Hormonal stimulation of Mg2+ uptake in hepatocytes. Regulation by plasma membrane and intracellular organelles

Collagenase dispersed rat liver hepatocytes release Mg2+ when stimulated with norepinephrine or accumulate Mg2+ when stimulated with vasopressin, respectively. Mg2+ fluxes in either direction account for a net loss or gain of approximately 10% of total cell magnesium and are rapidly reversible. Both stimulated Mg2+ efflux and Mg2+ influx require physiological concentration of extracellular NaCl and Ca2+. In the absence of extracellular Na+, Mg2+ efflux, but not influx, can be observed in the presence of extracellular Cl-. Under these conditions, the efflux is inhibited by the Cl-/HCO3- exchanger inhibitor 4,4'-dinitrostilbene-2,2'-disulfonic acid. In hepatocytes, Mg2+ influx, but not efflux, is completely inhibited by thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ ATPase. Several lines of evidence, such as measurements of cytosolic Ca2+ or of cytosolic Ca2+ buffering, indicate that the effect of thapsigargin in inhibiting Mg2+ influx could not be explained by an increase in cytosolic Ca2+. Instead, the inhibition of hepatocyte Mg2+ influx was found to be the result of the depletion of the Ca2+ stored within the endoplasmic reticulum.

Modulation by extracellular ATP of two distinct currents in rat myocytes

The whole cell voltage-clamp technique was used to study the electrophysiological effects induced by ATP in isolated ventricular myocytes. ATP or 2-methylthio-ATP evoked a transient inward current (IATP) when the transmembrane potential (Vm) was held at -70 mV and increased the Ca2+ current (ICa) when Vm was depolarized to 0 mV. The time course of IATP was fitted by a single exponential equation with a brief time constant (165 ms), whereas the time course of enhancement of ICa by ATP was also fitted by a single exponential equation with a much longer time constant (14 s). IATP was much less pronounced when extracellular Mg2+ was absent, and it was insensitive to dihydropyridines. In contrast, the enhancement of ICa by ATP was not affected by removing extracellular Mg2+, but it was suppressed by Ca2+ channel blockers. Both IATP and ICa were decreased by extracellular Cd2+. Internally applied guanosine 5'-O-(2-thiodiphosphate), which prevents the activation of G proteins, abolished the ATP-enhanced rise in ICa but did not inhibit IATP. These data suggest that ATP elicits IATP and increases ICa through two different mechanisms. IATP appears to be generated via receptor-operated channels that are activated by ATP. The ATP-induced increase of ICa appears to be mediated by G proteins via pathways that are independent of adenosine 3',5'-cyclic monophosphate and phosphoinositide turnover.

Regulation of magnesium uptake and release in the heart and in isolated ventricular myocytes

Perfused rat hearts release or accumulate approximately 10% of total Mg2+ content when stimulated with norepinephrine (NE) or carbachol, respectively. Collagenase-dispersed rat ventricular myocytes increase or decrease total cell Mg2+ by 1 mM within 5 minutes when stimulated with these same transmitters. Measurements of Mg2+ transport using 28Mg or atomic absorbance spectrophotometry indicate that the rate and the extent of both stimulated Mg2+ efflux and influx are independent of the concentration of extracellular Mg2+ (0 to 1.2 mM). Mg2+ release induced by NE is rapidly reversed by the addition of carbachol, and Mg2+ uptake induced by carbachol is reversed by NE. Decreasing extracellular Na+ or Ca2+ decreases or abolishes Mg2+ efflux from myocytes. Cd2+ or other Ca2+ channel blockers also inhibit Mg2+ efflux in the presence of a physiological concentration of extracellular Ca2+. Replacement of extracellular Ca2+ with Sr2+ or with Mn2+ decreases or abolishes both stimulated efflux and influx of Mg2+. Redistribution of 85Sr in myocytes and in the supernatant indicates that under those conditions Sr2+ is released or accumulated by NE or carbachol in a manner similar to that of Mg2+. Hence, at least in the case of Sr2+, the inhibition of Mg2+ fluxes can be explained by the transport of Sr2+ rather than Mg2+ through the transport(s) systems. By contrast, replacement of extracellular Ca2+ with Ba2+ inhibits stimulated Mg2+ uptake but not Mg2+ release. These results indicate that cardiac myocytes have a major pool of Mg2+ that can be rapidly mobilized upon hormonal stimulation. The net uptake and release of Mg2+ are quantitatively similar and appear to be independent of the extracellular Mg2+ concentrations but are affected, to various degrees, by the presence of other cellular or extracellular cations.

