The voltage-dependent anion channel (VDAC): function in intracellular signalling, cell life and cell death

Research over the last decade has extended the prevailing view of mitochondria to include functions well beyond the critical bioenergetics role in supplying ATP. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organelle communication, aging, many diseases, cell proliferation and cell death. Apoptotic signal transmission to the mitochondria results in the efflux of a number of potential apoptotic regulators to the cytosol that trigger caspase activation and lead to cell destruction. Accumulating evidence indicates that the voltage-dependent anion channel (VDAC) is involved in this release of proteins via the outer mitochondrial membrane. VDAC in the outer mitochondrial membrane is in a crucial position in the cell, forming the main interface between the mitochondrial and the cellular metabolisms. VDAC has been recognized as a key protein in mitochondria-mediated apoptosis since it is the proposed target for the pro- and anti-apoptotic Bcl2-family of proteins and due to its function in the release of apoptotic proteins located in the inter-membranal space. The diameter of the VDAC pore is only about 2.6-3 nm, which is insufficient for passage of a folded protein like cytochrome c. New work suggests pore formation by homo-oligomers of VDAC or hetero-oligomers composed of VDAC and pro-apoptotic proteins such as Bax or Bak. This review provides insights into the central role of VDAC in cell life and death and emphasizes its function in the regulation of mitochondria-mediated apoptosis and, thereby, its potential as a rational target for new therapeutics.

Mitochondria regulate inactivation of L-type Ca2+ channels in rat heart

1. L-type Ca2+ channels play an important role in vital cell functions such as muscle contraction and hormone secretion. Both a voltage-dependent and a Ca2+-dependent process inactivate these channels. Here we present evidence that inhibition of the mitochondrial Ca2+ import mechanism in rat (Sprague-Dawley) ventricular myocytes by ruthenium red (RR), by Ru360 or by carbonyl cyanide m-chlorophenylhydrazone (CCCP) decreases the magnitude of electrically evoked transient elevations of cytosolic Ca2+ concentration ([Ca2+]c). These agents were most effective at stimulus rates greater than 1 Hz. 2. RR and CCCP also caused a significant delay in the recovery from inactivation of L-type Ca2+ currents (I(Ca)). This suggests that sequestration of cytosolic Ca2+, probably near the mouth of L-type Ca2+ channels, into mitochondria during cardiac contractile cycles, helps to remove the Ca2+-dependent inactivation of L-type Ca2+ channels. 3. We conclude that impairment of mitochondrial Ca2+ transport has no impact on either L-type Ca2+ currents or SR Ca2+ release at low stimulation frequencies (e.g. 0.1 Hz); however, it causes a depression of cytosolic Ca2+ transients attributable to an impaired recovery of L-type Ca2+ currents from inactivation at high stimulation frequencies (e.g. 3 Hz). The impairment of mitochondrial Ca2+ uptake and subsequent effects on Ca2+ transients at high frequencies at room temperature could be physiologically relevant since the normal heart rate of rat is around 5 Hz at body temperature. The role of mitochondria in clearing Ca2+ in the micro-domain near L-type Ca2+ channels could be impaired during high frequencies of heart beats such as in ventricular tachycardia, explaining, at least in part, the reduction of muscle contractility.

Efflux of glutathione conjugate of monochlorobimane from striatal and cortical neurons

Evidence for the presence of a novel transporter in primary cultures of rat striatal neurons and mouse cortical neurons similar in function to the multidrug resistance-associated protein (MRP1) is presented. Functional activity was assessed by efflux studies with the glutathione conjugate of monochlorobimane (B-SG). The glutathione transferase-catalyzed formation of B-SG in rat striatal neurons and mouse cortical neurons was inhibited by ethacrynic acid. The efflux of B-SG from rat striatal neurons and mouse cortical neurons was lower at 20 degrees C than at 37 degrees C and was lower in cells with reduced ATP concentrations compared with cells with constitutive ATP concentrations. In addition, the efflux of B-SG was inhibited by MK-571 in both rat striatal and mouse cortical neurons and by probenecid in rat striatal neurons, but not in mouse cortical neurons. Verapamil did not inhibit B-SG efflux in either rat striatal or mouse cortical neurons. Although functionally similar to MRP1, Western blot analysis with commercially available antibodies directed against human and mouse MRP1 failed to show MRP1-like protein in either whole-cell homogenates of rat striatal neurons or mouse cortical neurons, indicating that the described neuronal transporter differs in structure from human or mouse MRP1 or lacks epitopes in common with MRP1.

Identification of a ryanodine receptor in rat heart mitochondria

Recent studies have shown that, in a wide variety of cells, mitochondria respond dynamically to physiological changes in cytosolic Ca(2+) concentrations ([Ca(2+)](c)). Mitochondrial Ca(2+) uptake occurs via a ruthenium red-sensitive calcium uniporter and a rapid mode of Ca(2+) uptake. Surprisingly, the molecular identity of these Ca(2+) transport proteins is still unknown. Using electron microscopy and Western blotting, we identified a ryanodine receptor in the inner mitochondrial membrane with a molecular mass of approximately 600 kDa in mitochondria isolated from the rat heart. [(3)H]Ryanodine binds to this mitochondrial ryanodine receptor with high affinity. This binding is modulated by Ca(2+) but not caffeine and is inhibited by Mg(2+) and ruthenium red in the assay medium. In the presence of ryanodine, Ca(2+) uptake into isolated heart mitochondria is suppressed. In addition, ryanodine inhibited mitochondrial swelling induced by Ca(2+) overload. This swelling effect was not observed when Ca(2+) was applied to the cytosolic fraction containing sarcoplasmic reticulum. These results are the first to identify a mitochondrial Ca(2+) transport protein that has characteristics similar to the ryanodine receptor. This mitochondrial ryanodine receptor is likely to play an essential role in the dynamic uptake of Ca(2+) into mitochondria during Ca(2+) oscillations.

Identification of a novel point mutation (S65T) in alpha-galactosidase A gene in Chinese patients with Fabry disease. Mutations in brief no. 169. Online

Fabry disease is an X-linked inborn error of sphingolipid catabolism resulting from deficient enzyme activity of alpha-galactosidase A. The molecular defects of human alpha-galactosidase A gene causing Fabry disease have been characterized, including gene rearrangement and point mutations, which show the genetic heterogeneity in Fabry disease. To characterize the molecular defects of these patients, each exon of alpha-galactosidase A gene including intron-exon junctions were PCR amplified using biotin-labelled primer and sequenced using magnetic beads solid-phase sequencing. A G to C transversion was identified in the last nucleotide of exon 1 in two unrelated Chinese patients. This mutation obliterates an EcoN1 restriction site. Family studies show close linkage with the affected family members. Screening of 100 alleles (22 males, 39 females) of unrelated normal Chinese can not find this mutation. This mutation not only changes the amino acid from serine to threonine, but also likely cause splicing defects. To our knowledge, this is the first report of mutation in Chinese patients with Fabry disease, and a novel mutation causing Fabry disease not reported in literature previously.

Transport of Ca2+ from sarcoplasmic reticulum to mitochondria in rat ventricular myocytes

Studies with electron microscopy have shown that sarcoplasmic reticulum (SR) and mitochondria locate close to each other in cardiac muscle cells. We investigated the hypothesis that this proximity results in a transient exposure of mitochondrial Ca2+ uniporter (CaUP) to high concentrations of Ca2+ following Ca2+ release from the SR and thus an influx of Ca2+ into mitochondria. Single ventricular myocytes of rat were skinned by exposing them to a physiological solution containing saponin (0.2 mg/ml). Cytosolic Ca2+ concentration ([Ca2+]c) and mitochondrial Ca2+ concentration ([Ca2+]m) were measured with fura-2 and rhod2, respectively. Application of caffeine (10 mM) induced a concomitant increase in [Ca2+]c and [Ca2+]m. Ruthenium red, at concentrations that block CaUP but not SR release, diminished the caffeine-induced increase in [Ca2+]m but not [Ca2+]c. In the presence of 1 mM BAPTA, a Ca2+ chelator, the caffeine-induced increase in [Ca2+]m was reduced substantially less than [Ca2+]c. Moreover, inhibition of SR Ca2+ pump with two different concentrations of thapsigargin caused an increase in [Ca2+]m, which was related to the rate of [Ca2+]c increase. Finally, electron microscopy showed that sites of junctions between SR and T tubules from which Ca2+ is released, or Ca2+ release units, CRUs, are preferentially located in close proximity to mitochondria. The distance between individual SR Ca2+ release channels (feet or ryanodine receptors) is very short, ranging between approximately 37 and 270 nm. These results are consistent with the idea that there is a preferential coupling of Ca2+ transport from SR to mitochondria in cardiac muscle cells, because of their structural proximity.

