Mechanisms of Na+ transport in human distal colonic apical membrane vesicles.

Apical membrane vesicles purified from mucosal scrapings obtained from distal segments of organ donor colons and a 22Na-uptake technique were used to characterize the mechanism(s) of Na+ transport into these vesicles. An outwardly directed H+ gradient (pH 5.5in/7.5out) markedly increased uptake of 22Na into these vesicles. Osmolarity studies demonstrated that 22Na was taken up into the intravesicular space with minimal binding observed to the surface of the vesicles. Voltage clamping in the presence of K+/valinomycin reduced the H+ gradient-dependent 22Na uptake into these vesicles by approximately 45% and generation of an inside negative membrane potential significantly increased 22Na uptake. Under non voltage clamped conditions, H+ gradient-dependent 22Na uptake into these vesicles was significantly inhibited by specific inhibitors of Na(+)-H+ exchange (DMA, HMA and EIPA) as well as by inhibitor of epithelial Na+ channels (phenamil). Under voltage clamped conditions, H+ gradient-dependent 22Na uptake, however, was unaffected by phenamil (20 microM), but was almost completely inhibited by DMA, HMA and EIPA (20 microM each). The mechanism of amiloride inhibition of electroneutral Na(+)-H+ exchange was noncompetitive with a Ki for amiloride of 340 microM. Electroneutral 22Na uptake exhibited saturation kinetics with an apparent Km for Na+ of 8.7 +/- 1.7 mM and a Vmax of 2.02 +/- 0.45 nmol/mg per 5 s. The Na(+)-H+ exchange demonstrated cation specificity similar to the Na(+)-H+ exchangers described in other epithelia. These studies demonstrate for the first time that Na+ transport across the apical membranes of human distal colon involves both conductive Na+ uptake and an electroneutral Na(+)-H+ exchange process.

Chloride transport in human proximal colonic apical membrane vesicles.

The mechanism(s) of Cl- transport across the human colonic apical membranes are not well understood. Apical membrane vesicles (AMV) purified from organ donor proximal colonic mucosa and a rapid millipore filtration technique were utilized to study 36Cl- uptake into these vesicles. Outwardly directed OH- and HCO3- gradient stimulated 36Cl- uptake into these vesicles demonstrating a transient accumulation over equilibrium uptake. Voltage clamping the membrane potential of the vesicles or making them inside positive with K+/valinomycin failed to influence chloride uptake, indicating that the conductive Cl- uptake pathway is minimal in proximal colonic AMV. Anion exchange inhibitors, DIDS and SITS (1 mM) inhibited OH- and HCO3- stimulated 36Cl- uptake by approximately 60%. Furosemide also demonstrated a small but significant inhibition of chloride uptake. Amiloride, bumetanide and acetazolamide (1 mM) failed to inhibit 36Cl uptake. HCO3- and pH gradient stimulated 36Cl- uptake exhibited saturation kinetics with an apparent Km for chloride of 4.0 +/- 0.7 mM and Vmax of 17.8 +/- 3.9 nmol/mg per min. Bromide, chloride, nitrate and acetate (50 mM each) inhibited 5 mM 36Cl uptake. Inwardly directed gradients of Na+, K+, or Na+ and K+ did not stimulate 36Cl- uptake into these vesicles, indicating that uptake of Na+ and Cl- in human proximal colonic AMV does not involve Na-Cl or Na-K-2Cl cotransport. The above findings indicate that chloride transport in human proximal colonic AMV involves an electroneutral Cl-HCO3- (OH-) exchange process. In view of the previous demonstration of Na+-H+ antiporter in these vesicles, dual ion exchange mechanism of Na+-H+ and Cl-HCO3- in apical membrane domain of human colonocytes is postulated to be the primary mechanism for NaCl absorption in the human proximal colon.

Tn-polyagglutinability associated with acute myelomonocytic leukemia.

A case of Tn polyagglutinability in a patient with acute myelomonocytic leukemia is described. This represents the third report in which Tn polyagglutination has been associated with a leukemic state. Serologic recognition of Tn polyagglutination and probable causes are discussed. Known cases of Tn-polyagglutinability are reviewed.

Cost-effectiveness studies of medical and chiropractic care for occupational low back pain. a critical review of the literature.

