Aging, cardiac proenkephalin mRNA and enkephalin peptides in the Fisher 344 rat

Cardiac proenkephalin (PENK) mRNA, methionine-enkephalin (ME) and leucine-enkephalin (LE) were determined from 2 days of age through senescence in Fisher 344 rats. Tissues were collected at 2 days, 2 weeks, 1, 2, 3, 7, 19, and either 22 or 27 months of age. Hearts were dissected, extracted and assayed for ME and LE by radioimmunoassay (RIA) or for PENK mRNA by Northern blot analysis with a cDNA probe. Relative left ventricular (LV) PENK mRNA was low in 2 day animals and increased slowly between 2 weeks and 3 months of age. LV PENK mRNA then rose five to six-fold between 3 and 27 months of age. LV ME measurements were high in neonatal animals, declined to a nadir during development and then rose again as the animals matured and advanced in age. The pattern for right ventricular (RV) ME was similar. Atrial ME, also high at 2 days, declined thereafter and remained low. LE measurements in LV, RV and the atria followed patterns similar to those described for ME. To evaluate for peptides contributed by cardiac nerves, 3, 7 and 22-month-old animals were acutely sympathectomized for 24 h with 6-hydroxydopamine. No decline in LV ME and LE was observed in the 6-hydroxydopamine treated animals. These data suggest several conclusions regarding myocardial enkephalinergic systems: (a) tissue enkephalin and PENK mRNA increase with advancing age, (b) tissue enkephalins may not strictly correlate with the relative abundance of PENK mRNA, and (c) most myocardial enkephalins are non-adrenergic in origin. The age-associated patterns in both PENK mRNA, ME and LE suggest that physiological, maturational or behavioral events between 3 and 7 months of age initiate the up-regulation and subsequent expansion of cardiac enkephalinergic systems.

Polysomy of chromosome 12 in 60 patients with non-Hodgkin's lymphoma assessed by fluorescence in situ hybridization: differences between follicular and diffuse large cell lymphoma

Sixty consecutive evaluable specimens from patients with non-Hodgkin's lymphoma (NHL) were studied for the incidence of polysomy of chromosome 12 by fluorescence in situ hybridization (FISH) with probes for the repetitive DNA sequence in the centromeric region of chromosome 12. Thirty-six samples were from follicular lymphomas (FLs), and twenty-four were from diffuse large cell lymphomas (DLCLs). Fifty-two specimens (86%) were obtained by fine-needle aspiration of a diseased node, seven (11.6%) were from involved bone marrows, and one specimen was from a pleural effusion. Twelve of the thirty-six (33%) cases with FL had trisomy 12 in 3-41% of the cells (median, 10%) (normal controls had three signals in 1.4 +/- 0.7% of cells). Trisomy 12 was found in 62% of the patients who had had FL for more than 5 years. Nine of the twenty-four cases (37%) with DLCL had more than two copies of chromosome 12 in 4-92% of the cells (median, 78%), and all nine cases were of B-cell phenotype. Unlike FL cells, some DLCL cells had 4-6 copies of chromosome 12. In previously untreated patients, 54% of DLCLs and 26% of FLs had subpopulations of cells containing more than two copies of chromosome 12 (P = 0.04). Only 2/7 cases of DLCL with polysomy 12 had rearrangement of the BCL2 gene, indicating that the majority of DLCL cases with polysomy 12 did not result from histologic transformation of low grade follicular lymphomas. These data demonstrate that FISH of interphase cells is a sensitive method for detecting numerical abnormalities of chromosome 12 in NHL.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth inhibition of follicular small-cleaved-cell lymphoma cells in short-term culture by interleukin-3

BACKGROUND

The role of interleukin-3 (IL-3) in stimulating the growth of early myeloid progenitor cells is very well established. Therefore, IL-3 has been incorporated into many post-bone-marrow transplantation and intensive chemotherapy programs for the treatment of solid tumors and hematologic malignancies, including lymphomas. However, the effect of IL-3 on normal and malignant lymphocytes has not been well studied.

PURPOSE

The purpose of this study was to evaluate the in vitro effect of IL-3 on the growth of follicular small-cleaved-cell lymphoma (FSCCL).

MATERIALS AND METHODS

IL-3 receptor expression on the surface of CD19+ cells was determined by two-color flow cytometry measuring the receptor-binding of biotinylated IL-3 to CD19+ B-cells. Seven cases of FSCCL were compared to six normal controls. Cell proliferation was evaluated by [3H]thymidine incorporation into cells grown in suspension cultures.