Mitochondrial calcium in heart cells: beat-to-beat oscillations or slow integration of cytosolic transients?

Mitochondria have been implicated in intracellular Ca2+ signaling in many cell types. The inner mitochondrial membrane contains Ca2+-transporting proteins, which catalyze Ca2+ uptake and extrusion. Intramitochondrial (matrix) Ca2+, in turn, regulates the activity of Krebs cycle dehydrogenases and, ultimately, the rate of ATP synthesis. In the myocardium, controversy remains whether the fast cytosolic Ca2+ transients underlying excitation-contraction coupling in beating cells are rapidly transmitted into the matrix compartment or slowly integrated by the mitochondrial Ca2+ transporters. This mini-review critically summarizes the recent experimental work in this field.

Rapid report: a novel technique for quantitative measurement of free Ca2+ concentration in rat heart mitochondria

1. The free mitochondrial Ca2+ concentration ([Ca2+]m) in rat heart mitochondria was measured quantitatively by loading the mitochondria with fura-2 and then injecting them into Xenopus laevis oocytes. 2. When oocytes were incubated with a physiological solution, the free cytosolic Ca2+ concentration ([Ca2+]c) in the oocytes was 82 +/- 11 nM (n = 20, mean +/- s.e.m.) and the [Ca2+]m of the injected rat heart mitochondria was 116 +/- 10 nM (n = 18, mean +/- s.e.m.). 3. Inhibition of the oocyte endoplasmic reticular Ca2+-ATPase with thapsigargin produced a transient increase in averaged [Ca2+]c at sub-micromolar concentrations. 4. Injection of cardiac mitochondria blunted the peak and prolonged the duration of thapsigargin-induced [Ca2+]c transients as a result of Ca2+ sequestration by the cardiac mitochondria. 5. These results demonstrate that the present technique provides a new approach for studying [Ca2+]m regulation quantitatively under physiological environments. Furthermore, it clearly shows that cardiac mitochondria can modify the shape of thapsigargin-induced cytosolic Ca2+ pulses in Xenopus oocytes.

Signaling mechanisms underlying muscarinic receptor-mediated increase in contraction rate in cultured heart cells

We have investigated the mechanisms by which stimulation of cardiac muscarinic receptors result in paradoxical stimulatory effects on cardiac function, using cultured neonatal rat ventricular myocytes as a model system. Application of low concentrations of carbachol (CCh) (EC50 = 35 nM) produced an atropine-sensitive decrease in spontaneous contraction rate, while, in cells pretreated with pertussis toxin, higher concentrations of CCh (EC50 = 26 microM) elicited an atropine-sensitive increase in contraction rate. Oxotremorine, an m2 muscarinic acetylcholine receptor (mAChR) agonist, mimicked the negative but not the positive chronotropic response to CCh. Reverse transcription followed by polymerase chain reaction carried out on mRNA obtained from single cells indicated that ventricular myocytes express mRNA for the m1, m2, and, possibly, m4 mAChRs. The presence of m1 and m2 mAChR protein on the surface membranes of the cultured ventricular myocytes was confirmed by immunofluorescence. The CCh-induced positive chronotropic response was significantly inhibited by fluorescein-tagged antisense oligonucleotides directed against the m1, but not the m2 and m4, mAChR subtypes. The response was also inhibited by antisense oligonucleotides against Gqalpha protein. Finally, inhibition of CCh-induced phosphoinositide hydrolysis with 500 microM neomycin or 5 microM U73122 completely abolished the CCh-induced positive chronotropic response. These results are consistent with the stimulatory effects of mAChR activation on the rate of contractions in cultured ventricular myocytes being mediated through the m1 mAChR coupled through Gq to phospholipase C-induced phosphoinositide hydrolysis.

3-Nitropropionic acid exacerbates N-methyl-D-aspartate toxicity in striatal culture by multiple mechanisms

We examined the effects of 3-nitropropionic acid-induced succinate dehydrogenase inhibition on neuronal ATP content, N-methyl-D-aspartate-induced neuronal death, resting membrane potential, and N-methyl-D-aspartate-induced changes in cytosolic calcium concentration ([Ca2+]c) in cultured rat striatal neurons. Exposure of cultures to 3 mM 3-nitropropionic acid for 3 h did not cause overt toxicity, but reduced ATP content by 35%. Treatment with 3-nitropropionic, or removal of Mg2+ from the medium, enhanced subsequent N-methyl-D-aspartate toxicity, reducing the LC50 from 250 microM to 12 microM or 30 microM, respectively. Even after Mg2+ removal, enhancement of N-methyl-D-aspartate toxicity by 3-nitropropionic acid remained pronounced, with the LC50 further decreasing to 3 microM. The mean resting membrane potential of neurons treated with 3-nitropropionic acid was -37 mV, while that in control neurons was -61 mV. Treatment with 3-nitropropionic did not affect baseline [Ca2+]c as determined by fura-2 microfluorimetry. N-methyl-D-aspartate (30 microM) caused a rapid rise in [Ca2+]c, the initial magnitude of which was not affected by 3-nitropropionic acid. However, after a 1-h treatment, [Ca2+]c was dramatically higher in 3-nitropropionic acid-treated neurons. This increased calcium load was washed out slowly and only partially, although calcium in control neurons washed out rapidly and almost completely. These results suggest that in striatal neurons, the enhancement of N-methyl-D-aspartate toxicity caused by succinate dehydrogenase inhibition may be due to synergism between partial relief of the Mg2+ blockade of the N-methyl-D-aspartate receptor and other mechanisms, including disruption of neuronal calcium regulation. This synergism may be relevant to the neuronal death observed in neurodegenerative disorders.

Visualization of NMDA receptor-induced mitochondrial calcium accumulation in striatal neurons

Ca2+ influx through NMDA receptor-gated channels and the subsequent rise in intracellular Ca2+ concentration ([Ca2+]i) have been implicated in cytotoxic processes that lead to irreversible neuronal injury. While many studies have focused on cytosolic Ca2+ homeostasis, much less is known about Ca2+ fluxes in subcellular organelles, such as mitochondria. The mitochondria play an important role in Ca2+ homeostasis by sequestering cytosolic Ca2+ loads. However, mitochondrial Ca2+ overload can impair ATP synthesis, induce free radical formation, and lead to lipid peroxidation. Thus, it is also important to understand the mitochondrial Ca2+ fluxes induced by NMDA. In this study, changes in mitochondrial Ca2+ concentration ([Ca2+]m) in cultured striatal neurons were monitored with a Ca(2+)-binding fluorescent probe, rhod-2, and laser scanning confocal microscopy. The rhod-2 fluorescence signal was highly localized in mitochondrial areas of confocal images. A rapid increase of [Ca2+]m was observed when neurons were treated with 100 microM NMDA. The increased [Ca2+]m induced by NMDA could not be observed in the presence of ruthenium red, an inhibitor of the mitochondrial Ca2+ uniporter, or CCCP, a protonophore that breaks down the mitochondrial membrane potential necessary for Ca2+ uptake. The magnitude and reversibility of changes in [Ca2+]m induced by NMDA were variable. In neurons receiving multiple pulses of NMDA, [Ca2+]m did not return to baseline. The elevated [Ca2+]m may persist indefinitely and may rise further after successive NMDA exposures. These data demonstrate that Ca2+ accumulates in mitochondria in response to NMDA receptor activation. This Ca2+ accumulation may play a role in the excitotoxic mitochondrial dysfunction induced by NMDA.

Phosphatidic acid: a potential signal transducer for cardiac hypertrophy

Phosphatidic acid (PA) is mainly formed by the hydrolysis of phosphatidylcholine due to the activation of phospholipase D (PLD). PA is also generated by phosphorylation of diacylglycerol (DAG) due to the action of DAG kinase and is converted to DAG under the action of PA phosphohydrolase. Most of the positive inotropic agents which are known to stimulate cardiac hypertrophy, have been shown to increase the level of PA in cardiac sarcolemma. Although the growth factor-like effect of PA has been recognized in a wide variety of tissues, there is a lack of similar information in adult cardiomyocytes. By using single cardiomyocytes, we have now shown that PA increased the basal [Ca2+]i level without significant effect on the amplitude of Ca2+ transients. PA (10-50 mu M) also increased the [Ca2+]i in cardiac cell suspension. PA has also been shown to stimulate protein synthesis in cardiomyocytes, which is inhibited by a PKC inhibitor as well as a Ca2+ chelator. PA at the concentration of 1-50 mu M was observed to stimulate the activity of PLC in cardiac sarcolemma; this effect was attenuated by a PLC inhibitor. Since DAG, formed due to the activation of PLC, is considered to play a crucial role in regulating the activity of protein kinase C (PKC), the positive feedback effect of PA on this pathway may be essential for maintaining the sustained elevation in the activity of PKC during the development of cardiac hypertrophy. In view of these observations and other facts available in the literature, it is suggested that PA may be a potential signal transducer for the development of cardiac hypertrophy.

