Effect of quinidine on Na,H+, and water transport by the turtle and toad bladders

Summary The effect of quinidine on Na and H⁺ transport by the turtle bladder and water transport by the toad bladder was examined. Quinidine inhibited the short-circuit current and the potential difference in a dose-dependent fashion. The effect of quinidine on the short-circuit was not dependent on extracellular calcium concentration and was not reversible with removal of the drug. Quinidine inhibited H⁺ secretion in a dose-dependent fashion. The effect of quinidine on H⁺ secretion also was not dependent on extracellular calcium concentration and was not reversible, either with removal of the drug or with stimulation of H⁺ secretion with 5% CO₂. The effect of quinidine on Na or H⁺ transport could not be elicited by an equivalent dose of tetracaine, suggesting that the inhibitory effect of quinidine is not dependent on its anesthetic properties. Quinidine also inhibited vasopressin and cyclic AMP stimulated water flow in the toad bladder. Quinidine did not alter calcium uptake by the turtle bladder but increased calcium efflux by the turtle and toad bladders. These observations suggest that a rise in cytosolic calcium is responsible for the inhibitory effect of quinidine on Na, H⁺, and water transport.     

Interrelationships among quinidine, amiloride, and lithium as inhibitors of the renal Na+-H+ exchanger

We examined the effects of quinidine, amiloride and Li+ on the kinetics of Na+-H+ exchange in microvillus membrane vesicles isolated from the rabbit renal cortex. Quinidine reversibly inhibited the initial rate of Na+-H+ exchange (I50 200 microM). The plot of 1/V versus [quinidine] was curvilinear, with Hill coefficient greater than 1.0, indicating that the drug interacts at two or more inhibitory sites or at a single site on at least two different conformations of the transporter. Quinidine decreased the Vmax for Na+-H+ exchange and increased the Km for Na+, indicating a mixed-type mechanism of inhibition. In contrast, plots of 1/V versus [amiloride] and 1/V versus [Li+] were linear, indicating single inhibitory sites; amiloride and Li+ each increased the Km for Na+ with no effect on Vmax, indicating a competitive mechanism of inhibition. Addition of Li+ increased the intercept with no change in slope of the 1/V versus [amiloride] plot, indicating that Li+ and amiloride are mutually exclusive inhibitors of Na+-H+ exchange. Addition of quinidine increased the slopes of the plots of 1/V versus [amiloride] and 1/V versus [Li+], indicating that the binding of quinidine is not mutually exclusive with the binding of amiloride and Li+. Results from this and previous studies are consistent with the concept that the inhibitor amiloride and the transportable substrates Na+, H+, Li+, and NH+4 all mutually compete for binding to a single site, the external transport site of the renal Na+-H+ exchanger. However, our findings indicate that quinidine interacts with the Na+-H+ exchanger on at least one additional site that is not shared by Na+, Li+, or amiloride.     

Demethylation of radiolabelled dextromethorphan in rat microsomes and intact hepatocytes.

Liver microsomal preparations are routinely used to predict drug interactions that can occur in vivo as a result of inhibition of cytochrome P450 (CYP)-mediated metabolism. However, the concentration of free drug (substrate and inhibitor) at its intrahepatic site of action, a variable that cannot be directly measured, may be significantly different from that in microsomal incubation systems. Intact cells more closely reflect the environment to which CYP substrates and inhibitors are exposed in the liver, and it may therefore be desirable to assess the potential of a drug to cause CYP inhibition in isolated hepatocytes. The objective of this study was to compare the inhibitory potencies of a series of CYP2D inhibitors in rat liver microsomes and hepatocytes. For this, we developed an assay suitable for rapid analysis of CYP-mediated drug interactions in both systems, using radiolabelled dextromethorphan, a well-characterized probe substrate for enzymes of the CYP2D family. Dextromethorphan demethylation exhibited saturable kinetics in rat microsomes and hepatocytes, with apparent Km and Vmax values of 2.1 vs. 2.8 microM and 0.74 nM x min(-1) per mg microsomal protein vs. 0.11 nM x min(-1) per mg cellular protein, respectively. Quinine, quinidine, pyrilamine, propafenone, verapamil, ketoconazole and terfenadine inhibited dextromethorphan O-demethylation in rat liver microsomes and hepatocytes with IC50 values in the low micromolar range. Some of these compounds exhibited biphasic inhibition kinetics, indicative of interaction with more than one CYP2D isoform. Even though no important differences in inhibitory potencies were observed between the two systems, most inhibitors, including quinine and quinidine, displayed 2-3-fold lower IC50 in hepatocytes than in microsomes. The cell-associated concentrations of quinine and quinidine were found to be significantly higher than those in the extracellular medium, suggesting that intracellular accumulation may potentiate the effect of these compounds. Studies of CYP inhibition in intact hepatocytes may be warranted for compounds that concentrate in the liver as the result of cellular transport.     