Pancreatic adenocarcinomas frequently show p53 gene mutations

Thirty-four pancreatic adenocarcinomas were studied for the presence of p53 gene mutations by the single-strand conformation polymorphism method and by direct sequencing of PCR-amplified fragments. p53 protein expression was immunohistochemically evaluated using monoclonal PAb1801 and polyclonal CM1 antibodies. Mutations were detected in 14 cases. The transitions were six G to A and two A to G; the transversions were one C to G and two A to C; the remaining three were frameshift mutations. Immunostaining results were identical with both antibodies. Nuclear immunohistochemical p53-positive cells were found in nine p53 mutated cases and in 12 cases in which no mutation was detected. In most of these latter cases only a minority of cancer cells showed immunohistochemical positivity. Twenty-nine cases, including all p53 mutated cancers, were known to contain codon 12 Ki-ras gene mutations. Also in the light of the demonstrated cooperation of ras and p53 gene alterations in the transformation of cultured cells, our data suggest that p53 mutation is one of the genetic defects that may have a role in the pathogenesis of a proportion of pancreatic cancers.

Neoplasia of the ampulla of Vater. Ki-ras and p53 mutations

Eleven tumors of the ampulla of Vater (5 stage IV and 2 stage II adenocarcinomas, 1 stage II papillary carcinoma, 1 neuroendocrine carcinoma, and 2 adenomas, one with foci of carcinoma) were examined for Ki-ras and p53 gene mutations by single-strand conformation polymorphism analysis and direct sequencing of polymerase chain reaction-amplified DNA fragments. Ki-ras mutations were found in one adenocarcinoma and in the adenoma with foci of carcinoma, both involving mainly the intraduodenal bile duct component of the ampulla. Seven cases showed p53 gene mutations: four advanced-stage adenocarcinomas, the papillary carcinoma, the neuroendocrine carcinoma, and the adenoma with foci of carcinoma. Nuclear accumulation of p53 protein was immunohistochemically detected in the morphologically high-grade areas of the five cancers harboring a p53 gene missense point mutation. The adenomas, the two frame shift-mutated cancers, and the adenomatous and low-grade cancer areas of mutated carcinomas were immunohistochemically negative. Our data suggest that in ampullary neoplasia 1) p53 mutations are common abnormalities associated with the transformation of adenomas and low-grade cancers into morphologically high-grade carcinomas, and 2) Ki-ras mutations are relatively less frequent and might be restricted to tumors originating from the bile duct component of the ampulla.

A CD5+ leukemic lymphoma with monocytoid features: an unusual B-cell lymphoma mimicking hairy-cell leukemia

A case of lymphoma presenting with features shared by hairy cell leukemia (HCL) and its variant, intermediate lymphocytic lymphoma (ILL) and monocytoid B-cell lymphoma (MBCL) is described. Clinical presentation and the morphological findings observed in peripheral blood and in bone marrow biopsy suggested an HCL; however, the tartrate-resistant acid phosphatase negativity of peripheral blood lymphocytes, the histologic pattern observed in the spleen, and the immunophenotyping of peripheral blood, spleen and bone marrow neoplastic lymphocytes (expression of CD5 and lack of CD25), were strongly against this hypothesis. The clinical course was aggressive. This report emphasizes the concept of the equivocal presentation of some 'low-grade' B-cell lymphomas. It is further pointed out that, even with complete and exhaustive morphological and immunological analyses, atypical cases may be encountered.

Immunodetection of proliferating cell nuclear antigen assesses the growth fraction and predicts malignancy in endocrine tumors of the pancreas

