Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “COS Cells”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27Linked to original sources
PubMed2002

Regulation of the human vitamin C transporters expressed in COS-1 cells by protein kinase C [corrected].

Protein kinase C (PKC) regulation of l-ascorbic acid transport mediated by the Na+/ascorbic acid transporters, hSVCT1 and hSVCT2, expressed in COS-1 cells was studied using recombinant carboxyl-terminal V5 epitope-tagged forms of the transporters. The PKC activator phorbol 12-myristate 13-acetate (PMA) caused a time-dependent and concentration-dependent decrease (40-60%) in ascorbic acid transport activity. Effects of PMA were not observed with the inactive phorbol ester 4 alpha-phorbol and were reversed by treatment of the cells with the PKC-specific inhibitor Ro-31-8220. Kinetically, the reduction in hSVCT1 and hSVCT2 activity arose from a decrease in maximal velocity with no change in the apparent affinity. Western blot and confocal microscopy analyses indicated that the total pool of hSVCT1 or hSVCT2 proteins expressed in the transfected COS-1 cells remained unaffected by PMA treatment. For hSVCT1 the decrease in L-ascorbic acid correlated with a redistribution of the transporter from the cell surface to intracellular membranes. However, for hSVCT2 there was no apparent change in transporter distribution, suggesting that the PKC-dependent modulation of L-ascorbic acid transport mediated by hSVCT2 was the result of reduced catalytic transport efficiency.

Animals↗
PubMed1999

Functional characterization of atherosclerosis-associated Ser128Arg and Leu554Phe E-selectin mutations.

The cellular adhesion molecule E-selectin is expressed on activated endothelial cells, and is involved in the process of adherence of blood cells to vessel endothelium in inflammatory events such as atherosclerosis. In a recent study we found a Ser128Arg mutation in the EGF domain as well as a Leu554Phe mutation in the membrane domain of E-selectin. We also established increased frequencies of both mutations among young patients with severe coronary atherosclerosis. In the present study we investigated the influence of these mutations on cell adhesion and on the release of soluble E-selectin. Mutants were created by site-directed mutagenesis and COS cells were transfected with E-selectin, either wild-type or mutant. Antibody-binding studies and cell-adhesion assays were performed on transfected COS cells and on interleukin-1 beta-stimulated HUVECs. Soluble E-selectin in supernatants of wild type and Leu554Phe mutant-transfected COS cells was measured by ELISA. We discovered significant differences in the strength of HL-60 cell adhesion for the Ser128Arg mutant: in comparison with the wild type, the strength of adhesion to the mutant was reduced on transfected COS cells (P < 0.01) as well as on stimulated HUVECs (P < 0.01). Significantly diminished release of soluble E-selectin was detected for the Leu554Phe membrane domain mutant, in comparison with the wild type. In summary, the mutations studied here influence the E-selectin function in vitro and may be considered as one of the risk factors involved in the complex pathogenesis of atherosclerosis.

Animals↗
PubMed1995

Production of bioactive enkephalin from the nonendocrine cell lines COS-7, NIH3T3, Ltk-, and C2C12.

Enkephalin is synthesized from proenkephalin in neuroendocrine cells. For the attempt to induce nonneuroendocrine origin cells to produce enkephalin, we used a mammalian expression vector for fusion peptides, pMEproCT beta, in which a fused peptide is designed to be cleaved by a yeast Kex2-like endoprotease furin. Met-Enkephalin was expressed in four nonneuroendocrine cell lines: COS-7, C2C12, Ltk-, and NIH3T3. The four cell lines produced a marked amount of Met-enkephalin, which appeared as a single peak on reverse-phase HPLC. Because transplantation of adrenal medullary cells to the subarachnoid space has been used to alleviate terminal cancer pain, and enkephalin appears to play a central role in relieving pain, this enkephalin expression vector may be useful for direct enkephalin expression in pericancerous tissues.

3T3 Cells↗
PubMed2002

Only the large soluble form of preadipocyte factor-1 (Pref-1), but not the small soluble and membrane forms, inhibits adipocyte differentiation: role of alternative splicing.

