Cell cycle-dependent calcium oscillations in mouse embryonic stem cells

NOS inhibition synchronizes calcium oscillations in human adipose tissue-derived mesenchymal stem cells by increasing gap-junctional coupling

Adipose tissue-derived mesenchymal stem cells (ASCs) are a promising stem cell source for cell transplantation. We demonstrate that undifferentiated ASCs display robust oscillations of intracellular calcium [Ca(2+) ](i) which may be associated with stem cell maintenance since oscillations were absent in endothelial cell differentiation medium supplemented with FGF-2. [Ca(2+) ](i) oscillations were dependent on extracellular Ca(2+) and Ca(2+) release from intracellular stores since they were abolished in Ca(2+) -free medium and in the presence of the store-depleting agent thapsigargin. They were inhibited by the phospholipase C antagonist U73,122, the inositol 1,4,5-trisphosphate (InsP(3) ) receptor antagonist 2-aminoethoxydiphenyl borate (2-APB) as well as by the gap-junction uncouplers 1-heptanol and carbenoxolone, indicating regulation by the InsP(3) pathway and dependence on gap-junctional coupling. Cells endogenously generated nitric oxide (NO), expressed NO synthase 1 (NOS 1) and connexin 43 (Cx 43). The nitric oxide NOS inhibitors NG-monomethyl-L-arginine (L-NMMA), N(G)-nitro-L-arginine methyl ester (L-NAME), 2-ethyl-2-thiopseudourea, and diphenylene iodonium as well as si-RNA-mediated down-regulation of NOS 1 synchronized [Ca(2+) ](i) oscillations between individual cells, whereas the NO-donors S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) as well as the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) were without effects. The synchronization of [Ca(2+) ](i) oscillations was due to an improvement of intracellular coupling since fluorescence recovery after photobleaching (FRAP) revealed increased reflow of fluorescent calcein into the bleached area in the presence of the NOS inhibitors DPI and L-NAME. In summary our data demonstrate that intracellular NO levels regulate synchronization of [Ca(2+) ](i) oscillations in undifferentiated ASCs by controlling gap-junctional coupling.

Transient receptor potential melastatin type 7 channel is critical for the survival of bone marrow derived mesenchymal stem cells

The transient receptor potential melastatin type 7 channel (TRPM7) is a member of the TRP family of ion channels that is essential for cell proliferation and viability. Mesenchymal stem cells (MSCs) from bone marrow are a potential source for tissue repair due to their ability to differentiate into specialized cells. However, the role of TRPM7 in stem cells is unknown. In this study, we characterized TRPM7 in mouse MSCs using molecular biology, immunocytochemistry, and patch clamp. We also investigated TRPM7 function using a lentiviral vector and specific shRNA to knockdown gene expression. By RT-PCR and immunocytochemistry, we identified TRPM7, but not TRPM6, a close family member with similar function. Electrophysiological recordings during depletion of intracellular Mg(2+) or Mg(2+)-ATP resulted in the development of currents typical for the channel. Furthermore, 2-aminoethoxydiphenyl borate (1 pM-100 microM) inhibited TRPM7 in a concentration-dependent manner. The molecular suppression of TRPM7 significantly decreased MSC proliferation and viability as determined by MTT assay. In addition, TRPM7 gene expression was up-regulated during osteogenesis. These findings demonstrate that TRPM7 is required for MSC survival and perhaps involved in the differentiation process.

Role of voltage-gated potassium channels in the fate determination of embryonic stem cells

Embryonic stem cells (ESCs) possess two unique characteristics: self-renewal and pluripotency. In this study, roles of voltage-gated potassium channels (K(v)) in maintaining mouse (m) ESC characteristics were investigated. Tetraethylammonium (TEA(+)), a K(v) blocker, attenuated cell proliferation in a concentration-dependent manner. Possible reasons for this attenuation, including cytotoxicity, cell cycle arrest and differentiation, were examined. Blocking K(v) did not change the viability of mESCs. Interestingly, K(v) inhibition increased the proportion of cells in G(0)/G(1) phase and decreased that in S phase. This change in cell cycle distribution can be attributed to cell cycle arrest or differentiation. Loss of pluripotency as determined at both molecular and functional levels was detected in mESCs with K(v) blockade, indicating that K(v) inhibition in undifferentiated mESCs directs cells to differentiate instead of to self-renew and progress through the cell cycle. Membrane potential measurement revealed that K(v) blockade led to depolarization, consistent with the role of K(v) as the key determinant of membrane potential. The present results suggest that membrane potential changes may act as a "switch" for ESCs to decide whether to proliferate or to differentiate: hyperpolarization at G(1) phase would favor ESCs to enter S phase while depolarization would favor ESCs to differentiate. Consistent with this notion, S-phase-synchronized mESCs were found to be more hyperpolarized than G(0)/G(1)-phase-synchronized mESCs. Moreover, when mESCs differentiated, the differentiation derivatives depolarized at the initial stage of differentiation. This investigation is the first study to provide evidence that K(v) and membrane potential affect the fate determination of ESCs.

