Cell physiology: cellular site of calcium regulation

Mitochondria-associated niches in health and disease

The appreciation of the importance of interorganelle contacts has steadily increased over the past decades. Advances in imaging, molecular biology and bioinformatic techniques allowed the discovery of new mechanisms involved in the interaction and communication between organelles, providing novel insights into the inner works of a cell. In this Review, with the mitochondria under the spotlight, we discuss the most recent findings on the mechanisms mediating the communication between organelles, focusing on Ca2+ signaling, lipid exchange, cell death and stress responses. Notably, we introduce a new integrative perspective to signaling networks that is regulated by interorganelle interactions – the mitochondria-associated niches – focusing on the link between the molecular determinants of contact sites and their functional outputs, rather than simply physical and structural communication. In addition, we highlight the neuropathological and metabolic implications of alterations in mitochondria-associated niches and outline how this concept might improve our understanding of multi-organelle interactions.

How does flecainide impact RyR2 channel function?

Flecainide, a cardiac class 1C blocker of the surface membrane sodium channel (NaV1.5), has also been reported to reduce cardiac ryanodine receptor (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ release. It has been introduced as a clinical antiarrhythmic agent for catecholaminergic polymorphic ventricular tachycardia (CPVT), a condition most commonly associated with gain-of-function RyR2 mutations. Current debate concerns both cellular mechanisms of its antiarrhythmic action and molecular mechanisms of its RyR2 actions. At the cellular level, it targets NaV1.5, RyR2, Na+/Ca2+ exchange (NCX), and additional proteins involved in excitation-contraction (EC) coupling and potentially contribute to the CPVT phenotype. This Viewpoint primarily addresses the various direct molecular actions of flecainide on isolated RyR2 channels in artificial lipid bilayers. Such studies demonstrate different, multifarious, flecainide binding sites on RyR2, with voltage-dependent binding in the channel pore or voltage-independent binding at distant peripheral sites. In contrast to its single NaV1.5 pore binding site, flecainide may bind to at least four separate inhibitory sites on RyR2 and one activation site. None of these binding sites have been specifically located in the linear RyR2 sequence or high-resolution structure. Furthermore, it is not clear which of the inhibitory sites contribute to flecainide's reduction of spontaneous Ca2+ release in cellular studies. A confounding observation is that flecainide binding to voltage-dependent inhibition sites reduces cation fluxes in a direction opposite to physiological Ca2+ flow from SR lumen to cytosol. This may suggest that, rather than directly blocking Ca2+ efflux, flecainide can reduce Ca2+ efflux by blocking counter currents through the pore which otherwise limit SR membrane potential change during systolic Ca2+ efflux. In summary, the antiarrhythmic effects of flecainide in CPVT seem to involve multiple components of EC coupling and multiple actions on RyR2. Their clarification may identify novel specific drug targets and facilitate flecainide's clinical utilization in CPVT.

Direct Measurements of Intracellular Elemental Composition Utilizing a New Approach to Freezing In Vivo

Many biological experiments can be successfully carried out on tissues isolated under well-defined experimental conditions, but for some physiological or pharmacological studies, knowledge of the in vivo elemental composition and/or structure is essential. We have developed a method for rapid freezing of organs in situ, with a new spring-loaded, hand held, clamping device that holds two apposing melting Freon popsicles (Fig. 5). The melting Freon surfaces that contact the tissue are at the melting point of Freon 22, and these mold to the unevenness of the tissue surface. Rapidly frozen samples of liver, pancreas or mesentery were obtained by placing an anesthetized rat in a cloth sling over an environmental chamber kept at 37°C, 100% humidity (Fig. 5). A small incision is made in the abdomen, and the abdominal wall is manipulated until a lobe of liver protrudes through the incision, while taking care not to touch the surface of the organ. Immediately, the Freon clamper is activated to snap freeze the tissue.

Molecular Mechanisms of Leucine Zipper EF-Hand Containing Transmembrane Protein-1 Function in Health and Disease

Mitochondrial calcium (Ca²⁺) uptake shapes cytosolic Ca²⁺ signals involved in countless cellular processes and more directly regulates numerous mitochondrial functions including ATP production, autophagy and apoptosis. Given the intimate link to both life and death processes, it is imperative that mitochondria tightly regulate intramitochondrial Ca²⁺ levels with a high degree of precision. Among the Ca²⁺ handling tools of mitochondria, the leucine zipper EF-hand containing transmembrane protein-1 (LETM1) is a transporter protein localized to the inner mitochondrial membrane shown to constitute a Ca²⁺/H⁺ exchanger activity. The significance of LETM1 to mitochondrial Ca²⁺ regulation is evident from Wolf-Hirschhorn syndrome patients that harbor a haplodeficiency in LETM1 expression, leading to dysfunctional mitochondrial Ca²⁺ handling and from numerous types of cancer cells that show an upregulation of LETM1 expression. Despite the significance of LETM1 to cell physiology and pathophysiology, the molecular mechanisms of LETM1 function remain poorly defined. In this review, we aim to provide an overview of the current understanding of LETM1 structure and function and pinpoint the knowledge gaps that need to be filled in order to unravel the underlying mechanistic basis for LETM1 function.

Wolfram syndrome: MAMs' connection?

Wolfram syndrome (WS) is a rare neurodegenerative disease, the main pathological hallmarks of which associate with diabetes, optic atrophy, and deafness. Other symptoms may be identified in some but not all patients. Prognosis is poor, with death occurring around 35 years of age. To date, no treatment is available. WS was first described as a mitochondriopathy. However, the localization of the protein on the endoplasmic reticulum (ER) membrane challenged this hypothesis. ER contacts mitochondria to ensure effective Ca²⁺ transfer, lipids transfer, and apoptosis within stabilized and functionalized microdomains, termed “mitochondria-associated ER membranes” (MAMs). Two types of WS are characterized so far and Wolfram syndrome type 2 is due to mutation in CISD2, a protein mostly expressed in MAMs. The aim of the present review is to collect evidences showing that WS is indeed a mitochondriopathy, with established MAM dysfunction, and thus share commonalities with several neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis, as well as metabolic diseases, such as diabetes.

