Basic fibroblast growth factor production and growth factor receptors as potential targets for melanoma therapy

A number of growth factors can stimulate the proliferation of human malignant melanoma cell lines. We investigated the effects of exogenous growth factors including basic fibroblast growth factor (bFGF), epidermal growth factor, transforming growth factor-alpha, insulin, insulin-like growth factor-I (IGF-I), acidic fibroblast growth factor, platelet-derived growth factor, transforming growth factor-beta and nerve growth factor on six human metastatic melanoma cell lines. The mitogenic activity of each growth factor was tested using the [3H]thymidine incorporation assay. There was a variable response of the different cell lines to most growth factors. All of the melanoma cell lines tested responded to IGF-I. Furthermore, the effects of growth factors were additive, a combination of bFGF and IGF-I having the greatest effect on three melanoma cell lines tested. The quantitative radioimmunoassay for bFGF and [125I]bFGF binding assay revealed that all of these melanoma cell lines produced bFGF and expressed high affinity receptors for bFGF. A 20-mer antisense oligodeoxynucleotide against the AUG initiation site of the bFGF coding region inhibited the proliferation of Mel-Tang by 40% (p less than 0.0001) and that of SK-MEL-5 by 20% (p less than 0.005), suggesting that these cell lines are at least under partial autocrine control of proliferation by bFGF. The presence of bFGF receptors on a high percentage of melanoma cell lines makes these a potential target for melanoma therapy.

Effects of analogs of 1,25(OH)2 Vitamin D3 on the proliferation and differentiation of the human chronic myelogenous leukemia cell line, RWLeu-4

We evaluated the proliferative and differentiative effects of analogs of 1,25(OH)2 vitamin D3 [1,25(OH)2D3] on a chronic myelogenous leukemia cell line, RWLeu-4, which is growth-inhibited and differentiates in response to 1,25(OH)2D3 (ED50 = 3-10 nM). Side-chain-fluorinated analogs were more potent (ED50 = 0.7-2 nM) while most of those with altered saturation of the D ring or side-chain carbon-carbon bonds were equally or less effective than 1,25(OH)2D3. However, the two analogs with either two additional double bonds or an extra double and triple bond in the D ring had greater antiproliferative activity [1,25(OH)2-16,23-diene D3 (ED50 = 2.7 nM) and 1,25(OH)2-16-ene-23-yne D3 (ED50 = 0.7 nM)]. Since the latter of these has been reported to be less potent at mobilizing calcium than 1,25(OH)2D3, it (or a similar compound) may be a candidate for clinical use as an antineoplastic agent.

Antitumor activity of basic fibroblast growth factor-saporin mitotoxin in vitro and in vivo

Many cancer cell lines express basic fibroblast growth factor (FGF) receptors, making them potential targets for the delivery of FGF-based cytotoxic compounds. To this end, we have investigated the antitumor activity of a novel mitotoxin, Fibroblast Growth Factor-saporin (FGF-SAP), a conjugate of FGF and the ribosome-inactivating protein, saporin. In vitro, FGF-SAP is cytotoxic for human melanoma, teratocarcinoma, and neuroblastoma cells expressing FGF-receptors. Mice treated with FGF-SAP i.v., on a variety of schedules, showed dramatic tumor growth inhibition with minimal toxicity. Thus, FGF-SAP appears to be a well-tolerated and potent antitumor agent. The potential of FGF-targeted cytotoxicity is discussed.