Frequent calcium oscillations lead to NFAT activation in human immature dendritic cells

Spontaneous Ca(2+) oscillations have been observed in a number of excitable and non-excitable cells, but in most cases their biological role remains elusive. In the present study we demonstrate that spontaneous Ca(2+) oscillations occur in immature human monocyte-derived dendritic cells but not in dendritic cells stimulated to undergo maturation with lipopolysaccharide or other toll like-receptor agonists. We investigated the mechanism and role of spontaneous Ca(2+) oscillations in immature dendritic cells and found that they are mediated by the inositol 1,4,5-trisphosphate receptor as they were blocked by pretreatment of cells with the inositol 1,4,5-trisphosphate receptor antagonist Xestospongin C and 2-aminoethoxydiphenylborate. A component of the Ca(2+) signal is also due to influx from the extracellular environment and may be involved in maintaining the level of the intracellular Ca(2+) stores. As to their biological role, our results indicate that they are intimately linked to the "immature" phenotype and are associated with the translocation of the transcription factor NFAT into the nucleus. In fact, once the Ca(2+) oscillations are blocked with 2-aminoethoxydiphenylborate or by treating the cells with lipopolysaccharide, NFAT remains cytoplasmic. The results presented in this report provide novel insights into the physiology of monocyte-derived dendritic cells and into the mechanisms involved in maintaining the cells in the immature stage.

Delayed enrichment of mesenchymal cells promotes cardiac lineage and calcium transient development

Bone marrow-derived mesenchymal stem cells (BM-MSCs) can be induced to differentiate into myogenic cells. Despite their potential, previous studies have not been successful in producing a high percentage of cardiac-like cells with a muscle phenotype. We hypothesized that cardiac lineage development in BM-MSC is related to cell passage, culture milieu, and enrichment for specific cell subtypes before and during differentiation. Our study demonstrated that Lin(-) BM-MSC at an intermediate passage (IP; P8-P12) expressed cardiac troponin T (cTnT) after 21 days in culture. Cardiac TnT expression was similar whether IP cells were differentiated in media containing 5-azacytidine+2% FBS (AZA; 14%) or 2% FBS alone (LS; 12%) and both were significantly higher than AZA+5% FBS. This expression was potentiated by first enriching for CD117/Sca-1 cells followed by differentiation (AZA, 39% and LS, 28%). A second sequential enrichment for the dihydropyridine receptor subunit alpha2delta1 (DHPR-alpha2) resulted in cardiac TnT expressed in 54% of cultured cells compared to 28% of cells after CD117/Sca-1(+) enrichment. Cells enriched for CD117/Sca-1 and subjected to differentiation displayed spontaneous intracellular Ca(2+) transients with an increase in transient frequency and a 60% decrease in the transient duration amplitude between days 14 and 29. In conclusion, IP CD117/Sca-1(+) murine BM-MSCs display robust cardiac muscle lineage development that can be induced independent of AZA but is diminished under higher serum concentrations. Furthermore, temporal changes in calcium kinetics commensurate with increased cTnT expression suggest progressive maturation of a cardiac muscle lineage. Enrichment with CD117/Sca-1 to establish lineage commitment followed by DHPR-alpha2 in lineage developing cells may enhance the therapeutic potential of these cells for transplantation.

In vitro electrophysiological mapping of stem cells

The use of stem cells for cardiac regeneration is a revolutionary, emerging research area. For proper function as replacement tissue, stem cell-derived cardiomyocytes (SC-CMs) must electrically couple with the host cardiac tissue. Electrophysiological mapping techniques, including microelectrode array (MEA) and optical mapping, have been developed to study cardiomyocytes and cardiac cell monolayers, and these can be applied to study stem cells and SC-CMs. MEA recordings take extracellular measurements at numerous points across a small area of cell cultures and are used to assess electrical propagation during cell culture. Optical mapping uses fluorescent dyes to monitor electrophysiological changes in cells, most commonly transmembrane potential and intracellular calcium, and can be easily scaled to areas of different sizes. The materials and methods for MEA and optical mapping are presented here, together with detailed notes on their use, design, and fabrication. We also provide examples of voltage and calcium maps of mouse embryonic stem cell-derived cardiomyocytes (mESC-CMs), obtained in our laboratory using optical mapping techniques.