BACKGROUND CONTEXT: Back pain is the single most costly work-related injury. Chiropractors and physicians are the main primary care providers for occupational low back pain (OLBP), but there is no consensus regarding the relative cost-effectiveness of these two modes of care. PURPOSE: To critically appraise and synthesize recent literature on the cost-effectiveness of medical and chiropractic care for OLBP, and to propose a cost-effectiveness methodology that integrates epidemiologic and economic methods for future studies. STUDY DESIGN: Literature review. MEDLINE was searched from 1990 through 1999. Nine articles that met the inclusion criteria were reviewed. The methodological quality of the articles was critically appraised independently by two epidemiologists using standardized review criteria. Two health economists reviewed the studies on cost-effectiveness. RESULTS: The current literature suggests that chiropractors and physicians provide equally effective care for OLBP but that chiropractic patients are more satisfied with their care. Evidence on the relative costs of medical and chiropractic care is conflicting. Several methodological deficiencies limit the validity of the reviewed studies. No studies combine high-quality cost data with adequate sample sizes and controls for confounding factors. CONCLUSION: Existing studies fail to clarify whether medical or chiropractic care is more cost effective. We suggest that future studies must combine epidemiologic and economic methods to answer the question adequately.

Assessing the clinical significance of anti-Cra and anti-M in a chronically transfused sickle cell patient.

An alloantibody to a high-incidence antigen, associated with multiple other alloantibodies, made it impossible to supply antigen-negative red blood cells (RBCs) for a chronically transfused sickle cell anemia patient. Anti-Cra,-E,K,-S, -Fya, -Fyb, as well as anti-M reactive at 37 degrees C and in the antiglobulin phase of testing, were identified in the patient's serum. An extensive search of rare donor files at the American Red Cross and at the American ASsociation of Blood Banks (AABB) failed to identify Cr(a-),M-,E-,K-,S-, Fy(a-b-) donors. Various studies were performed to predict the clinical significance of the anti-Cra and anti-M. Results of 51chromium survival studies showed 91.8 percent survival at 10 minutes and 87.2 percent survival at 60 minutes with Cr(a +),M-, K-,S-,Fy(a-b-) donors. Various studies were performed to predict the clinical significance of the anti-Cra and anti-M. Results of 51chromium survival studies showed 91.8 percent survival at 10 minutes and 87.2 percent survival at 60 minutes with Cr(a +),M-,E -,K-,S-,Fy(a-b-) red cells, suggesting that immediate destruction of transfused CrCa+) red cells would he unlikely. However, further analysis revealed diminished long-term survival of the donor's red cells with only 60.1 percent recovery at six days (T 1/2 = 12 days) and 10.8 percent at 14 days (T 1/2 = 4.5 days). A monocyte- monolayer assay (MMA) indicated that both the anti-Cra (5.9%) and anti-M (18%) would probably be clinically significant (normal value 0-3%). Mass screening continues at several blood centers for Cr(a-),M-, E-,K-,S-,Fy(a-b-) donors. However, if no suitable donors are found, the results of the 51chromium survival studies and the MMA support the decision to transfuse this patient with Cr(a+),M-,F(a- b-),S-,K- ,E- red cells, if necessary.

Why is the treatment of work-related injuries so costly? New evidence from California.

There is growing evidence that workers' compensation insurers are charged substantially more than health insurers for the treatment of similar injuries. The first study of the problem, conducted in Minnesota in 1987, found that both overutilization of services and price discrimination contributed to the charge differential. This article applies the Minnesota model to 1991-1993 data on health care charges and payments from California. Approximately 13,000 persons with work-related injuries are compared to approximately 3,600 persons with similar injuries that occurred off the job. Despite important differences in the populations and workers' compensation laws in California and Minnesota, workers' compensation insurers are charged more than health insurers for the treatment of similar injuries in both states. The difference in California's payments is attributable to using more health care providers and services to treat workers' compensation patients. The results do not support the hypothesis that work-related injuries cost more to treat because they are more severe than similar injuries occurring off the job.

The effects of impairments on employment and wages: estimates from the 1984 and 1990 SIPP.

Unlike the minority groups covered by civil rights laws in the past, the disabled population is a heterogeneous group. Because of differences in the nature and onset of health conditions, it is important to study the labor market experiences of different impairment groups separately, rather than treating "disabled workers" as a single group. This article uses data from the 1984 and the 1990 panels of the Survey of Income and Program Participation to analyze trends in the employment and wages of six impairment groups in the years immediately preceding the ADA. The results confirm the diversity of labor market experiences within the disabled population and suggest that policies designed to improve labor market outcomes for workers with disabilities in response to the ADA should be targeted to the different needs of different impairment groups.