We originally identified preadipocyte factor-1 (Pref-1) as an inhibitor of adipogenesis by the fact that constitutive expression of full-length Pref-1A inhibits differentiation of 3T3-L1 cells into adipocytes. Subsequently, we found that the membrane form of Pref-1 is proteolytically processed at two sites in the extracellular domain, resulting in the larger (50 kDa) and smaller (25 kDa) soluble forms. A specific form(s) of Pref-1, which is active in inhibiting adipocyte differentiation, has not been elucidated. Here, various artificial constructs and alternative-splicing variants of Pref-1 were stably transfected into 3T3-L1 cells, or conditioned media from COS cells transfected with the various forms were added into differentiating 3T3-L1 cells. Judging by Oil Red O staining for lipid accumulation and expression of adipocyte markers, we determined that, unlike the full-length Pref-1A and the constructed large soluble form, the artificial membrane form of Pref-1 lacking the processing site proximal to the membrane was not effective in inhibiting adipogenesis. Furthermore, conditioned media from COS cells transfected with the construct containing only the first three epidermal growth factor repeats, corresponding to the small soluble form, was not effective in inhibiting adipocyte differentiation. Of the four alternative-splicing products, Pref-1A and Pref-1B, which generate both large and small soluble forms, inhibited adipogenesis, whereas Pref-1C and Pref-1D, which lack the processing site proximal to the membrane and therefore generate only the smaller soluble form, did not show any effect. We conclude that only the large soluble form, and not the transmembrane or the small soluble form, of Pref-1 is biologically active and that alternative splicing therefore determines Pref-1 function in adipocyte differentiation.

3T3 Cells↗
PubMed1999

Caveolin-1 interacts with the insulin receptor and can differentially modulate insulin signaling in transfected Cos-7 cells and rat adipose cells.

Caveolae may function as microdomains for signaling that help to determine specific biological actions mediated by the insulin receptor (IR). Caveolin-1, a major component of caveolae, contains a scaffolding domain (SD) that binds to a caveolin-1 binding motif in the kinase domain of the IR in vitro. To investigate the potential role of caveolin-1 in insulin signaling we overexpressed wild-type (Cav-WT) or mutant (Cav-Mut; F92A/V94A in SD) caveolin-1 in either Cos-7 cells cotransfected with IR or rat adipose cells (low and high levels of endogenous caveolin-1, respectively). Cav-WT coimmunoprecipitated with the IR to a much greater extent than Cav-Mut, suggesting that the SD is important for interactions between caveolin-1 and the IR in intact cells. We also constructed several IR mutants with a disrupted caveolin-1 binding motif and found that these mutants were poorly expressed and did not undergo autophosphorylation. Interestingly, overexpression of Cav-WT in Cos-7 cells significantly enhanced insulin-stimulated phosphorylation of Elk-1 (a mitogen-activated protein kinase-dependent pathway) while overexpression of Cav-Mut was without effect. In contrast, in adipose cells, overexpression of either Cav-WT or Cav-Mut did not affect insulin-stimulated phosphorylation of a cotransfected ERK2 (but did significantly inhibit basal phosphorylation of ERK2). Furthermore, we also observed a small inhibition of insulin-stimulated translocation of GLUT4 when either Cav-WT or Cav-Mut was overexpressed in adipose cells. Thus, interaction of caveolin-1 with IRs may differentially modulate insulin signaling to enhance insulin action in Cos-7 cells but inhibit insulin's effects in adipose cells.

Adipocytes↗
PubMed2000

Characterization of the transport properties of organic anion transporting polypeptide 1 (oatp1) and Na(+)/taurocholate cotransporting polypeptide (Ntcp): comparative studies on the inhibitory effect of their possible substrates in hepatocytes and cDNA-transfected COS-7 cells.

In the present study, we compared the inhibitory effects of organic anions (including bile acids) on the uptake of taurocholate (TC) and estradiol 17beta-D-glucuronide (E(2)17betaG), typical substrates for sodium taurocholate cotransporting polypeptide (Ntcp) and organic anion transporting polypeptide (oatp1), respectively, using primary cultured rat hepatocytes and Ntcp- or oatp1-transfected COS-7 cells. The Na(+)-dependent uptake of TC was inhibited by nine bile acids and five nonbile acid organic anions in a concentration-dependent manner, and their inhibitory effects were similar in both primary cultured rat hepatocytes and Ntcp-transfected COS-7 cells. BQ-123 (1 microM) and indomethacin (10 microM), both of which exhibit no Ntcp-mediated transport, significantly inhibited the Na(+)-dependent uptake of TC mediated by Ntcp. In addition, the Na(+)-independent uptake of E(2)17betaG was inhibited by 15 organic anions in a concentration-dependent manner, and their inhibitory effects were similar between primary cultured rat hepatocytes and oatp1-transfected COS-7 cells. BQ-123 (1 microM), pravastatin (1 microM), and indomethacin (10 microM), all of which do not undergo oatp1-mediated transport, significantly inhibited the Na(+)-independent uptake of E(2)17betaG mediated by oatp1. These results are consistent with the hypothesis that the hepatic uptake of TC and E(2)17betaG is predominantly mediated by Ntcp and oatp1, respectively. In addition, it was clearly demonstrated that we cannot refer to the substrate specificity of transporters based on inhibition studies.