[3H]benzylpempidine, a new radioligand for probing a putative channel site on nicotinic cholinergic receptors

A study was undertaken to assess the receptor binding characteristics of [3H]4-benzylpempidine to an allosteric site on calf brain membranes associated with nicotinic cholinergic receptors and to compare the binding affinity of novel arylpempidine analogs with their ability to antagonize the behavioral effects of nicotine in mice. Scatchard analysis of the binding yielded a K(d) of 20 nM and a B(max) of 330 fmols/mg membrane protein. [3H]4-benzylpempidine appears to be a more satisfactory ligand than [3H]mecamylamine, since it possessed a 50-fold greater affinity and its binding was far less sensitive to inorganic ions and Tris. Among the arylpempidine analogs 4-m-chlorobenzylidenepempidine and 4-benzylidenepempidine had the lowest K(i) values (1.4 nM and 5.0 nM, respectively) and were the most potent in antagonizing nicotine-induced seizures in mice. Although the K(i) values for pempidine and mecamylamine were 1-2 orders of magnitude greater than any of the arylpempidines, the dose required to antagonize nicotine-induced seizures in mice was comparable to the arylpempidines. One explanation for this apparent discrepancy in the correlation of binding affinity and nicotine antagonism is the lower brain penetration of arylpempidines compared to mecamylamine, following their systemic administration to mice.

Does mammalian heart contain only the M2 muscarinic receptor subtype?

Five muscarinic acetylcholine receptor (mAChR) subtypes, m1-m5, have been cloned and sequenced to date. The question as to which mAChR subtypes exist in mammalian heart has been studied extensively and is still under considerable debate. We used the reverse transcriptase-polymerase chain reaction to amplify mRNA from adult rat ventricular myocytes, and found that these cells express mRNA for m1 and m2 mAChRs. Immunocytochemical analysis confirmed that m1 and m2, but not m3, mAChR proteins are present on the surface of these cells. Finally, the functional significance of these receptors was examined. Administration of the m1 mAChR antagonist pirenzepine inhibited the stimulatory effect of the muscarinic agonist carbachol on Ca transients. These findings are consistent with the presence of at least two mAChR subtypes in mammalian heart, m1 and m2, and suggest that activation of m1 mAChRs is involved in the stimulatory effects of muscarinic agonists in mammalian heart.

Histamine induces oscillations of mitochondrial free Ca2+ concentration in single cultured rat brain astrocytes

1. The free Ca2+ concentration of mitochondria ([Ca2+]m) in cultured rat brain astrocytes was measured with a fluorescent Ca2+ indicator, rhod-2, and laser confocal microscopy. 2. Confocal images revealed a rhod-2 distribution that matched mitochondrial localization. 3. Using a Ca2+ ionophore, ionomycin, to clamp the [Ca2+]m from 0 to 100 microM in order to obtain the minimal and maximal fluorescence of rhod-2 in situ, a 3.5 +/- 0.4-fold increase in fluorescence intensity was observed, suggesting that the fluorescence of intramitochondrial rhod-2 was responding in a Ca(2+)-sensitive manner, thereby allowing measurements of [Ca2+]m in single astrocytes. 4. Exposure of fura-2-loaded astrocytes to 100 microM histamine produced a rapid and transient increase in cytosolic Ca2+ concentration ([Ca2+]c) that lasted for several tens of seconds. The spike in [Ca2+]c was frequently followed by variable numbers of repetitive oscillations of Ca2+, which appeared to dampen in amplitude with time. 5. This pattern of histamine-induced [Ca2+]c oscillations was also observed in rhod-2-loaded cells suggesting that [Ca2+]m fluctuated with a similar frequency. 6. The oscillations of [Ca2+]m, but not of [Ca2+]c, were abolished by a proton ionophore, carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), and by Ruthenium Red, a mitochondrial Ca(2+)-uniporter inhibitor. 7. These results suggest that the mitochondrial Ca2+ transport systems in cultured rat brain astrocytes are able to relay receptor-mediated [Ca2+]m oscillations into mitochondria.

Molecular and functional identification of m1 muscarinic acetylcholine receptors in rat ventricular myocytes

The expression of muscarinic acetylcholine receptor (mAChR) subtypes in freshly isolated adult rat ventricular myocytes was investigated by reverse transcription of cellular mRNA followed by amplification of cDNA using the polymerase chain reaction (PCR). After reverse-transcriptase PCR, bands were obtained corresponding to the expected sizes for the m1 and m2 but not for the m3 to m5 mAChRs. The identity of the m1 and m2 bands was confirmed by single-cell PCR, restriction digest mapping, and Southern blot analysis. The presence of m1 and m2, but not m3, mAChR protein in these cells was shown by indirect immunofluorescence studies using subtype-specific antibodies. It was further investigated whether the identified m1 mAChR was responsible for the stimulatory effects on Ca2+ transients by high concentrations of carbachol ( > 10 mumol/L) known to occur in these cells. In pertussis toxin-treated ventricular myocytes electrically stimulated at 1 Hz, carbachol (300 mumol/L) increased the basal Ca2+ level from 96 +/- 7 to 118 +/- 8 nmol/L and the peak Ca2+ transient level from 519 +/- 32 to 640 +/- 36 nmol/L (mean +/- SEM P < .05 for both, n = 8). These effects of carbachol on Ca2+ transients were antagonized by 10 nmol/L pirenzepine, an m1 mAChR-selective antagonist. In contrast, the m2 mAChR-selective antagonist methoctramine (up to 100 nmol/L) did not inhibit the response. These results are the first to use single-cell PCR to probe cardiomyocyte-specific gene expression and indicate that m1 mAChRs are expressed on adult rat ventricular myocytes in addition to m2 mAChRs. The results further suggest that m1 mAChRs mediate the stimulatory responses on Ca2+ transients to high concentrations of cholinergic agonists seen in these cells.

Long-term clinical and morphological evaluation of primary membranoproliferative glomerulonephritis

BACKGROUND

Membranoproliferative glomerulonephritis (MPGN) is a relatively rare primary glomerulonephritis (GN). Its incidence has decreased progressively in the past two decades. To improve knowledge of this rare GN, a retrospective review of 22 patients during a 12-year period was undertaken.

METHODS

From November 1982 to December 1994, from a total 814 cases of primary GN, 22 patients with primary MPGN were diagnosed. Clinical data, medical records, renal pathology and outcome were reviewed.

RESULTS

Patients included 15 male and 7 females, aged from 11 to 67 years. The average follow-up period was 46.3 months, with a range of 1 to 140 months. Tissue was available for electromicroscopic study in 11 cases; of which 9 cases fulfilled morphologic criteria of Type I MPGN; the other 2 cases were Type II MPGN. The clinical presentations at diagnosis included nephrotic syndrome (86.4%), impaired renal function (63.6%), microhematuria (50%), gross hematuria (31.8%) and hypertension (50%). Low serum C3 was found in 40.9% cases, 44.4% in Type I and 50% in Type III MPGN. The positive rate of hepatitis B virus infection was 22.7% with 33.3% in Type I and none in Type III MPGN. All 22 patients received various combined antihypertensive agents, immunosuppressant, anticoagulant and antiplatelet agents at diagnosis, but 17 had progressive disease, 4 maintained normal renal function with proteinuria and only 1 had complete remission. Fifteen patients, including six Type I and no Type III MPGN, progressed to end-stage renal failure. Both patients with Type III MPGN maintained normal renal function and responded to treatment. The 5 and 10 year actuarial renal survival rates were 33.3% and 16.7% respectively. The median kidney survival time was 51.2 months.