Isolation of Mitochondria-Associated Membranes (MAM) from Mouse Brain Tissue

During the last decades, increasing evidence indicated that subcellular organelles do not exist as autarkic units but instead communicate constantly and extensively with each other in various ways. Some communication, for example, the exchange of small molecules, requires the marked convergence of two distinct organelles for a certain period of time. The cross talk between endoplasmic reticulum (ER) and mitochondria, two subcellular organelles of utmost importance for cellular bioenergetics and protein homeostasis, has been increasingly investigated under the last years. This development was significantly driven by the establishment of optimized subcellular fractionation techniques. In this chapter, we will describe and critically discuss the currently used protocol for the isolation of the membrane fraction containing mitochondria-associated membranes (MAM).

Mitochondria-Associated Membranes (MAMs): Overview and Its Role in Parkinson's Disease

Mitochondria-associated membranes (MAMs) are structures that regulate physiological functions between endoplasmic reticulum (ER) and mitochondria in order to maintain calcium signaling and mitochondrial biogenesis. Several proteins located in MAMs, including those encoded by PARK genes and some of neurodegeneration-related proteins (huntingtin, presenilin, etc.), ensure this regulation. In this regard, MAM alteration is associated with neurodegenerative diseases such as Parkinson's (PD), Alzheimer's (AD), and Huntington's diseases (HD) and contributes to the appearance of the pathogenesis features, i.e., autophagy dysregulation, mitochondrial dysfunction, oxidative stress, and lately, neuronal death. Moreover,, ER stress and/or damaged mitochondria can be the cause of these disruptions. Therefore, ER-mitochondria contact structure and function are crucial to multiple cellular processes. This review is focused on the molecular interaction between ER and mitochondria indispensable to MAM formation and on MAM alteration-induced etiology of neurodegenerative diseases.

Mitochondrial Oxidative Phosphorylation System (OXPHOS) Deficits in Schizophrenia: Possible Interactions with Cellular Processes

Mitochondria are key players in the generation and regulation of cellular bioenergetics, producing the majority of adenosine triphosphate molecules by the oxidative phosphorylation system (OXPHOS). Linked to numerous signaling pathways and cellular functions, mitochondria, and OXPHOS in particular, are involved in neuronal development, connectivity, plasticity, and differentiation. Impairments in a variety of mitochondrial functions have been described in different general and psychiatric disorders, including schizophrenia (SCZ), a severe, chronic, debilitating illness that heavily affects the lives of patients and their families. This article reviews findings emphasizing the role of OXPHOS in the pathophysiology of SCZ. Evidence accumulated during the past few decades from imaging, transcriptomic, proteomic, and metabolomic studies points at OXPHOS deficit involvement in SCZ. Abnormalities have been reported in high-energy phosphates generated by the OXPHOS, in the activity of its complexes and gene expression, primarily of complex I (CoI). In addition, cellular signaling such as cAMP/protein kinase A (PKA) and Ca(+2), neuronal development, connectivity, and plasticity have been linked to OXPHOS function and are reported to be impaired in SCZ. Finally, CoI has been shown as a site of interaction for both dopamine (DA) and antipsychotic drugs, further substantiating its role in the pathology of SCZ. Understanding the role of mitochondria and the OXPHOS in particular may encourage new insights into the pathophysiology and etiology of this debilitating disorder.

The mitochondrial calcium uniporter complex: molecular components, structure and physiopathological implications

Although it has long been known that mitochondria take up Ca2+, the molecular identities of the channels and transporters involved in this process were revealed only recently. Here, we discuss the recent work that has led to the characterization of the mitochondrial calcium uniporter complex, which includes the channel-forming subunit MCU (mitochondrial calcium uniporter) and its regulators MICU1, MICU2, MCUb, EMRE, MCUR1 and miR-25. We review not only the biochemical identities and structures of the proteins required for mitochondrial Ca2+ uptake but also their implications in different physiopathological contexts.

The endoplasmic reticulum-mitochondria connection: one touch, multiple functions

The endoplasmic reticulum (ER) and mitochondria are tubular organelles with a characteristic "network structure" that facilitates the formation of interorganellar connections. The ER and mitochondria join together at multiple contact sites to form specific domains, termed mitochondria-ER associated membranes (MAMs), with distinct biochemical properties and a characteristic set of proteins. The functions of these two organelles are coordinated and executed at the ER-mitochondria interface, which provides a platform for the regulation of different processes. The roles played by the ER-mitochondria interface range from the coordination of calcium transfer to the regulation of mitochondrial fission and inflammasome formation as well as the provision of membranes for autophagy. The novel and unconventional processes that occur at the ER-mitochondria interface demonstrate its multifunctional and intrinsically dynamic nature. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

Where the endoplasmic reticulum and the mitochondrion tie the knot: the mitochondria-associated membrane (MAM)

More than a billion years ago, bacterial precursors of mitochondria became endosymbionts in what we call eukaryotic cells today. The true significance of the word "endosymbiont" has only become clear to cell biologists with the discovery that the endoplasmic reticulum (ER) superorganelle dedicates a special domain for the metabolic interaction with mitochondria. This domain, identified in all eukaryotic cell systems from yeast to man and called the mitochondria-associated membrane (MAM), has a distinct proteome, specific tethers on the cytosolic face and regulatory proteins in the ER lumen of the ER. The MAM has distinct biochemical properties and appears as ER tubules closely apposed to mitochondria on electron micrographs. The functions of the MAM range from lipid metabolism and calcium signaling to inflammasome formation. Consistent with these functions, the MAM is enriched in lipid metabolism enzymes and calcium handling proteins. During cellular stress situations, like an altered cellular redox state, the MAM alters its set of regulatory proteins and thus alters MAM functions. Notably, this set prominently comprises ER chaperones and oxidoreductases that connect protein synthesis and folding inside the ER to mitochondrial metabolism. Moreover, ER membranes associated with mitochondria also accommodate parts of the machinery that determines mitochondrial membrane dynamics and connect mitochondria to the cytoskeleton. Together, these exciting findings demonstrate that the physiological interactions between the ER and mitochondria are so bilateral that we are tempted to compare their relationship to the one of a married couple: distinct, but inseparable and certainly dependent on each other. In this paradigm, the MAM stands for the intracellular location where the two organelles tie the knot. Resembling "real life", the happy marriage between the two organelles prevents the onset of diseases that are characterized by disrupted metabolism and decreased lifespan, including neurodegeneration and cancer. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.