RESULTS

All seven cases of FSCCL expressed the IL-3 receptor on the surface of CD19+ cells, whereas all six cases of CD19+ cells isolated from the peripheral blood of normal donors did not express IL-3 receptors. IL-3 had antiproliferative activity against FSCCL as manifested by a decrease in [3H]thymidine incorporation and a decrease in the total number of cells after 72 hours of culture.

CONCLUSION

IL-3 inhibits the growth of FSCCL cells in vitro. Clinical trials to evaluate the in vivo effect of IL-3 in patients with FSCCL are warranted.

Lymphomatous presentation of CD4+/CD8+ HTLV-1-related adult T-cell leukemia/lymphoma in an Iranian woman

Adult T-cell leukemia/lymphoma (ATLL) remains an uncommon disorder outside well-defined risk groups. We describe the case of an Iranian woman, who presented with isolated meningeal relapse of diffuse large-cell lymphoma. The malignant cells coexpressed CD4 and CD8 and HTLV-1 seropositivity was confirmed. Despite combination chemotherapy disseminated lymphoma developed. Preterminally the characteristic features of ATLL were noted; hypercalcemia, with normal parathyroid hormone-related protein and vitamin D levels, and peripheral blood leukemic involvement.

Intensive conventional-dose chemotherapy for stage IV low-grade lymphoma: high remission rates and reversion to negative of peripheral blood bcl-2 rearrangement

BACKGROUND

Advanced-stage low-grade lymphoma is characterized by initial responsiveness to many conventional therapies but ultimate relapse. Intensive therapy approaches with non-cross-resistant regimens have not been extensively explored. The polymerase chain reaction (PCR) can be used to monitor for the presence of cells with rearrangement of bcl-2, and provides a sensitive and stringent parameter of disease activity and treatment response that may have clinical utility.

PATIENTS AND METHODS

From 1988 to 1992, 138 evaluable patients were treated with 3 sequential chemotherapy regimens, as well as with interferon alfa 2b (IFN) in combination with corticosteroids. Nineteen patients had serial PCR monitoring for bcl-2 rearrangement.

RESULTS

Among a subset of 58 patients who had an initial phase of IFN plus prednisone, the response rate was 59%, mostly partial remissions (PR). With the chemotherapy program, 65% have achieved complete remission to date, and 30% PR. By PCR analysis, 13 of 19 tested achieved negative status ('molecular remission'), a much higher frequency of molecular remission than has been seen with standard therapies, and these molecular remissions appear to correlate with a lower likelihood of relapse.

CONCLUSIONS

Intensive conventional-dose chemotherapy can achieve high rates of remission, even when monitored by the stringent PCR technique.

Retinoic acid stimulates the protein kinase C pathway before activation of its beta-nuclear receptor during human teratocarcinoma differentiation

We previously reported that protein kinase C (PKC) stimulation through phorbol ester (TPA) treatment enhances the effects of all-trans retinoic acid (RA) on immunophenotypic differentiation and RA nuclear receptor (RAR) activation in the multipotential human teratocarcinoma (TC) cell line NTera-2/clone D1 (abbreviated NT2/D1). This study extends prior work in NT2/D1 cells by demonstrating that PKC pathway activation is an early effect of RA treatment which regulates RAR transcriptional activity. RA activated the PKC pathway prior to induction of RAR-beta expression at 6 h, which is an established early marker of RAR activation in NT2/D1 cells. RA caused a transient 1.3-fold increase in intracellular diacylglycerol (DG) at 2 min and a translocation of the gamma isozyme of PKC (PKC-gamma) within 5 min. Transient co-transfection studies provided evidence that PKC pathway activation plays a role in the regulation of RAR-beta expression. In these studies a constitutively active PKC-gamma augmented the RA-mediated transactivation of a luciferase reporter containing the native RAR-beta promoter which has a retinoic-acid-response element (RARE). These findings reveal that PKC pathway activation is an early step in RA-mediated human TC differentiation and that PKC-gamma can potentiate the effects of RA on RAR transcriptional activation.