CONCLUSIONS

A majority of cases with MPGN presenting with impaired renal function (86.7%) and hypertension (85%) at diagnosis progressed to end-stage renal failure. Delayed diagnosis and poor compliance were possible reasons for compared with for worse prognosis previous reports. But two patients with Type III MPGN had favorable prognosis previously described. Treatments generally failed to halt disease progression. Since at least five cases (22.7%) maintained normal renal function after treatments, a course of immunosuppressant is probably indicated if there is no contraindication. Further study with a larger population is warranted to clarify this issue.

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.

Association of sorcin with the cardiac ryanodine receptor

Sorcin is a 22-kDa calcium-binding protein initially identified in multidrug-resistant cells; however, its patterns of expression and function in normal tissues are unknown. Here we demonstrate that sorcin is widely distributed in rodent tissues, including the heart, where it was localized by immunoelectron microscopy to the sarcoplasmic reticulum. A > 500-kDa protein band immunoprecipitated from cardiac myocytes by sorcin antiserum was indistinguishable in size on gels from the 565-kDa ryanodine receptor/calcium release channel recognized by ryanodine receptor-specific antibody. Association of sorcin with a ryanodine receptor complex was confirmed by complementary co-immunoprecipitations of sorcin with the receptor antibody. Forced expression of sorcin in ryanodine receptor-negative Chinese hamster lung fibroblasts resulted in accumulation of the predicted 22-kDa protein as well as the unexpected appearance of ryanodine receptor protein. In contrast to the parental host fibroblasts, sorcin transfectants displayed a rapid and transient rise in intracellular calcium in response to caffeine, suggesting organization of the accumulated ryanodine receptor protein into functional calcium release channels. These data demonstrate an interaction between sorcin and the ryanodine receptor and suggest a role for sorcin in modulation of calcium release channel activity, perhaps by stabilizing the channel protein.

Extracellular heparin inhibits Ca2+ transients and contraction in mammalian cardiac myocytes

The effect of heparin on Ca2+ transients and cell shortening was studied in isolated cardiac myocytes from rat and guinea-pig ventricles. Ca2+ signals were measured with the fluorescent indicator fura-2. Heparin reversibly decreased Ca2+ transients and cell shortening in a dose-dependent manner. Half and complete blockade were obtained with 50microg/ml and 200microg/ml heparin, respectively. The dihydropyridine agonist BAY K 8644 (50nM) antagonized the effects of heparin. However, Ca2+ release elicited by caffeine (10mM) was not affected by heparin. The actions of heparin were also studied in multicellular preparations. In papillary muscle, heparin (5mg/ml) reversibly reduced the amplitude of the plateau of the action potential and the associated peak tension. BAY K 8644 (500nM) also antagonized these effects. It is proposed that heparin interacts with dihydropyridine-sensitive Ca2+ channels to cause a decrease of Ca2+ transients and contractility in heart.

Serum immunoglobulin E in primary IgA nephropathy

The biosynthesis of human immunoglobulin E (IgE) is regulated by a complex network involving T and B lymphocytes. Diseases associated with high serum IgE (sIgE) levels are usually characterized by T cell disorders. Total sIgE level has been found to be of clinical relevance in minimal change nephrotic syndrome. However, the clinical significance has rarely been studied in primary IgA nephropathy (IgA N). We retrospectively studied 99 cases of primary IgA N. There were 59 males and 40 females with a mean age of 30.0 +/- 12.1 years. The mean follow-up duration was 45.9 +/- 31.1 months. Pathological grading was done according to the criteria of Meadow et al. Median sIgE for the entire group was 122.0 IU/ml (range: 2.8-5805 IU/ml) which was significantly higher than the healthy control group (median: 43,7 IU/ml, range: 5.0-1003 IU/ml, p < 0.001). However, when the IgA N cases were stratified into grades, only grade I (median: 514 IU/ml, range: 72.1-5805.0 IU/ml) and grade II (median: 229 IU/ml, range: 5.0-5464 IU/ml) patients had significantly higher sIgE than the control group (p < 0.0005 and p < 0.001 respectively). Patients with nephrotic ranged proteinuria (32 cases) were further classified into "stable" and "progressive" groups. The "stable" group had a significantly higher sIgE level (median: 922.0 IU/ml, range: 2.8-5805 IU/ml), compared to that of the "progressive" group (median: 55.3 IU/ml, range: 5.0-1600 IU/ml, p < 0.02). The effect of aggressive treatment (including corticosteroid and/or cyclophosphamide, cyclosporine) was also assessed.(ABSTRACT TRUNCATED AT 250 WORDS)

Autonomic modulation of action potential and tension in guinea pig papillary muscles

The effects of alpha 1-adrenoceptor and muscarinic acetylcholine receptor stimulation on action potential and tension were studied in guinea pig papillary muscles obtained from both right and left ventricles. Stimulation of muscarinic acetylcholine receptors with carbachol produced a reduction of the action potential duration and a positive inotropic effect in papillary muscles from both ventricles. Both effects were concentration dependent and atropine sensitive. However, differential responsiveness was found upon alpha 1-adrenoceptor activation in muscles obtained from left and right ventricles. In right side papillary muscles, the alpha 1-adrenoceptor agonist, methoxamine, decreased the action potential duration and produced a positive inotropic effect. In contrast, methoxamine decreased the action potential duration but failed to produce a positive inotropic effect in left side papillary muscles. All methoxamine effects were antagonized by prazosin. Responses to maximum concentration of carbachol and methoxamine on the action potential duration and contractility were additive in right side papillary muscles. Phorbol 12,13-dibutyrate (PDB), a direct protein kinase C activator, also decreased the action potential duration in a manner that was additive to both carbachol and methoxamine. However, PDB reversed the positive inotropic effect of carbachol and methoxamine. The methoxamine-induced shortening of the action potential duration was prevented by pretreatment with indomethacin and nordihydroguaiaretic acid, blockers of arachidonic acid metabolism, but not by the protein kinase C antagonist, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7).(ABSTRACT TRUNCATED AT 250 WORDS)

Fabry's disease: clinical, pathologic and biochemical manifestations in two Chinese males

Fabry's disease is a rare hereditary disease transmitted as an X-linked recessive trait with the primary metabolic defect of an enzyme alpha-galactosidase A, resulting in deposition of glycolipids (ceramide trihexoside) in various tissues, including the kidneys. Two sibling cases of Chinese adult male patients in a family with Fabry's disease were completely evaluated including the clinical, pathologic and biochemical studies. Both of the patients had the similar clinical manifestations such as telangiectases, proteinuria, acral pains, corneal opacities, tortuous renal vessels and recurrent fever. Chronic renal insufficiency was noted in Case 1, whereas Case 2 had normal renal function. Microscopic hematuria was noted in Case 1. In renal biopsy, LM showed foamy vacuolation of the glomerular visceral epithelial cells and EM showed widespread myelin bodies (Zebra bodies) in kidney tissues, most numerous in visceral epithelia in both cases. Those findings are diagnostic for Fabry's disease. The plasma activity of alpha-galactosidase of Case 1 was 0.8 and that of Case 2 was 1.0 (normal reference range: 8.5-18.5 nmol/hr/min). The plasma activity of alpha-galactosidase A of Case 1 was 0.4 and that of Case 2 was 0.8 (normal reference range: 7.9-16.9 nmol/hr/min). All the enzyme activities in both cases were much lower than those of normal subjects. In addition to clinical presentations, pathologic study and biochemical study with assays of plasma or serum activities of alpha-galactosidase and alpha-galactosidase A are important steps in the diagnosis of Fabry's disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial free Ca2+ concentration in living cells

Evidence has accrued during the past two decades that mitochondrial Ca2+ plays an important role in the regulation of numerous cell functions such as energy metabolism. This implies that mitochondrial Ca2+ transport systems might be able to relay the changes of cytosolic Ca2+ concentration ([Ca2+]c) into mitochondrial matrix for regulating biochemical activities. To substantiate this idea, measurements of intramitochondrial free Ca2+ concentration ([Ca2+]m) become essential. In this article, we review the results from recent studies attempting to measure [Ca2+]m in living cells. In addition, the significance of each study is discussed.

The clinicopathological spectrum of renal amyloidosis

BACKGROUND

Renal amyloidosis is an uncommon cause of nephrotic syndrome. The clinical conditions in Chinese people remain obscure. This is a retrospective review of the clinicopathological spectrum of 12 cases diagnosed in Taichung Veterans General Hospital from October 1982 to November 1993.

METHODS

Charts and renal pathological slides were reviewed retrospectively. A scoring system was set to evaluate the degree of amyloid deposition in the renal tissue. The clinical profile, immunological data, pathological picture and final outcomes are presented and discussed, with literature review.

