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C W Taylor

Publications and source records attributed to C W Taylor.

At least 109 records · Page 6Linked to original sources
PubMed1993

Suppression of human lymphoma development in the severe combined immune-deficient mouse by imexon therapy.

Severe combined immune-deficient (SCID) mice accept human cell xenografts. SCID mice inoculated intraperitoneally with human peripheral blood mononuclear cells (PBMCs) from EBV-seropositive donors develop a form of disseminated human large cell lymphoma similar to that seen in transplant recipients and AIDS patients. Imexon (4-imino-1,4-diazobicyclo-3.1.0-hexan-2-one), a cyanoziridine immunomodulator with a selective inhibitory effect on B cells, was tested for its effect on this lymphoma development. Imexon started 2 weeks after PBMC inoculation significantly reduced the number of animals with lymphoma and the number of lymphomas sites per animal. The lymphoma was widely metastatic in the control animals but limited to the peritoneal cavity in the treated animals. Morphological and molecular parameters were used to confirm the reduced takes in the treated animals. Imexon was not myelosuppressive as determined by measurement of white blood cell counts. Imexon did not significantly suppress human IgG levels in the animals' serum. The tumor growth and lethality of human B lymphoblastoid cell lines in SCID mice was also suppressed by imexon treatment. A relatively nontoxic class of drugs that can suppress the development and/or growth of EBV-related lymphoma should be explored with priority for potential use in patients at high risk of this type of lymphoma.

Animals↗
PubMed1993

Evaluation of cisplatin intensity in metastatic non-small-cell lung cancer: a phase III study of the Southwest Oncology Group.

PURPOSE: To test the concept that cisplatin dose-intensity is important in the treatment of non-small-cell lung cancer (NSCLC), the Southwest Oncology Group (SWOG) performed a randomized trial comparing standard-dose cisplatin (SDCP) 50 mg/m2 days 1 and 8 on a 28-day cycle for eight cycles, high-dose cisplatin (HDCP) 100 mg/m2 days 1 and 8 for four cycles, and high-dose cisplatin plus mitomycin (HDCP-M) 8 mg/m2 day 1. To isolate the effects of dose-intensity versus total dose, the planned cumulative cisplatin dose was 800 mg/m2 in each arm. PATIENTS AND METHODS: Between July 1988 and April 1990, 356 patients were enrolled and 323 were eligible and assessable. All patients had metastatic, measurable disease, were chemotherapy-naive, and had a performance status (PS) of 0 to 2. RESULTS: Confirmed complete plus partial response rates were SDCP, 12%; HDCP, 14%; and HDCP-M, 27% (P < .05). Complete responses were uncommon (HDCP, 3%; HDCP-M, 4%) and were observed only in the high-dose arms. Progressive disease occurred more frequently in the SDCP arm (57%) compared with HDCP (38%) or HDCP-M (34%) (P < .05). However, there were no significant differences in median survival times (SDCP, 6.9 months; HDCP, 5.3 months; HDCP-M, 7.2 months; P = .53). The mean delivered dose-intensity for cisplatin was significantly greater in the high-dose arms: HDCP 41 mg/m2/wk and HDCP-M 39 mg/m2/wk, versus SDCP 23 mg/m2/wk (P = .05). The high-dose arms resulted in an increased incidence of ototoxicity, emesis, and myelosuppression, but similar degrees of renal toxicity and neuropathy compared with SDCP. CONCLUSION: This study does not confirm evidence of a steep clinical dose-response curve for cisplatin in NSCLC at the cisplatin dose-intensities achieved. The addition of mitomycin increases the response rate, but does not improve survival. Continued evaluation of new agents in this disease is warranted.

Adult↗
PubMed1993

Regulation of inositol 1,4,5-trisphosphate receptors.

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] is a soluble second messenger responsible for the generation of highly organized Ca2+ signals in a variety of cell types. These Ca2+ signals control many cellular responses, including cell growth, fertilization, smooth muscle contraction and secretion. Ins(1,4,5)P3 is produced at the plasma membrane following receptor activation, but rapidly diffuses into the cytosol, where it binds to specific receptors through which it mobilizes intracellular Ca2+ stores. The actions of Ins(1,4,5)P3 within cells are tightly controlled: enzymes control the rapid generation and metabolism of Ins(1,4,5)P3 following receptor activation; multiple Ins(1,4,5)P3 receptor subtypes and splice variants exist, some of which are differentially expressed between cell types and at different stages of development; and Ins(1,4,5)P3 receptors are the targets for a number of allosteric regulators, including protein kinases, ATP and divalent cations. Understanding how cells control the Ca(2+)-mobilizing activity of Ins(1,4,5)P3 will be important if we are to unravel the mechanisms that underlie the complex arrangements of Ca2+ signals.