Reassessment of the electrophysiological effects of the antiarrhythmic agent quinidine in canine Purkinje fibers

Microelectrode techniques were used to re-examine the direct effects of quinidine on isolated canine Purkinje fibers. Our results confirmed the previous findings that quinidine in a dose-related manner suppressed spontaneous activity, prolonged effective refractory period and depressed the maximal rate of upstroke (dv/dt). Quinidine shifted the membrane response curves to the right of the voltage axis regardless of external K⁺ concentrations. The drug diminished the phase 2 plateau of the action potential indicating a possible inhibitory effect on calcium influx. Quinidine generally lengthened the action potential duration (ADP) but the extent of the increase varied with the site of recording along the Purkinje conduction system. The shorter APDs were lengthened far more than the longer APDs, thus dissimilar APDs became more uniform. Following the alteration of external K⁺ between 2 mM and 6 mM, the effects of quinidine on transmembrane characteristics, including the depression of dv/dt, remain constant with the exception of APD.     

Quinidine is a strong perturber of acidic phospholipid bilayer order and fluidity

The effect of an antiarrhythmic drug, quinidine, on the organization of model phospholipid membranes was studied by the spin-labeling technique. Quinidine strongly perturbs the molecular organization of lipid bilayers prepared from acidic phospholipids (phosphatidylserine, phosphatidic acid) and has only a slight effect on neutral phosphatidylcholine membranes. The interaction of the drug with acidic phospholipids manifests itself in a pronounced increase in the order parameter of the region close to the polar surface of the bilayer and in some decrease in its inner hydrocarbon core fluidity. It is suggested that the perturbation in the organization of membrane lipids may contribute to the mechanisms by which quinidine exerts its pharmacological effects.     

Competitive inhibition of sparteine oxidation in human liver by beta-adrenoceptor antagonists and other cardiovascular drugs

The rate of oxidation of sparteine by the 9000 x g supernatant fraction of a human liver was measured in the presence of various drugs which exert cardiovascular effects. Hexamethonium, ouabain, caffeine and isoproterenol had no effect on this rate, while alprenolol, metoprolol, oxprenolol, propranolol, timolol, pindolol, lidocaine, mexiletine, 17-n-pentyl-sparteine, tolazoline, quinine, quinidine, cinchonine and cinchonidine inhibited the in vitro reaction competitively. Stereoselective inhibition was observed between quinine (Ki = 15 microM) and quinidine (Ki = 0.06 microM). Genetic evidence suggests that the primary metabolism of sparteine depends on a single species of cytochrome P450. In vitro competitive inhibition of sparteine oxidation by a drug indicates that this drug is capable of occupying the same enzymatic site as sparteine. This may mean that the competing drug is also metabolized at that site and thereby subject to the same genetic variation as sparteine's oxidation; absence of inhibition excludes this possibility.     

Role of monocytes in the inhibitory effect of calcitriol on PHA-stimulated lymphocytes

The possible role played by monocytes in the inhibitory effect of calcitriol on phytohemagglutinin (PHA)-stimulated lymphocyte proliferation was assessed by testing the effect of this sterol under different cell culture conditions. Calcitriol had a dose-dependent inhibitory effect on lymphocyte proliferation in concentrations ranging from 10(-10) up to 10(-8) M. The effect of 10(-9) M calcitriol was almost completely abolished by: a) monocyte depletion, b) inhibition of prostaglandin (PG) synthesis by indomethacin, and c) addition of exogenous interleukin-2 (IL-2). These results suggested that the inhibitory effect of calcitriol was mediated through monocytes. This possibility was substantiated by the following observations: a) the calcitriol inhibitory effect was restored when autologous adherent cells were added to monocyte-depleted PBM cells; b) the supernatant of adherent cells cultured for 24 hours in the presence of calcitriol exerted a marked inhibitory effect on lymphocyte proliferation; and c) this effect was not longer evident when adherent cells were cultured in the presence of calcitriol plus indomethacin. These data support the hypothesis that calcitriol acts, at least partially, through the monocytes, inducing an increased release of PG, with subsequent inhibition of IL-2 synthesis, then resulting in a decreased lymphocyte proliferation.     

