Endothelial cells require functional FLVCR1a during developmental and adult angiogenesis.

The Feline Leukemia Virus Subgroup C Receptor 1a (FLVCR1a) is a transmembrane heme exporter essential for embryonic vascular development. However, the exact role of FLVCR1a during blood vessel development remains largely undefined. Here, we show that FLVCR1a is highly expressed in angiogenic endothelial cells (ECs) compared to quiescent ECs. Consistently, ECs lacking FLVCR1a give rise to structurally and functionally abnormal vascular networks in multiple models of developmental and pathologic angiogenesis. Firstly, zebrafish embryos without FLVCR1a displayed defective intersegmental vessels formation. Furthermore, endothelial-specific Flvcr1a targeting in mice led to a reduced radial expansion of the retinal vasculature associated to decreased EC proliferation. Moreover, Flvcr1a null retinas showed defective vascular organization and loose attachment of pericytes. Finally, adult neo-angiogenesis is severely affected in murine models of tumor angiogenesis. Tumor blood vessels lacking Flvcr1a were disorganized and dysfunctional. Collectively, our results demonstrate the critical role of FLVCR1a as a regulator of developmental and pathological angiogenesis identifying FLVCR1a as a potential therapeutic target in human diseases characterized by aberrant neovascularization.

COVID-19-associated cardiovascular morbidity in older adults: a position paper from the Italian Society of Cardiovascular Researches.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects host cells following binding with the cell surface ACE2 receptors, thereby leading to coronavirus disease 2019 (COVID-19). SARS-CoV-2 causes viral pneumonia with additional extrapulmonary manifestations and major complications, including acute myocardial injury, arrhythmia, and shock mainly in elderly patients. Furthermore, patients with existing cardiovascular comorbidities, such as hypertension and coronary heart disease, have a worse clinical outcome following contraction of the viral illness. A striking feature of COVID-19 pandemics is the high incidence of fatalities in advanced aged patients: this might be due to the prevalence of frailty and cardiovascular disease increase with age due to endothelial dysfunction and loss of endogenous cardioprotective mechanisms. Although experimental evidence on this topic is still at its infancy, the aim of this position paper is to hypothesize and discuss more suggestive cellular and molecular mechanisms whereby SARS-CoV-2 may lead to detrimental consequences to the cardiovascular system. We will focus on aging, cytokine storm, NLRP3/inflammasome, hypoxemia, and air pollution, which is an emerging cardiovascular risk factor associated with rapid urbanization and globalization. We will finally discuss the impact of clinically available CV drugs on the clinical course of COVID-19 patients. Understanding the role played by SARS-CoV2 on the CV system is indeed mandatory to get further insights into COVID-19 pathogenesis and to design a therapeutic strategy of cardio-protection for frail patients.

SiO2 nanoparticles modulate the electrical activity of neuroendocrine cells without exerting genomic effects.

Engineered silica nanoparticles (NPs) have attracted increasing interest in several applications, and particularly in the field of nanomedicine, thanks to the high biocompatibility of this material. For their optimal and controlled use, the understanding of the mechanisms elicited by their interaction with the biological target is a prerequisite, especially when dealing with cells particularly vulnerable to environmental stimuli like neurons. Here we have combined different electrophysiological approaches (both at the single cell and at the population level) with a genomic screening in order to analyze, in GT1-7 neuroendocrine cells, the impact of SiO2 NPs (50 ± 3 nm in diameter) on electrical activity and gene expression, providing a detailed analysis of the impact of a nanoparticle on neuronal excitability. We find that 20 µg mL-1 NPs induce depolarization of the membrane potential, with a modulation of the firing of action potentials. Recordings of electrical activity with multielectrode arrays provide further evidence that the NPs evoke a temporary increase in firing frequency, without affecting the functional behavior on a time scale of hours. Finally, NPs incubation up to 24 hours does not induce any change in gene expression.

Activation of P2X7 and P2Y11 purinergic receptors inhibits migration and normalizes tumor-derived endothelial cells via cAMP signaling.