Influence of spontaneous calcium events on cell-cycle progression in embryonal carcinoma and adult stem cells

Spontaneous Ca(2+) events have been observed in diverse stem cell lines, including carcinoma and mesenchymal stem cells. Interestingly, during cell cycle progression, cells exhibit Ca(2+) transients during the G(1) to S transition, suggesting that these oscillations may play a role in cell cycle progression. We aimed to study the influence of promoting and blocking calcium oscillations in cell proliferation and cell cycle progression, both in neural progenitor and undifferentiated cells. We also identified which calcium stores are required for maintaining these oscillations. Both in neural progenitor and undifferentiated cells calcium oscillations were restricted to the G1/S transition, suggesting a role for these events in progression of the cell cycle. Maintenance of the oscillations required calcium influx only through inositol 1,4,5-triphosphate receptors (IP(3)Rs) and L-type channels in undifferentiated cells, while neural progenitor cells also utilized ryanodine-sensitive stores. Interestingly, promoting calcium oscillations through IP(3)R agonists increased both proliferation and levels of cell cycle regulators such as cyclins A and E. Conversely, blocking calcium events with IP(3)R antagonists had the opposite effect in both undifferentiated and neural progenitor cells. This suggests that calcium events created by IP(3)Rs may be involved in cell cycle progression and proliferation, possibly due to regulation of cyclin levels, both in undifferentiated cells and in neural progenitor cells.

Spontaneous calcium oscillations regulate human cardiac progenitor cell growth

RATIONALE

The adult heart possesses a pool of progenitor cells stored in myocardial niches, but the mechanisms involved in the activation of this cell compartment are currently unknown.

OBJECTIVE

Ca2+ promotes cell growth raising the possibility that changes in intracellular Ca2+ initiate division of c-kit-positive human cardiac progenitor cells (hCPCs) and determine their fate.

METHODS AND RESULTS

Ca2+ oscillations were identified in hCPCs and these events occurred independently from coupling with cardiomyocytes or the presence of extracellular Ca2+. These findings were confirmed in the heart of transgenic mice in which enhanced green fluorescent protein was under the control of the c-kit promoter. Ca2+ oscillations in hCPCs were regulated by the release of Ca2+ from the endoplasmic reticulum through activation of inositol 1,4,5-triphosphate receptors (IP3Rs) and the reuptake of Ca2+ by the sarco-/endoplasmic reticulum Ca2+ pump (SERCA). IP3Rs and SERCA were highly expressed in hCPCs, whereas ryanodine receptors were not detected. Although Na+-Ca2+ exchanger, store-operated Ca2+ channels and plasma membrane Ca2+ pump were present and functional in hCPCs, they had no direct effects on Ca2+ oscillations. Conversely, Ca2+ oscillations and their frequency markedly increased with ATP and histamine which activated purinoceptors and histamine-1 receptors highly expressed in hCPCs. Importantly, Ca2+ oscillations in hCPCs were coupled with the entry of cells into the cell cycle and 5-bromodeoxyuridine incorporation. Induction of Ca2+ oscillations in hCPCs before their intramyocardial delivery to infarcted hearts was associated with enhanced engraftment and expansion of these cells promoting the generation of a large myocyte progeny.

CONCLUSION

IP3R-mediated Ca2+ mobilization control hCPC growth and their regenerative potential.

Cell density-dependent changes in intracellular Ca2+ mobilization via the P2Y2 receptor in rat bone marrow stromal cells

Bone marrow stromal cells (BMSCs) are an interesting subject of research because they have characteristics of mesenchymal stem cells. We investigated intracellular Ca(2+) signaling in rat BMSCs. Agonists for purinergic receptors increased intracellular Ca(2+) levels ([Ca(2+)](i)). The order of potency followed ATP = UTP > ADP = UDP. ATP-induced rise in [Ca(2+)](i) was suppressed by U73122 and suramin, but not by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suggesting the functional expression of G protein-coupled P2Y(2) receptors. RT-PCR and immunohistochemical studies also showed the expression of P2Y(2) receptors. [Ca(2+)](i) response to UTP changed with cell density. The UTP-induced rise in [Ca(2+)](i) was greatest at high density. V(max) (maximum Ca(2+) response) and EC(50) (agonist concentration that evokes 50% of V(max)) suggest that the amount and property of P2Y(2) receptors were changed by cell density. Note that UTP induced Ca(2+) oscillation at only medium cell density. Pharmacological studies indicated that UTP-induced Ca(2+) oscillation required Ca(2+) influx by store-operated Ca(2+) entry. Carbenoxolone, a gap junction blocker, enhanced Ca(2+) oscillation. Immunohistochemical and quantitative real-time PCR studies revealed that proliferating cell nuclear antigen (PCNA)-positive cells declined but the mRNA expression level of the P2Y(2) receptor increased as cell density increased. Co-application of fetal calf serum with UTP induced Ca(2+) oscillation at high cell density. These results suggest that the different patterns observed for [Ca(2+)](i) mobilization with respect to cell density may be associated with cell cycle progression.