Animals↗
PubMed1992

The thiol reagent, thimerosal, evokes Ca2+ spikes in HeLa cells by sensitizing the inositol 1,4,5-trisphosphate receptor.

The thiol reagent, thimerosal, has been shown to cause an increase in intracellular Ca2+ concentration ([Ca2+]i) in several cell types, and to cause Ca2+ spikes in unfertilized hamster eggs. Using single cell video-imaging we have shown that thimerosal evokes repetitive Ca2+ spikes in intact Fura-2-loaded HeLa cells that were similar in shape to those stimulated by histamine. Both thimerosal- and histamine-stimulated Ca2+ spikes occurred in the absence of extracellular (Ca2+ o), suggesting that they result from mobilization of Ca2+ from intracellular stores. Whereas histamine stimulated formation of inositol phosphates, thimerosal, at concentrations that caused sustained Ca2+ spiking, inhibited basal and histamine-stimulated formation of inositol phosphates. Thimerosal-evoked Ca2+ spikes are therefore not due to the stimulated production of inositol 1,4,5-trisphosphate (InsP3). The effects of thimerosal on Ca2+ spiking were probably due to alkylation of thiol groups on intracellular proteins because the spiking was reversed by the thiol-reducing compound dithiothreitol, and the latency between addition of thimerosal and a rise in [Ca2+]i was greatly shortened in cells where the intracellular reduced glutathione concentration had been decreased by preincubation with DL-buthionine (S,R)-sulfoximine. In permeabilized cells, thimerosal caused a concentration-dependent inhibition of Ca2+ accumulation, which was entirely due to inhibition of Ca2+ uptake into stores because thimerosal did not affect unidirectional 45Ca2+ efflux from stores preloaded with 45Ca2+. Thimerosal also caused a concentration-dependent sensitization of InsP3-induced Ca2+ mobilization: half-maximal mobilization of Ca2+ stores occurred with 161 +/- 20 nM InsP3 in control cells and with 62 +/- 5 nM InsP3 after treatment with 10 microM thimerosal. We conclude that thimerosal can mimic the effects of histamine on intracellular Ca2+ spiking without stimulating the formation of InsP3 and, in light of our results with permeabilized cells, suggest that thimerosal stimulates spiking by sensitizing cells to basal InsP3 levels.

Calcium↗
PubMed1992

Antitumor activity and clinical pharmacology of sulofenur in ovarian cancer.

BACKGROUND: Sulofenur is a diarylsulfonylurea with demonstrated antitumor activity in patients with advanced epithelial ovarian cancer refractory to standard chemotherapy. The dose-limiting toxic effects observed in phase I clinical trials have been anemia and methemoglobinemia, resulting in cyanosis. PURPOSE: The purposes of this study were to further define the response rate, toxic effects, and pharmacokinetics and pharmacodynamics of sulofenur in patients with advanced ovarian cancer. METHODS: We conducted a phase II trial of sulofenur at a dose of 800 mg/m2 per day in 35 patients with stage III or IV ovarian cancer refractory to standard chemotherapy. Pharmacokinetics and pharmacodynamics were analyzed by comparing sulofenur parent and metabolite plasma levels with methemoglobin levels. RESULTS: Partial responses lasting 6.5-18 weeks occurred in four (15%; 95% confidence interval = 4%-35%) of the 26 patients assessable for response. In addition, 42% (11) of the assessable patients had prolonged stable disease (median, 20 weeks). The first nine patients received sulofenur as a daily oral dose for 14 days, with a 21-day treatment cycle. However, they developed substantial anemia and methemoglobinemia. As a result, the next 26 patients received sulofenur daily for 5 days followed by 2 days of rest for 3 consecutive weeks, with a 28-day treatment cycle (5/2-day schedule). Preclinical models predicted that 2 days of rest would decrease toxicity while maintaining antitumor activity. Patients treated with the 5/2-day schedule had relatively less severe anemia and methemoglobinemia and needed fewer red blood cell transfusions (31% versus 78% of patients), but 31% still required dose reductions because of these toxic effects. The hydroxy and keto metabolites of sulofenur had prolonged plasma half-lives relative to the parent compound, and the difference was statistically significant. In addition, the correlations of metabolite concentrations with methemoglobin levels were higher than the correlation of sulofenur concentrations with methemoglobin levels, and those differences were statistically significant. CONCLUSION: We conclude that sulofenur has modest clinical activity in heavily pretreated patients with ovarian cancer. IMPLICATIONS: The toxic effects of anemia and methemoglobinemia may limit the ultimate clinical utility of diarylsulfonylureas until less toxic derivatives with alternate metabolic pathways can be identified.