A model of beta-adrenergic effects on calcium and potassium current in bullfrog atrial myocytes

A model of beta-adrenergic and muscarinic cholinergic effects on the bullfrog atrial myocyte has been developed to simulate the dose-dependent effects of isoprenaline (Iso) on the action potential duration (APD); i.e., low doses of Iso lengthen the APD, whereas high doses shorten the APD. In this model, the reduction in APD is the result of 1) calcium-dependent inactivation of calcium current (ICa) resulting from the enhancement of ICa by Iso and 2) an enhancement of potassium current (IK) due to both an Iso-induced increase in the rate of activation of IK and an increase in peak action potential height. The effect of acetylcholine (ACh) is simulated by a reduction in the Iso-induced increase in ICa and IK through a reduction in relative adenosine 3',5'-cyclic monophosphate concentration ([cAMP]), as well as activation of the ACh-sensitive potassium current. At low [Iso] levels in the presence of a high [ACh], the muscarinic cholinergic effects dominate the beta-adrenergic change. However, for a large [Iso] and a small [ACh], this pattern of changes in transmembrane currents is different; in this case the model predicts that ACh can actually increase APD.

Simulation of the bursting activity of neuron R15 in Aplysia: role of ionic currents, calcium balance, and modulatory transmitters

1. An equivalent circuit model of the R15 bursting neuron in Aplysia has been combined with a fluid compartment model, resulting in a model that incorporates descriptions of most of the membrane ion channels that are known to exist in the somata of R15, as well as providing a Ca2+ balance on the cell. 2. A voltage-activated, calcium-inactivated Ca2+ current (denoted the slow inward current ISI) was sufficient to produce bursting activity without invoking any other calcium-dependent currents (such as a nonspecific cation current, INS, or a calcium-activated K+ current, IK,Ca). Furthermore, many characteristics of a typical R15 burst could be simulated, such as a parabolic variation in interspike interval, the depolarizing afterpotential (DAP), and the progressive decrease in the undershoots of spikes during a burst. 3. The dynamic activity of R15 was analyzed by separately characterizing two different temporal domains; the fast dynamics associated with action potentials and the slow dynamics associated with low-amplitude oscillations lasting tens of seconds ("slow waves"). The slow dynamics were isolated by setting the Na+ conductance (gNa) to zero and then studied by the use of a system of equations reduced to two variables: intracellular concentration of Ca2+ and membrane potential. The fixed point of the system was located at the intersection of the nullclines for these two variables. A stability analysis of the fixed point was then used to determine whether a given set of parameters would produce slow-wave activity. 4. If the reduced model predicted slow-wave oscillations for a given set of parameters with gNa set to zero, then bursting activity was observed for the same set of parameters in the full model with gNa reset to its control value. However, for certain sets of parameters with gNa at its usual value, the full model exhibited bursting activity because of a slow oscillation produced by the activation of INS by action potentials. This oscillation resulted from an interaction between the fast and slow dynamics that the reduced model alone could not predict and was not observed when gNa was subsequently set to zero. If gNS was also set to zero, this discrepancy disappeared.(ABSTRACT TRUNCATED AT 400 WORDS)

Prolonged, continuous treatment of hairy cell leukemia patients with recombinant interferon-alpha 2a

Interferons are not curative in hairy cell leukemia (HCL), and retreatment is necessary in most patients whose therapy is stopped. In an attempt to maintain or improve responses, we administered recombinant interferon-alpha 2a (rIFN-alpha 2a) continuously to patients with HCL who initially responded to this therapy. Of 53 evaluable patients enrolled in this study, 32 have received rIFN-alpha 2a continuously for a median of 5 years. Patients received 3 million units of rIFN-alpha 2a subcutaneously (SC) daily for 6 months, followed, in responding patients, by the same dose three times weekly. Twenty-one patients (40%) discontinued IFN after a median of 29 months, seven of whom developed resistant disease in association with anti-IFN antibodies. Treatment produced high response rates: complete response plus partial response (CR + PR) = 40 of 53 (76%), CR + PR + minor response (MR) = 43 of 53 (82%), with no differences in response rates between patients with and without splenectomy. Sixteen patients who had MR at 18 months had PR with prolonged treatment, nine of whom had a significant further reduction in the hairy cell infiltrate in the bone marrow (BM). The median granulocyte and platelet counts have continued to increase and the median serum soluble interleukin-2 receptor (sIL-2R) level has continued to decrease with prolonged treatment. Two patients developed erythrocytosis that may be treatment related, but no other new toxicities were noted with prolonged treatment. We conclude that prolonged, continuous rIFN-alpha 2a treatment has acceptable toxicity, is not associated with late development of IFN resistance, and results in continued hematologic improvement with time on treatment.