Animals↗
PubMed1999

N-linked glycosylation is required for optimal AT1a angiotensin receptor expression in COS-7 cells.

The nature and role of glycosylation in AT1 angiotensin receptor (AT1-R) function were investigated by expressing glycosylation-deficient influenza hemagglutinin (HA) epitope-tagged rat AT1a-Rs (HA-AT1a-Rs) in COS-7 cells. All three asparagine residues (Asn4, Asn176, Asn188) contained within consensus sites for N-linked glycosylation could be glycosylated in Cos-7 cells and appeared to be glycosylated on the endogenous AT1-R in bovine adrenal glomerulosa cells. Heterogeneity of glycosylation at each site accounted for the broad migration pattern of the AT1-R in SDS-PAGE. Mutation at each glycosylation site, either alone or in combination, had little effect on ligand binding parameters (although the N4K mutant had higher affinity) or signaling activity. However, an increasing number of mutated glycosylation sites was associated with decreasing cell surface receptor expression, which was minimal for the unglycosylated N4K/N176Q/N188Q receptor. Decreased surface expression of mutant HA-AT1a-Rs was correlated with decreased total cell receptor content as revealed by immunoblotting with an anti-HA antibody. These findings suggest that glycosylation enhances receptor stability, possibly by protecting nascent receptors from proteolytic degradation.

Affinity Labels↗
PubMed1991

The hydrophobic amino-terminal sequence of bovine 17 alpha-hydroxylase is required for the expression of a functional hemoprotein in COS 1 cells.

The endoplasmic reticulum is a major site of localization for eukaryotic cytochrome P-450 mixed-function oxidase complexes. Previous studies have shown that the microsomal forms of P-450 insert into the membrane via their hydrophobic amino terminus through the signal recognition particle-dependent pathway. We have examined the insertion of bovine 17 alpha-hydroxylase (P45017 alpha) into the endoplasmic reticulum of COS 1 cells to evaluate the functional role of its hydrophobic amino-terminal sequence and membrane insertion. An NH2-terminal truncated protein, P450 delta 2-17, which lacked amino acids 2-17 was expressed in COS 1 cells, subcellular fractions were isolated, and P450 delta 2-17 was localized by immunoblot analysis. Compared to the full-length P45017 alpha, the NH2-terminal truncation resulted in a 2.5-fold decrease in P45017 alpha protein recovered with the microsomal fraction, 50% of which was an integral membrane protein as defined by resistance to Na2CO3 extraction. Despite correct membrane localization, P450 delta 2-17 was not a functional enzyme in COS 1 cells. A CO difference spectrum of microsomes containing P450 delta 2-17 did not give a typical 450 nm absorbance. We conclude that the hydrophobic amino terminus is required for the expression of a functionally competent P45017 alpha in COS 1 cells and suggest that the insertion of the amino terminus into the membrane is necessary for the folding of this protein into its correct structural form.

Amino Acid Sequence↗
PubMedDate not supplied

Functional analysis of Ran/TC4 as a protein regulating T-cell costimulation.