The yeast multidrug transporter Qdr3 (Ybr043c): localization and role as a determinant of resistance to quinidine, barban, cisplatin, and bleomycin

Saccharomyces cerevisiae ORF YBR043c, predicted to code for a transporter of the major facilitator superfamily required for multiple drug resistance, encodes a plasma membrane protein that confers resistance to quinidine and barban, as observed before for its close homologues QDR1 and QDR2. This ORF was, thus, named the QDR3 gene. The increased expression of QDR3, or QDR2, also leads to increased resistance to the anticancer agents cisplatin and bleomycin. However, no evidence for increased QDR3 expression in yeast cells exposed to all these inhibitory compounds was found. Transport assays support the concept that Qdr3 is involved, even if opportunistically, in the active export of quinidine out of yeast cell. A correlation was established between the efficiency of quinidine active export mediated by Qdr3p, Qdr2p or Qdr1p, and the efficacy of the expression of the encoding genes in alleviating the deleterious action of quinidine, as well as of the other compounds (QDR2>QDR3>QDR1).     

The Effect of Quinidine on Calcium Accumulation by Isolated Sarcoplasmic Reticulum of Skeletal and Cardiac Muscle

Quinidine potentiates twitch tension and (at higher concentrations) causes contracture of skeletal muscle whereas the same drug reduces tension development of cardiac muscle. To gain insight into the possible differences in the excitation-contraction coupling mechanism of the two types of muscle the effect of quinidine on calcium accumulation by isolated sarcoplasmic reticulum from skeletal and cardiac muscle was investigated. In a medium containing ATP, Mg⁺⁺, oxalate, and ⁴⁵Ca, pharmacologically active concentrations of the drug inhibited calcium accumulation by both skeletal and cardiac sarcoplasmic reticulum. The inhibition of the rates of calcium, uptake by the skeletal muscle preparation ranged from 11% with 10^-4 M quinidine to 90% with 10^-3 M quinidine. With the cardiac muscle preparation the inhibition ranged from 16% with 3 x 10^-6 M quinidine to 100% with 10^-3 M quinidine. With both preparations the inhibition of calcium transport was accompanied by an inhibition of the Ca⁺⁺-activated ATPase activity of the sarcoplasmic reticulum. The effect of quinidine on the skeletal sarcoplasmic reticulum supports the hypothesis that this compound produces twitch potentiation and contracture by interfering with intracellular calcium, sequestration. Its effect on cardiac sarcoplasmic reticulum. has been interpreted in terms of the hypothesis that cardiac contractility is a function of the amount of calcium released from the sarcoplasmic reticulum which is in turn dependent upon the absolute calcium content of the reticulum. Hence, following inhibition of calcium transport there would be less calcium available for coupling.     

Inhibition of phytohemagglutinin-, periodate- and A23187-induced lymphocyte transformation by Vinca alkaloids

The effect of the Vinca alkaloids, vincristine and vinblastine, on mitogen-induced transformation of isolated human peripheral blood lymphocytes has been investigated. Cells were subjected to a variety of mitogens (PHA, ionophore A23187 and sodium periodate) whose mechanism and site of action differ. Addition of vincristine or vinblastine to lymphocyte cultures prior to mitogen produced a concentration-dependent inhibition of cell transformation as determined by measurement of DNA synthesis and blast formation. The inhibitory effects were not due to decreased cell viability, since the drugs had little or no effect on cell viability. Vincristine and vinblastine were also found to impair [³H]thymidine incorporation by prestimulated blast cells at the higher drug concentrations tested. The results presented in this communication show that the Vinca alkaloids block lymphocyte transformation induced by either lectin or non-lectin mitogens. This suggests that the inhibitory step(s) may occur after mitogen stimulation.     