The in vivo immunomodulatory effects of recombinant interferon gamma plus recombinant tumor necrosis factor-alfa

We conducted a phase I study in which an intramuscular injection of interferon gamma (IFN gamma) at 10, 50, or 100 micrograms/m2 was followed 5 minutes later by an intramuscular injection of 10, 50, or 100 micrograms/m2 of tumor necrosis factor-alfa (TNF alpha) at another site every other day for 20 days (10 doses). The addition of TNF alpha to IFN gamma reduced both the magnitude and duration of IFN gamma-mediated effects on peripheral blood monocyte expression of Fc receptors (FcRs) and HLA-DR and production of hydrogen peroxide. This inhibition was related to the dose of TNF alpha. On the other hand, TNF alpha and IFN gamma appeared to have an additive stimulatory effect on the production of neopterin by monocytes. The highest serum levels of neopterin were detected in patients who received the highest doses of both IFN gamma and TNF alpha. Thus, conflicting conclusions regarding the effect of the combination on immune activation are possible. If the activation of peripheral blood monocytes is the appropriate surrogate measure of the immune enhancement of the combination, then the simultaneous administration of IFN gamma and TNF alpha is ineffective, and future attempts to exploit the potential additive or synergistic effects of this combination of cytokines in humans may need to explore sequential administration schemata. On the other hand, if serum neopterin levels are a more reliable index of immune activation, simultaneous administration of 100 micrograms/m2 IFN gamma and 50 micrograms/m2 TNF alpha every other day (the maximally tolerated dose [MTD]) should be used in phase II testing. This dilemma points out the limitations of currently available methods of human immune assessment and the inadequacies in our capacity to gauge what particular immune measure or set of measures predict for in vivo antitumor effects.

Phase I evaluation of recombinant tumor necrosis factor given in combination with recombinant interferon-gamma

In light of in vitro and preclinical animal model data suggesting potential additive or synergistic antitumor effects from the combined use of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha), we conducted a phase I study employing escalating doses of each agent in 36 patients with solid tumors to determine the maximum tolerated dose (MTD). Patients were given an intramuscular (i.m.) injection of IFN-gamma, followed 5 min later by an i.m. injection of TNF-alpha, each agent in different sites, every other day for ten doses over 20 days. Patients received 10, 50, or 100 micrograms/m2 of each agent throughout the treatment course. No dose modifications were made. Patients suffering serious toxicity had therapy stopped and were considered to be off-study. All patients experienced fatigue, and 36% spent over half their time in bed on treatment days. Fever and chills were nearly universal. Mild to moderate elevations in serum transaminase levels were noted in 44% of patients, and 44% developed transient microscopic hematuria. Although 81% of patients had anorexia, only 17% of patients lost more than 3 kg of body wt during the 3 weeks of therapy. Because two of three patients receiving 100 micrograms/m2 of both agents developed serious toxicity (one fever greater than 105 degrees F, one thrombocytopenia 43,000/mm3), the MTD was established to be 100 micrograms/m2 of IFN-gamma plus 50 micrograms/m2 of TNF-alpha. The use of aspirin did not significantly alter the toxic effects of the agents. One patient with melanoma had a mixed response and one patient with mesothelioma transiently cleared his ascites of malignant cells.