Purinergic signaling is involved in inflammation and cancer. Extracellular ATP accumulates in tumor interstitium, reaching hundreds micromolar concentrations, but its functional role on tumor vasculature and endothelium is unknown. Here we show that high ATP doses (>20 µM) strongly inhibit migration of endothelial cells from human breast carcinoma (BTEC), but not of normal human microvascular EC. Lower doses (1-10 µM result ineffective. The anti-migratory activity is associated with cytoskeleton remodeling and is significantly prevented by hypoxia. Pharmacological and molecular evidences suggest a major role for P2X7R and P2Y11R in ATP-mediated inhibition of TEC migration: selective activation of these purinergic receptors by BzATP mimics the anti-migratory effect of ATP, which is in turn impaired by their pharmacological or molecular silencing. Downstream pathway includes calcium-dependent Adenilyl Cyclase 10 (AC10) recruitment, cAMP release and EPAC-1 activation. Notably, high ATP enhances TEC-mediated attraction of human pericytes, leading to a decrease of endothelial permeability, a hallmark of vessel normalization. Finally, we provide the first evidence of in vivo P2X7R expression in blood vessels of murine and human breast carcinoma. In conclusion, we have identified a purinergic pathway selectively acting as an antiangiogenic and normalizing signal for human tumor-derived vascular endothelium.

Cytokine, chemokine, and growth factor profile of platelet-rich plasma.

During wound healing, biologically active molecules are released from platelets. The rationale of using platelet-rich plasma (PRP) relies on the concentration of bioactive molecules and subsequent delivery to healing sites. These bioactive molecules have been seldom simultaneously quantified within the same PRP preparation. In the present study, the flexible Bio-Plex system was employed to assess the concentration of a large range of cytokines, chemokines, and growth factors in 16 healthy volunteers so as to determine whether significant baseline differences may be found. Besides IL-1b, IL-1ra, IL-4, IL-6, IL-8, IL-12, IL-13, IL-17, INF-γ, TNF-α, MCP-1, MIP-1a, RANTES, bFGF, PDGF, and VEGF that were already quantified elsewhere, the authors reported also on the presence of IL-2, IL-5, IL-7, IL-9, IL-10, IL-15 G-CSF, GM-CSF, Eotaxin, CXCL10 chemokine (IP-10), and MIP 1b. Among the most interesting results, it is convenient to mention the high concentrations of the HIV-suppressive and inflammatory cytokine RANTES and a statistically significant difference between males and females in the content of PDGF-BB. These data are consistent with previous reports pointing out that gender, diet, and test system affect the results of platelet function in healthy subjects, but seem contradictory when compared to other quantification assays in serum and plasma. The inconsistencies affecting the experimental results found in literature, along with the variability found in the content of bioactive molecules, urge further research, hopefully in form of randomized controlled clinical trials, in order to find definitive evidence of the efficacy of PRP treatment in various pathologic and regenerative conditions.

Overcoming physical constraints in bone engineering: 'the importance of being vascularized'.

Bone plays several physiological functions and is the second most commonly transplanted tissue after blood. Since the treatment of large bone defects is still unsatisfactory, researchers have endeavoured to obtain scaffolds able to release growth and differentiation factors for mesenchymal stem cells, osteoblasts and endothelial cells in order to obtain faster mineralization and prompt a reliable vascularization. Nowadays, the application of osteoblastic cultures spans from cell physiology and pharmacology to cytocompatibility measurement and osteogenic potential evaluation of novel biomaterials. To overcome the simple traditional monocultures in vitro, co-cultures of osteogenic and vasculogenic precursors were introduced with very interesting results. Increasingly complex culture systems have been developed, where cells are seeded on proper scaffolds and stimulated so as to mimic the physiological conditions more accurately. These bioreactors aim at enabling bone regeneration by incorporating different cells types into bio-inspired materials within a surveilled habitat. This review is focused on the most recent developments in the organomimetic cultures of osteoblasts and vascular endothelial cells for bone tissue engineering.

Systems biology of ion channels and transporters in tumor angiogenesis: An omics view.