RESULTS

There were 12 cases of primary amyloidosis including 1 case of multiple myeloma. All were confirmed by renal biopsy. The cases were all male with mean age of onset as 53.3 +/- 11.3 (range: 32 to 65 years). The mean follow-up duration was 22.9 +/- 32.8 months. The initial average creatinine clearance was 66.7 +/- 42.7 ml/min; mean daily urine protein was 7.0 +/- 4.1 grams. Nephrotic syndrome was the main clinical manifestation, present in all 12 cases. Other presenting symptoms and signs included: malaise in 7 cases; hypertension and anorexia in 4 cases; limb numbness in 3 cases; low back pain, dizziness and microhematuria in 2 cases each; anemia, headache, stroke, restrictive cardiomyopathy, hepatomegaly, syncope, body weight loss, dysphagia and skin itching in one case individually. Amyloid cardiomyopathy was present in 4 of the 8 patients who received echocardiography. The mean serum albumin level was 1.9 +/- 0.7 mg/dl, globulin level 3.1 +/- 1.1 mg/dl. Urinary Bence-Jones protein examination was performed in 8 cases; none revealed positive response. The mean immunoglobulin (Ig) level for the patients included: IgG 1018 +/- 901 mg/dl, IgA 262.0 +/- 313.8 mg/dl, IgM 104.8 +/- 84.2 mg/dl. There were at least 4 cases with high levels of one Ig but depressed levels in the others. M-component was shown by immunoelectrophoresis (IEP) in 90% of the cases. IEP impression in 10 cases revealed 2 cases of IgA lambda, 2 cases IgA kappa, 3 cases IgG lambda and 1 case IgG kappa monogammopathy, 1 case free lambda myeloma and 1 case negative. The ratio of kappa to lambda chain was 3:6. Bone marrow biopsy performed in 8 cases found only 1 case with multiple myeloma, one with amyloidosis; the other 6 cases were unremarkable. Mesangium was the site of heaviest amyloid deposition, followed by tubular basement membrane, artery and interstitium. The median survival time for those whose total score was lower than 3 points was 97.5 months; 3 to 5 points, 14 months; 6 points or more, 18.5 months. The median survival time was 21.6 months and 3-year-survival rate was 32.7%. The 2 cases with long-term survival were of 111 months and 84 months. The possible reason included: 1) Organ-limited renal amyloidosis; 2) Light amyloid deposition; 3) Younger age; 4) Other undetected favorable factors.

CONCLUSIONS

1) Renal amyloidosis is not a frequent diagnosis of nephrotic syndrome in Taiwan, but it should be suspected in every patient over 50 years old with a recent onset of proteinuria. 2) Renal amyloidosis can be diagnosed only by renal biopsy. 3) Primary renal amyloidosis is a disease of poor prognosis.

Differences in ligand binding and phosphoinositide turnover between M1 muscarinic receptor gene transfected cells and mouse and rat brain membranes

The present study describes some unexpected receptor mediated effects of N-methylcarbamylcholine on mouse M1 muscarinic receptor gene transfected cell line (M1Y1) that were not evident from biochemical studies with mouse and rat brain tissue where N-methylcarbamylcholine exhibited only nicotinic properties. Although N-methylcarbamycholine was devoid of muscarinic properties in mouse and rat brain preparations, as determined by phosphoinositide turnover and inhibition of [3H]QNB binding, it exhibited significant muscarinic characteristics in the transfected M1Y1 cell line. At a concentration of 10(-6) M or greater, N-methylcarbamycholine caused a transient increase in intracellular Ca2+ of 50 s duration that was reversible by atropine or pirezepine. The Ca(2+)-transient was not elicited by other nicotinic agents such as nicotine and N,N-dimethylcarbamylcholine, a close analogue of N-methylcarbamylcholine, with comparable affinity for nicotinic receptors and devoid of muscarinic activity. N-Methylcarbamylcholine also stimulated phosphoinositide turnover in M1Y1 cells with an estimated EC50 value 10 times greater than that of carbachol, and the effect was blocked by atropine. Both carbachol and N-methylcarbamycholine inhibited [3H]QNB binding in a concentration-dependent manner; however, the IC50 for carbachol was over two orders of magnitude greater than that observed in mouse and rat brain membranes. In considering possible explanations for the differential characteristics of N-methylcarbamylcholine in mouse and rat brain as compared to the transfected M1Y1 cells, it was concluded that the difference may be attributable to differences in the receptor-transduction coupling efficiency and the microenvironment of the muscarinic receptors.

Mitochondrial calcium transport: physiological and pathological relevance

Since the initiation of work on mitochondrial Ca2+ transport in the early 1960s, the relationship between experimental observations and physiological function has often seemed enigmatic. Why, for example, should an organelle dedicated to the crucial task of producing approximately 95% of the cell's ATP sequester Ca2+, sometimes in preference to phosphorylating ADP? Why should there be two separate efflux mechanisms, the Na+ independent and the Na+ dependent, both thought until recently to be driven exclusively either directly or indirectly by the energy of the pH gradient? Does intramitochondrial free Ca2+ concentration control metabolism? Is there evidence for any separate function of the mitochondrial Ca2+ transport mechanisms under pathological conditions? What is the relationship between mitochondrial Ca2+ transport, the mitochondrial membrane permeability transition, and irreversible cell damage under pathological conditions? First, we review what is known about control of metabolism, evidence for a role for intramitochondrial Ca2+ in control of metabolism, the cellular conditions under which mitochondria are exposed to Ca2+, characteristics of the mitochondrial Ca2+ transport mechanisms including the permeability transition, and evidence for and against mitochondrial Ca2+ uptake in vivo. Then the questions listed above and others are addressed from the perspective of the characteristics of the mechanisms of mitochondrial Ca2+ transport.

Primary IgA nephropathy: a nine-year clinicopathologic study in the Veterans General Hospital-Taichung

IgA nephropathy (IgA N) is the most common type of primary glomerulonephritis (GN) diagnosed in Taiwan. From February 1983 to May 1992, 194 patients with primary IgA N, representing 25.3% of the primary GN, were diagnosed by renal biopsy at this hospital. Clinicopathologic correlation was made in 175 cases of IgA N with adequate clinical and pathologic data including light-(LM), immunofluorescent (IF) and/or electron-(EM) microscopy. Modified classification of Meadow et al. was adopted for the histologic grading of glomerular lesions. Forty-nine biopsies (28.0%) showed Grade IV and V lesions (Grade IV, 10.9%; Grade V, 17.1%, respectively) in association with a high level of serum creatinine and a lower frequency of gross hematuria when compared with lesions of histologic Grades I to III. Patients with Grade V lesions revealed a high frequency of hypertension as compared with those with Grades I to IV. The frequencies of nephrotic range proteinuria in those with various grades of IgA N was not statistically significant in this study. One hundred and thirty patients were followed up for one to eight and half years or until end-stage renal disease (ESRD) developed (mean 3.9 years), excluding the biopsies done at ESRD or from the graft kidney. Forty-two patients (32.3%) had chronic renal insufficiency, of those 25 (19.2%) eventually developed ESRD. Seventy-five percent of the patients with histologic Grades IV and V showed progressive renal disease, while only 16% of patients with Grades I to III lesions revealed progressive disease, the latter indicating a more benign course (P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Na+ efflux mechanisms in ventricular myocytes: measurement of [Na+]i with Na(+)-binding benzofuran isophthalate

We characterized the Na(+)-sensitive dye benzofuran isophthalate (SBFI) with fluorescence microscopy in isolated, adult rat ventricular myocytes. When cells were loaded with SBFI by incubation with 10 microM of the acetoxymethyl ester, fluorescence excitation spectra were markedly attenuated below 340 nm and the isoemissive point was blue shifted by approximately 25 nm when compared with spectra from SBFI acid in buffered solutions. Fluorescence intensity (49 +/- 3%) was partially released by permeabilization of the sarcolemma with digitonin, suggesting that one-half of the dye molecules are sequestered subcellularly in a compartment shown most likely to be mitochondrial. Intracellular Na+ concentration ([Na+]i) was determined by in situ calibration using cation-selective ionophores and was found to be 14 +/- 2 mM in cells studied at 37 degrees C. The relative importance of Na+ efflux mechanisms in myocytes was investigated. Substitution of Ca2+ and Mg2+ with EGTA in the superfusing medium resulted in a reversible rise of [Na+]i from 13 +/- 2 to 31 +/- 5 mM, which was blocked by 1 microM verapamil. Cellular efflux of Na+ after loading in this manner was found to be insensitive to blockade of Na(+)-Ca2+ exchange but was abolished when Na(+)-K(+)-ATPase was inhibited with zero extracellular K+ concentration. We conclude that SBFI can be used to measure [Na+]i in ventricular cells nondestructively and without impalement. Na+ efflux after loading by Ca2+ and Mg2+ withdrawal is mediated by the Na+ pump with no measurable contribution from Na(+)-Ca2+ exchange.

