^– channels by Ca²⁺ flowing only through I_CaT channels. The resulting current flow through I_CaL results in release of Ca²⁺ from internal stores and the summation of Cl^–-selective spontaneous transient depolarizations (STDs) to form pacemaker potentials that propagate passively into the prostatic stroma to evoke regenerative action potentials and excitation-contraction coupling.

Calcium signalling in Cajal-like interstitial cells of the lower urinary tract

Interstitial cells of Cajal (ICC) serve several critical physiological roles in visceral smooth muscle organs, including acting as electrical pacemakers to modulate phasic contractile activity and as intermediaries in motor neurotransmission. The major roles of ICC have been described in the gastrointestinal tract, however, ICC-like cells (ICC-LC) can also be found in other visceral organs, including those of the lower urinary tract (LUT), where they provide similar functions, acting as electrical pacemakers and as intermediary cells involved in the modulation of neurotransmission to adjacent smooth muscle cells. The physiological functions of ICC-LC, in particular their role as pacemakers, relies on their ability to generate transient and propagating intracellular Ca(2+) events. The role of ICC-LC as pacemakers and neuromodulators in the LUT is increasingly apparent and the study of their intracellular Ca(2+) dynamics will provide a better understanding of their role in LUT excitability.

Voltage dependence of slow wave frequency in the guinea pig prostate

PURPOSE

Spontaneous phasic contractions of the guinea pig prostate stroma result from the generation of slow waves that appear to primarily rely on spontaneous Ca(2+) release from the endoplasmic/sarcoplasmic reticulum and subsequent opening of Ca(2+) activated chloride channels. We investigated voltage dependent mechanisms in the regulation of slow wave frequency.

MATERIALS AND METHODS

Changes in membrane potential were recorded using conventional intracellular recording techniques while simultaneously measuring the isometric tension of guinea pig prostate lobes. Fluorescence immunohistochemistry was done to determine the cellular composition of the prostate stroma.

RESULTS

Depolarization induced by high K(+) solution, K(+) free solution or outward current injection was associated with increased slow wave frequency. In contrast, hyperpolarization induced by the re-addition of K(+), adenosine triphosphate sensitive K(+) channel openers or inward current injection prevented slow wave generation. K(+) channel openers induced hyperpolarization and the cessation of slow waves was reversed by glibenclamide (10 μM). Nifedipine (1 to 10 μM) shortened the duration of slow waves and pacemaker potentials but often failed to prevent their generation and associated contractions. Subsequently Ni(2+) (100 μM) or mibefradil (1 μM) largely suppressed slow waves and abolished residual contractions. Immunohistochemistry revealed small interconnected smooth muscle bundles as well as vimentin positive interstitial cells but failed to show a network of Kit positive interstitial cells.

CONCLUSIONS

Prostate slow wave frequency is voltage dependent due to the significant contribution of L-type and T-type Ca(2+) channels. Prostate slow waves may arise from cooperation between spontaneous Ca(2+) release from internal stores and plasmalemmal voltage dependent Ca(2+) channels.

Tamsulosin modulates, but does not abolish the spontaneous activity in the guinea pig prostate gland

AIMS

To examine the effects of the α1A -adrenoceptor antagonist, tamsulosin, on spontaneous contractile and electrical activity in the guinea-pig prostate gland.

METHODS

The effects of tamsulosin (0.1 and 0.3 nM) were investigated in adult and ageing male guinea pig prostate glands using conventional tension recording and electrophysiological intracellular microelectrode recording techniques.

RESULTS

Tamsulosin reduced spontaneous activity, and had different age-dependent effects on adult and ageing guinea pigs at different concentrations. 0.1 nM tamsulosin caused a significantly greater reduction of spontaneous contractile and electrical activity in ageing guinea pigs in comparison to adult guinea pigs. In contrast, 0.3 nM tamsulosin had a significantly greater reduction of spontaneous contractile and electrical activity in adult guinea pigs in comparison to ageing guinea pigs.