Conduction in bullfrog atrial strands: simulations of the role of disc and extracellular resistance

A number of fundamental properties of intercellular conduction in simulated cylindrical strands of cardiac tissue are examined. The paper is based on recent biophysical information describing the transmembrane ionic currents in bullfrog atrial cells as well as anatomical data on the structures (gap junctions) responsible for the coupling between cells in that tissue. A mathematical model of the single bullfrog atrial cell based on suction microelectrode single-cell voltage clamp data is employed, as well as a modified version of the well-known model of Heppner and Plonsey, to characterized the resistive connections between adjacent cells in a cardiac strand. In addition, the simulated cellular strand is assumed to be encased in a cylindrical, resistive endothelial sheath, thus forming an idealized atrial trabeculum; the trabeculum is immersed in an extensive volume conductor. It is possible to simulate both uniform and discontinuous conduction in this atrial strand model by appropriately changing the resistance of the intercalated discs that occur at cell boundaries. The conduction velocity achieved in the normal or control case is within the range of conduction velocities that have been measured for bullfrog atrial trabeculae using optical methods. Extracellular resistance is shown to have a significant effect on both conduction velocity and the critical value of disc resistance at which discontinuous conduction first occurs. Since the atrial cell model employed in this study is based on experimental data and can accurately simulate the atrial action potential, the transmembrane ionic currents generated by the model are capable of providing detailed information concerning the mechanisms of intercellular current spread, particularly in the region of the intercalated disc.

A model of slow conduction in bullfrog atrial trabeculae

The success or failure of the propagation of electrical activity in cardiac tissue is dependent on both cellular membrane characteristics and intercellular coupling properties. This paper considers a linear arrangement of individual bullfrog atrial cells that are resistively coupled end to end to form a cylindrical strand. The strand, in turn, is encased by an endothelial sheath that provides a restricted extracellular space and an ion diffusion barrier to the outer bathing medium. This encased strand serves as an idealized model of an atrial trabeculum. Excitable membrane characteristics of the atrial cell are specified in terms of a Hodgkin-Huxley type of model that is quantitatively based on single-microelectrode voltage clamp data from bullfrog atrial myocytes. This membrane model can simulate the behavior of normal cells as well as of ischemic cells that exhibit depressed electrophysiological behavior (e.g., decreased resting potential, upstroke velocity, peak height, and action potential duration). Depressed activity can be easily simulated with variation of a single model parameter, the gain of the Na+/K+ pump current (INaK). Intercellular coupling properties are specified in terms of a lumped resistive T-type network between adjacent cells. The atrial strand model provides a means for studying the theoretical aspects of slow conduction in a "hybrid" strand that consists of a central region of cells having abnormal membrane or coupling properties, flanked on either side by normal atrial cells. Both uniform and discontinuous conduction are simulated by means of appropriate changes in the coupling resistance between cells. In addition, by varying either the degree of depressed electrical activity or the intercalated disc resistance in the central zone of the strand, slow conduction or complete conduction block in that region is demonstrated. Since the cellular model used in this study is based on experimental data and closely mimics both the atrial action potential and the underlying membrane currents, it has the potential to (1) accurately represent the current and voltage wave-forms occurring in the region of intercalated discs and (2) provide detailed information regarding the mechanisms in intercellular current spread in the region of slow conduction.

A model of the phase-sensitivity of the pacemaking cell in the bullfrog heart

In this study, mathematical models of the bullfrog sinus venosus (SV) pacemaker cell (Rasmusson et al., 1990, Am. J. Physiol. 259, H352-H369) and the ACh-sensitive K+ channel (Shumaker et al., 1990, Biophys. J. 57, 567-576) are combined to simulate the response of the SV myocyte to brief hyperpolarizing currents or acetylcholine (ACh) pulses. These simulations provide an ionic basis for the interpretation of the response of this pacemaker cell to either single perturbation or periodic stimuli. The model predicts that the effects of ACh stimulation on the pacemaker cycle length are dependent both on the phase and temporal characteristics of the [ACh] waveform. For example, the simulations show that (1) although ACh normally has an inhibitory effect on the pacemaker model, for cases where the rise time and duration of the [ACh] waveform are sufficiently brief, ACh can paradoxically accelerate the beat in which a single stimulus is given; (2) the SV pacemaker normally exhibits type 1 (odd) phase-resetting in response to ACh delivery, however type 0 (even) phase-resetting behavior may be exhibited when the [ACh] waveform is large enough and has a very fast rise time; and (3) the SV pacemaker may become phase-locked to a repetitive ACh stimulus applied with either a constant period or coupling interval. In the latter case, this entrainment phenomenon has implications for the control of the cardiac pacemaker by a neural oscillator (e.g. located in the medullary cardiovascular control center) which provides input to the pacemaker cell via the vagus nerve. In these regions of capture, repetitive ACh stimulation produces a well-known paradoxical accelerative effect on the SV pacemaker cell, similar to that seen in a variety of other species.