Streptomycin and EDTA decrease the number of desmin-negative fibers following stretch injury

Streptomycin and ethylene diamine tetraacetic acid (EDTA) were used to examine the role of extracellular calcium in stretch-induced muscle injury. Streptomycin was injected in one group of rats, three times daily for 8 days (S, 300 mg.kg(-1).day(-1) intraperitoneally). In another group, EDTA was administered (150 mg.kg(-1) IP) 20 min before and 24 h after the injury protocol. Untreated rats (C) served as controls. Muscle injury was produced by 40 stretches of active dorsiflexor muscles by ankle rotation from 80 degrees to 130 degrees (velocity 1.75 rad.s(-1)). Ten minutes after the injury protocols, all animals lost the same amount of isometric force at both low and high stimulation frequencies (20 HZ; S, 56 +/- 6%; EDTA, 47 +/- 7%; C, 55 +/- 4%) and 120 HZ; S, 11 +/- 3%, EDTA, 13 +/- 3%; C, 11 +/- 3%). Tibialis anterior (TA) muscles were removed after 48 h for morphometric analysis. In both streptomycin-and EDTA-treated rats, the percent of injured (i.e., desmin-negative) myofibers in TA was reduced compared to untreated, injured muscles (S, 0.35 +/- 0.08%; EDTA, 0.64 +/- 0.19%; C, 1.81 +/- 0.43%). Thus, streptomycin and EDTA treatment did not alter the development of muscle weakness (i.e., isometric force deficit), but almost abolished the histopathologic changes. This study shows that the mechanisms for muscle weakness and histopathologic changes (inflammation) following repeated muscle strains can largely be dissociated from each other and helps explain why there is no correlation between isometric force deficits and the number of pathologic cells.

A simulation analysis on mechanisms of damped oscillation in retinal rod photoreceptor cells

The different actions of two I(h) channel blockers, zatebradine (UL-FS 49) and ZD7288, on rod photoresponses were analysed by computer simulation using a newly revised ionic current model of the rod photoreceptor, based on Hodgkin-Huxley equations. The model, adjusted to fit the experimental results of amphibian rods, shows that both of the blockers enhance the light-induced membrane hyperpolarization. Our model can also predict a mechanism of a damped oscillation arising during the recovery phase appeared only in the presence of zatebradine which, unlike ZD7288, reduces both I(h) and I(Kv). We suggest that the oscillation can appear due to the alternative activation of voltage-dependent Ca(2+) current (I(Ca)) and calcium-dependent current (I(K(Ca)) and I(Cl(Ca))) when I(Kv) is blocked, with I(K(Ca)) having a stronger effect than I(Cl(Ca)).

Regulation of plasma membrane Ca2+-ATPase by small GTPases and phosphoinositides in human platelets

We have investigated the restoration of [Ca(2+)](i) in human platelets following the discharge of the intracellular Ca(2+) stores. We found that the plasma membrane Ca(2+)-ATPase is the main mechanism involved in Ca(2+) extrusion in human platelets. Treatment of platelets with the farnesylcysteine analogs, farnesylthioacetic acid and N-acetyl-S-geranylgeranyl-l-cysteine, inhibitors of activation of Ras proteins, accelerated the rate of decay of [Ca(2+)](i) to basal levels after activation with thapsigargin combined with a low concentration of ionomycin, indicating that Ras proteins are involved in the negative regulation of Ca(2+) extrusion. Rho A, which is involved in actin polymerization, was not responsible for this effect. Consistent with this, the actin polymerization inhibitors, cytochalasin D and latrunculin A, did not alter the recovery of [Ca(2+)](i). Activation of human platelets with thapsigargin and ionomycin stimulated the tyrosine phosphorylation of the plasma membrane Ca(2+)-ATPase, a mechanism that was inhibited by farnesylcysteine analogs, suggesting that Ras proteins could regulate Ca(2+) extrusion by mediating tyrosine phosphorylation of the plasma membrane Ca(2+)-ATPase. Treatment of platelets with LY294002, a specific inhibitor of phosphatidylinositol 3- and phosphatidylinositol 4-kinase, resulted in a reduction in the rate of recovery of [Ca(2+)](i) to basal levels, suggesting that the products of these kinases are involved in stimulating Ca(2+) extrusion in human platelets.

Calmodulin and cyclic ADP-ribose interaction in Ca2+ signaling related to cardiac sarcoplasmic reticulum: superoxide anion radical-triggered Ca2+ release

Reactive oxygen species (ROS) are often shown to damage cellular functions. The targets of oxidative damage depend on the nature of ROS produced and the site of generation. In contrast, ROS can also regulate signal transduction. In this case, ROS may either induce or enhance events, which lead to forward directions of cellular signaling. The consequences of regulation of signal transduction can be observed in physiological processes such as muscle contraction. Here, we discuss the concentration-dependent effects of superoxide anion radical (*O2-) on Ca2+ release from the cardiac sarcoplasmic reticulum (SR). Recent studies suggest that the ADP-ribosyl cyclase pathway, through its production of cyclic adenosine 5'-diphosphoribose (cADPR), may control Ca2+ mobilization in cardiac muscle cells. *O2- has dual effects that are concentration dependent. At low concentrations (nearly nanomolar levels), *O2- induces Ca2+ release by stimulating synthesis of cADPR, which requires calmodulin for sensitization of ryanodine-sensitive Ca2+-release channels (RyRC). At these low concentrations, *O2- is responsible for regulation of cellular signal transduction. At higher concentrations (micromolar levels), *O2- produces a loss in the function of calmodulin that is to inhibit RyRC. This results in an increase in Ca2+ release, which is linked to cell injury. The difference in the functions of low and high concentrations of *O2- may result in two distinct physiological roles in cardiac muscle Ca2+ signaling.