Adult↗
PubMed1992

Long-term engraftment of fresh human myeloma cells in SCID mice.

Using highly purified myeloma cells from patient bone marrow, we established human-murine myeloma chimeras in severe combined immunodeficiency (SCID) mice and documented secretion of monoclonal human immunoglobulins (Hulgs) in the mice for up to 299 days. Monoclonality of circulating Hulgs was found only when highly purified myeloma cells were injected intraperitoneally. In contrast, injection of unfractionated myeloma marrow led to the development of polyclonal Hulgs in the SCID mice. The criteria for myeloma engraftment included prolonged presence of monoclonal Hulgs in the sera of SCID mice and/or detection of human myeloma cells in their tissues by immunohistochemical examination. Ninety-one percent (10/11) of the fresh purified myeloma specimens engrafted in the SCID mice. Fifty-five percent (6/11) of the patient samples resulted in human B-cell grafts, and 45% (5/11) were identifiable as human myeloma chimeras. Pathologic studies showed that most human plasmacytes were located in the peritoneal cavity but metastatic infiltrates were also found in other organs in 69% of the SCID-human myeloma chimeras. This chimeric model should provide a useful tool for characterization of growth modulation and microenvironmental interactions as well as a means of testing new therapeutic approaches to multiple myeloma.

Adult↗
PubMed1992

Antiproliferative and antitumor activity of the 2-cyanoaziridine compound imexon on tumor cell lines and fresh tumor cells in vitro.

BACKGROUND: Imexon, a 2-cyanoaziridine, is therapeutic and reverses lymphadenopathy and splenomegaly in the LP-BM5 murine retrovirus-induced immunodeficiency disease (murine AIDS). It can restore chemotherapy-induced immunosuppression. Imexon reduced the incidence of lymphoma in severe combined immune deficient mice inoculated with human lymphocytes. PURPOSE: To determine its antitumor activity, we screened imexon against fresh human tumor cells and tumor cell lines. To determine the time-concentration relationships of its cytotoxicity, we studied the effects of imexon on macromolecular synthesis and on the cell cycle. METHODS: Imexon was incubated at 1-200 micrograms/mL with various tumor cell lines, mitogen-stimulated peripheral blood lymphocytes, and fresh tumor cells. Cell survival, macromolecular synthesis, and cell cycle progression were studied. RESULTS: The concentration of imexon that caused 50% inhibition of growth was under 10 micrograms/mL for lymphocytes stimulated with mitogens. It was about 3-10 micrograms/mL for B-cell lymphomas and both multi-drug-resistant and -sensitive myeloma cell lines. Imexon inhibited four of seven fresh lymphoma and 11 of 16 fresh myeloma biopsy specimens to less than 40% of the control. A 1-hour exposure of lymphoma cells to 50-100 microgram/mL followed by removal of drug by washing the cells and continuing culture resulted in greater than 95% inhibition during the next 48-72 hours. Imexon selectively inhibited protein synthesis during the first 24-48 hours of exposure of lymphoma and myeloma cells. Cells exposed to inhibitory concentrations of imexon were blocked in cell cycle progression. CONCLUSION: Imexon may be a potentially useful agent in the treatment of malignant disease, particularly lymphoid malignancies, and should be explored further.

Antineoplastic Agents↗
PubMed1992

Prolonged exposure to inositol 1,4,5-trisphosphate does not cause intrinsic desensitization of the intracellular Ca(2+)-mobilizing receptor.