A distributed-parameter model of the myelinated nerve fiber

This paper presents a new model for the characterization of electrical activity in the nodal, paranodal and internodal regions of isolated amphibian and mammalian myelinated nerve fibers. It differs from previous models in the following ways: (1) in its ability to incorporate detailed anatomical and electrophysiological data; (2) in its approach to the myelinated nerve fiber as a multi-axial cable; and (3) in the numerical algorithm used to obtain distributed model equation solutions for potential and current. The morphometric properties are taken from detailed electron microscopic anatomical studies (Berthold & Rydmark, 1983a, Experientia 39, 964-976). The internodal axolemma is characterized as an excitable membrane and model-generated nodal and internodal membrane action potentials are presented. A system of describing equations for the equivalent network model is derived, based on the application of Kirchoff's Current Law, which take the form of multiple cross-coupled parabolic partial differential equations. An implicit numerical integration method is developed and the numerical solution implemented on a parallel processor. Non-uniform spatial step sizes are used, enabling detailed representation of the nodal region while minimizing the number of total segments necessary to represent the overall fiber. Conduction velocities of 20.2 m sec-1 at 20 degrees C for a 15 microns diameter amphibian fiber and 57.6 m sec-1 at 37 degrees C for a 17.5 microns diameter mammalian fiber are achieved, which agrees qualitatively with published experimental data at similar temperatures (Huxley & Stämpfli, 1949, J. Physiol., Lond. 108, 315-339; Rasminsky, 1973, Arch, Neurol. 28, 287-292). The simulation results demonstrate the ability of this model to produce detailed representations of the transaxonal, transmyelin and transfiber potentials and currents, as well as the longitudinal extra-axonal, periaxonal and intra-axonal currents. Also indicated is the potential contribution of the paranodal axolemma to nodal activity as well as the presence of significant longitudinal currents in the periaxonal space adjacent to the node of Ranvier.

Cryopreservation and long-term liquid nitrogen storage of peripheral blood mononuclear cells for flow cytometry analysis: effects on cell subset proportions and fluorescence intensity

The effect of cryopreservation and long-term liquid nitrogen storage on peripheral blood mononuclear cell (PBMC) subsets was prospectively analyzed using monoclonal antibodies and flow cytometry. Brief cryopreservation did not significantly alter the proportion of positively stained cells for CD3+, CD4+, CD8+, CD14+, CD16+, and CD19+ cells. A small but statistically significant increase in the proportion of positive cells was observed for HLA-DR+ and HLe-1+ cells. Brief cryopreservation was associated with a decrease in the mean fluorescence intensity (MFI) values for CD3+, CD4+, and CD8+ cells; an increase in MFI values for CD14+ and HLA-DR+ cells; and no change for CD16+, CD19+, and HLe-1+ cells. There was no significant change in the proportion of CD3+, CD4+, or CD16+ cells during 20 months of storage in liquid nitrogen. Small but statistically significant decreases in the proportion of CD8+ and CD19+ cells were observed over the same interval, and the proportion of CD14+ cells (monocytes) was highly variable. Chronologic changes in fluorescence intensity during long-term storage were observed for all cell subsets except CD16+ and CD19+ cells. Cryopreservation is a valuable technique for long-term storage of viable cells. For many laboratory applications, the small changes noted in the present study will have no practical importance. However, for clinical and epidemiological investigations encompassing large numbers of samples, statistical techniques to adjust for small changes during storage should be considered.