Depolarization-induced mitochondrial Ca accumulation in sympathetic neurons: spatial and temporal characteristics

Several lines of evidence suggest that neuronal mitochondria accumulate calcium when the cytosolic free Ca(2+) concentration ([Ca(2+)](i)) is elevated to levels approaching approximately 500 nM, but the spatial, temporal, and quantitative characteristics of net mitochondrial Ca uptake during stimulus-evoked [Ca(2+)](i) elevations are not well understood. Here, we report direct measurements of depolarization-induced changes in intramitochondrial total Ca concentration ([Ca](mito)) obtained by x-ray microanalysis of rapidly frozen neurons from frog sympathetic ganglia. Unstimulated control cells exhibited undetectably low [Ca](mito), but high K(+) depolarization (50 mM, 45 sec), which elevates [Ca(2+)](i) to approximately 600 nM, increased [Ca](mito) to 13.0 +/- 1.5 mmol/kg dry weight; this increase was abolished by carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP). The elevation of [Ca](mito) was a function of both depolarization strength and duration. After repolarization, [Ca](mito) recovered to prestimulation levels with a time course that paralleled the decline in [Ca(2+)](i). Depolarization-induced increases in [Ca](mito) were spatially heterogeneous. At the level of single mitochondria, [Ca](mito) elevations depended on proximity to the plasma membrane, consistent with predictions of a diffusion model that considers radial [Ca(2+)](i) gradients that exist early during depolarization. Within individual mitochondria, Ca was concentrated in small, discrete sites, possibly reflecting a high-capacity intramitochondrial Ca storage mechanism. These findings demonstrate that in situ Ca accumulation by mitochondria, now directly identified as the structural correlate of the "FCCP-sensitive store, " is robust, reversible, graded with stimulus strength and duration, and dependent on spatial location.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

Novel mechanisms involved in superoxide anion radical-triggered Ca2+ release from cardiac sarcoplasmic reticulum linked to cyclic ADP-ribose stimulation

It has been suggested that cyclic adenosine 5'-diphosphoribose (cADPR) directly activates the cardiac isoform of the ryanodine receptor (RyR)/Ca2+ release channel. We have previously shown that selective activation of RyR/Ca2+ release channel by superoxide anion radical (O2.-) is dependent of the presence of calmodulin and identified calmodulin as a functional mediator of O2.- -triggered Ca2+ release through the RyR/Ca2+ release channel of cardiac sarcoplasmic reticulum (SR). We now demonstrate that although the effect of O2.- on Ca2+ efflux from RyR/Ca2+ release channel at higher concentrations ( >5 microM) is due to its ability to produce a loss in function of calmodulin thereby decreasing calmodulin inhibition, O2.- radicals at lower concentrations (<5 microM) may be able to stimulate Ca2+ release only in the presence of calmodulin from the SR via increased cADPR synthesis; it is also shown that cADPR is a modulator that can activate the Ca2+-release mechanism when it is in a sensitized state by the presence of calmodulin, possibly, at physiological concentration. In addition, the SR vesicles immediately upon addition of cADPR, but not NAD+, did exhibit Ca2+ efflux stimulation. When heart homogenate was incubated with O2.-, conversion of NAD+ into cADPR was stimulated; the reduction of homogenate Ca2+ uptake (by increasing Ca2+ efflux through RyR/Ca2+ release channel) occurred. Thus O2.- radical is responsible for cADPR formation from NAD+ in the cellular environment outside of the SR of heart muscle. The results presented here provide the first evidence of a messenger role for O2.- radical in cADPR-mediated Ca2+ mobilization in myocardium.

Stress-reactive response of the gerbil pineal gland: concretion genesis

The reaction of pinealocytes and glia cells to an acute immobilization stress and their poststress recovery was studied in gerbils. Pinealocytes responded to immobilization with an increased peptidergic activity and formation of new concretions, whereas glia cells with an increased growth of interstitial concretions. The occurrence of degenerating pinealocytes indicated deleterious actions of immobilization stress on functionally stimulated cells. The pyroantimonate method to detect Ca2+ demonstrated enlarged crystalline profiles (Ca2+ crystallization into hydroxyapatite) in functionally stimulated pinealocytes and the accumulation of Ca2+ in the interstitial concretion. The pinealocyte concretions did not show the Ca2+ accumulation. The pineal gland poststress recovery was manifested by a reduced functionally stimulated pinealocyte activity and a protracted increase in glia cell activity. It is suggested that the physiological relevance of the crystallization of Ca2+ into hydroxyapatite is to maintain a noradrenalin-stimulated Ca2+ influx at an optimal level during attentuated pinealocyte turnover. The interstitial concretions may lower the extracellular Ca2+ concentrations and thereby stimulate pinealocytes and restrict an increased Ca2+ influx.

Hepatoprotective effect of a nonselective endothelin receptor antagonist (TAK-044) in the transplanted liver

This study was designed to investigate whether or not a novel nonselective endothelin A/B (ETA/ETB) receptor antagonist (TAK-044) provides hepatoprotection during porcine liver transplantation. The grafts were stored in chilled Euro-Collins solution and recirculated following reflush with lactated Ringer's with (TAK group) or without (control group) TAK-044 (10 mg/kg). Intracellular (cytoplasma, mitochondria, and nucleus) calcium (Ca) concentrations were measured in the hepatic biopsy materials obtained serially at varying time point from donor laparotomy to recipient closure using an electron probe X-ray microanalyzer. Liver function tests also were determined. The cold and warm ischemia times of the grafts were comparable between the two groups. The peak endothelin-1 T-1) concentration after recirculation was significantly higher in the TAK group than in the control group (129 +/- 30 pg/ml vs 26 +/- 6.5 pg/ml). However, release of liver enzymes, increases in total bile acid, and deterioration of indocyanine green retention rate were significantly suppressed in the TAK group. In the control group, the intracellular Ca concentrations, especially in the mitochondrial fraction, were elevated markedly following recirculation of the hepatic arterial flow. In the TAK group, this effect was suppressed. Thus, the supplementary use of the nonselective ETA/ETB receptor antagonist TAK-044 via a rinse route may alleviate an early postreperfusion microcirculatory disturbance of the liver grafts without adverse effects by the increased ET-1 on the systemic circulation.