The rapid release of Ca2+ from intracellular stores stimulated with inositol 1,4,5-trisphosphate (InsP3) has required superfusion or stopped-flow techniques to resolve the kinetics of Ca2+ mobilization and made it difficult to determine whether the InsP3 receptor desensitizes during prolonged stimulation. Here we have overloaded the intracellular Ca2+ stores of permeabilized rat hepatocytes by incubating them with ATP and 45Ca2+ in the presence of pyrophosphate, which precipitates Ca2+ within the lumen of the stores. Subsequent ATP removal initiated slow 45Ca2+ efflux that followed zero-order kinetics, allowing us to examine the effects of InsP3 over a prolonged time course. InsP3 produced a concentration-dependent increase in the 45Ca2+ efflux rate that was sustained for several min. The rate rapidly returned to the unstimulated level after the addition of decavanadate, a competitive antagonist of InsP3 at its receptor. Prior incubation with a submaximal concentration of InsP3 (1 microM) did not affect the subsequent enhanced rate of 45Ca2+ efflux stimulated by a higher, but still submaximal, concentration of InsP3 (3 microM). We conclude that prolonged exposure to InsP3 does not desensitize the InsP3 receptor and that intrinsic receptor desensitization cannot provide an explanation for the quantal responses to InsP3 observed in several cell types.

Animals↗
PubMed1992

Effects of suramin on in vitro growth of fresh human tumors.

BACKGROUND: Suramin is a polysulfonated urea recently tested in clinical trials as an anticancer agent. PURPOSE: To define tumor types for further clinical testing of suramin, we assessed the in vitro activity of suramin against fresh human tumor specimens. METHODS: Inhibition of tumor colony formation (human tumor clonogenic assay [HTCA] method) and inhibition of tritiated thymidine incorporation (TTI method) were used as indicators of drug sensitivity. RESULTS: With the use of the HTCA method, 80% or more of carcinomas of the colon, endometrium, kidney, lung (non-small-cell), and ovary as well as malignant melanoma and mesothelioma were sensitive to 200 micrograms/mL of suramin by continuous exposure. Suramin's antitumor activity was dose dependent, and it was less effective when tested at concentrations of 50 micrograms/mL or less. With the TTI method, the more slowly growing tumors (breast cancer, colon cancer, multiple myeloma, non-Hodgkin's lymphoma, prostate cancer, and sarcoma) appeared to be less sensitive to suramin. However, when the two assay methods were directly compared in melanoma and ovarian cancer specimens, individual tumors were generally more sensitive to suramin (greater inhibition relative to control) using the HTCA method, whereas the TTI method appeared to underestimate suramin's antitumor activity. There was no significant difference in the activity of suramin when tested in the presence of 10% versus 50% serum. CONCLUSION: These results provide an experimental basis for clinical evaluations of suramin therapy in patients with colon, endometrial, kidney, non-small-cell lung, and ovarian cancers as well as malignant melanoma and mesothelioma.

Humans↗
PubMed1992

Sensitivity of nuclear c-myc levels and induction to differentiation-inducing agents in human colon tumor cell lines.

Six human colon tumor cell lines were analyzed for their constitutive levels of the c-myc protein. The nuclear proto-oncogene, c-myc, was detected as an expressed product in all of the human colon tumor cell lines analyzed. The poorly differentiated cell lines HCT116, RKO and C showed c-myc levels that averaged 2-fold greater than their well-differentiated counterparts, i.e., GEO, CBS and FET. When c-myc levels and responses to serum induction were analyzed in the presence of inducers of differentiation, i.e., dimethylformamide, retinoic acid, sodium butyrate and TGF-beta, distinct patterns of sensitivity and resistance emerged. Nuclear c-myc levels were reduced in all the colon cell phenotypes treated with dimethylformamide or sodium butyrate. Only the well-differentiated human colon tumor cell lines were responsive to transforming growth factor-beta. Only one of the human colon tumor cell lines (GEO) responded to retinoic acid. Increased levels of c-myc protein were found to correlate well with greater growth rates and with poor differentiation class. Similarly, a parallel sensitivity to down-regulation of c-myc levels and attenuation of c-myc induction curves for inducers of differentiation were observed in growth sensitive human colon tumor cell lines.

Butyrates↗
PubMed1992

Calcium and inositol 1,4,5-trisphosphate receptors: a complex relationship.

Increases in intracellular free Ca2+ concentration ([Ca2+]i), whether initiated by changes in plasma membrane potential or receptor-stimulated polyphosphoinositide hydrolysis, can be astonishingly complex, often occurring as repetitive Ca2+ spikes and regenerative Ca2+ waves that propagate through the cell and sometimes into neighbouring cells. The key to understanding these complex Ca2+ signals lies in understanding the interactions between the different pools from which Ca2+ can rapidly enter the cytosol and the activities of the various Ca(2+)-transporting systems that reverse the process.

Animals↗
PubMed1992

Inhibition of colon tumor cell growth by 8-chloro-cAMP is dependent upon its conversion to 8-chloro-adenosine.