Age-associated increase in rat cardiac opioid production

Several lines of evidence suggest that opioid peptide production in the heart may increase during aging from adulthood through senescence. We tested the hypothesis that cardiac opioid peptides and preproenkephalin (PNK) mRNA would increase with advancing age. Ventricles and atria from male Wistar rats, aged 2, 6, 18, and 22-24 mo of age, were acid extracted and assayed for methionine enkephalin (ME) and leucine enkephalin (LE). Total RNA was extracted from hearts of age-matched rats and probed for PNK mRNA by Northern blot analysis using a cDNA probe. ME and LE peptides were significantly elevated with advancing age in both ventricles [P < 0.001; by analysis of variance (ANOVA)]. Ventricular ME concentration exhibited a biphasic increase with approximately twofold higher peaks at 6 and 22-24 mo of age compared with the 2 mo value of 1.04 +/- 0.05 (SE) pmol/g wet wt. In contrast, ventricular LE concentration was largely unchanged until 22-24 mo of age when it increased approximately threefold over the 2-mo value of 2.19 +/- 0.14 pmol/g wet wt. Left ventricular PNK mRNA increased approximately fivefold between 2 and 18 mo of age (P < 0.01; ANOVA). Thus both enkephalins and the mRNA coding for them were increased in hearts of older vs. younger rats. Because opioid receptor stimulation can negatively modulate several characteristics of cardiac myocyte contraction, these results may have important functional implications for the senescent heart.

Activation of the sphingomyelin signaling pathway in intact EL4 cells and in a cell-free system by IL-1 beta

The mechanism of interleukin-1 (IL-1) signaling is unknown. Tumor necrosis factor-alpha uses a signal transduction pathway that involves sphingomyelin hydrolysis to ceramide and stimulation of a ceramide-activated protein kinase. In intact EL4 thymoma cells, IL-1 beta similarly stimulated a rapid decrease of sphingomyelin and an elevation of ceramide, and enhanced ceramide-activated protein kinase activity. This cascade was also activated by IL-1 beta in a cell-free system, demonstrating tight coupling to the receptor. Exogenous sphingomyelinase, but not phospholipases A2, C, or D, in combination with phorbol ester replaced IL-1 beta to stimulate IL-2 secretion. Thus, IL-1 beta signals through the sphingomyelin pathway.

A rise and fall in 1,2-diacylglycerol content signal hexamethylene bisacetamide-induced erythropoiesis

Previous studies have suggested a role for protein kinase C (PKC) during induction of murine erythroleukemia cell (MELC) differentiation by hexamethylene bisacetamide (HMBA) (Melloni, E., Pontremoli, S., Viotti, P. L., Patrone, M., Marks, P. A., and Rifkind, R. A. (1989) J. Biol. Chem. 264, 18414-18418). The present studies assess the effect of HMBA on the content of 1,2-diacylglycerol (DG), the physiologic activator of PKC, in MELC variants. Exposure of parental Sc9 cells to HMBA induced a rapid rise and fall in DG content. The DG level increased within seconds from 225 pmol.10(6) cells-1 to a maximum of 305 pmol.10(6) cells-1 at 5 min. Thereafter, DG content fell reaching control levels at 30 min and 46% of control at 4 h. Similar DG elevations were detected in HMBA-resistant, phorbol ester-resistant, and vincristine-resistant MELC lines. Early DG elevation was followed by the characteristic rapid fall in both the phorbol ester-resistant and vincristine-resistant lines, both of which differentiate rapidly in response to HMBA. In contrast, in an HMBA-resistant MELC the DG level failed to fall for at least 10 h. Selection of HMBA-resistant cells for vincristine resistance restores both HMBA sensitivity and the rapid fall in DG content. Addition of a synthetic DG, 1-oleyl-2-acetyl glycerol (OAG), along with HMBA and every 2 h for the next 48 h blocked differentiation, as measured by accumulation of benzidine-reactive cells or by the commitment assay in methyl-cellulose. However, if addition of OAG was delayed for just a few minutes, until endogenous DG levels began to fall, differentiation was no longer inhibited. Rapid elevation of DG content is the earliest reported event during HMBA action and a subsequent fall in the DG content appears to be a critical step in the process of commitment to terminal differentiation.