Thirty-five endocrine tumors of the pancreas, 17 functioning and 18 nonfunctioning, were immunohistochemically studied for the expression of proliferating cell nuclear antigen (PCNA) using 19A2 and PC10 monoclonal antibodies. The proportion of PCNA-reactive cells (PCNA index) ranged from 0.2 to 27% in functioning tumors and from 0.1% to 55% in nonfunctioning tumors. PCNA index showed a statistically significant correlation with mitotic and Ki67 indexes. The median values of PCNA index identified three groups of patients: group A (PCNA < or = 2%), including 13 functioning and six nonfunctioning tumors; group B (PCNA between 2 and 5%), including three functioning and three nonfunctioning tumors; group C (PCNA > 5%), including one functioning and nine nonfunctioning tumors. All group A tumors were confined to the pancreas. In group B, the functioning tumors were limited to the pancreas, and the nonfunctioning tumors extended to extrapancreatic tissues. All group C patients had extrapancreatic extension of the disease. At follow-up, a PCNA index higher than 5% correlated to a decreased mean survival. Our data suggest that PCNA index is a reliable tool to assess the growth fraction, discern local from advanced diseases, and predict malignancy in pancreatic endocrine tumors.

p53 gene mutation spectrum in hepatocellular carcinoma

In order to clarify the significance of mutation of the p53 tumor suppressor gene in the genesis and development of human hepatocellular carcinoma (HCC) in an aflatoxin B1 low-exposure area, the spectrum, i.e., incidence, type, and site, of p53 gene mutations was examined in 169 tissue samples resected mainly from Japanese patients using single-strand conformation polymorphism analysis and direct sequencing. Forty-nine tumors (29%) showed a p53 mutation (39 point mutations and 10 frameshifts). The point mutations comprised 18 transitions, only 4 of which occurred at CpG sites, and 21 transversions. Two evolutionarily conserved domains, IV and V, contained 65% of all mutations and codon 249 was the most frequent mutation site (7/49). The spectrum of p53 mutation did not differ among HCCs in relation to the type of hepatitis virus infection, sex, age, and background liver disease of patients, tumor size, or presence of metastasis, but incidence and site were significantly associated with the degree of differentiation of cancer cells. In poorly differentiated HCC, p53 mutation was frequent (54%) and clustered on domains IV and V, whereas in well or moderately differentiated HCC, the mutation was less frequent (21%) and equally distributed on domains II to V. Restriction fragment length polymorphism analysis revealed loss of heterozygosity on chromosome 17p in 55 (69%) of 80 informative cases and in 34 (95%) of 36 cases with p53 mutation. Therefore, p53 gene mutation is suggested to occur independently of the type of viral infection or status of preexisting liver disease and to occur preferentially in moderately and poorly differentiated HCCs in association with or after loss of another p53 allele as a late event of HCC progression.

Ca2+ mobilization by extracellular ATP in rat cardiac myocytes: regulation by protein kinase C and A

Activation of protein kinase C (PKC) modulates the mobilization of intracellular Ca2+ induced by extracellular ATP in rat ventricular myocytes. Pretreatment of myocytes with PKC activators attenuated both the ATP-induced Ca2+ transient and the noradrenergic potentiation of the Ca2+ response. Various PKC activators decreased both the basal cAMP level and the cAMP levels that had been elevated by norepinephrine, forskolin, or 3-isobutyl-1-methylxanthine. The inhibitory effects of PKC activators were reversed by the PKC inhibitor staurosporine. The ATP-induced Ca2+ response is an integrated response resulting from ATP eliciting an inward cation current (IATP), cellular depolarization, Ca2+ influx through Ca2+ channels, and Ca2+ release from the sarcoplasmic reticulum. We used the whole cell voltage-clamp technique to investigate which steps of this integrated response are affected by PKC. PKC activators did not significantly affect the IATP. In contrast, PKC activators decreased the basal Ca2+ current (ICa) or Ba2+ current and the beta-adrenergic-stimulated ICa. These results suggest that PKC-induced suppression of the ATP-induced Ca2+ response and the beta-adrenergic-potentiated Ca2+ response is achieved at least partially by decreasing the intracellular cAMP level and ICa.

Neoplastic epithelial cells in a subset of human thymomas express the B cell-associated CD20 antigen

A series of 36 human thymomas have been immunohistochemically analyzed using a panel of antibodies recognizing B-cell markers including CD20. Most thymomas exhibiting the cortical pattern, according to the criteria of Marino and Muller-Hermelink, were characterized by areas of medullary differentiation containing variable numbers of CD20+ B lymphocytes, thus mimicking the medulla of normal thymus. On the other hand, B cells were absent or rare in thymomas recognized as mixed using the same morphological criteria. Surprisingly, we observed in most mixed thymomas variable numbers of CD20+ spindle cells, characterized by long slender processes. Using double-marker analysis we could demonstrate the epithelial nature of these cells (expression of keratin and lack of lymphoid and B-cell-related markers). The immunoreactivity of thymoma epithelial cells with L26, an antibody widely used in the characterization of B-cell lymphomas, can represent a drawback of practical relevance in the differential diagnosis of mediastinal tumors.