Solid tumors require the formation of new blood vessels to support their growth, invasiveness and metastatic potential. Tumor neovascularization is achieved by vasculogenesis from endothelial precursors and by sprouting angiogenesis from preexisting vessels. The complex sequence of events driving these processes, including endothelial activation, proliferation, migration and differentiation, is associated with fluxes of ions, water and other small molecules mediated by a great pool of ion channels and transporters (ICT). This 'transportome' is regulated by environmental factors as well as intracellular signaling molecules. In turn, ICT play a prominent role in the response to angiogenesis-related stimuli through canonical and 'unconventional' activities: indeed, there is an increasing recognition of the multifunctionality of several ion channels that could also be annotated as receptors, enzymes, scaffolding proteins, mechanical and chemical sensors. The investigation of ICT structure and function has been far from the experimental oncology for long time and these two domains converged only very recently. Furthermore, the systems biology viewpoint has not received much attention in the biology of cancer transportome. Modulating angiogenesis by interference with membrane transport has a great potential in cancer treatment and the application of an 'omics' logic will hopefully contribute to the overall advancement in the field. This review is an attempt to apply the systems biology approach to the analysis of ICT involved in tumor angiogenesis, with a particular focus on endothelial transportome diversity. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Endothelial remodelling and intracellular calcium machinery.

Rather being an inert barrier between vessel lumen and surrounding tissues, vascular endothelium plays a key role in the maintenance of cardiovascular homeostasis. The de-endothelialization of blood vessels is regarded as the early event that results in the onset of severe vascular disorders, including atherosclerosis, acute myocardial infarction, brain stroke, and aortic aneurysm. Restoration of the endothelial lining may be accomplished by the activation of neighbouring endothelial cells (ECs) freed by contact inhibition and by circulating endothelial progenitor cells (EPCs). Intracellular Ca(2+) signalling is essential to promote wound healing: however, the molecular underpinnings of the Ca(2+) response to injury are yet to be fully elucidated. Similarly, the components of the Ca(2+) toolkit that drive EPC incorporation into denuded vessels are far from being fully elucidated. The present review will survey the current knowledge on the role of Ca(2+) signalling in endothelial repair and in EPC activation. We propose that endothelial regeneration might be boosted by intraluminal release of specific Ca(2+) channel agonists or by gene transfer strategies aiming to enhance the expression of the most suitable Ca(2+) channels at the wound site. In this view, connexin (Cx) channels/hemichannels and store-operated Ca(2+) entry (SOCE) stand amid the most proper routes to therapeutically induce the regrowth of denuded vessels. Cx stimulation might trigger the proliferative and migratory behaviour of ECs facing the lesion site, whereas activation of SOCE is likely to favour EPC homing to the wounded vessel.

TRPV4 mediates tumor-derived endothelial cell migration via arachidonic acid-activated actin remodeling.

Changes in intracellular calcium [Ca(2+)](i) levels control critical cytosolic and nuclear events that are involved in the initiation and progression of tumor angiogenesis in endothelial cells (ECs). Therefore, the mechanism(s) involved in agonist-induced Ca(2+)(i) signaling is a potentially important molecular target for controlling angiogenesis and tumor growth. Several studies have shown that blood vessels in tumors differ from normal vessels in their morphology, blood flow and permeability. We had previously reported a key role for arachidonic acid (AA)-mediated Ca(2+) entry in the initial stages of tumor angiogenesis in vitro. In this study we assessed the mechanism involved in AA-induced EC migration. We report that TRPV4, an AA-activated channel, is differentially expressed in EC derived from human breast carcinomas (BTEC) as compared with 'normal' EC (HMVEC). BTEC display a significant increase in TRPV4 expression, which was correlated with greater Ca(2+) entry, induced by AA or 4αPDD (a selective TRPV4 agonist) in the tumor-derived ECs. Wound-healing assays revealed a key role of TRPV4 in regulating cell migration of BTEC but not HMVEC. Knockdown of TRPV4 expression completely abolished AA-induced BTEC migration, suggesting that TRPV4 mediates the pro-angiogenic effects promoted by AA. Furthermore, pre-incubation of BTEC with AA induced actin remodeling and a subsequent increase in the surface expression of TRPV4. This was consistent with the increased plasma membrane localization of TRPV4 and higher AA-stimulated Ca(2+) entry in the migrating cells. Together, the data presented herein demonstrate that: (1) TRPV4 is differentially expressed in tumor-derived versus 'normal' EC; (2) TRPV4 has a critical role in the migration of tumor-derived but not 'normal' EC migration; and (3) AA induces actin remodeling in BTEC, resulting in a corresponding increase of TRPV4 expression in the plasma membrane. We suggest that the latter is critical for migration of EC and thus in promoting angiogenesis and tumor growth.

Endothelial calcium machinery and angiogenesis: understanding physiology to interfere with pathology.