Mitochondria contribute to Ca2+ removal in smooth muscle cells

Recent evidence, from a variety of cell types, suggests that mitochondria play an important role in shaping the change in intracellular calcium concentration ([Ca2+]i) that occurs during physiological stimulation. In the present study, using a range of inhibitors of mitochondrial Ca2+ uptake, we have examined the contribution of mitochondria to Ca2+ removal from the cytosol of smooth muscle cells following stimulation. In voltage-clamped single smooth muscle cells, we found that following a 8-s train depolarizing pulses, the rate of Ca2+ extrusion from the cytosol was reduced by more than 50% by inhibitors of cytochrome oxidase or exposure of cells to the protonophore carbonyl cyanide P-trifluoromethoxy-phenylhydrazone. Using the potential-sensitive indicator-tetramethyl rhodamine ethyl ester, we confirmed that the effect of these agents was associated with depolarization of the mitochondrial membrane. Since, the primary function of the mitochondria is to provide the cell's ATP, it could be argued that it is the ATP supply to the ion pumps which is limiting the rate of Ca2+ removal. However, experiments carried out with the mitochondrial Ca2+ uniporter inhibitor ruthenium red produced similar results, while the ATP synthetase inhibitor oligomycin had no effect, suggesting that the effect was not due to ATP insufficiency. These results establish that mitochondria in smooth muscle cells play a significant role in removing Ca2+ from the cytosol following stimulation. The uptake of Ca2+ into mitochondria is proposed to stimulate mitochondrial ATP production, thereby providing a means for matching increased energy demand, following the cell's rise in [Ca2+]i, with increased cellular ATP production.

Mitochondrial calcium uptake from physiological-type pulses of calcium. A description of the rapid uptake mode

A controversy in the field of bioenergetics has been whether mitochondria are capable of sequestering enough Ca2+ from cytosolic Ca2+ pulses to raise their intramitochondrial free Ca2+ level ([Ca2+]m). This is significant because an increase in [Ca2+]m has been linked to an increase in cellular metabolic rate through various mechanisms. To resolve this question, we exposed isolated liver mitochondria to physiological type pulses of Ca2+ produced using a pulse-generating system (Sparagna, G. C., Gunter, K. K., and Gunter, T. E. (1994) Anal. Biochem. 219, 96-103). We then measured the resulting mitochondrial Ca2+ uptake. The uniporter was previously thought to be the only specific Ca2+ uptake mechanism in mitochondria. Our studies have uncovered an additional uptake mechanism, the rapid mode of uptake or RaM, which functions at the beginning of each pulse and allows mitochondria to sequester a considerable amount of Ca2+ from short pulses. We have shown that the RaM is reset by decreasing the [Ca2+] between pulses for a very short time, making this uptake mode ideally suited for Ca2+ sequestration from Ca2+ pulse sequences. With rapid Ca2+ uptake occurring at the beginning of each pulse, liver mitochondria may be able to sequester sufficient Ca2+ from a short sequence of pulses to activate the cellular metabolic rate.

Calcium concretions in the pineal gland of aged rats: an ultrastructural and microanalytical study of their biogenesis

The genesis of calcium concretions in aged rats was studied by means of transmission and scanning electron microscopy. The potassium pyroantimonate method, combined with X-ray microanalysis, allowed us to study the distribution of cations and calcium. Notable accumulations of calcium (associated with phosphorus) were localized in vesicles, vacuoles, lipid droplets, lipopigments, and mitochondria of dark pinealocytes. The results obtained in the present investigation suggest that these organelles are involved in the genesis of the concretions. The presence of sulfur indicates the existence of an organic matrix. We propose that genesis takes place in dark pinealocytes, which contain more calcium than light pinealocytes. Mineralization foci are sometimes associated with cellular debris and enlarge by further apposition of material. Two types of concretions, as determined by electron microscopy and confirmed by electron diffraction, could be observed: the "amorphous" type with concentric layers and the crystalline type with needle-shaped crystals. Once formed, the concretions reach the extracellular space and the cell breaks down. Possible extracellular calcification is suggested in the extracellular calcium-rich floculent material. The mineralization process is interpreted as being an age-related phenomenon and mainly a consequence of the degeneration of pinealocytes.

The pineal gland of the aging rat: calcium localization and variation in the number of pinealocytes

In the present study, we investigated the population of pinealocytes in the pineal gland of aging rats. Dark and light pinealocytes were analyzed as to their calcium content. Calcium localization was realized in dark and light cells by means of cytochemistry and X-ray microanalysis. Calcium was mainly localized in dark pinealocytes characterized by many ultrastructural signs of degeneration. The number of pinealocytes per square surface of aged rats (28 months) was compared to young ones (3-4 months). While there is a significant increase in the number of dark pinealocytes there is a decrease in the total number of pinealocytes in aged rats. This age-related loss of pinealocytes may explain the age-related functional decline of the pineal gland activity (e.g., the decrease of the nocturnal melatonin production).