Characterization of myocardial extracellular ATP receptors by photoaffinity labelling and functional assays

Extracellular ATP receptors in rat ventricular myocytes were investigated through intact cell photolabelling followed by protein isolation. 8-Azido-ATP (8Az-ATP) was used for labelling under specific conditions determined by parallel functional studies. In those studies ATP-induced cytosolic Ca2+ transients were irreversibly and specifically inhibited by UV-photolyzed 8Az-ATP, but not by 2-azido-ATP (2Az-ATP), even in the presence of high concentrations of phosphonucleotides not affecting myocardial ATP receptors. Under those conditions background labelling is minimized and radioactive 8Az-ATP specifically labels a band of 45-48 kDa on a SDS gel. Labelling under the above conditions in the presence of ATP gamma S or 2-methylthio-ATP (2-meSATP), which are distinct for two functionally different cardiac ATP receptors, shows two different proteins within the same band consistent with the possible labelling of these two receptors.

Mutation pattern of the p53 gene as a diagnostic marker for multiple hepatocellular carcinoma

Hepatocellular carcinoma, sometimes shows multiple tumor nodules, therefore poses a problem of differential diagnosis between cancers of multifocal and those of metastatic origin. Conventionally, pathological criteria have been used for this purpose, but these are largely subjective. In order to facilitate more objective differential diagnosis of multiple hepatocellular carcinoma, we used the pattern of mutation of the p53 gene as a marker for each tumor nodule. We studied 58 nodules from 26 cases of multiple hepatocellular carcinoma using polymerase chain reaction-single strand conformation polymorphism analysis, a simple method for detecting mutations. p53 gene mutations were detected in 65% (17 of 26) of cases. The internodule mutation patterns were heterogeneous in 11 cases and homogeneous in 6, enabling a multifocal origin to be diagnosed in the former and a metastatic origin in the latter at the genetic level. Moreover, the origin of recurrent tumors was determined from the mutation pattern. It is concluded that analysis of p53 mutations seems to be useful for differentiating the origin of multiple cancers, since the information it yields is essentially objective.

cAMP control of Mg2+ homeostasis in heart and liver cells

The increase of intracellular cAMP level in intact myocytes and hepatocytes induced by beta-adrenergic stimulation or by forskolin or permeant cyclic AMP analogues induces a sizeable Mg2+ efflux from the cells. Approximately 10-15% of total cell Mg2+ is mobilized over 5-8 min stimulation. The addition of cAMP to digitonin-permeabilized hepatocytes also mobilizes Mg2+ from intracellular compartment(s). Furthermore, cAMP stimulates a major Mg2+ extrusion from isolated rat liver mitochondria, indicating this compartment as the main source of Mg2+ mobilization in liver cells.

Association of Epstein-Barr virus genome with mixed cellularity and cellular phase nodular sclerosis Hodgkin's disease subtypes

Association of Epstein-Barr virus (EBV) genome with Hodgkin's disease (HD) histological subtypes was investigated using the polymerase chain reaction (PCR). A highly significant association of EBV genome with the mixed cellularity (MC) subtype (10 positive out of 15 cases; 66%) was observed, suggesting a possible etiopathogenetic role of EBV in the induction of this subset. By contrast, a markedly lower frequency of association with EBV genome was found in nodular sclerosis (NS) (12 positive out of 46 cases; 26%) and in nodular lymphocytic predominance (NLP) (0 positive out of 5 cases) HD subtypes. In addition, in the NS series, the presence of EBV genome was mainly restricted to the 'cellular phase' NS subset. This finding strengthens the possibility, suggested by clinico-pathological features and survival rates, that 'cellular phase NS' is a disease more akin to MC than to typical NS HD.

Regulation of Mg2+ uptake in isolated rat myocytes and hepatocytes by protein kinase C

A large Mg2+ cell uptake against concentration gradients is stimulated in collagenase-dispersed rat myocytes by carbachol and in hepatocytes by carbachol or vasopressin. The signalling pathway(s) responsible for this stimulation of Mg2+ uptake was investigated by using various activators or inhibitors of protein kinase C (PKC) and by correlating Mg2+ uptake with cell PKC activity and cAMP content. In both cell preparations, the direct stimulation of PKC by diacylglycerol analogs or phorbol esters reproduce the same pattern of Mg2+ uptake as that induced by carbachol or vasopressin. These data indicate that the activation of PKC is responsible for a stimulation of Mg2+ uptake by myocytes or hepatocytes, whereas increase in cAMP in these cells stimulates Mg2+ release.