CONCLUSIONS

This study demonstrates that tamsulosin can modulate spontaneous myogenic stromal contractility and the underlying spontaneous electrical activity; tamsulosin does not block spontaneous activity. This reduction in spontaneous activity suggests that downstream cellular mechanisms underlying smooth muscle tone are being targeted, and these may represent novel therapeutic targets to better treat benign prostatic hyperplasia.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Calcium oscillations and pacemaking

Calcium plays important role in biological systems where it is involved in diverse mechanisms such as signaling, muscle contraction and neuromodulation. Action potentials are generated by dynamic interaction of ionic channels located on the plasma-membrane and these drive the rhythmic activity of biological systems such as the smooth muscle and the heart. However, ionic channels are not the only pacemakers; an intimate interaction between intracellular Ca(2+) stores and ionic channels underlie rhythmic activity. In this review we will focus on the role of Ca(2+) stores in regulation of rhythmical behavior.

Novel drug targets for the pharmacotherapy of benign prostatic hyperplasia (BPH)

Benign prostatic hyperplasia (BPH) is the major cause of lower urinary tract symptoms in men aged 50 or older. Symptoms are not normally life threatening, but often drastically affect the quality of life. The number of men seeking treatment for BPH is expected to grow in the next few years as a result of the ageing male population. Estimates of annual pharmaceutical sales of BPH therapies range from $US 3 to 10 billion, yet this market is dominated by two drug classes. Current drugs are only effective in treating mild to moderate symptoms, yet despite this, no emerging contenders appear to be on the horizon. This is remarkable given the increasing number of patients with severe symptoms who are required to undergo invasive and unpleasant surgery. This review provides a brief background on prostate function and the pathophysiology of BPH, followed by a brief description of BPH epidemiology, the burden it places on society, and the current surgical and pharmaceutical therapies. The recent literature on emerging contenders to current therapies and novel drug targets is then reviewed, focusing on drug targets which are able to relax prostatic smooth muscle in a similar way to the α(1) -adrenoceptor antagonists, as this appears to be the most effective mechanism of action. Other mechanisms which may be of benefit are also discussed. It is concluded that recent basic research has revealed a number of novel drug targets such as muscarinic receptor or P2X-purinoceptor antagonists, which have the potential to produce more effective and safer drug treatments.

Spontaneous Ca2+ signaling of interstitial cells in the guinea pig prostate

PURPOSE

We investigated whether prostate interstitial cells generate spontaneous Ca(2+) oscillation, a proposed mechanism underlying pacemaker potentials to drive spontaneous activity in stromal smooth muscle cells.

MATERIALS AND METHODS

Intracellular free Ca(2+) in portions of guinea pig prostate and freshly isolated, single prostate interstitial cells were visualized using fluo-4 Ca(2+) fluorescence. Spontaneous electrical activity was recorded in situ with intracellular microelectrodes.

RESULTS

In whole tissue preparations spontaneous Ca(2+) flashes firing synchronously across all smooth muscle cells within the field of view resulted in muscle wall contractions. Nonpropagating Ca(2+) waves were also recorded in individual smooth muscle cells. Nifedipine (Sigma®) (1 μM) largely decreased or abolished these Ca(2+) flashes and suppressed slow wave discharge upon blockade of their superimposed action potentials. Isolated prostate interstitial cells were readily distinguished from smooth muscle cells by their spiky processes and lack of contraction during intracellular Ca(2+) increases. Prostate interstitial cells generated spontaneous Ca(2+) transients in the form of whole cell flashes, intracellular Ca(2+) waves or localized Ca(2+) sparks. All 3 Ca(2+) signals were abolished by nicardipine (1 μM), cyclopiazonic acid (10 μM), caffeine (Sigma) (10 mM) or extracellular Ca(2+) removal.