Immunogold localization of inositol 1,4,5-trisphosphate receptors and characterization of ultrastructural features of the sarcoplasmic reticulum in phasic and tonic smooth muscle

Although agonist stimulation leads to an increase in inositol 1,4,5-trisphosphate (InsP3) and decreased calcium in peripherally and centrally located sarcoplasmic reticulum in smooth muscle, the distribution of InsP3 receptors is unknown. InsP3 receptor and the calcium binding protein, calsequestrin were localized by immunolabelling in a tonic and a phasic smooth muscle. InsP3 receptor labelling was predominantly localized at the cell periphery, where most of the sarcoplasmic reticulum is localized in vas deferens (phasic muscle). Elements of central sarcoplasmic reticulum, where present, were also labelled. Distribution of calsequestrin in vas deferens was similar to that of the InsP3 receptor. In aorta (tonic muscle) the InsP3 receptor labelling was proportional to sarcoplasmic reticulum distribution: predominantly central. No labelling of sections or immunoblots was observed with the anti-calsequestrin antibody in aorta. InsP3 and caffeine, but not cyclic ADP-ribose, released intracellular Ca2+ in permeabilized vas deferens and aorta. The ultrastructure of the sarcoplasmic reticulum, investigated in stereo views of semi-thick and thin sections of osmium ferricyanide stained tissue, is shown to have several distinctive features, such as fenestrated sheets (single or in stacks), as well as numerous regions of continuity between central and peripheral sarcoplasmic reticulum, suggesting a single compartment within the smooth muscle cell. Regions of the sarcoplasmic reticulum were closely apposed to and often ensheathed mitochondria. We conclude that InsP3 receptors are present in both the central and the peripheral sarcoplasmic reticulum of tonic and phasic smooth muscle, consistent with electron probe analysis results showing calcium release from both regions.

Evidence for calcium-binding proteins and calcium-dependent regulatory proteins in sensory cells of the organ of Corti

Calcium is thought to play a major signaling role in outer hair cells to control metabolism, cytoskeletal integrity, cell shape and cell excitability. For this to happen, in resting cells the concentration of free calcium ions must be maintained at low levels so that focal increases can trigger specific events. In this paper, the localization of calcium, calcium-binding and calcium-dependent regulatory proteins in sensory cells from the guinea pig inner ear was demonstrated using immunocytochemical and histochemical techniques. We found the calcium buffer and/or calcium sensor proteins calmodulin, calbindin and calsequestrin predominantly in sensory cells and that when present, these proteins can be enriched in the outer hair cells. Calmodulin is found in the stereocilia, in the cuticular plate and in the cytoplasm and calbindin is found only in the cuticular plate and cytoplasm of both the inner and outer hair cells. The staining for these proteins in the outer hair cells is homogeneous, with no apparent compartmentalization along the lateral wall. Calsequestrin, thought to store and release calcium from membrane bound intracellular storage sites is found only in the cytoplasm of outer hair cells. There, it has a more punctuate staining pattern than does calmodulin or calbindin suggesting that it may be present in calciosomes rather than soluble in the cytoplasm. We did not detect caldesmon and S-100. Using the potassium pyroantimonate technique, we found precipitates containing calcium ions distributed throughout the cytoplasm of outer hair cells, with no evidence that the subsurface cisterns along the lateral wall act as calcium storage sites. Thus, calcium in resting cells is found in the cytoplasm along with calbindin and calmodulin and appears to have a punctate distribution consistent with a co-localization with calsequestrin. The implications of this distribution with respect to the slow shortening and elongation seen in outer hair cells are discussed.

Anti-ryanodine receptor antibody binding sites in vascular and endocardial endothelium

The ryanodine receptor (RyR) functions as the calcium release channel of the sarcoplasmic reticulum activated by electromechanical coupling in skeletal and cardiac muscles. In smooth muscle, inositol trisphosphate releases calcium from internal stores during pharmacomechanical coupling, but these cells also contain ryanodine-sensitive calcium stores. In this study, we establish the presence of anti-RyR antibody binding sites in vascular and endocardial endothelium. Both types of endothelia also contain messenger RNA, which hybridizes to a cardiac RyR isoform cDNA probe. Western blots of endothelial cell homogenates demonstrate the presence of a single, high molecular weight band of protein that corresponds to the cardiac RyR isoform. Confocal micrographs of endothelial cells labeled with a specific anti-RyR antibody reveal an intense fluorescent signal surrounding the nucleus and distributed in a nonhomogeneous pattern throughout the cytoplasm. This pattern of fluorescence is consistent with the electron microscopic distribution of the endoplasmic reticulum. The pattern of immunofluorescence seen with the anti-RyR antibody is distinctly different from that seen with the mitochondrial fluorophore rhodamine 123. Our findings suggest that the RyR plays a role in endothelial signaling.

Long-term sensitization training in Aplysia leads to an increase in calreticulin, a major presynaptic calcium-binding protein

Long-term memory for sensitization in Aplysia requires new protein and RNA synthesis. Here, we identify a late protein as calreticulin, the major Ca(2+)-binding protein of the lumen of the endoplasmic reticulum. An antiserum against Aplysia calreticulin reveals an enrichment of calreticulin immunoreactivity in presynaptic varicosities. Quantitative S1 nuclease analysis indicates that the steady-state level of calreticulin mRNA in Aplysia sensory neurons increases during the maintenance phase of long-term sensitization. The finding that this mRNA increases in expression late, some time after training, is consistent with the idea that long-term neuromodulatory changes underlying sensitization may depend on a cascade of gene expression in which the induction of early regulatory genes leads to the expression of late effector genes.

Structure and organization of membrane organelles along distal microtubule segments in growth cones

Advance and stabilization of organelle-rich cytoplasm within the neuronal growth cone is coupled to axon elongation (Goldberg and Burmeister, 1986; Aletta and Greene, 1988), and this involves forward movement of organelles from the growth cone base along distinct tracks toward the leading edge. Membrane-bound organelles that advance first within the growth cone often make transient excursions toward the leading edge, and at the light microscope level these leading organelles appear to colocalize with distal microtubule (MT) segments (Dailey and Bridgman, 1989). We have used electron microscopy (EM) to identify the membranous organelles adjacent to distal MT segments, and to examine their structural interactions with MTs. In both glutaraldehyde-fixed and rapid frozen whole-mount growth cones, attenuated endoplasmic reticulum (ER)-like membrane elements were the most common organelle type located adjacent to distal MT segments. These ER-like membrane elements coursed roughly parallel to MTs and frequently terminated within an electron-dense bulb at the MT tip. Blind-ended membrane tubes, dense-core vesicles, clear vesicles, and vacuoles were also found adjacent to distal MT segments. Quantitative analyses of organelle-MT associations suggest that elements of the ER-like membrane system may frequently advance ahead of other membrane-bound organelles. Freeze-etch EM revealed crossbridging structures between MTs and membranous organelles, which is consistent with the idea that advance of leading membrane organelles into the growth cone periphery is mediated by microtubule-based motor transport mechanisms. The results suggest that distal microtubule segments serve as transport elements for advance of membrane organelles into more peripheral growth cone regions, and together MTs and ER-like membrane organelles may initiate the conversion of dynamic F-actin-rich cytoplasm to more stable organelle-rich cytoplasm (i.e., axoplasm).