Interleukin 2 and lymphokine-activated killer cell therapy: analysis of a bolus interleukin 2 and a continuous infusion interleukin 2 regimen

Several groups have described the efficacy of interleukin 2 (IL-2) plus lymphokine-activated killer (LAK) cells in the treatment of cancer patients with significant response rates noted in patients with renal cell cancer and malignant melanoma; however, the optimum regimen remains undefined. The Biological Response Modifiers Program of the National Cancer Institute conducted two consecutive Phase I/II studies evaluating the toxicity and clinical efficacy of different methods of IL-2 and LAK cell therapy. In the first trial, we modified the standard Rosenberg regimen by decreasing the duration of priming in an attempt to reduce the toxicity related to this phase of the therapy and thereby administer more IL-2 doses with the LAK cells. In the second trial, we used a continuous i.v. infusion IL-2 regimen and altered both the leukapheresis procedure and the LAK cell culture techniques based on our in vitro and preclinical studies suggesting that 2-day LAK cells were superior. Thirty cancer patients received i.v. bolus IL-2 at 100,000 units/kg every 8 h for 3 days during priming and for 5 days during LAK cell administration. A second group of 22 cancer patients received IL-2 by continuous i.v. infusion at 3 x 10(6) units/m2 for 5 days during priming and an additional 5 days of IL-2 with the LAK cell phase of the treatment. The timing of the start of the leukapheresis procedures, their duration and number, and the LAK cell culture techniques differed in the two trials. Overall, 52 patients with various cancers were treated. The toxicities associated with each regimen were similar to those seen in other IL-2 plus LAK cell trials. Four patients (one each with melanoma and diffuse large cell lymphoma and two with renal cell cancer) exhibited partial responses lasting 2, 4, 10, and 15+ mo. Serial tumor biopsies from treated patients demonstrated that therapy can produce a marked mononuclear cell infiltrate and an increase in HLA-DR expression on tumor cells. There was no difference in the overall response rate between the two regimens, but toxicity was less with continuous i.v. infusion IL-2. The 5-day continuous i.v. infusion regimen resulted in significantly higher rebound lymphocytosis, cell yield from leukapheresis, and number of LAK cells harvested from culture.

Intraperitoneal lymphokine-activated killer-cell and interleukin-2 therapy for malignancies limited to the peritoneal cavity

Autologous lymphokine-activated killer (LAK) cells and recombinant human interleukin-2 (rIL-2) were administered intraperitoneally (IP) to 24 patients with malignancies limited to the peritoneal space. Ten patients had ovarian cancer, 12 had colorectal cancer, and one patient each had endometrial carcinoma and primary small-bowel adenocarcinoma. All ovarian cancer patients, three of twelve colorectal cancer patients, and one patient with endometrial carcinoma had received prior therapy. Patients received IL-2 100,000 U/kg every 8 hours intravenously (IV) for 3 days, and 2 days later underwent daily leukapheresis for 5 days. LAK cells were generated in vitro by incubating the peripheral blood mononuclear cells in IL-2 for 7 days and were then administered IP daily for 5 days through a Tenckhoff catheter (Davol, Inc, Cranston, RI) together with IL-2 25,000 U/kg IP every 8 hours. All but one patient completed at least one cycle of therapy. Toxic side effects included minor to moderate hypotension, fever, chills, rash, nausea, vomiting, abdominal pain and distension, diarrhea, oliguria, fluid retention, thrombocytopenia, and minor elevations of liver function tests; all of these rapidly improved after discontinuation of IL-2. One patient had a grand mal seizure, and one suffered a colonic perforation; these were felt to be treatment-related. IP fibrosis developed in 14 patients and limited repeated cyclic administration of this therapy in five patients. Two of 10 (20%) ovarian cancer patients and five of 12 (42%) colorectal cancer patients had laparoscopy- or laparotomy-documented partial responses. We conclude that LAK cells and rIL-2 can be administered IP to cancer patients, resulting in moderate to severe short-term toxicity and modest therapeutic efficacy. Further investigation of this form of adoptive immunotherapy modified to address the problem of IP fibrosis and with lower IP IL-2 doses is justified by these initial results.