CONCLUSIONS

Prostate interstitial cells generate spontaneous Ca(2+) transients that occur at a frequency comparable to Ca(2+) flashes in situ or slow waves relying on functional internal Ca(2+) stores. However, unlike other interstitial cells in the urinary tract, Ca(2+) influx through L-type Ca(2+) channels is fundamental to Ca(2+) transient firings in prostate interstitial cells. Thus, it is not possible to conclude that prostate interstitial cells are responsible for pacemaker potential generation.

Role of mitochondria in contraction and pacemaking in the mouse uterus

BACKGROUND AND PURPOSE

Uterine spontaneous contraction and pacemaking are poorly understood. This study investigates the role of the mitochondrial Ca(2+) store in uterine activity.

EXPERIMENTAL APPROACH

We investigated the effects of mitochondrial and sarco-endoplasmic reticulum (SER) inhibitors on contraction, membrane potential (Vm) and cytosolic Ca(2+) concentration ([Ca(2+) ](c) ) in longitudinal smooth muscle of the mouse uterus.

KEY RESULTS

The mitochondrial agents rotenone, carbonylcyanide-3-chlorophenylhydrazone (CCCP), 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157) and kaempferol decreased the force of contractions. The ATP synthase inhibitor oligomycin had no significant effect. The effects of these agents were compared with those of SER inhibitors cyclopiazonic acid (CPA), 2-amino ethoxyphenylborate (2-APB) and caffeine. All agents, except CPA and oligomycin, decreased contractile force. CPA and CCCP transiently increased contraction frequency, which returned to control levels, whereas rotenone, CGP37157, kaempferol and 2-APB decreased frequency and caffeine had no significant effect. Application of the mitochondrial agents when CPA functionally inhibited stores did not change contraction frequency but, with the exception of kaempferol, decreased force. CCCP caused depolarization and maintained increase in [Ca(2+) ](c) or depolarization/transient hyperpolarization and transient increase in [Ca(2+) ](c) for oestrus and di-oestrus tissues respectively. Rotenone caused hyperpolarization and maintained increase in [Ca(2+) ](c) . CGP37157 and kaempferol caused hyperpolarization but no measurable change in [Ca(2+) ](c) . Application of a range of K(+) channel blockers indicated a role of Ca(2+) -activated K(+) (K(Ca) ) channels in the CCCP- and CGP37157-induced actions.

CONCLUSIONS AND IMPLICATIONS

Mitochondria have a modulatory role on uterine contractions, with mitochondrial inhibition reducing contraction amplitude and pacemaker frequency by changes in Vm, [Ca(2+) ](c) and/or Ca(2+) influx.

Role of connexin 43 in the maintenance of spontaneous activity in the guinea pig prostate gland

BACKGROUND AND PURPOSE

To investigate the role of connexin 43 in the maintenance of spontaneous activity in prostate tissue from young and old guinea pigs.

EXPERIMENTAL APPROACH

Conventional intracellular microelectrode and tension recording techniques, coupled with Western blot analysis and immunohistochemistry for connexin 43 (CX43) were used. The effects of three gap junction uncouplers, 18β glycyrrhetinic acid (10 µM, 40 µM), carbenoxolone (10 µM, 50 µM) and octanol (0.5 mM, 1 mM), were studied in cells displaying slow wave activity and on spontaneously contracting tissue from prostate glands of young (2-5 months) and old (9-16 months) guinea pigs.

KEY RESULTS

18β Glycyrrhetinic acid (40 µM), carbenoxolone (50 µM) or octanol (0.5 mM) abolished slow wave activity in prostate tissue from young and old guinea pigs and depolarized membrane potential by approximately 5 mV. These treatments also abolished all contractions in both sets of prostate tissue. These effects were reversed upon washout. Western blot analysis and CX43 immunohistochemistry showed that there was no age-related difference in the expression and distribution of CX43 in prostate tissues.

CONCLUSION AND IMPLICATIONS

When gap junctional communication via CX43 was disrupted, spontaneous activity was abolished at a cellular and whole tissue level; CX43 is therefore essential for the maintenance of spontaneous slow wave activity and subsequent contractile activity in the guinea pig prostate gland.