Synergism between cAMP and ATP in signal transduction in cardiac myocytes

ATP transiently increases the intracellular Ca2+ concentration in cardiac myocyte suspensions. Pretreatment with norepinephrine (NE) greatly potentiates the ATP response. We performed experiments on adult rat myocyte suspensions loaded with fura-2 to investigate the mechanism of NE potentiation. We found that forskolin (an activator of adenylate cyclase), 3-isobutyl-1-methylxanthine (an inhibitor of phosphodiesterase), and permeative adenosine 3',5'-cyclic monophosphate (cAMP) analogues potentiate the increase in cytosolic Ca2+ concentration induced by ATP. NE, forskolin, and 8-(4-chlorophenylthio)-cAMP all increase Vmax of the Ca2+ response curve of ATP. Measurement of cAMP by radioimmunoassay confirmed that the changes in the ATP response were accompanied by an increase in cAMP. These results suggest that the noradrenergic potentiation of the ATP-induced Ca2+ mobilization involves cAMP as a second messenger. Patch-clamp studies of isolated myocytes showed that neither NE nor forskolin alters the inward current elicited by ATP, but rather they increase the duration of secondary slow action potentials elicited by ATP. NE also increases the Ca2+ current through L-type Ca2+ channels in the myocytes. We conclude that NE potentiates the ATP-induced Ca2+ transient by increasing cAMP levels and that one of the early events is the increase of the inward Ca2+ current during the action potential.

Cyclic AMP-induced Mg2+ release from rat liver hepatocytes, permeabilized hepatocytes, and isolated mitochondria

The addition of norepinephrine, epinephrine, or forskolin to collagenase-dispersed rat liver hepatocytes increase cAMP and result in a 15% loss in total cell Mg2+ within 5 min. Conversely, carbachol and vasopressin induce a 10-15% increase of total cell Mg2+. Permeabilized hepatocytes also mobilize a large pool of Mg2+ when stimulated by ADP or cAMP. This stimulation is completely inhibited by atractyloside and bongkrekic acid, two different specific inhibitors of the mitochondrial adenine nucleotide translocase. cAMP directly mobilizes Mg2+ efflux from isolated rat liver mitochondria. 50 nM cAMP or 250 microM ADP induces in 5 min a mitochondrial loss of about 6 nmol of Mg2+/mg of protein and a stimulation of ATP efflux. The effect of cAMP is specific, is not reproduced by other cyclic or noncyclic nucleotides, and is inhibited by inhibitors of the adenine nucleotide translocase. These data indicate that cAMP is a messenger for a major mobilization of Mg2+ in hepatocytes. A major target for the effect of cAMP are mitochondria, which lose up to 20-25% of their total Mg2+ in 5 min, both within the cell and after isolation. Evidence is presented suggesting that the adenine nucleotide translocase is the target of the cAMP-dependent Mg2+ efflux and that cAMP may change the operation of the translocase. This, in turn, could change within the matrix the substrate of choice of the translocase from ATP to ATP.Mg.

Calcium currents in the A7r5 smooth muscle-derived cell line. Increase in current and selective removal of voltage-dependent inactivation by intracellular trypsin

We studied the effects of trypsin on L-type calcium current in the A7r5 smooth muscle cell line. Intracellular dialysis with trypsin increased the whole-cell current up to fivefold. The effect was concentration dependent, and was prevented by soybean trypsin inhibitor. Ensemble analysis indicated an increase in the number of functional channels, and possibly a smaller increase in the open probability, with no change in the single channel current. The shape of the current-voltage curve was unaffected. Trypsin also nearly eliminated inactivation of currents carried by Ba2+, but had little or no effect on the rapid inactivation process in Ca2+, This indicates that trypsin removes voltage-dependent but not Ca(2+)-dependent inactivation, suggesting the existence of distinct protein domains for these two mechanisms of calcium channel inactivation.