Modulation of cytosolic free calcium concentration by ?1-adrenoceptors in rat atrial cells

The effects of alpha 1-adrenoceptor stimulation by phenylephrine (PE) and beta-adrenoceptor stimulation by isoprenaline (ISO) on Ca2+ current (ICa) and free intracellular Ca2+ concentration ([Ca2+]i) were studied in isolated atrial myocytes from rat hearts. PE did not significantly affect the magnitude of ICa, whereas large increases of peak ICa were observed in response to ISO. In electrically driven cells, PE evoked a concentration-dependent, gradual increase in diastolic [Ca2+]i and, initially, an increase in the height of peak [Ca2+]i transients. When the diastolic [Ca2+]i was increased to a greater extent, the amplitude of [Ca2+]i transients was decreased. Simultaneous measurements of [Ca2+]i and membrane potential showed that the increase in diastolic [Ca2+]i was associated with a depolarization of the membrane, and the greater amplitude of [Ca2+]i transients with a prolongation of the action potential (AP). The PE-induced increase in diastolic [Ca2+]i was eliminated when the cells were voltage-clamped at the original resting membrane potential (RP); under these conditions, an increase in [Ca2+]i transients was observed in response to PE. ISO usually caused larger increases in the amplitude of [Ca2+]i transients with only minor changes in diastolic [Ca2+]i. These results suggest that PE and ISO increase the amplitude of [Ca2+]i transients in rat atrium in different ways. The increase in [Ca2+]i transients in response to beta-adrenoceptor stimulation is commonly thought to be mediated by a greater conductance of voltage-dependent Ca2+ channels causing a greater Ca2+ influx and a release of more Ca2+ from the sarcoplasmic reticulum during the AP. The increase in diastolic [Ca2+]i in response to PE is probably a consequence of the depolarization of the membrane, possibly involving the voltage-dependent Na(+)-Ca2+ exchange mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of extracellular ATP-induced increase of cytosolic Ca2+ concentration in isolated rat ventricular myocytes

1. Changes in the cytosolic Ca2+ concentration ([Ca2+]i) of isolated rat ventricular myocytes in suspension were measured in response to extracellular ATP using the fluorescent Ca2+ indicators Quin-2 and Fura-2. 2. ATP produced a concentration-, time- and Mg(2+)-dependent, biphasic increase of [Ca2+]i whereas slowly hydrolysable ATP analogues produced a slow, monophasic increase of [Ca2+]i and the non-hydrolysable ATP analogues were without effect. 3. Extracellular Ca2+ was required for the ATP-induced increase of [Ca2+]i and pre-treatment of the cells with caffeine, ryanodine, verapamil or nimodipine partially inhibited the [Ca2+]i increase. 4. Whole-cell patch-clamp experiments revealed that ATP activated an ionic current that had a linear current-voltage relationship with a reversal potential near O mV. Quinidine, a putative P2 purinergic receptor blocker, abolished the ATP-activated current. The ATP-activated current was Mg2+ dependent. 5. Associated with the ATP-activated current was cellular depolarization. In a physiological solution, ATP depolarized cells to the threshold for the firing of action potentials. In the presence of the voltage-activated ion channel blockers tetrodotoxin, 4-aminopyridine, caesium and nitrendipine, ATP depolarized cells to -44 +/- 6 mV from a resting potential of -66 +/- 4 mV (n = 11). 6. Sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis and autoradiography demonstrated that extracellular ATP stimulated the phosphorylation of several extracellular membrane-bound proteins. The phosphorylation of these proteins was concentration, time and Mg2+ dependent. Pre-treatment of cells with the slowly hydrolysable ATP analogues inhibited the ATP-induced phosphorylation. Adenosine 5'-O-3-thiotriphosphate (ATP gamma S) thiophosphorylated proteins with the same apparent molecular weight as the proteins phosphorylated by ATP. 7. These results suggest that the ATP-induced increase of [Ca2+]i is a result of the activation, possibly by protein phosphorylation, of a novel ion channel carrying inward current. The ATP-activated channel may be permeable to Na+ and Ca2+ and causes [Ca2+]i to rise. More importantly, this inward current depolarizes the cell to the threshold of inducing spontaneous firing of action potentials. The firing of action potentials results in the influx of Ca2+ through L-type Ca2+ channels which would trigger Ca2+ release from the sarcoplasmic reticulum and lead to the increase in [Ca2+]i.

Ca2+ current and Ca2+ transients under action potential clamp in guinea pig ventricular myocytes

Precise characterization of the magnitude and kinetics of transsarcolemmal Ca2+ influx during an action potential (AP) is essential for a complete understanding of excitation-contraction coupling in heart. Using a voltage-clamp protocol that simulated a physiological AP (AP clamp), we characterized the properties of the Ca2+ current (ICa) in guinea pig ventricular myocytes. The AP-generated ICa showed a complex time course that was different from ICa generated by a square pulse. ICa activated rapidly during the upstroke of the AP and then partially inactivated during the plateau. The fast component of ICa reached a peak value of -7.6 +/- 1.0 pA/pF at 2.40 +/- 0.30 ms after depolarization, followed by a slow component with a peak value of -2.9 +/- 0.4 pA/pF during the plateau. ICa generated by an AP was composed of both L- and T-type Ca2+ channels. T-type Ca2+ current contributed to the fast component of ICa and L-type Ca2+ current contributed to both fast and slow components of ICa. Activation of beta-adrenoceptors enhanced ICa with a maximal effect lasting throughout the entire plateau of the AP. Measurements of cytosolic Ca2+ transients using fura-2 indicated that the ICa was responsible for triggering Ca2+ release from the sarcoplasmic reticulum. The AP clamp provides a new approach for investigation of the relationship between ICa and Ca2+ transients under more physiological conditions.

Perturbations of intracellular calcium distribution in kidney cells by nephrotoxic haloalkenyl cysteine S-conjugates

The Ca2(+)-sensitive dye fura-2 was used to investigate the disturbances in intracellular Ca2+ concentration [( Ca2+]i) and distribution induced by the nephrotoxic cysteine S-conjugate S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and its homocysteine analog S-(1,2-dichlorovinyl)-L-homocysteine (DCVHC) in LLC-PK1 cells. After 24-hr treatment with DCVC, the average [Ca2+]i increased from 88 +/- 23 nM to 415 +/- 92 nM. Digital image analysis revealed that the mitochondrial region, which was stained with rhodamine-123, contained lower Ca2+ concentration ([Ca2+]) than other cell areas. This distribution was different from the higher [Ca2+] in the nuclear and mitochondrial regions observed in control cells. In DCVHC-treated cells, there was also an increase in [Ca2+]i to 355 +/- 85 nM, but the increase in [Ca2+] was greater in the mitochondrial region, compared with the rest of the cell. After 72-hr treatment with DCVC or DCVHC, the average [Ca2+]i was 410 +/- 85 nM and 340 +/- 90 nM, respectively, and blebs with markedly higher [Ca2+] (600-1000 nM) than the rest of the cell appeared in both DCVC- and DCVHC-treated cells. Moreover, in DCVC-treated cells the mitochondria could not be stained with rhodamine-123, indicating severe mitochondrial damage and loss of membrane potential. All changes described above took place in viable (propidium iodide-negative) cells. The experiments demonstrate that severe perturbations of intracellular Ca2+ distribution, particularly in the mitochondrial region, precede bleb formation and cell death in the course of development of toxicity by DCVC and DCVHC.