Alfa-2A interferon and 5-fluorouracil for advanced colorectal carcinoma: the Memorial Sloan-Kettering experience

In a phase II study, 38 previously untreated patients with metastatic colorectal carcinoma were treated with continuous intravenous infusion of 5-fluorouracil (5-FU) 750 mg/m2 daily for 5 days, followed by weekly bolus 5-FU at 750 mg/m2 and subcutaneous interferon (IFN) at 9 million units three times per week. Of 35 evaluable patients, nine (26%) achieved a partial response (95% confidence limit, 11% to 41%), with a median response duration of 7.5 months (range, 4.4 to 17+ months). Seven patients (20%) had a minor response, and 10 (28%) had stable disease. The median length of survival was 13 months (range, 2 to 19+ months). The most common toxicities observed were stomatitis (52%) and diarrhea (43%). Neurotoxicity was seen in 34% of patients and consisted of gait disturbance, dizziness, confusion, memory loss, and dementia. Because of toxicity, 84% of patients required a reduction of the IFN dose by at least 50%, and 63% required reduction of 5-FU by at least 25%. We conclude that while the combination of 5-FU and IFN in patients with advanced colorectal carcinoma has some activity, the regimen is toxic and the observed response rate (26%) is not substantially superior to alternative 5-FU programs.

Dimethylthiourea, an oxygen radical scavenger, protects isolated cardiac myocytes from hypoxic injury by inhibition of Na(+)-Ca2+ exchange and not by its antioxidant effects

Myocardial reoxygenation injury may be attenuated by oxygen free radical scavengers, arguing for a role of oxygen radicals in this process. To determine whether free radical scavengers affect reoxygenation injury in isolated cardiac myocytes, resting rat ventricular myocytes were exposed to hypoxic (PO2 less than 0.02 mm Hg) glucose-free buffer alone (n = 50) or with the addition of the oxygen radical scavengers 1,3-dimethyl-2-thiourea (DMTU, 25 mM, n = 46), human recombinant superoxide dismutase (SOD, 1,000 units/ml, n = 40), or the combination of these agents (n = 41). All cells responded by undergoing contracture to a rigor form. Hypoxia was then continued for a second period (T2), the duration of which correlates inversely with survival. After reoxygenation, cells either retained their rectangular shape (survival) or hypercontracted to a rounded form (death). For the group of cells with a T2 period greater than 30 minutes, no cell exposed to buffer alone (n = 20) or to SOD (n = 16) survived, in contrast to 15 of 24 (63%) cells exposed to DMTU. The addition of SOD to DMTU offered no advantage to DMTU alone. The protective effect of DMTU was not observed when it was added at reoxygenation, suggesting that this agent has an important effect during the hypoxic period when intracellular Ca2+ is known to rise, most likely because of the reversal of Na(+)-Ca2+ exchange. Therefore, the effects of DMTU on Ca2+ regulation (indexed by indo-1 fluorescence) during hypoxia were studied. DMTU significantly blunted the [Ca2+] rise during the hypoxic period.(ABSTRACT TRUNCATED AT 250 WORDS)

Ceramide is a competitive inhibitor of diacylglycerol kinase in vitro and in intact human leukemia (HL-60) cells

Prior studies demonstrated that ceramide was phosphorylated by a novel Ca(2+)-dependent kinase distinct from diacylglycerol (DG) kinase in human myelogenous leukemia (HL-60) cells (Kolesnick, R. N., and Hemer, M. R. (1990) J. Biol. Chem. 265, 10900-10904). The present studies were initiated to determine whether mammalian DG kinase purified to homogeneity possessed phosphotransferase activity toward ceramide. A high molecular weight rat brain DG kinase demonstrated Mg(2+)-(but not Ca(2+)-) dependent DG kinase activity and did not phosphorylate ceramide in the presence of either cation. In contrast, ceramide served as a competitive inhibitor with an inhibition constant (Ki) 2-6-fold greater than the Km for DG. Inhibition was noncompetitive with respect to ATP and Mg2+. A cell-permeable ceramide, N-octanoyl sphingosine (C8-cer), was used to study effects of ceramide on DG kinase in intact HL-60 cells. C8-cer induced dose- and time-dependent increases in cellular DG levels. As little as 1 microM C8-cer increased DG from a basal level of 103 to 177 pmol.10(6) cells-1, and a maximal 2.9-fold elevation to 292 pmol.10(6) cells-1 occurred with 10 microM C8-cer. DG elevation was detected after 1 min, maximal by 7.5 min, and sustained for 30 min. The DG elevation was accompanied by a reduction in 32P incorporation in phosphatidic acid in cells short term-labeled with [32P]orthophosphoric acid, consistent with inhibition of DG kinase. In contrast, a similar elevation in the DG level induced by exogenous phospholipase C increased 32P incorporation into phosphatidic acid. C8-cer was not metabolized to sphingomyelin, indicating that DG was not generated through the phosphatidylcholine:ceramide cholinephosphotransferase reaction. DG elevation after C8-cer or phospholipase C treatment was sufficient to redistribute protein kinase C from cytosol to membrane. These findings provide evidence that ceramide may serve as a competitive inhibitor of DG kinase.