Lymphatic pump-conduit duality: contraction of postnodal lymphatic vessels inhibits passive flow

Lymphangions, the segments of lymphatic vessels between valves, exhibit structural characteristics in common with both ventricles and arteries. Although once viewed as passive conduits like arteries, it has become well established that lymphangions can actively pump lymph against an axial pressure gradient from low-pressure tissues to the great veins of the neck. A recently reported mathematical model, based on fundamental principles, predicted that lymphangions can transition from pump to conduit behavior when outlet pressure falls below inlet pressure. In this case, the axial pressure gradient becomes the major source of energy for the propulsion of lymph, despite the presence of cyclical contraction. In fact, flow is augmented when cyclical contractions are abolished. We therefore used an in vitro preparation to confirm these findings and to test the hypothesis that lymphangion contraction inhibits flow when outlet pressure falls below inlet pressure. Bovine postnodal mesenteric lymphatic vessels harvested from an abattoir were subjected to an inlet pressure of 5.0 cmH(2)O and an outlet pressure that decreased from 6.5 to 3.5 cmH(2)O under control conditions, stimulated with U-46619 (a thromboxane analog) and relaxed with calcium-free solution. Under control conditions, lymphatic flow markedly increased as outlet pressure fell below inlet pressure. In this case, the slopes of the flow versus axial pressure gradient increased with calcium-free conditions (61%, n = 8, P = 0.016) and decreased with U-46619 stimulation (21%, n = 5, P = 0.033). Our findings indicate that the stimulation of lymphatic contractility does indeed inhibit lymphatic flow when vessels act like conduits.

Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release by reversible phosphorylation and dephosphorylation

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) is a universal intracellular Ca2+-release channel. It is activated after cell stimulation and plays a crucial role in the initiation and propagation of the complex spatio-temporal Ca2+ signals that control cellular processes as different as fertilization, cell division, cell migration, differentiation, metabolism, muscle contraction, secretion, neuronal processing, and ultimately cell death. To achieve these various functions, often in a single cell, exquisite control of the Ca2+ release is needed. This review aims to highlight how protein kinases and protein phosphatases can interact with the IP3R or with associated proteins and so provide a large potential for fine tuning the Ca2+-release activity and for creating efficient Ca2+ signals in subcellular microdomains.

Amphetamine and methamphetamine differentially affect dopamine transporters in vitro and in vivo

The psychostimulants d-amphetamine (AMPH) and methamphetamine (METH) release excess dopamine (DA) into the synaptic clefts of dopaminergic neurons. Abnormal DA release is thought to occur by reverse transport through the DA transporter (DAT), and it is believed to underlie the severe behavioral effects of these drugs. Here we compare structurally similar AMPH and METH on DAT function in a heterologous expression system and in an animal model. In the in vitro expression system, DAT-mediated whole-cell currents were greater for METH stimulation than for AMPH. At the same voltage and concentration, METH released five times more DA than AMPH and did so at physiological membrane potentials. At maximally effective concentrations, METH released twice as much [Ca(2+)](i) from internal stores compared with AMPH. [Ca(2+)](i) responses to both drugs were independent of membrane voltage but inhibited by DAT antagonists. Intact phosphorylation sites in the N-terminal domain of DAT were required for the AMPH- and METH-induced increase in [Ca(2+)](i) and for the enhanced effects of METH on [Ca(2+)](i) elevation. Calmodulin-dependent protein kinase II and protein kinase C inhibitors alone or in combination also blocked AMPH- or METH-induced Ca(2+) responses. Finally, in the rat nucleus accumbens, in vivo voltammetry showed that systemic application of METH inhibited DAT-mediated DA clearance more efficiently than AMPH, resulting in excess external DA. Together these data demonstrate that METH has a stronger effect on DAT-mediated cell physiology than AMPH, which may contribute to the euphoric and addictive properties of METH compared with AMPH.