Blockade of low and high threshold Ca2+ channels by diphenylbutylpiperidine antipsychotics linked to inhibition of prolactin gene expression

The effects of diphenylbutylpiperidine (DPBP) antipsychotics on Ca2+ currents and prolactin (PRL) synthesis were studied in rat pituitary growth hormone (GH) cell lines (GH3 and GH4C1). In whole-cell patch-clamp experiments, DPBPs including fluspirilene, penfluridol, and pimozide at concentrations ranging from 0.25 to 5 microM each blocked current through low threshold T-type as well as high threshold L-type channels. Each of the drugs preferentially blocked T-type current, and complete inhibition was observed at concentrations as low as 1 microM. Inhibition of L-type channels by DPBPS was enhanced at depolarized holding potentials and promoted by prolonged channel activation. At concentrations similar to those which blocked Ca2+ currents, each of the three DPBPs markedly reduced basal PRL production by GH cells. PRL synthesis stimulated by the dihydropyridine Ca2+ agonist R5417 or thyrotropin releasing hormone (TRH) was also inhibited. The inhibitory effects of the DPBPs were observed at the level of gene transcription. Penfluridol and fluspirilene inhibited basal, Ca2(+)- and TRH-stimulated expression of a fusion gene construct containing the 5'-flanking sequence of the rat PRL gene linked to the luciferase gene. The effect was concentration-dependent with the IC50 values for both drugs of less than 1 microM. Nimodipine also reduced basal, R5417, and TRH-stimulated expression of the reporter gene construct. Similar results were obtained with a reporter gene construct containing the 5'-flanking sequence of the rat GH gene. The GH luciferase construct was only slightly responsive to R5417 and TRH; however, these responses were reduced by fluspirilene and nimodipine at concentrations of less than 1 microM. These studies demonstrate that the DPBP antipsychotics block T- as well as L-type Ca2+ channels in GH cells and inhibit PRL production at the level of transcription. They also indicate that functioning Ca2+ channels are necessary for TRH-stimulated PRL gene transcription.

Antipsychotic pimozide is a potent Ca2+ channel blocker in heart

The diphenylbutylpiperidine (DPBP) antipsychotic pimozide was identified as a potent new Ca2+ channel antagonist in heart. In whole-cell patch-clamp experiments, pimozide blocked Ca2+ current through L type channels of rat ventricular myocytes, in a voltage-dependent manner. At holding potentials positive to -40 mV, approximately 90% of current was blocked by 200 nM pimozide. Nearly half of this block, 48 +/- 5% (mean +/- SE, n = 5), was relieved by 5-min hyperpolarization to -100 mV. In quin2-loaded myocytes, pimozide blocked 50 mM KCl-induced increases in intracellular Ca2+ concentration [( Ca2+]i) half maximally at a concentration of 75 +/- 15 nM (n = 5). Two other DPBPs, penfluridol and fluspirilene, also blocked the KCl-induced response at similar concentrations. The contractile force of cardiac tissue was also depressed by pimozide. One micromolar pimozide reduced twitch tension in rat papillary muscles by an average of 36 +/- 8% (n = 3). These results demonstrate that the DPBPs comprise a potent new class of Ca2+ antagonists in heart, which will be useful in studying cardiac Ca2+ channels. They also suggest that these agents may have therapeutic value outside the field of psychiatry.

Modulation of ventricular action potential by alpha 1-adrenoceptors and protein kinase C

The effects of the alpha 1-adrenoceptor agonist methoxamine (MTX) and the direct protein kinase C (PKC) activator phorbol 12,13-dibutyrate (PDBU) on action potentials from guinea pig papillary muscles were studied. Measured with conventional microelectrodes, MTX (1 x 10(-7) to 3 x 10(-4) M) and PDBU (1 x 10(-9) to 1 x 10(-6) M) both caused a dose-dependent and reversible decrease in the action potential duration measured at 90% repolarization (APD90) at 36.5 degrees C. The MTX-mediated response was blocked by both prazosin (3 x 10(-6) M) and phentolamine (1 x 10(-6) M) and mimicked by phenylephrine. Maximal concentrations of the two agents together resulted in only a partial (50%) additive decrease in the APD90. At a lower temperature (27.5 degrees C), PDBU no longer produced a shortening in the APD90 and MTX produced a prolongation in the APD90. These results demonstrate that although alpha 1-adrenoceptor stimulation and PKC activation in guinea pig papillary muscle both lead to a decrease in the APD90, the differences in their effects as related to the magnitude, partial additivity, and temperature dependence suggest that the mechanism of action are not identical. These subtle differences may help to delineate the exact physiological implications of alpha 1-adrenoceptors in cardiac excitation and contraction.

Spatial heterogeneity of intracellular Ca2+ concentration in nonbeating guinea pig ventricular myocytes

The spatial distribution of intracellular Ca2+ concentration was determined by fluorescent digital imaging microscopy in fura-2-loaded quiescent cardiac myocytes isolated from guinea pig ventricle. Fluorescent ratio images revealed discrete as well as clustered bright fluorescent spots ("hot spots"), which occupied approximately 20-50% of an individual cell's area. The fluorescent intensity and the area of the hot spots were increased by agents that deplete Ca2+ in the sarcoplasmic reticulum, namely, ryanodine (20-40 nM) and caffeine (5-15 mM). However, when cells were exposed to agents that deplete mitochondrial Ca2+, such as the protonophore, carbonyl cyanide m-chlorophenyl-hydrazone (CCCP, 100-300 nM), or the inhibitor of electron transport, antimycin A (4-40 nM), the fluorescent intensity and the area of the hot spots were reduced. These results indicate that the spatial distribution of intracellular Ca2+ concentration in the ventricular myocytes of guinea pig is quite heterogeneous. The ability of CCCP and antimycin A, but not of caffeine and ryanodine, to reduce the fluorescent intensity in the hot spots implies that Ca2+ compartmentation in the mitochondria is largely responsible for the intracellular Ca2+ heterogeneity seen in the present study.

Cytosolic sodium concentration regulates contractility of cardiac muscle

The present chapter provides experimental evidence to show that intracellular Na+ concentration regulates cardiac contractility effectively by altering intracellular Ca2+ concentration via the Na-Ca exchange. This steep coupling between the Na+ and Ca2+ electrochemical gradients implies that a change in intracellular Na+ concentration is accompanied by a concomitant change in intracellular Ca2+ concentration (and, therefore, contractility). Under the physiologic conditions, each cardiac action potential alters intracellular Na+ concentration in a dynamic manner. Therefore, Na-Ca exchange can regulate cardiac contraction from a beat-to-beat basis.

Stimulation of muscarinic receptors raises free intracellular Ca2+ concentration in rat ventricular myocytes

The effect of carbachol on free intracellular calcium concentration, ([Ca2+]i) and on intracellular hydrogen concentration (pHi) was determined from fluorescence signals obtained from rat ventricular myocytes. Application of carbachol (300 mumol/l) to quin2-loaded myocytes bathed in 2 mmol/l Ca2+-containing solution caused [Ca2+]i to increase within 7-10 minutes from 182 +/- 9 to 212 +/- 11 nmol/l (n = 4). Carbachol acted via stimulation of muscarinic receptors because atropine (1 mumol/l) either prevented or abolished the increase in [Ca2+]i. Carbachol also produced a positive inotropic effect in rat papillary muscles contracting isometrically at a frequency of 0.5 Hz and enhanced contracture in resting preparations in the presence of high extracellular Ca2+ concentration ([Ca2+]o) (20 mmol/l). The effect of carbachol on [Ca2+]i was dependent on [Ca2+]o. In the presence of 10 mmol/l [Ca2+]o, the increase in [Ca2+]i was about two times that elicited by carbachol when bath [Ca2+]o was 2 mmol/l. Reduction of [Ca2+]o to 50 mumol/l abolished the carbachol effect but did not prevent caffeine-induced Ca2+ release. The carbachol-induced rise in [Ca2+]i remained unchanged in the presence of either 10 mmol/l caffeine or 1 mumol/l ryanodine. In the absence of extracellular Na+ concentration [( Na+]o), carbachol no longer produced an increase in [Ca2+]i of cardiomyocytes and failed to enhance Na+-withdrawal contracture of the rat papillary muscle. In contrast to the effect on [Ca2+]i, carbachol did not produce any change in pHi as determined from fluorescence signals obtained from rat ventricular myocytes loaded with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of forskolin on intracellular sodium activity in resting and stimulated cardiac Purkinje fibers from sheep

In the present investigation, the effects of forskolin on intracellular sodium activity were studied in quiescent and electrically stimulated cardiac Purkinje fibers from sheep using Na+-sensitive microelectrodes. Also assessed, were the effects of this promoter of cytosolic cAMP production on resting membrane potential, action potential and twitch tension. In the quiescent fibers, forskolin (12 microM) caused intracellular sodium activity to decrease in the face of cellular depolarization. This cellular depolarization was occasionally accompanied by spontaneous firing of action potentials. In the stimulated fibers, forskolin (10 microM) also caused intracellular sodium activity to decrease. Moreover, it caused a marked acceleration of phase 4 pacemaker depolarization, an elevation of the plateau of the action potential and an increase in twitch tension. When the Na+ pump was inhibited by either strophanthidin (1 microM) or by 0 mM extracellular K+, forskolin had no effect on intracellular sodium activity. In summary, the results of the present study indicate that forskolin, presumably by increasing intracellular cAMP, causes the following to occur in cardiac Purkinje fibers from sheep: (a) a decrease in intracellular sodium activity when the Na+ pump is functioning normally; (b) a promotion of membrane depolarization in quiescent fibers; (c) an increase in the steepness of the pacemaker potential in electrically stimulated fibers, and (d) an increase in the force of contraction. Therefore, forskolin will be a useful tool for investigating the role of cAMP in physiological function of cardiac cells.