A phase I-II trial of continuous-infusion cisplatin, continuous-infusion 5-fluorouracil, and VP-16 in colorectal carcinoma

Twenty-nine evaluable patients with colorectal adenocarcinoma were treated in a phase I-II trial of combination chemotherapy with a 72-h continuous infusion of cisplatin (CDDP) and 5-fluorouracil (5-FU) with an infusion of VP-16 given at 24 and 48 h after the start of therapy. There were five (17 +/- 14%) partial responses lasting 2-6 months (median, 3). Three of these responses occurred among the 10 previously untreated patients. The toxicity of this regimen was pronounced. Four of eight patients with severe neutropenia required hospitalization for infections, two of which were life-threatening; one of six patients with severe thrombocytopenia had a life-threatening hemorrhagic complication; and four patients experienced severe, dose-limiting fatigue. These complications occurred principally with CDDP and VP-16 at doses above 27.5 mg/m2/day and 110 mg/m2/dose, respectively. Mucositis occurred in six patients and limited the dose of 5-FU to 1,300 mg/m2/day. Although the response rate appeared to be high in previously untreated patients, the minimal palliative benefit of treatment and the brief duration of the responses do not compensate for the toxicity observed.

A mathematical model of electrophysiological activity in a bullfrog atrial cell

We have developed a model of cardiac atrial electrical activity based on voltage-clamp measurements obtained from single cells isolated from the bullfrog atrium. These measurements have allowed us to simulate a number of processes thought to be important in action potential initiation, repolarization, and the excitation-contraction (EC) coupling process. In this atrial model, the cell membrane contains both channel-mediated (Na+, Ca2+, inward rectifier K+, delayed rectifier K+, linear background leak) and transporter-mediated (Na(+)-K+ pump, Na(+)-Ca2+ exchanger, Ca2+ pump) currents. The cell is surrounded extracellularly by a diffusion-limited space. The intracellular volume contains Ca2(+)-binding proteins (calmodulin, troponin). The model makes several important predictions. 1) Incomplete inactivation of the Ca2+ current provides an inward current the maintains the plateau of the action potential. 2) Activation of the delayed rectifier K+ current initiates repolarization. 3) Due to Ca2+ buffering by myoplasmic proteins the Na(+)-Ca2+ exchanger current is relatively small and has little influence on repolarization. 4) The Na(+)-K+ pump current does not play a major role in repolarization. 5) K+ accumulation and Ca2+ depletion may occur in the extracellular spaces. 6) Modulation of EC coupling is governed by interactions between the myoplasmic Ca2(+)-binding proteins; specifically, the inotropic "positive staircase effect" may be explained by interactions between Ca2+ and Mg2+ at a competitive binding site on troponin. When considered in conjunction with the results of our model of primary pacemaking in the sinus venosus [Rasmusson et al., Am. J. Physiol. 259 (Heart Circ. Physiol. 28): H352-H369, 1990], this atrial model shows how the presence or absence of certain transmembrane currents can change action potential characteristics and consequently alter the relative influence of the various transporter-mediated and channel-mediated currents.