HlyX, the FNR homologue of Actinobacillus pleuropneumoniae, is a [4Fe-4S]-containing oxygen-responsive transcription regulator that anaerobically activates FNR-dependent class I promoters via an enhanced AR1 contact.

The hlyX gene of the pig pathogen Actinobacillus pleuropneumoniae encodes HlyX, a homologue of FNR, the anaerobic transcription regulator of Escherichia coli. The hlyX gone complements the anaerobic respiratory deficiencies of E. coli fnr mutants but also induces the expression of an otherwise latent haemolysin. Therefore, FNR and HlyX have distinct but overlapping regulons. The hlyX gene has been overexpressed as a gst::hlyX fusion and the HlyX protein purified. Similar to FNR, HlyX can acquire a [4Fe-4S] cluster, which promotes binding to the FNR box (Kd of 20-30 nM) under anaerobic conditions. Expression of hlyX in E. coli induced the anaerobic production of at least five polypeptides, including the yfiD gene product, which were not induced by fnr. Analysis of the yfiD promoter region revealed the presence of two FNR boxes situated at -61.5 and -114.5. Consistent with this observation, expression from the semi-synthetic Class I promoter FF + 20pmelR was efficiently activated by HlyX but not by FNR. The weaker level of FNR-mediated activation of Class I promoters suggests that there is a poorer activating contact (activating region 1 (AR1) equivalent) between FNR and RNA polymerase at these promoters and that HlyX possesses an additional or improved AR1. The AR1 of HlyX is partially characterized by a surface-exposed region around amino acid A187, which confers the altered specificity and provides an explanation for the existence of distinct but overlapping HlyX and FNR regulons.

Detection of cytomegalovirus antibody in stored blood products using latex agglutination.

The detection of antibody to cytomegalovirus (CMV) in donor sera is one effective method for the prevention of posttransfusion CMV infection in seronegative recipients. A passive latex agglutination test (CMV Scan, Hynson, Wescott & Dunning, Baltimore, MD) has been shown to be an acceptable method of screening sera from the blood donor population. The procedure for this test, however, does not permit the testing of stored donor blood products. To evaluate the feasibility of testing blood components at various times during storage, the authors examined 25 CMV-positive and 25-CMV negative samples of CPDA-1 plasma from 50 units each of platelets and red cells. Plasma samples from platelets stored at 22 degrees C were tested on each of the 5 days of storage. Samples from red cell units stored at 2 to 6 degrees C were tested on storage Days 1, 2, 4, 6, 8, 10, 12, and 14. Of 250 tests done on plasma from platelet units, there were 123 true-positive and 123 true-negative results (sensitivity and specificity, 98.4%). Of 400 tests on plasma from red cell units, there were 200 true-positive and 198 true-negative results (sensitivity, 100%; specificity, 99.5%). These data show that the CMV Scan test can be used reliably to test segments of CPDA-1 plasma from platelets stored for up to 5 days and from red cells stored for up to 14 days.

The first example of a Raddon-like antibody as a cause of a transfusion reaction.

A serum Raddon defining a low-frequency red blood cell antigen FR, and its association with the Miltenberger complex of antigens, were reported in 1977. An antibody of the same serological specificity was the cause of a transfusion reaction. The patient's reaction and serological response, as well as the frequency of Raddon and its inheritance, are described.

Clinical high-risk designation does not predict excess fetal-maternal hemorrhage.

During a 5-year period, an enzyme-linked antiglobulin test was used to screen and quantitate fetal-maternal hemorrhage in 789 consecutive D-negative mothers who were delivered of D-positive babies. Six hundred seventy-two patients (85.2%) had no detectable fetal-maternal hemorrhage, and 117 patients (14.8%) had a detectable fetal-maternal hemorrhage. Eight of the 789 (1%) had a fetal-maternal hemorrhage greater than 30 ml and required more than one vial of Rh immune globulin. Two patients with fetal-maternal hemorrhage of 29 and 30 ml also received additional Rh immune globulin. Each case was reviewed for the presence of high-risk features that are thought to predict patients at risk for fetal-maternal hemorrhage. Patients having a cesarean section or complicated vaginal delivery were considered to be in a group at high risk for fetal-maternal hemorrhage, while those with a spontaneous vaginal delivery were considered not to be at risk for fetal-maternal hemorrhage. Thirty-two of 237 patients (13.5%) in the risk group and 82 of 552 patients (15.3%) in the group not at risk had detectable fetal-maternal hemorrhage. The incidence of fetal-maternal hemorrhage for these two groups was not statistically different (p greater than 0.50 by chi 2 analysis). If only patients in the risk group had been screened for fetal-maternal hemorrhage, then five of 10 (50%) who required more than one vial of Rh immune globulin would have been undertreated and at risk for developing anti-D antibodies. In addition, newborn birth weight, Apgar scores, and hematocrits were examined for 13 cases of fetal-maternal hemorrhage of greater than or equal to 21 ml, and none of these characteristics could be used to predict patients at risk for fetal-maternal hemorrhage. Therefore, no maternal or newborn characteristics could be identified that would reliably predict patients at risk for fetal-maternal hemorrhage. We conclude that all D-negative patients with D-positive babies should continue to be screened for fetal-maternal hemorrhage to identify those patients requiring more than one vial of Rh immune globulin.