Optimal postnodal lymphatic network structure that maximizes active propulsion of lymph

The lymphatic system acts to return lower-pressured interstitial fluid to the higher-pressured veins by a complex network of vessels spanning more than three orders of magnitude in size. Lymphatic vessels consist of lymphangions, segments of vessels between two unidirectional valves, which contain smooth muscle that cyclically pumps lymph against a pressure gradient. Whereas the principles governing the optimal structure of arterial networks have been identified by variations of Murray's law, the principles governing the optimal structure of the lymphatic system have yet to be elucidated, although lymph flow can be identified as a critical parameter. The reason for this deficiency can be identified. Until recently, there has been no algebraic formula, such as Poiseuille's law, that relates lymphangion structure to its function. We therefore employed a recently developed mathematical model, based on the time-varying elastance model conventionally used to describe ventricular function, that was validated by data collected from postnodal bovine mesenteric lymphangions. From this lymphangion model, we developed a model to determine the structure of a lymphatic network that optimizes lymph flow. The model predicted that there is a lymphangion length that optimizes lymph flow and that symmetrical networks optimize lymph flow when the lymphangions downstream of a bifurcation are 1.26 times the length of the lymphangions immediately upstream. Measured lymphangion lengths (1.14 +/- 0.5 cm, n = 74) were consistent with the range of predicted optimal lengths (0.1-2.1 cm). This modeling approach was possible, because it allowed a structural parameter, such as length, to be treated as a variable.

Myogenic constriction and dilation of isolated lymphatic vessels

We tested the hypothesis that lymphatics would exhibit myogenic constrictions and dilations to intraluminal pressure changes. Collecting lymphatic vessels were isolated from rat mesentery, cannulated, and pressurized for in vitro study. The lymphatic diameter responses to controlled intraluminal pressure steps of different magnitudes were tested in the absence and presence of the inflammatory mediator substance P, which is known to enhance lymphatic contractility. Myogenic constriction, defined as a time-dependent decrease in end-diastolic diameter over a 1- to 2-min period following pressure elevation (after initial distension), was observed in the majority of rat mesenteric lymphatic vessels in vitro and occurred over a relatively wide pressure range (1-15 cmH2O). Myogenic dilation, a time-dependent rise in end-diastolic diameter following pressure reduction, was observed in over half the vessels equilibrated at a low baseline pressure. Myogenic constrictions were independent of the cardiac-like and time-dependent compensatory decline in end-systolic diameter and increase in amplitude observed in almost all vessels following pressure elevation. Substance P increased the percentage of vessels exhibiting myogenic constriction, the magnitude and rate of constriction, and the pressure range over which constriction occurred. Our results demonstrate that myogenic responses occur in collecting lymphatic vessels and suggest that the response may aid in preventing vessel overdistension during inflammation/edema.

Kinetics of fluid flux in the rat diaphragmatic submesothelial lymphatic lacunae

The specific role of loops and/or linear segments in pleural diaphragmatic submesothelial lymphatics was investigated in seven anesthetized, paralyzed, and mechanically ventilated rats. Lymphatic loops lay peripherally above the diaphragmatic muscular plane, whereas linear vessels run over both the muscular and central tendineous regions. Lymph vessel diameter, measured by automatic software analysis, was significantly greater ( P < 0.01) in linear vessels [103.4 ± 8.5 μm (mean ± SE), n = 18] than in loops (54.6 ± 3.3 μm, n = 21). Conversely, the geometric mean of intraluminal flow velocity, obtained from the speed of distribution of a bolus of fluorescent dextrans injected into the vessel, was lower ( P < 0.01) in linear vessels (26.3 ± 1.4 μm/s) compared with loops (51.3 ± 3.2 μm/s). Lymph flow, calculated as the product of flow velocity by vessel cross-sectional area, was similar in linear vessels and in individual vessels of a loop, averaging 8.6 ± 1.6 nl/min. Flow was always directed from the diaphragm periphery toward the medial tendineous region in linear vessels, whereas it was more complex and evidently controlled by intraluminal unidirectional valves in loops. The results suggest that loops might be the preferential site of lymph formation, whereas linear vessels would be mainly involved in the progression of newly formed lymph toward deeper collecting diaphragmatic ducts. Within the same hierarchic order of diaphragmatic lymphatic vessels, the spatial organization and geometrical arrangement of the submesothelial lacunae seem to be finalized at exploiting the alternate contraction/relaxation phases of diaphragmatic muscle fibers to optimize fluid removal from serosal cavities.