Antigen (Ag)-triggered activation of T cells requires engagement of both the T-cell Ag receptor and a costimulatory receptor, for which CD28 can function as a prototypical example. CD80 and CD86 represent ligands for this receptor, and although they are present on professional Ag-presenting cells, these molecules are absent from most tumors. Yet some tumors are still able to costimulate a T-cell response, while others cannot. Therefore, a key question concerns the molecular basis for the costimulation of T cells by those tumor cells not expressing the CD28 ligands CD80 and CD86. Upon screening a cDNA library of such a tumor cell line in a transient COS cell transfection assay for costimulatory activity, we identified Ran/TC4 as a protein whose overexpression results in costimulatory activity. Ran/TC4 is a ubiquitously expressed member of the Ras gene superfamily of small guanosine triphosphate-binding proteins and is involved in nuclear transport; Ran/TC4 cDNA-transfected COS cells specifically costimulate CD8 T cells and not CD4 T cells. Transfection of Ran/TC4 into the costimulation-deficient murine RMA lymphoma cell line introduced costimulatory capacity for CD8 T cells and resulted in markedly elevated levels of nuclear Ran/TC4 protein expression. In addition, in vivo priming of mice with Ran/TC4-transfected RMA cells induced protection against wild-type (wt) RMA tumor cells. Ran/TC4-transfected RMA cells and wt RMA tumor cells exhibit comparable in vivo growth rates in mice lacking T and B cells, and Ran/TC4-mediated tumor rejection thus involves B and/or T cells. This possibility is substantiated by the observation that T cells from normal mice challenged with Ran/TC4-transfected RMA cells can mount a cytotoxic T-cell response not only against the Ran/TC4-transfected tumor cells but also against wt RMA tumor cells. Based on these results, we conclude that gene transfer-mediated elevations in Ran/TC4 can confer costimulatory function for CD8 T cells to tumor cells. This finding suggests a novel application of Ran/TC4 as a protein capable of regulating costimulation in tumor cells.

Amino Acid Sequence↗
PubMed1996

Fidelity of translesional synthesis past benzo[a]pyrene diol epoxide-2'-deoxyguanosine DNA adducts: marked effects of host cell, sequence context, and chirality.

We have used a site-specific approach to investigate the mutagenic potential of (+)- and (-)-trans-anti-benzo[a]pyrene diol epoxide (BPDE) DNA adducts. Oligodeoxyribonucleotides (5'TCCTCCTG1G2-CCTCTC), modified at the exocyclic amino groups of G1 or G2, were incorporated into a single-stranded shuttle vector and introduced into Escherichia coli or simian kidney (COS) cells. This experimental system permits translesional synthesis to proceed in the absence of DNA repair. The presence of (+)- or (-)-BPDE-N2-dG adducts strongly inhibited translesional synthesis in E. coli; induction of cellular SOS functions reduced this blocking effect. Vectors containing (+)-BPDE adducts at G1 or G2 generated mutation frequencies of 19% and 3%, respectively; these values were not altered significantly by induction of SOS functions. In COS cells, (+)-BPDE-modified vectors generated mutation frequencies of 13% at G1 and 45% at G2. In E. coli, the (-)-BPDE adduct generated mutation frequencies of < or = 2% at G1 and G2 and, in COS cells, 13% at G1 and 21% at G2. The predominant mutations in E. coli and COS cells were G-->T transversions targeted to the site of the lesion; however, when G2 was modified, a significant number of targeted G-->A and G-->C mutations were observed in COS cells. We conclude from this study that (+)-and (-)-BPDE-N2-dG adducts pair preferentially to dCMP and dAMP during translesional synthesis in a process that is strongly influenced by the stereochemistry of the adduct, by the bases flanking the lesion, and by host cell factors.

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗
PubMed2002

Ca(2+) signals in Pmr1-GFP-expressing COS-1 cells with functional endoplasmic reticulum.

We studied the role of the Pmr1-containing Ca(2+) store in COS-1 cells endowed with a functional endoplasmic reticulum. Transfected cells could be recognized by using a green-fluorescent-protein (GFP)-tagged form of Pmr1. Pmr1-GFP fluorescence showed a typical juxtanuclear Golgi-like distribution. Pmr1-GFP-containing cells with functional endoplasmic reticulum responded to 100 microM ATP with baseline Ca(2+) spiking, while non-transfected cells produced an initial Ca(2+) peak followed by a long-lasting plateau. The Ca(2+) signal often appeared after a long latency in Pmr1-GFP-expressing cells. ATP-stimulated Pmr1-GFP-expressing cells with functional endoplasmic reticulum responded after a latency period to extracellular Ca(2+) with a regenerative Ca(2+) signal, while non-transfected control cells responded with an immediate slow rise in free cytosolic Ca(2+) concentration. These results demonstrate the importance of the Pmr1-containing Ca(2+) store in generating or modifying cellular Ca(2+) signals.

Adenosine Triphosphate↗
PubMed1996

[High efficiency eukaryotic expression of rhIL-12 in COS-7 cells].