Ca(2+)-storage organelles

Intracellular Ca(2+)-storage organelles are found in virtually all eukaryotic cells. They play an important role in the regulation of the cytosolic free Ca2+ concentration and, thereby, in the regulation of cellular activity. Ca(2+)-storage organelles consist, in the simplest model of a Ca2+ pump, of a Ca(2+)-storage protein and a Ca(2+)-release channel. The primary structure of these functionally important proteins of Ca(2+)-storage organelles is similar in different cell types and conserved through evolution. In contrast, their spatial arrangement and, thus, the architecture of Ca(2+)-storage organelles may vary dramatically from one cell type to another.

Pinealocyte subsurface cisterns. III: Storage of calcium ions and their probable role in cell stimulation

Different techniques for the ultrastructural demonstration of calcium have been applied to the pineal gland of Meriones unguiculatus, attention being focussed on the endoplasmic reticulum (ER) and its subsurface cisterns (ssc). By means of a "loading" method [Walz, 1982; Wakasugi et al., 1982] it is shown that the pinealocyte ER-ssc system sequesters calcium with dependency on ATP. Furthermore, a modification of the method of Duce and Keen [1978] is presented which turned out a) to be sensitive enough to demonstrate the cell's own low amounts of calcium as fine granular precipitates, and b) to preserve ultrastructure sufficiently. This method rendered possible comparison of the calcium distribution inside pinealocytes of the following groups: animals fixed during daytime, animals fixed at night, animals fixed at night with prior exposure to bright white light, animals fixed at night but injected at the end of the preceding light period with a pharmacon known to prevent the release of calcium from the ER of muscle fibers (Dantrolen). In contrast to the daytime findings, the pinealocyte ER-ssc system at night is free of precipitable calcium; nocturnal illumination induces reacquisition, Dantrolen hinders nocturnal depletion. From the nocturnal coincidence of pinealocyte activity and calcium release from ssc, and from other cytological and experimental data, it is concluded that the functional significance of ssc refers to the regulation of pinealocyte sensitivity. Vice versa, pinealocyte activity may influence ER expansion and ssc size via the calcium-dependent stability of microtubules.

[3H]ryanodine binding sites in rat brain demonstrated by membrane binding and autoradiography

High affinity [3H]ryanodine binding sites were characterized in P1 (crude nuclear), P2 (mitochondrial/synaptosomal) and P3 (microsomal) subcellular fractions of rat brain. Binding in each of the fractions was highest at 37 degrees C and pH 8-9, optimal in the presence of 100 microM Ca2+, 550 microM ATP and 1.0 M KCl, and increased linearly as a function of protein. Saturation analyses revealed a single class of binding sites with mean KD values (nM) of 8.9, 1.6 and 5.7 and Bmax values (fmol/mg protein) of 122, 69 and 106 for the P1, P2 and P3 fractions, respectively. The levels of [3H]ryanodine binding in P1 and P2 fractions of 4 brain regions were fairly uniform while those in P3 fractions were 5-fold greater in cerebral cortex than in the other areas examined. By autoradiography, a high concentration of [3H]ryanodine binding sites was seen in the dentate gyrus and CA3 subregions of the hippocampus. The results suggest that [3H]ryanodine binding sites, perhaps similar to [3H]ryanodine receptors in muscle, are associated with various subcellular structures and are heterogeneously distributed in the CNS.

Electron probe microanalysis of calcium release and magnesium uptake by endoplasmic reticulum in bee photoreceptors

Honey bee photoreceptors contain large sacs of endoplasmic reticulum (ER) that can be located unequivocally in freeze-dried cryosections. The elemental composition of the ER was determined by electron probe x-ray microanalysis and was visualized in high-resolution x-ray maps. In the ER of dark-adapted photoreceptors, the Ca concentration was 47.5 +/- 1.1 mmol/kg (dry weight) (mean +/- SEM). During a 3-sec nonsaturating light stimulus, approximately 50% of the Ca content was released from the ER. Light stimulation also caused a highly significant increase in the Mg content of the ER; the ratio of Mg uptake to Ca released was approximately 0.7. Our results show unambiguously that the ER is the source of Ca2+ release during cell stimulation and suggest that Mg2+ can nearly balance the charge movement of Ca2+.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

The endoplasmic reticulum and calcium storage

Calcium storage is one of the functions commonly attributed to the endoplasmic reticulum (ER) in nonmuscle cells. Several recent studies have added support to this concept. Analysis of reticuloplasm, the luminal ER content, has shown that it contains several proteins (reticuloplasmins) which are prospective calcium storage proteins. One of these, calreticulin, is also present in the sarcoplasmic reticulum (SR). In sea urchin eggs, a calsequestrin-like protein has been clearly localised to the ER. The recent demonstration that the IP3 receptor, which has similarities with the calcium release channel in the SR is also localised in the ER membrane suggests that calcium stored in the ER is important for intracellular signalling. The alternative view, that the physiologically important calcium store is a specialised organelle, the calciosome, is not supported by these observations. Recent evidence also suggests that ER calcium might be important in ER structure and in the retention of the luminal ER proteins.