Recent interest in site-selective cAMP analogs has focused on the role of 8-chloro-adenosine (8-Cl-adenosine) in the inhibition of tumor cell growth by 8-chloro-cAMP (8-Cl-cAMP) (Van Lookeren Campagne, et al. Cancer Res 1991; 51: 1600-5). We have evaluated 8-Cl-cAMP and 8-Cl-adenosine for their growth inhibitory activity against two human colon adenocarcinoma cell lines, HCT116 and FET. Because these cell lines have been adapted to grow in chemically defined medium we were able to evaluate the effect of serum on 8-Cl-cAMP's growth inhibitory activity. In addition, cells grown in serum-free medium were tested for their sensitivity to 8-Cl-cAMP, serum-activated 8-Cl-cAMP and 8-Cl-adenosine. IC50 values, determined by measuring cell growth using a MTT colorimetric assay, showed that 'serum activation' of 8-Cl-cAMP was required to achieve inhibition of HCT116 (IC50 = 1.3 +/- 0.1 microM) and FET (IC50 = 2.0 +/- 0.1 microM) cell growth. IC50 values were not reached at the highest concentrations tested (IC50 > 500 microM) in the absence of serum, permitting us to conclude that 8-Cl-cAMP does not have growth inhibitory activity between 1.0 and 500 microM doses. HCT116 and FET cells grown in media containing serum and in the presence of 8-Cl-adenosine had IC50 values of 0.6 +/- 0.1 and 0.9 +/- 0.2 microM, respectively. HCT116 and FET cells grown in chemically defined medium containing 8-Cl-adenosine exhibited IC50 values of 1.0 +/- 0.1 and 3.1 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Chloroadenosine↗
PubMed1992

A Southwest Oncology Group Phase I study of the sequential combination of recombinant interferon-gamma and recombinant interleukin-2 in patients with cancer.

Thirty-seven patients with advanced malignancies were treated sequentially with recombinant interferon-gamma (rIFN-gamma) and recombinant interleukin-2 (rIL-2) in an outpatient dose escalation clinical trial. rIFN-gamma (0.1 or 0.25 mg/m2/day) was administered by intramuscular injection, days 1-7 and rIL-2 (12, 18, or 24 x 10(6) IU/m2/day) was administered by a 15-min intravenous bolus, days 8-12. Common toxicities encountered included fever, chills, fatigue, neutropenia, and elevations of SGOT, bilirubin, or creatinine. Hypotension and cardiac and pulmonary toxicities were rare. With repeated cycles of therapy, nausea/vomiting and diarrhea associated with the administration of rIL-2 were seen in greater frequency. There were no treatment-related deaths, and no patient required intensive care unit admission for toxicity management. A complete response was observed in one of 11 patients with renal cancer and a partial response was observed in one of seven patients with malignant melanoma. Due to problems with drug supply, further dose escalation could not be continued, and maximum tolerated doses (MTD) were not determined by strict criteria. However, the combination of rIFN-gamma, 0.25 mg/m2/day, and rIL-2, 24 x 10(6) IU/m2/day, appeared to be beyond the MTD, as three of six patients at this dose level could not complete one cycle of therapy due to toxicity. It is unlikely that higher doses of either agent would be tolerated, and for further study using this schedule, we recommend the doses: rIFN-gamma, 0.1 mg/m2/day, and rIL-2, 24 x 10(6) IU/m2/day.

Adult↗
PubMed1992

Phase I study of low-dose cyclophosphamide and recombinant interleukin-2 for the treatment of advanced cancer.

We conducted a phase I study of low-dose cyclophosphamide and recombinant interleukin-2 (rIL-2) in 66 patients with advanced cancer resistant to standard therapy. All patients were evaluable for toxicity and 46 patients were evaluable for antitumor response. Patients evaluable for antitumor response included 23 with malignant melanoma, 10 with renal cell carcinoma, 4 with colon cancer, and 9 with various other solid tumors. All patients received i.v. cyclophosphamide (350 mg/m2) on day 1 followed by rIL-2 via 15 min i.v. infusion on days 4-8 and 11-15. The doses of rIL-2 ranged from 6.0 to 36.0 x 10(6) IU/m2. Each treatment cycle consisted of 21 days and a total of 113 cycles was administered. The number of treatment cycles administered per patient ranged from 1 to 8. The dose-limiting toxicities associated with rIL-2 included altered mental status, arthralgias, diarrhea, fatigue, fever, hypotension, nausea/vomiting, and peripheral edema. Twelve patients (18%) were removed from the study secondary to toxicity. Among the evaluable patients, 2 (4%) (malignant melanoma, renal cell carcinoma) developed a partial remission, 13 (29%) maintained stable disease, and 31 (67%) developed progressive disease. We conclude that the combination of low-dose cyclophosphamide and rIL-2 is tolerable in most patients but our data do not suggest an improved response rate for the combination vs. rIL-2 alone.