Global self-organization of the cellular metabolic structure

BACKGROUND

Over many years, it has been assumed that enzymes work either in an isolated way, or organized in small catalytic groups. Several studies performed using "metabolic networks models" are helping to understand the degree of functional complexity that characterizes enzymatic dynamic systems. In a previous work, we used "dissipative metabolic networks" (DMNs) to show that enzymes can present a self-organized global functional structure, in which several sets of enzymes are always in an active state, whereas the rest of molecular catalytic sets exhibit dynamics of on-off changing states. We suggested that this kind of global metabolic dynamics might be a genuine and universal functional configuration of the cellular metabolic structure, common to all living cells. Later, a different group has shown experimentally that this kind of functional structure does, indeed, exist in several microorganisms.

METHODOLOGY/PRINCIPAL FINDINGS

Here we have analyzed around 2.500.000 different DMNs in order to investigate the underlying mechanism of this dynamic global configuration. The numerical analyses that we have performed show that this global configuration is an emergent property inherent to the cellular metabolic dynamics. Concretely, we have found that the existence of a high number of enzymatic subsystems belonging to the DMNs is the fundamental element for the spontaneous emergence of a functional reactive structure characterized by a metabolic core formed by several sets of enzymes always in an active state.

CONCLUSIONS/SIGNIFICANCE

This self-organized dynamic structure seems to be an intrinsic characteristic of metabolism, common to all living cellular organisms. To better understand cellular functionality, it will be crucial to structurally characterize these enzymatic self-organized global structures.

Modeling flow in collecting lymphatic vessels: one-dimensional flow through a series of contractile elements

The lymphatic system comprises a series of elements, lymphangions, separated by valves and possessed of active, contractile walls to pump interstitial fluid from its collection in the terminal lymphatics back to the main circulation. Despite its importance, there is a dearth of information on the fluid dynamics of the lymphatic system. In this article, we describe linked experimental and computational work aimed at elucidating the biomechanical properties of the individual lymphangions. We measure the static and dynamic mechanical properties of excised bovine collecting lymphatics and develop a one-dimensional computational model of the coupled fluid flow/wall motion. The computational model is able to reproduce the pumping behavior of the real vessel using a simple contraction function producing fast contraction pulses traveling in the retrograde direction to the flow.

Modulation of lymphatic muscle contractility by the neuropeptide substance P

Substance P (SP) is a neuropeptide associated with sensory innervation of lymphoid tissue and a suspected modulator of lymphatic function in inflammation. Only a few studies have examined the effects of SP on lymphatic contraction, and it is not clear to what extent SP acts directly on the lymphatic muscle and/or endothelium or indirectly through changes in intraluminal filling pressure secondary to increases in capillary permeability/filtration. We tested the effects of SP on the spontaneous contractions of rat isolated mesenteric lymphatic vessels under isometric and isobaric conditions, hypothesizing that low concentrations would stimulate lymphatic pumping by enhancing lymphatic muscle contraction in a manner complementary to the effect of increased preload. Under isometric conditions, SP (10 nM) dramatically enhanced lymphatic chronotropy and inotropy. Unlike guinea pig lymphatics, SP actions were not blocked by cyclooxygenase or PLA(2) inhibition. In the absence of SP, ramp increases in isometric preload resulted in x approximately 1.6 increases in contraction amplitude (Amp) and x approximately 1.7 increases in frequency (Freq). SP increased Freq by x approximately 2.4, Amp by x approximately 1.9, and the Amp-Freq product (AFP) by x approximately 3.5. Under isobaric conditions, the pressure elevation from 0.5 to 10 cmH(2)O in the absence of SP decreased Amp by x approximately 0.6 and increased Freq by x approximately 1.8. SP caused a modest increase in Amp, a robust increase in Freq at all pressures, and shifted the AFP-pressure relationship upward and leftward. Therefore, SP has substantial positive inotropic and chronotropic effects on rat lymphatic muscle, improving pump efficiency independent of the effects of preload and broadening of the working range of the lymphatic pump.