Active involvement of Ca2+ in mitotic progression of Swiss 3T3 fibroblasts

Global Ca2+ transients have been observed to precede nuclear envelope breakdown and the onset of anaphase in Swiss 3T3 fibroblasts in 8% (vol/vol) FBS. The occurrence of these Ca2+ transients was dependent on intracellular stores. These Ca2+ transients could be (a) abolished by serum removal without halting mitosis, and (b) eliminated by increasing intracellular Ca2+ buffering capacity through loading the cells with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) buffer, via the tetra(acetoxymethyl) ester, without hindering the transition into anaphase. Microinjection of sufficient concentrations of BAPTA buffer could block nuclear envelope breakdown. Pulses of Ca2+ generated by flash photolysis of intracellularly trapped nitr-5, a "caged" Ca2+, could precipitate precocious nuclear envelope breakdown in prophase cells. In metaphase cells, photochemically generated Ca2+ pulses could cause changes in the appearance of the chromosomes, but the length of time required for cells to make the transition from metaphase to anaphase remained essentially unchanged regardless of whether a Ca2+ pulse was photoreleased during metaphase. The results from these photorelease experiments were not dependent on the presence of serum in the medium. Discharging intracellular Ca2+ stores with ionomycin in the presence of 1.8 mM extracellular Ca2+ doubled the time for cells to pass from late metaphase into anaphase, whereas severe Ca2+ deprivation by treatment with ionomycin in EGTA-containing medium halted mitosis. Our results collectively indicate that Ca2+ is actively involved in nuclear envelope breakdown, but Ca2+ signals are likely unnecessary for the metaphase-anaphase transition in Swiss 3T3 fibroblasts. Additional studies of intracellular Ca2+ concentrations in mitotic REF52 and PtK1 cells revealed that Ca2+ transients are not observed at all mitotic stages in all cells. The absence of observable global Ca2+ transients, where calcium buffers can block and pulses of Ca2+ can advance mitotic stages, may imply that the relevant Ca2+ movements are too local to be detected.

Distribution and function of organized concentrations of actin filaments in mammalian spermatogenic cells and Sertoli cells

Actin filaments are concentrated in specific regions of spermatogenic cells and Sertoli cells. In spermatogenic cells they occur in intercellular bridges and in the subacrosomal space. In Sertoli cells they are abundant in ectoplasmic specializations and in regions adjacent to tubulobulbar processes of spermatogenic cells. At all of these sites, the filaments are morphologically related to the plasma membrane and+or intercellular membranes, and, as in many other cell types, are arranged in either bundles or networks. In at least two of the locations just indicated (ectoplasmic specializations and intercellular bridges), elements of the ER are closely related to the actin filaments. In tubulobulbar complexes, ER is present but is more distantly related to the filaments. Elements of the ER, when present, may serve a regulatory function. The filaments in ectoplasmic specializations and in regions adjacent to tubulobulbar processes of spermatogenic cells are suspected to be involved with the mechanism by which intercellular junctions are established, maintained, and degraded. In intercellular bridges, actin filaments may serve to reinforce and perhaps regulate the size of the cytoplasmic connections between differentiating germ cells. Filaments in the subacrosomal space may serve as a linking network between the acrosome and nucleus and may also be involved in the capping process. Because of the possibility that the actin filaments discussed before may be related to specific membrane domains involved with intercellular or interorganelle attachment, and that changes in these membrane domains are prerequisite to processes such as sperm release, turnover of the blood-testis barrier, formation of the acrosome, and coordination of spermatogenic cell differentiation, an understanding of exactly how these actin filaments are related to elements in the membrane and how this interaction is controlled is fundamental to our understanding, and perhaps our manipulating, of male fertility. I suspect that working out the molecular organization of these actin filament-containing sites and determining how their organization is controlled will be the major focus of research in this field over the next few years.

Identification of a Ca2+-ATPase in cerebellar Purkinje cells

The expression of a sarcoplasmic reticulum (SR)-like Ca2+-ATPase was studied in the adult chicken cerebellum. A monoclonal antibody. CaS/C1-IgG, specific for the cardiac/slow-twitch skeletal muscle SR Ca2+-ATPase, was used as a probe of protein expression. An immunoblot analysis showed that CaS/C1-IgG recognized similar size polypeptides in adult chicken heart and cerebellum. CaS/C1-IgG recognized fragments of similar size after limited tryptic digestion of cardiac and cerebellar membranes. A two-dimensional alpha-chymotryptic peptide map analysis demonstrated that the cardiac and cerebellar Ca2+-ATPases were structurally very similar. Immunofluorescence microscopy localized the cerebellar Ca2+-ATPase to Purkinje cell bodies and dendritic trees. These results suggest that the well-known Ca2+ uptake system of skeletal and cardiac muscle SR has a remarkably similar counterpart in some neurons.

Retinomotor movements in the frog retinal pigment epithelium: dependence of pigment migration on Na+ and Ca2+

The ionic dependence of the screening-pigment migrations in the frog retinal epithelium (RPE) was quantitatively studied with eyecups incubated in media of different compositions. Typical migrations in response to light and darkness, equivalent to those observed in the intact animal, were fully accomplished and maintained for up to 6 hr by the isolated organ bathed in Ringer solution rich with O2. Pigment migration in either direction was completed under the appropriate illumination conditions at any time during the day, indicating that circadian influences, if present in the intact animal, can be overridden in the isolated organ by light or darkness alone. Pigment aggregation toward the dark-adapted position was inhibited by: (a) low external Ca2+, (b) high external Na+, and (c) drugs expected to increase the cytoplasmic levels of either Na+, or Ca2+, like ouabain, caffeine and the ionophore A23187. However, the inhibition caused by low Ca2+ did not occur if Na+ was also reduced in the incubation medium. On the other hand, an increase in the concentration of external Ca2+ or the addition of Co2+ to the normal Ringer facilitated pigment aggregation in the dark. Pigment dispersion to the light-adapted position was unaffected by any of the above conditions. This is the first report of full and stable pigment responses in the RPE of vertebrate eyes incubated under simple physiological conditions. The results seem to conciliate a discrepancy of previous reports on the Ca2+ dependence of RPE movements, and are compatible with current views on ionic mechanisms in analogous systems of intracellular transport.