A mathematical model of a bullfrog cardiac pacemaker cell

Previous models of cardiac cellular electrophysiology have been based largely on voltage-clamp measurements obtained from multicellular preparations and often combined data from different regions of the heart and a variety of species. We have developed a model of cardiac pacemaking based on a comprehensive set of voltage-clamp measurements obtained from single cells isolated from one specific tissue type, the bullfrog sinus venosus (SV). Consequently, sarcolemmal current densities and kinetics are not influenced by secondary phenomena associated with multicellular preparations, allowing us to realistically simulate processes thought to be important in pacemaking, including the Na(+)-K+ pump and Na(+)-Ca2+ exchanger. The membrane is surrounded extracellularly by a diffusion-limited space and intracellularly by a limited myoplasmic volume containing Ca2(+)-binding proteins (calmodulin, troponin). The model makes several predictions regarding mechanisms involved in pacing. 1) Primary pacemaking cannot be attributed to any single current but arises from both the lack of a background K+ current and a complex interaction between Ca2+, delayed-rectifier K+, and background leak currents. 2) Ca2+ current displays complex behavior and is important during repolarization. 3) Because of Ca2+ buffering by myoplasmic proteins, the Na(+)-Ca2+ exchanger current is small and has little influence on action potential repolarization but may modulate the maximum diastolic potential. 4) The Na(+)-K+ pump current does not play an active role in repolarization but is of sufficient size to modulate the rate of diastolic depolarization. 5) K+ accumulation and Ca2+ depletion may occur in the extracellular spaces but play no role in either the diastolic depolarization or repolarization of a single action potential. This model illustrates the importance of basing simulations on quantitative measurements of ionic currents in myocytes and of including both electrogenic transporter mechanisms and Ca2+ buffering by myoplasmic Ca2(+)-binding proteins.

Exacerbation of symptoms of autoimmune disease in patients receiving alpha-interferon therapy

The occurrence of autoimmune disease in patients receiving alpha-interferon (alpha-IFN) therapy has been reported in several studies; these include autoimmune thyroiditis, thrombocytopenia, anemia, exacerbation of psoriasis, and the occurrence of sarcoidosis. The primary mechanism presumably is the emergence of autoantibodies to various structural proteins or receptors. Two studies have recently shown that a significant percentage of patients treated with recombinant alpha-interferon (r alpha-IFN) do form autoantibodies. The authors report six additional cases of development or exacerbation of autoimmune phenomena in patients receiving alpha-IFN therapy. Five of these patients developed symmetric polyarthropathies and the sixth had thyroiditis. The presence of a history of underlying autoimmune disease or baseline serologic abnormalities in five of these patients, including the patient who developed thyroiditis, suggests that alpha-IFN treatment can lead to the exacerbation of an underlying subclinical autoimmune process.

T-cell subsets in healthy teenagers: transition to the adult phenotype

Little is known about the normal range and variability of T-cell subsets in older children. We analyzed peripheral blood mononuclear cell subsets in 112 healthy children, ages 12-19 years (mean +/- SD: 15.4 +/- 1.9 years), using monoclonal antibodies and flow cytometry. The study population included 28 blacks and 84 whites, with 59 boys and 53 girls. The mean +/- SD cell subset values were: CD3+ T cells, 74.0 +/- 7.8%; CD4+ helper-inducer T cells, 46.8 +/- 6.9%; CD8+ suppressor-cytotoxic T cells, 27.3 +/- 5.7%; CD4:CD8 helper:suppressor ratio, 1.81 +/- 0.57; CD16+ natural killer cells, 4.4 +/- 3.1%; CD19+ B cells, 10.0 +/- 5.3%; CD14+ monocytes, 20.0 +/- 6.5%; and HLA-DR cells, 15.4 +/- 4.8%. Overall, boys had a higher proportion of HLA-DR+ cells than girls, attributable to an increase in CD19+ B cells. Blacks tended to have a higher proportion of HLA-DR+ cells than whites, apparently due to an increase in activated T cells. Detailed analysis by age group revealed a striking transition in the pattern of CD4+ and CD8+ cell populations. The CD4:CD8 ratio, higher in boys than girls for ages 12-16, was reversed to the "adult" pattern in 17-19 year olds, with a higher CD4:CD8 ratio in girls. These data provide important baseline values for healthy children and stress the importance of establishing normative ranges for pediatric subjects separately from adults.