Intracellular ionized calcium changes in squid giant axons monitored by Fura-2 and aequorin

Squid giant axons were injected simultaneously with Ca indicators Fura-2 and aequorin. Fura-2 was calibrated in situ by measuring fluorescence at 510 nm upon UV excitation at 340 nm, 360 nm, and 380 nm with a time-sharing multiple wavelength spectrofluorimeter. Limiting values for dye fluorescence were obtained by allowing a massive load of Ca to enter the axon with the aid of procedures such as prolonged depolarization in the presence of CN (for saturation) and by sequestration of all Ca present in the axoplasm accomplished with injection of EGTA into the axon (for a zero-Ca signal). The average intracellular Ca concentration obtained with Fura-2 was 184 nM. The sensitivity of Fura-2 to intracellular Ca is at least as great as that of aequorin, thus permitting its use in the characterization of Ca homeostasis mechanisms such as Na-Ca exchange. It was found, however, that for voltage-clamp experiments requiring an internal current electrode, Fura-2 is not a convenient Ca probe because electrode reactions in the axoplasm denature the dye, thereby restricting its use in characterization of Ca movements associated with electrically induced changes in membrane potential. A comparison of aequorin luminescence with Fura-2 fluorescence demonstrated that light output by aequorin is linear with intracellular Ca concentrations up to values of 750 nM, changing to a square law relationship from 750 nM up to 10 microM Ca.

Calcium currents in the A7r5 smooth muscle-derived cell line. Calcium-dependent and voltage-dependent inactivation

Inactivation of a dihydropyridine-sensitive calcium current was studied in a cell line (A7r5) derived from smooth muscle of the rat thoracic aorta. Inactivation is faster with extracellular Ca2+ than with Ba2+. In Ba2+, inactivation increases monotonically with depolarization. In Ca2+, inactivation is related to the amount of inward current, so that little inactivation is seen in Ca2+ for brief depolarizations approaching the reversal potential. Longer depolarizations in Ca2+ reveal two components of inactivation, the slower component behaving like that observed in Ba2+. Furthermore, lowering extracellular Ca2+ slows inactivation. These results are consistent with the coexistence of two inactivation processes, a slow voltage-dependent inactivation, and a more rapid current-dependent inactivation which is observable only with Ca2+. Ca(2+)-dependent inactivation is decreased but not eliminated when intracellular Ca2+ is buffered by 10 mM BAPTA, suggesting that Ca2+ acts at a site on or near the channel. We also studied recovery from inactivation after either a short pulse (able to produce significant inactivation only in Ca2+) or a long pulse (giving similar inactivation with either cation). Surprisingly, recovery from Ca(2+)-dependent inactivation was voltage dependent. This suggests that the pathways for recovery from inactivation are similar regardless of how inactivation is generated. We propose a model where Ca(2+)- and voltage-dependent inactivation occur independently.

Characterization of calcium channel forming activity of Entamoeba histolytica in model biological membranes

Entamoeba histolytica is an invasive enteric protozoan parasite whose cytolytic activity is associated with irreversible increases in target cell intracellular calcium. We studied the effect of homogenates of virulent E. histolytica on calcium permeability of bovine chromaffin granules and rat liver mitochondria, model membrane systems whose mechanisms of ion transport and permeability are well characterized. Treatment of chromaffin granules, with an extract of E. histolytica resulted in a dose-dependent increase in calcium uptake. These effects were similar to that seen with the calcium ionophore A23187 and indicate that the homogenate acts as a divalent cation ionophore. However, unlike what is observed with A23187, the Ca2+ uptake was greater in the presence of the permeable anion Cl- than in its absence. Also, in contrast to the calcium ionophore A23187, a homogenate of E. histolytica caused no calcium release from rat liver mitochondria but resulted in a dramatic increase in the rate of calcium accumulation. The amebic homogenate did not bind calcium in the absence of mitochondria. The increase in mitochondria calcium uptake occurred only in the presence of respiration and with normal state 3 and 4 oxygen consumption. Calcium accumulation occurred both in the presence and absence of 5 mM Pi. Substitution of NaCl or KCl for sucrose in the medium did not alter the enhanced mitochondria calcium uptake. Most data are consistent with the hypothesis that the E. histolytica homogenate is acting as a calcium ionophore transporting charged calcium electrophoretically across the chromaffin granules and the mitochondria inner membrane. This activity may contribute to the cytopathogenicity by E. histolytica.