In this study, two high-efficiency eukaryotic expression vectors, PED35 and PED40 containing p35 and p40 cDNA of rhIL-12 respectively, were co-transfected into COS-7 cells by modified DEAE-Dextran method. The COS-7 cell culture supernatant had been certified to contain high level of expressed rhIL-12 protein, nearly 600 IU/ml. Furthermore, in the two biological assays, the expressed rhIL-12 protein can cause the proliferation of human PHA-activated lymphoblasts and augmentation of NK cells mediated cytotoxic activity. This study, therefore, could help further exploration of biological activities and potential usage of rhIL-12.

Animals↗
PubMed1993

A spike protein-dependent cellular factor other than the viral receptor is required for mouse hepatitis virus entry.

Previous studies have shown that some mouse strains are resistant to mouse hepatitis virus (MHV) infection despite the presence of functional viral receptors (K. Yokomori and M. M. C. Lai, J. Virol. 66, 6931-6938, 1992). To determine the molecular requirement for MHV infection, several cell lines derived from both susceptible and resistant mouse strains were tested for their ability to support infection by two different MHV strains, JHM and A59. Most of the cell lines tested, including ones from susceptible mouse strains, exhibited selective resistance to JHM, but were susceptible to A59, suggesting that there is an additional cellular factor(s) discriminating JHM from A59 infection. Both RNA and protein syntheses of JHM were inhibited in the resistant cells; however, transfection of JHM genomic RNA into these cells led to the production of infectious virus, suggesting that the restriction step(s) is during an early stage of viral replication cycle. The mRNA for the MHV receptor (the murine homolog of the carcinoembryonic antigen) is expressed in all cell lines, and expression in COS cells of the receptor isolated from the resistant murine cell lines rendered the COS cells susceptible to both A59 and JHM infections. Furthermore, the transfection of additional MHV receptors into the resistant cells did not overcome the resistance to JHM virus infection. These results suggested that the viral receptor is functional; nevertheless, the JHM infection is restricted at an early step of infection in these cells. The study of the growth properties of the various recombinant viruses between A59 and JHM revealed that one of the viral genes determining viral replication in these cell lines is the S protein gene; thus, the second factor required for viral infection may interact directly or indirectly with the S protein at an early step of infection. Taken together, these studies suggest that expression of a functional viral receptor is not sufficient to establish MHV infection, and that an additional factor(s) is required for an early step of viral infection, possibly during virus entry.

Animals↗
PubMed1999

Promotion of tyrosinase folding in COS 7 cells by calnexin.

To understand the process of expression of tyrosinase, a key enzyme of melanogenesis, we examined its maturation in the endoplasmic reticulum (ER) by using a heterogeneous expression system. When human tyrosinase cDNA was introduced into COS 7 cells, tyrosinase activity was minimally detected. Immunofluorescence study revealed that tyrosinase was immunolocalized in the nuclear rim, the reticular network, and the punctuated structures. Because a cytoplasmic tail of tyrosinase-gene family protein functions as a lysosomal targeting signal in non-melanocytic cells, and immature and/or misfolded molecules are selectively retained in the ER, the observed localization suggested the inefficient maturation in the COS 7 cells. We thus examined if supplementation of calnexin, a membrane-bound chaperone with affinity for oligosaccharide-processing intermediates containing monoglucose, could improve the process. As expected, the activity was enhanced approximately 2-fold by co-transfection of cDNA encoding calnexin. In contrast, co-transfection of the cytosolic tail-free calnexin, which inhibits calnexin function by allowing premature egress of its ligands from the ER, suppressed expression of this enhanced tyrosinase activity. When alpha-glucosidase activity, which is required for calnexin function, was inhibited by castanospermine (CST) treatment, expression of tyrosinase activity was completely abolished. To confirm the direct involvement of calnexin in tyrosinase maturation, the interaction of calnexin with tyrosinase was examined. Immunoprecipitation of calnexin from extracts of [35S]methionine labeled cells with anti-calnexin antibody revealed that the association is highest immediately after the pulse and that nascent tyrosinase is gradually dissociated upon chase. The association was completely inhibited when CST was included in the medium. Hence, we suggest that the proper folding of tyrosinase is largely dependent on its direct interaction with calnexin for the determined duration in the ER.

Animals↗
PubMed2006

[Expression of alpha-defensin-1 with J chain in the transfected COS-7 cells].