First-order approximation for the pressure-flow relationship of spontaneously contracting lymphangions

To return lymph to the great veins of the neck, it must be actively pumped against a pressure gradient. Mean lymph flow in a portion of a lymphatic network has been characterized by an empirical relationship (Pin − Pout = −Pp + RLQL), where Pin − Pout is the axial pressure gradient and QL is mean lymph flow. RL and Pp are empirical parameters characterizing the effective lymphatic resistance and pump pressure, respectively. The relation of these global empirical parameters to the properties of lymphangions, the segments of a lymphatic vessel bounded by valves, has been problematic. Lymphangions have a structure like blood vessels but cyclically contract like cardiac ventricles; they are characterized by a contraction frequency ( f) and the slopes of the end-diastolic pressure-volume relationship [minimum value of resulting elastance ( Emin)] and end-systolic pressure-volume relationship [maximum value of resulting elastance ( Emax)]. Poiseuille's law provides a first-order approximation relating the pressure-flow relationship to the fundamental properties of a blood vessel. No analogous formula exists for a pumping lymphangion. We therefore derived an algebraic formula predicting lymphangion flow from fundamental physical principles and known lymphangion properties. Quantitative analysis revealed that lymph inertia and resistance to lymph flow are negligible and that lymphangions act like a series of interconnected ventricles. For a single lymphangion, Pp = Pin ( Emax − Emin)/ Emin and RL = Emax/ f. The formula was tested against a validated, realistic mathematical model of a lymphangion and found to be accurate. Predicted flows were within the range of flows measured in vitro. The present work therefore provides a general solution that makes it possible to relate fundamental lymphangion properties to lymphatic system function.

Calcium oscillations

Changes in cellular Ca2+ concentration control a wide range of physiological processes, from the subsecond release of synaptic neurotransmitters, to the regulation of gene expression over months or years. Ca2+ can also trigger cell death through both apoptosis and necrosis, and so the regulation of cellular Ca2+ concentration must be tightly controlled through the concerted action of pumps, channels and buffers that transport Ca2+ into and out of the cell cytoplasm. A hallmark of cellular Ca2+ signalling is its spatiotemporal complexity: stimulation of cells by a hormone or neurotransmitter leads to oscillations in cytoplasmic Ca2+ concentration that can vary markedly in time course, amplitude, frequency, and spatial range. In this chapter we review some of the biological roles of Ca2+, the experimental characterisation of complex dynamic changes in Ca2+ concentration, and attempts to explain this complexity using computational models. We consider the 'toolkit' of cellular proteins which influence Ca2+ concentrarion, describe mechanistic models of key elements of the toolkit, and fit these into the framework of whole cell models of Ca2+ oscillations and waves. Finally, we will touch on recent efforts to use stochastic modelling to elucidate elementary Ca2+ signal events, and how these may evolve into global signals.

Slow spontaneous [Ca2+] i oscillations reflect nucleotide release from renal epithelia

Renal epithelia can be provoked mechanically to release nucleotides, which subsequently increases the intracellular Ca(2+) concentration [Ca(2+)](i) through activation of purinergic (P2) receptors. Cultured cells often show spontaneous [Ca(2+)](i) oscillations, a feature suggested to involve nucleotide signalling. In this study, fluo-4 loaded Madin-Darby canine kidney (MDCK) cells are used as a model for quantification and characterisation of spontaneous [Ca(2+)](i) increases in renal epithelia. Spontaneous [Ca(2+)](i) increases occurred randomly as single cell events. During an observation period of 1 min, 10.9 +/- 6.7% (n = 23) of the cells showed spontaneous [Ca(2+)](i) increases. Spontaneous adenosine triphosphate (ATP) release from MDCK cells was detected directly by luciferin/luciferase. Scavenging of ATP by apyrase or hexokinase markedly reduced the [Ca(2+)](i) oscillatory activity, whereas inhibition of ecto-ATPases (ARL67156) enhanced the [Ca(2+)](i) oscillatory activity. The association between spontaneous [Ca(2+)](i) increases and nucleotide signalling was further tested in 132-1N1 cells lacking P2 receptors. These cells hardly showed any spontaneous [Ca(2+)](i) increases. Transfection with either hP2Y(6) or hP2Y(2) receptors revealed a striking degree of oscillations. Similar spontaneous [Ca(2+)](i) increases were observed in freshly isolated, perfused mouse medullary thick ascending limb (mTAL). The oscillatory activity was reduced by basolateral apyrase and substantially lower in mTAL from P2Y(2) knock out mice (0.050 +/- 0.020 events per second, n = 8) compared to the wild type (0.147 +/- 0.018 events per second, n = 9). These findings indicate that renal epithelia spontaneously release nucleotides leading to P2-receptor-dependent [Ca(2+)](i) oscillations. Thus, tonic nucleotide release is likely to modify steady state renal function.