Quinidine and melittin both decrease the fluidity of liver plasma membranes and both inhibit hormone-stimulated adenylate cyclase activity

Increasing concentrations of either quinidine or melittin gave a dose-dependent inhibition of both the glucagon- and fluoride-stimulated activities of adenylate cyclase in the liver plasma membranes. At similar concentrations these agents increased the order of liver plasma membranes as detected by a fatty acid ESR probe, doxyl stearic acid. This increase in bilayer order (decrease in 'fluidity') is suggested to explain the inhibitory action of quinidine on adenylate cyclase activity but only in part contributes to the inhibitory action of melittin on adenylate cyclase. Arrhenius plots of fluoride-stimulated activity became non-linear in the presence of either quinidine or melittin, with a single well-defined break occurring at around 12 degrees C in each instance. Arrhenius plots of the glucagon-stimulated activity also exhibited such a novel break at around 12 degrees C when either quinidine or melittin were present as well as exhibiting a break at around 28 degrees C, as was seen in the absence of these ligands. The fatty acid spin probe inserted into liver plasma membranes detected a novel lipid phase separation occurring at around 12 degrees C when either quinidine or melittin was present and showed that the lipid phase separation occurring at around 28 degrees C in native membranes was apparently unaffected by these ligands.     

Separation of the gonadotropic activity of crude choriogonadotropin from the inhibitory effect on lymphocyte transformation.

The effect of choriogonadotropin of different purities on the transformation of peripheral human lymphocytes was studied. Various crude hormone batches inhibited lymphocyte transformation in a dose-dependent manner, both in the mixed lymphocyte reaction and in the phytohemagglutinin-induced stimulation. The inhibitory activity, however, was found not to be correlated with the gonadotropic activity of the crude hormone batches (2660-4300 IU/mg). Choriogonadotropin (13 000 IU/mg), which was purified in 3 steps, showed no inhibitory effect except at high doses (greater than 5000 IU/ml final dilution). More detailed investigations provided evidence that in the first step of the choriogonadotropin purification procedure (batch adsorption of crude choriogonadotropin on SP-Sephadex C-50), the inhibitory activity can be enriched in a fraction (Fract. I) which displays a very low gonadotropic activity (less than 500 IU/mg). A further separation of Fract. I was achieved by isoelectric focusing as well as by chromatography on DEAE-Sephadex A-25. By these means, the inhibitory potency could be enriched more than 100-fold. The substances which display inhibition of DNA synthesis in lymphocytes were proven to act in a nontoxic way. A preliminary characterization of the strongly inhibiting substances which show a dose-dependent suppression of lymphocyte transformation by about 99%, showed that this effect is probably exerted via non-dialysable sialoglycoproteins. By a fourth purification step entailing a chromatography of purified choriogonadotropin (13 000 IU/mg) on SP-Sephadex C-50, a highly purified choriogonadotropin (14000 IU/mg) could be obtained which showed no inhibitory effect on lymphocyte transformation (in both mixed lymphocyte reaction and in phytohemagglutinin-induced stimulation) up to a dose of 43 000 IU/ml. The components which were removed from choriogonadotropin in this step seem to be immunologically identical with the strongly inhibiting substances isolated by isoelectric focusing. These investigations demonstrate that biologically active, highly purified choriogonadotropin is unable to inhibit lymphocyte transformation. The inhibitory activity of crude hormone can be enriched in choriogonadotropin-free fractions. Therefore, it is concluded that the inhibitory activity of crude hormone is not a property of choriogonadotropin itself.     

Computational prediction of drug response in short QT syndrome type 1 based on measurements of compound effect in stem cell-derived cardiomyocytes

Short QT (SQT) syndrome is a genetic cardiac disorder characterized by an abbreviated QT interval of the patient’s electrocardiogram. The syndrome is associated with increased risk of arrhythmia and sudden cardiac death and can arise from a number of ion channel mutations. Cardiomyocytes derived from induced pluripotent stem cells generated from SQT patients (SQT hiPSC-CMs) provide promising platforms for testing pharmacological treatments directly in human cardiac cells exhibiting mutations specific for the syndrome. However, a difficulty is posed by the relative immaturity of hiPSC-CMs, with the possibility that drug effects observed in SQT hiPSC-CMs could be very different from the corresponding drug effect in vivo. In this paper, we apply a multistep computational procedure for translating measured drug effects from these cells to human QT response. This process first detects drug effects on individual ion channels based on measurements of SQT hiPSC-CMs and then uses these results to estimate the drug effects on ventricular action potentials and QT intervals of adult SQT patients. We find that the procedure is able to identify IC₅₀ values in line with measured values for the four drugs quinidine, ivabradine, ajmaline and mexiletine. In addition, the predicted effect of quinidine on the adult QT interval is in good agreement with measured effects of quinidine for adult patients. Consequently, the computational procedure appears to be a useful tool for helping predicting adult drug responses from pure in vitro measurements of patient derived cell lines.     