Endothelial cells (ECs) play a pivotal role in physiological and altered tissue neovascularization. They face multiple morphological, biochemical and functional changes during the different phases of angiogenesis, under the regulation of a great number of proangiogenic and antiangiogenic signals, including soluble and insoluble factors, cell-cell and cell-matrix interactions. ECs mutual contacts (and also interactions with other cell types, such as pericytes and smooth vascular muscle cells), motility, proliferation, apoptosis and differentiation are all calcium-dependent events finely tuned in space and time. Most of the angiogenic-related peptidic factors (VEGF, bFGF and others) promote an increase of cytosolic free calcium concentration in ECs, giving rise to calcium-activated intracellular cascades engaged in the different steps of the angiogenic process. A better knowledge of such signals could allow to set new diagnostic and therapeutical approaches aimed to interfere with altered neovascularization, particularly during cancer progression. This review reports the state of the art about endothelial angiogenic-related calcium signaling and discusses the most attractive perspectives for the future.

Zinc-containing bioactive glasses: surface reactivity and behaviour towards endothelial cells.

This paper reports a physico-chemical study devoted to reactivity towards hydroxo-carbonate apatite (HCA) formation of bioactive glass 45S5 (H glass; commercially known as Bioglass) and of two preparations of zinc-doped 45S5-derived systems (HZ5, HZ20), immersed in Tris(hydroxymethyl)aminomethane (Tris) and Dulbecco's modified Eagle's medium (DMEM) buffer solutions. The activity/toxicity of the glasses was also tested using endothelial cells (EC). Zn caused a drastic reduction in the overall leaching activity of glasses and, at high Zn concentration (HZ20), the formation of HCA on the glass surface was thoroughly inhibited. The presence of Zn also decreased the increment of pH after glass immersion in both Tris and DMEM solution. EC are known to be very sensitive to pH changes and, for this reason, the rapid increase in pH brought about by H glass dissolution is likely to affect cell adhesion and spreading, whereas the high zinc release from HZ20 causes a drastic reduction in cell proliferation after a long contact time (approximately 1 week). This study shows that only HZ5 glass containing 5 wt.% Zn presents at the same time: reduced solubility, bioactivity (monitored by HCA formation) and conditions allowing EC growth over a 6-day period.

Intracellular calcium signals and control of cell proliferation: how many mechanisms?

The progression through the cell cycle in non-transformed cells is under the strict control of extracellular signals called mitogens, that act by eliciting complex cascades of intracellular messengers. Among them, increases in cytosolic free calcium concentration have been long realized to play a crucial role; however, the mechanisms coupling membrane receptor activation to calcium signals are still only partially understood, as are the pathways of calcium entry in the cytosol. This article centers on the role of calcium influx from the extracellular medium in the control of proliferative processes, and reviews the current understanding of the pathways responsible for this influx and of the second messengers involved in their activation.

Calcium influx, arachidonic acid,and control of endothelial cell proliferation.

Recent evidences suggest a role for arachidonic acid (AA) in the triggering of store-independent, ornon-capacitative, calcium entry in different cell types. Here, using patch clamp and fluorimetric single-cell calcium measurements, we provide evidence for AA-activated calcium influx in bovine aortic endothelial cells (BAEC). AA-activated calcium entry is independent from intracellular calcium stores depletion at low doses of the fatty acid (< 5 microM) and insensitive to a decrease of pH to 6.7. Single-channel analysis in inside-out configuration reveals the presence of a family of AA-activated calcium-permeable channels, with different conductances and reversal potentials. Treatment with AA or ETYA induces a proliferative effect, significantly affected by external EGTA application during the early period (up to 2h) of stimulation with the agonists. We conclude that low concentrations of arachidonic acid are able to evoke a store-independent calcium influx, exerting a mitogenic role in BAECs.

Activation of functional oxytocin receptors stimulates cell proliferation in human trophoblast and choriocarcinoma cell lines.