Mitochondrial nucleotide translocase from skeletal muscle of halothane sensitive pigs: an electrophoretic study

ATP translocation into mitochondria isolated from halothane-sensitive pig (HP) muscle was dramatically reduced compared with normal pigs (NP). To determine if this was due to a decreased amount of ATP translocase in the mitochondrial membranes, or a structural modification of this protein, an electrophoretic study was undertaken. Total proteins and purified translocase preparations from (NP) and (HP) mitochondria were analyzed by SDS gel electrophoresis. In the two types of mitochondria no significant differences were observed either in the amount of ATP translocase or in the molecular weight. Also, neither nonequilibrium pH gradient gel electrophoresis nor the analysis of peptides produced by limited proteolysis revealed any structural difference between the two types of protein. On the basis of these results, the depressed translocase activity observed in (HP) mitochondria cannot be explained by a reduced amount of the nucleotide translocase, nor a structural alteration of this protein. Possible inhibition of (HP) translocase activity by Ca2+ accumulation or by other mechanisms is discussed.

Interferon alpha-2a and 5-fluorouracil for advanced colorectal carcinoma. Assessment of activity and toxicity

Preclinical data showed that the cytotoxic effects of 5-fluorouracil (5-FU) are augmented by interferon (IFN). In a small study, 13 of 17 patients with advanced colorectal cancer responded to a regimen of 5-FU with IFN. Using the same dose and schedule as in this pilot study, 38 previously untreated patients with metastatic colorectal carcinoma were treated with continuous intravenous (IV) infusion of 5-FU 750 mg/m2 daily for 5 days, followed by weekly bolus 5-FU at 750 mg/m2 and subcutaneous IFN at 9 million units three times per week. Of 35 evaluable patients, nine (26%) had a partial response (95% confidence limit, 11% to 41%), with a median response duration of 7.5 months (range, 4.4 to greater than 11.7 months). Seven patients (20%) had a minor response, and ten (28%) had stable disease. The most common toxicities observed were stomatitis (52%) and diarrhea (43%). Neurotoxicity was seen in 34% of patients and consisted of gait disturbance, dizziness, confusion, memory loss, and dementia. Because of toxicity, 84% of patients required a reduction of the IFN dose by at least 50%, and 63% required reduction of the 5-FU dose by at least 25%. Although the combination of 5-FU and IFN in patients with advanced colorectal carcinoma has some activity, the regimen was toxic, and the observed response rate (26%) was not substantially superior to alternative 5-FU programs.

Impairment of ATP-linked reactions in mitochondria isolated from skeletal muscle of halothane-sensitive pigs

Oxygen consumption was depressed in mitochondria isolated from halothane sensitive pig (HP) muscle. The calculation of the respiratory control ratio (RCR) indicated that mitochondria were more affected at the site-I level of the respiratory chain. Calcium accumulation in these mitochondria was not altered when driven by the oxidation of succinate. This process was abolished when linked to ATP as a source of energy. ATP transport was completely inhibited in (HP) mitochondria.

Binding of bepridil to isolated rat heart mitochondria

We investigated mass action of isolated rat heart mitochondria with the calcium antagonist bepridil. At pH 7.20 bepridil in basic form b associates rapidly with the mitochondrial membrane but the amount fixed is higher in non-energized mitochondria than in mitochondria energized by succinate or ATP Mg2+. This effect is related to the dissociation state of the drug since conditions favoring the acidic form bH+ suppress this difference. Tritiated bepridil bound to mitochondrial membrane is only partially displaced by high concentrations of unlabeled drug (greater than or equal to 510(-5) M). No membrane energization effect is noted on this displacement. Binding values of bepridil to mitochondrial membrane (KD 1.710(-5) M; Bmax 23.8 nmol.mg-1 protein) show only low affinity receptor sites. Bepridil binding to the lipid part of the inner membrane surface is postulated. This interaction is used to explain some of the in vitro effects of this calcium antagonist on membrane bound enzyme activities.