Correlative effects of quinidine on ECG pattern and serum cholesterol concentration in Acridotheres tristis.

Antiarrhythmic drug quinidine, administered daily at the rate of 10 mg/kg for 7 days in A. tristis produced an increasing effect on the amplitude and duration of different waves and intervals. Heart rate was decreased from 478.40 to 444.47 beats/min. Serum cholesterol level was reduced from 86 to 54.30 mg/100 ml. The data of the effect of quinidine on the ECG pattern and serum cholesterol were analysed and the values of the correlation coefficient and their significance were computed. The values of the correlation coefficient computed between the level of serum cholesterol (Y) and P-R interval (X) comes out to be significant at 5% level of significance. A linear regression line Y on X was fitted to the above data and the line is found to be: Y = 2535.897 X -62.858. This regression line may be used to determine the level of serum cholesterol on the basis of changes in the P-R interval of the ECG tracing in the quinidine treated birds.     

Regulation of lymphocyte motility by macrophages: characterization of a lymphocyte migration inhibitory factor derived from a macrophage-like cell line

An inhibitory factor on lymphocyte migration was detected using a capillary random migration assay in the culture supernatant of peritoneal exudate macrophages cultured at concentrations greater than 8 x 10(6) cells/ml. After examining different macrophage-like cell lines, J774A.1 cells were found to produce this inhibitory factor, which was termed lymphocyte migration inhibitory factor (LMIF). The inhibitory effect of LMIF on the migration of spleen lymphocytes, thymocytes, and bone marrow cells was determined. The migration of thymocytes was more sensitive to LMIF than was the migration of spleen lymphocytes and bone marrow cells. Interestingly, when the effect of LMIF was tested on the migration of spleen T cells and B cells, T cells were more sensitive than B cells. When the thymocytes were separated by peanut agglutinin into mature and immature thymocytes, the migration of mature thymocytes was more sensitive than that of immature thymocytes, the migration of mature thymocytes was more sensitive than that of immature thymocytes to the effect of LMIF, suggesting that the greatest effect of LMIF was on the migration of mature T cells. Partial purification of LMIF by ion-exchange and gel-filtration chromatography revealed that it is approximately 14,000 in molecular weight and could exist in either monomeric or dimeric forms. The possible role of this factor in an immune response is discussed.     

Role of surface phenomena in the mechanism of action of antiarrhythmic substances]

The surface activity of the antiaarhythmic drugs and their influence on the lipid-containing interphasics were studied. It was found that drug No. 7351 (diphenyl isopropyl acetic acid diethyl aminopropyl ester), fubromegan, metamysil, propranolol, quinidine, novocalinamide, xylocaine, and trimecaine possessed surface activity. There exists a symbate relationship between the surface and especially the interphasic activity and the antiarrhythmic effect. The most active antiarrhythmic drug No. 7351 increased the electric conductivity of the lecithine bilayer membrane much more than novocainamide.     

电剌大鼠的血清中淋巴细胞转化抑制因子的作用机制分析

Previous reports showed that EA stimulation (3V, 2Hz, 30 min/d, 5 d) induced the production of one or more lymphocyte proliferation-inhibitory factor(s) in the rat serum. In this paper, the mechanisms of the action for the inhibitory factor(s) to suppress lymphocyte proliferation were studied. (1) the lymphocytes from different immune organs of the mice were prepared and cultured with the rat serum stimulated by EA. The results show that the serum not only inhibited the mouse lymph node T cell proliferation induced by Con A, but also inhibited the mouse thymocyte and spleen T cell proliferation induced by Con A. When B cells were stimulated by LPS, the proliferative effect can also be inhibited significantly by the rat serum stimulated by EA. This implies that the effect of the lymphocyte proliferation-inhibitory factor(s) has no specificity. (2) Incubation of the mouse lymph node cell with serum for one hour is enough to cause an inhibitory effect on Con A stimulated lymphocyte proliferation. However, no inhibitory effect was observed if the mouse lymph node cells were incubated with Con A for 15 min or 30 min before the addition of rat serum. The results demonstrate that the lymphocyte proliferation-inhibitory factor(s) act on the early events of T lymphocyte activation induced by Con A. (3) Protein kinase C (PKC) is a key link in the activation of T and B lymphocyte proliferation by Con A and LPS respectively. So it would be interesting to learn whether the inhibitory effect of the lymphocyte proliferation-inhibitory factor(s) is caused by the inhibition of PKC activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