Despite oxytocin receptors (OTR) being present in human chorio-decidual tissues, their expression and role in placental trophoblast cells in the context of tumor growth or physiological functions related to cell proliferation have never been examined. In the present study we demonstrate the presence and functionality of OTR in normal human trophoblast cell lines (ED77 and ED27) and a choriocarcinoma cell line (BeWo). RT-PCR and immunofluorescence analysis revealed the presence of OTR messenger RNA and protein in these cells. Binding studies using [(125)I]oxytocin ([(125)I]OT) antagonist confirmed the presence of specific binding sites in ED27, ED77, and BeWo cells. OTR functionality was demonstrated by measuring the OT-induced increase in the intracellular calcium concentrations. This effect was dose dependent and was blocked by the selective OT antagonist d(CH(2))(5)[Tyr(Me)(2),Thr(4), Tyr-NH(2)(9)]OVT (OT antagonist). Furthermore, two proteins with apparent molecular masses of 125 and 60 kDa became tyrosine phosphorylated in all of the cell lines after OT stimulation (and an additional protein of 45 kDa in BeWo choriocarcinoma cells), suggesting that this peptide can stimulate tyrosine kinase activity. Finally, we observed a dose-dependent OT stimulation of cell proliferation associated with OTR activation that was completely abolished by the selective OT antagonist. These findings provide the first evidence of the presence of functional OTR in normal trophoblast cell lines as well as in choriocarcinoma cells and show that a specific effect of OT on normal and neoplastic trophoblast is to promote cellular proliferation.

Calcium influx induced by activation of tyrosine kinase receptors in cultured bovine aortic endothelial cells.

We studied the ionic currents activated by basic fibroblast growth factor (bFGF) and insulin-like growth factor-I (IGF-I) in cultured bovine aortic endothelial cells (BAE-1) by using patch-clamp and single-cell fluorimetric calcium measurements. In whole-cell, voltage-clamp experiments at V(h) = -50 mV, the addition of either bFGF (20 ng/ml) or IGF-I (50 ng/ml) induced an inward current with similar amplitude, time course, and permeation properties. The response was dependent on receptor occupancy and showed a desensitisation in the continued presence of the factors. Ionic substitutions in whole-cell experiments indicated that the current barely discriminated among Na(+), Ca(+), and K(+) ions. Accordingly, stimulation with bFGF or IGF-I induced a dose-dependent [Ca(2+)](i) elevation completely due to entry from the extracellular medium, whereas no detectable release from internal stores was observed. Calcium influx was dependent on protein tyrosine kinase (PTK) activity; it was significantly inhibited by treatment with genistein or tyrphostin 47, two PTK inhibitors, and not affected by inactive analogues, daidzein, and tyrphostin 1. Moreover, addition of 200 microM Na(3)VO(4), an inhibitor of protein tyrosine phosphatase (PTP) activity, evoked the responses to the factors both in patch-clamp and in fluorimetric measurements. Cell-attached recordings using 100 mM CaCl(2) in the pipette showed that bFGF and IGF-I activate calcium-permeable channels with similar properties. These results provide evidence for a calcium influx induced by two factors that bind to tyrosine kinase receptors (RTK) in endothelial cells.

Neuronal survival and calcium influx induced by basic fibroblast growth factor in chick ciliary ganglion neurons.

Basic fibroblast growth factor (bFGF/FGF2) exhibits widespread biological activities in the nervous system. However, little is known about the cascade of intracellular events that links the activation of its tyrosine kinase receptors to these effects. Here we report that, in ciliary ganglion neurons from chick embryo, this trophic factor significantly enhanced neuronal survival. The percentage of surviving neurons was reduced when intracellular calcium was chelated by adding a membrane-permeable BAPTA ester to the culture medium, while antagonists of L- and N-type voltage-dependent calcium channels were ineffective. The ionic signals in response to bFGF stimulation have been studied using cytofluorimetric and patch-clamp techniques. In single-cell Fura-2 measurements, bFGF elicited a long lasting rise of the cytosolic calcium concentration that was dependent on [Ca2+]o. In whole-cell experiments, we observed a reversible depolarization of the membrane resting potential and an inward cationic current. Single channel experiments, performed in the cell-attached configuration, provide evidence for the activation of two families of Ca2+-permeable cationic channels. Moreover, inositol 1,4,5-trisphosphate opens channels with similar properties, suggesting that this cytosolic messenger can be responsible for the calcium influx induced by bFGF.

Arachidonic acid mediates calcium influx induced by basic fibroblast growth factor in Balb-c 3T3 fibroblasts.