Adolescent↗
PubMed1992

Alteration of human tumor cell adhesion by high-strength static magnetic fields.

RATIONALE AND OBJECTIVES: The authors investigated whether the ability of human malignant melanoma cells to increase in cell number or their ability to remain viable was compromised by high-field strength static magnetic fields. Normal human fibroblasts also were studied to determine if any magnetic field-related alterations were unique to tumor populations. METHODS: Human cell lines were grown in monolayer culture in vitro and subjected to a static magnetic field using a 4.7-Tesla superconducting magnetic resonance imaging (MRI) magnet with the gradient coils removed. The number of cells within the total population was determined using an electronic particle counter. Cell viability was estimated by trypan blue exclusion, and the cellular morphology of the attached cells was documented using microscopy. RESULTS: Both the human malignant melanoma cells and the normal human cells were unaffected by the presence of a high-strength magnetic field in terms of increasing their cell numbers or their viability. However, the ability of the malignant melanoma cells to remain attached to the tissue culture surface was impaired. Normal fibroblasts were not affected in this regard. CONCLUSION: High-strength static magnetic fields alter the ability of human malignant melanoma cells to remain adherent to the tissue culture surface, but have no effect on normal human fibroblasts. This may affect the ability of tumor cells to successfully interact with their environment.

Cell Adhesion↗
PubMed1992

All-or-nothing Ca2+ mobilization from the intracellular stores of single histamine-stimulated HeLa cells.

1. Histamine-stimulated mobilization of intracellular Ca2+ stores was monitored in intact and permeabilized populations of HeLa cells using both the fluorescent Ca2+ indicator Fura-2 and 45Ca2+ measurements. Digital video imaging of Fura-2-loaded cells was used to measure the intracellular calcium concentration ([Ca2+]i) of single cells. 2. In populations of HeLa cells, histamine caused a concentration-dependent increase in cytoplasmic [Ca2+]. The initial transient increase was independent of extracellular Ca2+ (Ca2+o) and was followed by a sustained increase that was abolished by removal of Ca2+o. 3. In Ca(2+)-free medium ([Ca2+]o < 1 microM), a maximal histamine concentration (25 microM) caused a transient increase in [Ca2+]i, and a subsequent challenge with histamine failed to evoke a further response indicating that the inositol 1,4,5-trisphosphate (InsP3)-sensitive Ca2+ stores had been completely emptied. Lower concentrations of histamine (0.5-10 microM) caused smaller, concentration-dependent increases in [Ca2+]i that were also transient. After exposure to these low histamine concentrations, where [Ca2+]i returned to baseline within 2 min, addition of a higher histamine concentration evoked a further increase in [Ca2+]i. The second increase in [Ca2+]i was inversely proportional to the increase caused by the first exposure to histamine, indicating that Ca2+ released in the initial response was not substantially resequestered into histamine-sensitive stores. 4. Single HeLa cells challenged with low concentrations of histamine in Ca(2+)-free medium responded with transient increases in [Ca2+]i, but individual cells differed in their sensitivity with 51% of cells responding to 1 microM, and 98% responding to 25 microM-histamine. 5. When single cells in Ca(2+)-free medium were challenged with stepwise increases in histamine concentration, they responded to each step with a transient [Ca2+]i increase after which [Ca2+]i returned to baseline within 1 min. Prolonging the interval between histamine additions by up to 25 min did not affect the [Ca2+]i increase evoked by a subsequent histamine addition. 6. Unidirectional 45Ca2+ efflux from saponin-permeabilized HeLa cells showed that, under conditions that prevented Ca2+ resequestration, submaximal concentrations of InsP3 rapidly emptied only a fraction of the InsP3-sensitive Ca2+ stores. The failure of low InsP3 concentrations to fully mobilize the InsP3-sensitive Ca2+ stores was not a consequence of InsP3 degradation. 7. We conclude that within single HeLa cells, intracellular Ca2+ stores are heterogeneous in their sensitivity to InsP3, and the fraction of Ca2+ stores mobilized by InsP3 increases as the InsP3 concentration increases.

Biological Transport↗