电针大鼠的血清中淋巴细胞转化抑制因子的作用机制分析

本室以前的工作表明:电针(2H_z,3V,30min/d)刺激 SD 大鼠双侧足三里-三阴交,5d后,大鼠血清中产生出淋巴细胞转化抑制因子,本工作对此抑制因子的作用机制进行了初步研究,主要结果如下:(1)电针大鼠的血清不仅显著抑制 Con A 刺激的小鼠淋巴结 T 淋巴细胞转化,还可显著抑制 Con A 刺激的小鼠胸腺细胞和脾脏 T 淋巴细胞转化;同时也发现电针大鼠的血清能显著抑制脂多糖(LPS)刺激的小鼠淋巴结 B 淋巴细胞转化。提示此淋巴细胞转化抑制因子对不同淋巴器官及不同类型的淋巴细胞无选择性作用。(2)将电针大鼠的血清同小鼠淋巴结细胞培养1h,电针大鼠的血清就可显著抑制 Con A 刺激的 T 淋巴细胞转化;将小鼠淋巴结细胞同 Con A 预培养30min,电针大鼠的血清的抑制作用便消失,提示电针大鼠血清中淋巴细胞转化抑制因子作用于 Con A 刺激 T 淋巴细胞活化的早期阶段,同时也排除了此抑制因子的细胞毒作用。(3)电针大鼠的血清显著抑制蛋白激酶 C(PKC)激活剂 PMA和 PMA 加 ca~(2+)通道 A23187刺激的小鼠淋巴结细胞转化,提示淋巴细胞转化抑制因子通过抑制 PKC 的活性或抑制 PKC 介导的细胞活化通路,抑制有丝分裂原刺激的淋巴细胞转化。     

Simple sugars inhibit proliferation of human T lymphocytes in autologous and allogeneic mixed lymphocyte reactions

Several oligo- and monosaccharides were studied for their capacity to modulate lymphocyte proliferation in human allogeneic and autologous mixed lymphocyte reactions (MLR). A defined subset of sugars showed a marked inhibitory effect on lymphocyte proliferative response in the majority of the allogeneic MLR combinations studied. The inhibitory effect disappeared when sugars were added to allogeneic MLR 96 hr after the beginning of culture. These sugars also showed a significant inhibitory power on autologous MLR, performed by using T- and non-T-enriched lymphocytes from the same donor. The reported data suggest that carbohydrate determinants are involved in the proliferative response of human lymphocytes in both autologous and allogeneic MLR.     

Cyclosporin A does not inhibit the PHA-stimulated increase in intracellular Ca2+ concentration but inhibits the increase in E-rosette receptor (CD2) expression and appearance of interleukin-2 receptors (CD25)

The immunosuppressive drug cyclosporin A (CsA) inhibits mixed lymphocyte responses, blocks the generation of cytotoxic T lymphocytes, and inhibits the T lymphocyte proliferative response stimulated by polyclonal activators such as phytohemagglutinin (PHA). Nevertheless, there have been contradictory reports attempting to explain the mechanism(s) for this immunosuppressive activity. In the current studies, human peripheral blood mononuclear cells (PBM) were stimulated with PHA in the presence or absence of CsA. Flow cytometric examination of PBM loaded with the Ca2+-sensitive dye Indo-1 showed that concentrations of CsA sufficient to inhibit 90-100% of tritiated thymidine incorporation had no effect on the PHA-stimulated increase in the intracellular Ca2+ concentration ([Ca2+]i). Likewise, inhibitory amounts of CsA had virtually no effect on the increase in cell volume that occurs during T lymphocyte activation. These results were not altered by pretreating the PBM with CsA for 30 min at 37 degrees C prior to adding the PHA. On the other hand, inhibitory concentrations of CsA prevented the expression of receptors for T cell growth factor (interleukin-2, IL-2), as measured by monoclonal antibodies to CD25 after 16-24-hr incubation. In like manner, CsA also prevented the increase in the expression of the E-rosette receptor (CD2) on these same cells. If cultures containing PHA and inhibitory amounts of CsA were incubated for 40-72 h, there was partial recovery both of proliferative activity and of the expression of CD25 and CD2. Thus, CsA does not appear to affect the initial activation signal(s), but does interfere with one or more subsequent events necessary to initiate the appearance of "activation antigens."