OBJECTIVE: To recombine the alpha-defensin-1 (HNP-1) and J chain cDNA fragment into a new germicidal molecule J-HNP-1, and investigate its expression in the transfected COS-7 cells. METHODS: J chain and HNP-1 cDNA were amplified from the plasmids pCH-J and pBabeNeo-HNP-1 respectively, then the J-HNP-1 DNA fragment was obtained by recombinant polymerase chain reaction (PCR). The J-HNP-1 fragments were inserted into pcDNA3.1 (-)/Myc-HisC, which subsequently was made to transfect the COS-7 cell by lipid transfection method. The expression was confirmed by Western blot. RESULTS: Determination with His antibody, a dense band at 24 ku was displayed by the cell lysate and the cell culture supernatant in concordance with that of the predicted extent, but not in the control. CONCLUSION: The new germicidal peptide J-HNP-1 is found to be expressed intracellularly and could be secreted. The results lay the foundation for further study on the germicidal activity of J-HNP-1.

Animals↗
PubMed2000

Expression and functional characterization of a pHis-tagged human bradykinin B2 receptor in COS-7 cells.

A polyHis-tagged bradykinin (BK) B2 receptor (pHis-BKR) cDNA was constructed and expressed in COS-7 cells. The pHis-BKR is suitable for both immunoprecipitation and immunoblotting with anti-polyHis antibodies and can be easily purified using Ni-NTA columns. Immunochemical detection revealed a molecular mass of approximately 66 kDa. The pHis-BKR is capable of mediating BK-induced stimulation of inositol phosphate formation as well as of mitogen-activated protein kinase (MAPK) activity. Compared with the wild-type receptor (WT-BKR) the tagged receptor showed a slightly enhanced affinity towards BK but a reduced expression level. Despite these modified pharmacological properties the pHis-tagged BKR may be a useful tool for studying BKR modifications and signaling.

Adrenergic beta-Antagonists↗
PubMed2006

Human RAB24, interestingly and predominantly distributed in the nuclei of COS-7 cells, is colocalized with cyclophilin A and GABARAP.

As an atypical member of the Rab family, Rab24 has several attributes distinguishing this protein from the other members. Based on the yeast two-hybrid system, interaction between human RAB24 and two proteins, cyclophilin A (CyP-A) and gamma-aminobutyric acid type A receptor-associated protein (GABARAP), was detected and identified in COS-7 cells. Interestingly, RAB24 is predominantly localized in the nuclei of COS-7 cells, which is different from previous reports using other cell lines. RAB24 (D123I) can trigger the accumulation of intracellular inclusions, with small quantities of intranuclear inclusions in some cells. The GTPase activity of RAB24 and its two mutants was detected.

Adaptor Proteins, Signal Transducing↗
PubMed1995

Quinine sensitive changes in cellular Na+ and K+ homeostasis of COS-7 cells caused by a lipophilic phenol red impurity.

An impurity of phenol red (PRI) has been shown to markedly alter the intracellular Na+ and K+ homeostasis of several cell types. The effect of PRI seems to involve intracellular Ca(++)-dependent mechanisms. Using COS-7 cells as a model, we further characterized the mechanism of action of PRI by measuring cellular Na+/K+ contents and 86Rb+ efflux. Similar to human skin fibroblasts, in COS-7 cells calmodulin inhibition moderated the cationic transport effects of PRI. A TMB-8 dependent intracellular Ca++ pool does not seem to be involved in these transport events. We found no evidence for participation of the transcriptional-translational machinery in the effect of PRI. Both quinine and quinidine are able to prevent nearly all changes caused by PRI in the cellular Na+/K+ contents and 86Rb+ efflux. Although phenol red contained multiple impurities by high performance liquid chromatography (HPLC), phenolphthalein, a structurally close relative of phenol red, was free of any detectable contamination. Phenolphthalein elicited qualitatively similar transport changes to those observed during exposure to PRI. Regardless of the exact mechanism of action, we propose that the as yet unidentified substance is not a cellular toxin, rather it is a cationic transport modulator. Directly or indirectly, it may interact with the cellular Ca++/calmodulin system and activate some quinine/quinidine sensitive transport processes. This transport process is likely to be a Ca(++)-sensitive K+ channel but, due to the lack of specificity of quinine and quinidine, other transport mechanisms must be also considered. The chemical nature of PRI may be similar to phenolphthalein.

Animals↗