Basic fibroblast growth factor (bFGF), a peptide acting as a mitogen in different cell types, is able to induce a long lasting non capacitative calcium influx from the extracellular medium in Balb-c 3T3 mouse fibroblasts. This effect is mediated by the tyrosine kinase activity of bFGF receptors and the opening of voltage independent, agonist activated calcium channels. In this paper we investigate the signal transduction steps involved in this process using single cell calcium fluorimetry and electrophysiological techniques. One of the pathways initiated by the binding of growth factors to their tyrosine kinase receptors is the activation of cytosolic phospholipase A2 (cPLA2) and the release of arachidonic acid (AA) from the plasma membrane with the subsequent production of eicosanoids. We show here that, in our preparation, this pathway is involved in the opening of the bFGF-activated calcium permeable channels, through the activation of mitogen activated protein kinase (MAPK) and cPLA2. Evidence for direct involvement of AA is given by the finding that: (i) bFGF induces AA release from Balb-c 3T3 cells; (ii) blockers of AA metabolism are not effective; and (iii) the application of either arachidonic acid or its non metabolizable analogue 5,8,11,14-eicosatetraynoic acid (ETYA) reproduces the responses described for bFGF. Finally, single channel analysis indicates that bFGF, AA and ETYA can activate the same calcium permeable channel.

Oxytocin inhibits the proliferation of MDA-MB231 human breast-cancer cells via cyclic adenosine monophosphate and protein kinase A.

Oxytocin (OT) inhibits the proliferation of breast-cancer cells in vitro via a specific G-coupled receptor. To elucidate the intracellular mechanism involved in this biological effect, different G-coupled receptor mediators have been investigated in untreated and OT-treated MDA-MB231 breast-carcinoma cells. In these cells, after OT treatment, a significant cAMP increase was observed using a radioimmunoassay procedure, whereas the Ca2+ (determined with the fluorescent probe fura-2) and the inositol phosphate (determined after cell labeling with myo(2-(3)H)-inositol) concentrations were not modified, contrary to what has been observed in myometrial and myo-epithelial cells. The PKA inhibitor PKI (6-22) amide reverted the effect of OT, indicating that the anti-proliferative effect of the peptide is strictly related to the cAMP-PKA pathway. OT treatment did not modify tyrosine phosphorylation either. Our results indicate that in breast epithelial cells devoid of contractile activity, cAMP is the intracellular mediator of OT action, whereas the Ca2+-phosphoinositide system is not involved.

Role of mitogen-induced calcium influx in the control of the cell cycle in Balb-c 3T3 fibroblasts.

The role of mitogen-activated calcium influx from the extracellular medium in the control of cell proliferation was studied in Balb-c 3T3 fibroblasts. Stimulation of serum-deprived, quiescent cells with 10% foetal calf serum (FCS) induced a long-lasting (up to 70 min) elevation of intracellular free calcium concentration ([Ca2+]i). Both the sustained [Ca2+]i increase and the related inward current, described in a previous paper [Lovisolo D. Munaron L. Baccino FM. Bonelli G. (1992) Potassium and calcium currents activated by foetal calf serum in Balb-c 3T3 fibroblasts. Biochim. Biophys. Acta, 1104, 73-82], could be abolished either by chelation of extracellular calcium with EGTA or by SK&F 96365, an imidazole derivative that can block receptor-activated calcium channels. The effect of the abolition of these ionic signals on FCS-induced proliferation was investigated by adding either EGTA or SK&F 96365 to the culture medium during the first hours of stimulation of quiescent cells with 10% FCS. As measured after 24 h, a 22% inhibition of growth was observed when SK&F 96365 was added for the first hour, and stronger inhibitions, up to 56%, were obtained by adding the blocker for the first 2 or 4 h. Similar effects were observed with addition of 3 mM EGTA, though the inhibition was less marked for the 4 h treatment. By contrast, incubation with either substance in the next 4 h of serum stimulation did not influence cell growth, except for a slight inhibition observed when SK&F 96365 was applied from the 4th to the 8th hour. The reduction in growth resulting from the abolition of the early calcium influx was paralleled by an accumulation of cells in the G2/M phase. Both growth inhibition and G2/M accumulation were reversible, since after further 24 h in 10% FCS cells had fully recovered the exponential growth. These data indicate that the early calcium influx seen in response to mitogen stimulation develops on a timescale long enough to play a significant role in cell cycle progression, and that its block in the early G1 phase can lead to a reduction of proliferation by arresting cells in later stages of the cycle.