Contractility and pacemaker cells in the prostate gland

PURPOSE

We focused on the current opinion on mechanisms generating stromal tone in the prostate gland.

MATERIALS AND METHODS

We selected the guinea pig as the main species for investigation since its prostate has a high proportion of smooth muscle that undergoes age related changes similar in many respects to that in humans. The main techniques that we used were tension recording and electrophysiology.

RESULTS

We previously reported distinct electrical activity and cell types in the prostate, and speculated on their functional roles. We believe that a specialized group of c-kit immunoreactive prostatic interstitial cells that lie between glandular epithelium and smooth muscle stroma have a role similar to that of gastrointestinal interstitial cells of Cajal, generating the pacemaker signal that manifests as slow wave activity and triggers contraction in smooth muscle cells in guinea pig prostates.

CONCLUSIONS

Since changes in muscle tone are involved in the etiology of age dependent prostate specific conditions such as benign prostatic hyperplasia, knowledge of the electrical properties of the various prostatic cell types and their interactions with each other, with nerves and with the hormonal environment, and how these factors change with age is of considerable medical importance.

Functions of ICC-like cells in the urinary tract and male genital organs

Interstitial cells of Cajal (ICC)-like cells (ICC-LCs) have been identified in many regions of the urinary tract and male genital organs by immunohistochemical studies and electron microscopy. ICC-LCs are characterized by their spontaneous electrical and Ca²⁺ signalling and the cellular mechanisms of their generation have been extensively investigated. Spontaneous activity in ICC-LCs rises from the release of internally stored Ca²⁺ and the opening of Ca²⁺-activated Cl⁻ channels to generate spontaneous transient depolarizations (STDs) in a manner not fundamentally dependent on Ca²⁺ influx through L-type voltage-dependent Ca²⁺ channels. Since urogenital ICC-LCs have been identified by their immunoreactivity to Kit (CD117) antibodies, the often-used specific marker for ICC in the gastrointestinal tract, their functions have been thought likely to be similar. Thus ICC-LCs in the urogenital tract might be expected to act as either electrical pacemaker cells to drive the smooth muscle wall or as intermediaries in neuromuscular transmission. However, present knowledge of the functions of ICC-LCs suggests that their functions are not so predetermined, that their functions may be very region specific, particularly under pathological conditions. In this review, we summarize recent advances in our understanding of the location and function of ICC-LCs in various organs of the urogenital system. We also discuss several unsolved issues regarding the identification, properties and functions of ICC-LCs in various urogenital regions in health and disease.

Synchronization of Ca2+ oscillations: a coupled oscillator-based mechanism in smooth muscle

Entrained oscillations in Ca(2+) underlie many biological pacemaking phenomena. In this article, we review a long-range signaling mechanism in smooth muscle that results in global outcomes of local interactions. Our results are derived from studies of the following: (a) slow-wave depolarizations that underlie rhythmic contractions of gastric smooth muscle; and (b) membrane depolarizations that drive rhythmic contractions of lymphatic smooth muscle. The main feature of this signaling mechanism is a coupled oscillator-based synchronization of Ca(2+) oscillations across cells that drives membrane potential changes and causes coordinated contractions. The key elements of this mechanism are as follows: (a) the Ca(2+) release-refill cycle of endoplasmic reticulum Ca(2+) stores; (b) Ca(2+)-dependent modulation of membrane currents; (c) voltage-dependent modulation of Ca(2+) store release; and (d) cell-cell coupling through gap junctions or other mechanisms. In this mechanism, Ca(2+) stores alter the frequency of adjacent stores through voltage-dependent modulation of store release. This electrochemical coupling is many orders of magnitude stronger than the coupling through diffusion of Ca(2+) or inositol 1,4,5-trisphosphate, and thus provides an effective means of long-range signaling.