Lymphangion coordination minimally affects mean flow in lymphatic vessels

The lymphatic system returns interstitial fluid to the central venous circulation, in part, by the cyclical contraction of a series of “lymphangion pumps” in a lymphatic vessel. The dynamics of individual lymphangions have been well characterized in vitro; their frequencies and strengths of contraction are sensitive to both preload and afterload. However, lymphangion interaction within a lymphatic vessel has been poorly characterized because it is difficult to experimentally alter properties of individual lymphangions and because the afterload of one lymphangion is coupled to the preload of another. To determine the effects of lymphangion interaction on lymph flow, we adapted an existing mathematical model of a lymphangion (characterizing lymphangion contractility, lymph viscosity, and inertia) to create a new lymphatic vessel model consisting of several lymphangions in series. The lymphatic vessel model was validated with focused experiments on bovine mesenteric lymphatic vessels in vitro. The model was then used to predict changes in lymph flow with different time delays between onset of contraction of adjacent lymphangions (coordinated case) and with different relative lymphangion contraction frequencies (noncoordinated case). Coordination of contraction had little impact on mean flow. Furthermore, orthograde and retrograde propagations of contractile waves had similar effects on flow. Model results explain why neither retrograde propagation of contractile waves nor the lack of electrical continuity between lymphangions adversely impacts flow. Because lymphangion coordination minimally affects mean flow in lymphatic vessels, lymphangions have flexibility to independently adapt to local conditions.

Lymphatic vessels transition to state of summation above a critical contraction frequency

Although behavior of lymphatic vessels is analogous to that of ventricles, which completely relax between contractions, and blood vessels, which maintain a tonic constriction, the mixture of contractile properties can yield behavior unique to lymphatic vessels. In particular, because of their limited refractory period and slow rate of relaxation, lymphatic vessels lack the contractile properties that minimize summation in ventricles. We, therefore, hypothesized that lymphatic vessels transition to a state of summation when lymphatic vessel contraction frequency exceeds a critical value. We used an isovolumic, controlled-flow preparation to compare the time required for full relaxation with the time available to relax during diastole. We measured transmural pressure and diameter on segments of spontaneously contracting bovine mesenteric lymphatic vessels during 10 isovolumic volume steps. We found that beat-to-beat period (frequency−1) decreased with increases in diameter and that total contraction time was constant or slightly increased with diameter. We further found that the convergence of beat-to-beat period and contraction cycle duration predicted a critical transition value, beyond which the vessel does not have time to fully relax. This incomplete relaxation and resulting mechanical summation significantly increase active tension in diastole. Because this transition occurs within a physiological range, contraction summation may represent a fundamental feature of lymphatic vessel function.

Inositol-1,4,5-trisphosphate-mediated spontaneous activity in mouse embryonic stem cell-derived cardiomyocytes

Embryonic stem cell-derived cardiomyocytes (ESdCs) have been proposed as a source for cardiac cell-replacement therapy. The aim of this study was to determine the Ca2+-handling mechanisms that determine the frequency and duration of spontaneous Ca2+ transients in single ESdCs. With laser scanning confocal microscopy using the Ca2+-sensitive dye Fluo-4/AM, we determined that spontaneous Ca2+ transients in ESdCs at the onset of beating (day 9) depend on Ca2+ entry across the plasma membrane (50%) whereas Ca2+-induced Ca2+ release is the major contributor to Ca2+ transients in ESdCs after 16 days (72%). Likewise, Ca2+ extrusion in 9-day-old ESdCs depends on Na+-Ca2+ exchange (50.0+/-8%) whereas Ca2+ reuptake by the sarco(endo)plasmic Ca2+ ATPase (72+/-5%) dominates in further differentiated cells. Spontaneous Ca2+ transients were suppressed by the inositol-1,4,5-trisphosphate (IP3) receptor (IP3R) blocker 2-aminoethoxydiphenyl borate (2-APB) and the phospholipase C blocker U73122 but continued in the presence of caffeine. Stimulation of IP3 production by phenylephrine or endothelin-1 had a positive chronotropic effect that could be reversed by U73122 and 2-APB. The presence of Ca2+-free solution and block of L-type Ca2+ channels by nifedipine also resulted in a cessation of spontaneous activity. Overall, IP3R-mediated Ca2+ release in ESdCs is translated into a depolarization of the plasma membrane and a whole-cell Ca2+ transient is subsequently induced by voltage-dependent Ca2+ influx. Although ryanodine receptor-mediated Ca2+ release amplifies the IP3R-induced trigger for the Ca2+ transients and modulates its frequencies, it is not a prerequisite for spontaneous activity. The results of this study offer important insight into the role of IP3R-mediated Ca2+ release for pacemaker activity in differentiating cardiomyocytes.