Effects of bepridil on calcium release from rat heart mitochondria

Bepridil at concentrations above 10 microM, and at pH 7.2 stimulates calcium release from rat heart mitochondria. However this action is different from that of ClCCP, an uncoupler of oxidative phosphorylations, since it is ruthenium red insensitive. At lower concentrations bepridil may inhibit the Na-induced calcium release. The effects of bepridil depend on the pH and indicate that the protonated form of the drug is more efficient on calcium release than the basic form.

Protective effects of bepridil on calcium injury of anoxic myocytes isolated from adult rat heart ventricular muscle

Cardiac myocytes allowed to recover after the isolation procedure in a well-oxygenated medium were resistant to an extracellular Ca++ concentration of 1.5 mM. At least 90% of the isolated myocytes maintained their initial rod-shaped form after a 1-hr incubation, as well as 85% of their ATP and a low cellular content of calcium. At concentrations less than 10(-5) M, bepridil had no effect on these cells. On the other hand, when the myocytes were incubated in anaerobic conditions, 1.5 mM Ca++ was deleterious: the number of viable cells decreased by about 50%, ATP levels were lowered by 70% and the total cell calcium content increased by more than 100%. Bepridil had a biphasic effect on anaerobic cells. At concentrations below 10(7) M, the drug had a beneficial action. It restored cell viability and cellular ATP to 77 and 100% when compared to their respective level in the absence of Ca++. However, at concentrations higher than 10(-6) M, bepridil decreased the number of viable cells as well as their ATP content. At optimal concentrations, bepridil lowered the cellular calcium to its level in aerobic conditions. Without calcium addition, anoxic cells accumulated Na+ and lost K+. Calcium addition decreased Na+ accumulation by 68% and prevented the loss of K+. The Na+ and K+ content of the cells were not affected by bepridil. However, it is noteworthy that, at 10(-4) M, bepridil decreased dramatically the potassium content of the cells. In aerobic conditions, the calcium resistance of isolated ventricular myocytes may still be enhanced by Fluozol, a fluorocarbon compound which increases oxygen solubility in water.

Redistribution of creatine kinase isoenzymes in chronically overloaded myocardium

Chronic overload due to an experimental abdominal aortic stenosis in rats causes hypertrophy of ventricles and a parallel increase of the MB and BB isoforms of creatine kinase (CK) in cardiac tissue. The CK isoenzyme profile was determined using a new two-step method combining anion exchange chromatography and electrophoresis. In overloaded ventricles a shift was observed from the MM isoform which decreased (from 407 +/- 10 mumol X min-1 X g-1 ww in sham-operated to 370 +/- 0.13, in the overloaded group, p less than 0.05) towards the MB and BB forms whose activity was enhanced (from 56 +/- 4 and 6.5 +/- 0.7 to 77 +/- 6 and 10.1 +/- 1.2, respectively, p less than 0.02). These modifications were more pronounced (318 +/- 15, 83 +/- 15 and 15.1 +/- 2.5, for the MM, MB and BB forms respectively, p less than 0.01) in rats having a very marked hypertrophy and whose ventricular/body weight ratio (expressed in mg of ventricles per g) was above 3. Moreover this ratio correlates both with the amount of the MB form (r = 0.32, p less than 0.05) and with the percentage of the B monomer (r = 0.51, p less than 0.01). This shift, like that previously described for lactate dehydrogenase and myosin, favoured the fetal form and this supports the hypothesis that overloaded myocytes improve their efficiency by expressing some of the isoforms normally present in the fetus.

Effects of bepridil on Ca2+ uptake by cardiac mitochondria

Isolated rat heart mitochondria accumulate large amounts of Ca2+ at the expense of respiration-linked energy or of that provided by the hydrolysis of ATP by the mitochondrial ATPase. At concentrations below 10 microM bepridil has no effect on the first mechanism but inhibits the second. At higher concentrations bepridil depresses both. At low concentrations bepridil decreases proton influx into mitochondria in ADP-stimulated respiration while it has no effect on proton ejection in Ca2+-stimulated respiration. A preliminary study shows that bepridil inhibits ATP hydrolysis linked to Ca2+ absorption by mitochondria. The calcium antagonists verapamil, nifedipine and diltiazem exhibit none of these effects.