Pituitary Ca2+ channels: blockade by conventional and novel Ca2+ antagonists

We have identified several new agents that block Ca2+ channels in the rat pituitary GH4C1 cell line. These drugs, which include the diphenylbutylpiperidine antipsychotic pimozide, the calmodulin antagonist calmidazolium, and the steroidal Na+ channel toxin veratridine, were compared with several conventional Ca2+ antagonist in 45Ca2+ uptake, prolactin secretion, and whole cell patch voltage-clamp experiments. Pimozide, the most potent of these novel Ca2+ antagonists, inhibited depolarization-dependent 45Ca2+ uptake and prolactin secretion half maximally at a concentration of 100 nM, whereas calmidazolium and veratridine produced 50% inhibition at concentrations of 500 nM and 1 microM. In comparison, the three organic Ca2+ antagonists nitrendipine, verapamil, and diltiazem blocked 45Ca2+ uptake half maximally at concentrations of 2.5 nM, 1 microM, and 2.5 microM, respectively. All of the antagonists inhibited Ca2+ uptake and prolactin secretion stimulated by the dihydropyridine Ca2+ agonist BAY-K 8644 less potently than KCl-stimulated responses. In patch-clamp experiments, pimozide, veratridine, and nitrendipine blocked Ca2+ current through the slowly inactivating Ca2+ channels of GH4C1 cells. These results demonstrate that Ca2+ channels in an endocrine cell line can be blocked by a variety of molecules including sodium channel toxins and calmodulin antagonists. The data extend the pharmacological similarity between Ca2+ channels in pituitary and other excitable cells and suggest a structural similarity among several cellular proteins.

Voltage-dependent effects of isoproterenol on cytosolic Ca concentration in rat heart

The effect of the beta-adrenoceptor agonist, isoproterenol, on cytosolic calcium concentration ([Ca2+]i) was studied with the Ca2+-sensitive fluorescent indicator quin 2 in enzymatically dissociated rat ventricular myocytes. Under conditions in which cells have normal polarized resting membrane potential, isoproterenol (1 microM) produced a decrease in [Ca2+]i. In contrast, in the depolarized cells (by raising extracellular K+ concentration to 50 mM), isoproterenol (1 microM) caused an increase in [Ca2+]i. This isoproterenol-induced increase in [Ca2+]i in depolarized cells could be reversed by prior exposure of the cells to the Ca2+ channel blocker, verapamil (5 microM). The results indicate that isoproterenol can either decrease or increase [Ca2+]i depending on membrane potential. The actual effect of isoproterenol on [Ca2+]i at any given membrane potential probably reflects the relative contributions of isoproterenol-induced stimulation of Ca2+ buffering or effluxing activities (which favor a decrease in [Ca2+]i) and enhancement of Ca2+ influx through voltage-sensitive Ca2+ channels (which favors an increase in [Ca2+]i).

Dihydropyridine modulators of voltage-sensitive Ca2+ channels specifically regulate prolactin production by GH4C1 pituitary tumor cells

To determine whether hormone synthesis by the GH4C1 pituitary cell line could be regulated by specifically modulating the movement of Ca2+ through voltage-sensitive channels, we have compared the effects of the dihydropyridine Ca2+ channel agonist BAY K8644 and the antagonist nimodipine on hormone production and Ca2+ current in these cells. BAY K8644 elicited, after a 10-15-h lag, a dose-dependent increase in prolactin (PRL) production as determined by measurements of total intracellular and secreted hormone. Over a 72-h period, GH4C1 cells incubated with 300 nM BAY K8644 produced 2-3 times as much total PRL as control cells. The effect on PRL was specific, since BAY K8644 did not increase growth hormone production, cell growth rate, or total cell protein. Exposing GH4C1 cells to BAY K8644 for short periods, up to 90 min, did not induce the delayed increase in PRL production observed with longer incubations. The effects of nimodipine were opposite to those of the Ca2+ channel agonist. PRL production was reduced 85% during 48-h treatment with 200 nM nimodipine, whereas growth hormone production was decreased less than 15%, and cell growth and total protein were unaffected. The actions of these two drugs on PRL production were well correlated with their effects on GH4C1 Ca2+ currents as measured by whole-cell patch-clamp recordings. BAY K8644 enhanced the magnitude of the peak Ca2+ current and shifted the current-voltage relationship such that Ca2+ channels were activated at less depolarized potentials. Nimodipine potently inhibited Ca2+ movement through the non-inactivating channel, while it antagonized the increases elicited by BAY K8644. These results indicate that PRL synthesis by GH4C1 cells can be specifically regulated by agents that enhance or block the movement of Ca2+ through voltage-sensitive channels. They also suggest that hormone synthesis by a secretory cell may be coupled to electrical activity by the opening of Ca2+ channels.

Na+-Ca2+ exchange contributes to increase of cytosolic Ca2+ concentration during depolarization in heart muscle

The possible role of Na+-Ca2+ exchange in contributing to depolarization-induced increase in cytosolic Ca2+ concentration ([Ca2+]i) of isolated rat ventricular myocytes was investigated. Measured with the Ca2+-sensitive indicator quin 2, [Ca2+]i increased from 177 +/- 12 (mean +/- SE, n = 11) to 468 +/- 41 nM when cells were depolarized with solutions containing 50 mM KCl [high extracellular K+ concentration ([K+]o)]. Approximately 73% of this high-[K+]o-induced increase in [Ca2+]i was abolished by the Ca2+ channel blocker verapamil (5 microM). For cells pretreated with 10 mM caffeine to deplete the Ca2+ stored in sarcoplasmic reticulum, 50 mM KCl still produced an increase in [Ca2+]i, even in the presence of 5 microM verapamil. However, if extracellular Na+ was replaced by Li+ or tris(hydroxymethyl)aminomethane, this increase was completely abolished. The results suggest that, in addition to voltage-sensitive Ca2+ channels, voltage-sensitive Na+-Ca2+ exchange can also contribute to the increase in [Ca2+]i on depolarization. Therefore both Ca2+ transport systems may play important roles in regulating cardiac excitation and contraction.

Lidocaine's negative inotropic and antiarrhythmic actions. Dependence on shortening of action potential duration and reduction of intracellular sodium activity

The mechanisms by which lidocaine brings about negative inotropic effects and antiarrhythmic actions in the heart have been examined. Using sheep cardiac Purkinje fibers, we studied the effects of "therapeutic" concentrations of lidocaine (20 microM lidocaine = 5.4 micrograms/ml) on electrical activity, intracellular sodium activity, and tension. For the preparation with a normal, physiological level of intracellular sodium activity (5-9 mM), the application of lidocaine leads to the following actions: action potential duration is decreased, intracellular sodium activity falls, and twitch tension is reduced. If intracellular sodium activity is first elevated (e.g., by sodium pump inhibition) so that arrhythmogenic transient depolarizations (and the underlying transient inward current are seen) then lidocaine has the following actions: The action potential duration is reduced. There is a reduction of the magnitude of the arrhythmogenic transient depolarization (or the underlying membrane current transient inward current). The magnitude of the aftercontraction that accompanies the transient depolarization (or transient inward current) is reduced. If, after sodium pump inhibition, intracellular sodium activity is still rising-then, the application of lidocaine leads to a reduction of the rate of rise of intracellular sodium. From these results, parallel voltage-clamp experiments, and recent work by others, we conclude that lidocaine leads to the reduction of the arrhythmogenic transient depolarization and a reduction of twitch tension by decreasing the inward sodium current. These actions are mediated by a reduction in action potential duration and a reduction of intracellular sodium activity and of intracellular calcium activity (by the sodium-calcium exchange mechanism).