Downregulation of Escherichia coli yfiD expression by FNR occupying a site at -93.5 involves the AR1-containing face of FNR.

The promoter of the FNR-activated yfiD gene of Escherichia coli has an unusual architecture because it contains two FNR sites, an arrangement usually associated with FNR-mediated repression. Investigation of yfiD promoter derivatives with altered FNR sites revealed that occupation of the far upstream FNR site (FNR II) downregulated expression, despite the presence of a FNR dimer activating expression from the promoter proximal site (FNR I). Transcript mapping by primer extension, and mutagenesis of potential -10 elements, indicated that yfiD expression is driven from a single FNR-dependent promoter with FNR sites at -40.5 (FNR I) and -93.5 (FNR II). However, yfiD mRNA is processed in stationary-phase cultures independently of rne, rpoS, ihfA and fis to yield transcripts lacking 12 and 21 bases from their respective 5' ends. Single amino acid substitutions (G74-->C, F92-->S, A95-->P, R184-->P, P188-->A or L193-->P) in the surface of FNR that contains activating region 1 (AR1 contacts the alpha-subunit of RNA polymerase to promote transcription activation) reduced the inhibitory effect of FNR at FNR II, indicating that this region of the protein may have a role in repression as well as activation. The FNR variant F92-->S was notable because, although it activated transcription of yfiD (two FNR sites), it was unable to activate transcription from model Class I and II promoters, which contain only a single FNR site.

The error of using returns-to-work to measure the outcomes of health care.

This article uses data from The Survey of Ontario Workers With Permanent Impairments. the world's largest survey of injured workers, to show that, as currently used, return-to-work is a misleading measure of the effectiveness of health care. The article discusses examples of two serious limitations on the use of return-to-work to measure the outcomes of health care, where health care refers to all the medical and rehabilitative services provided to a worker following a workplace injury. The first limitation is that return-to-work, like many other outcomes of health care, is influenced by factors that are not directly related to health care. Using a logit model to estimate the determinants of first absences from work after an injury, we find that socioeconomic characteristics, economic incentives, and job characteristics have a significant influence on return-to-work. The second limitation on return-to-work as an outcome measure is that the first return-to-work after an injury, like a hospital discharge, frequently marks the end of only the first of several episodes of work disability caused by the original injury. Using first post-injury returns-to-work as a proxy for recovery, we would assume that 85% of the Ontario workers recovered from their injury when, in fact, 61% had subsequent spells of work disability. We identified four mutually exclusive patterns of post-injury work and work disability. Multinomial logit estimates of the determinants of the patterns show that health care is only one of several influences on return-to-work. The results also demonstrate that if return-to-work is used to measure outcomes, it must be evaluated over a time horizon that permits multiple spells of work disability.

Alloimmunization to D antigen and HLA in D-negative immunosuppressed oncology patients.

D-negative patients may be divided into responders and nonresponders when immunized with D-positive red cells (RBC). Forty-nine D-negative oncology patients who received D-positive RBCs via platelet and white cell transfusions were studied to determine if nonresponders to D were likely to form lymphocytotoxic antibody (LCA). Nine patients developed anti-D in 16 to 390 days (mean = 112) after 2.6 to 481 ml (mean = 106) of D-positive RBCs. Forty patients had no evidence of anti-D after 0.8 to 535 ml (mean = 98) of D-positive RBCs and were followed for 14 to 1275 days (mean = 192). The anti-D group had no prior D-positive RBC transfusions, and two of five women making anti-D had previous pregnancies but no record of anti-D. LCA was found in four of nine (44%) patients with anti-D and in 12 of 40 (30%) patients without anti-D (p less than 0.50). Since both D and antigens HLA are considered highly immunogenic, it is of interest that the ability to form anti-D or LCA does not correlate. In fact, more patients (16/49; 32%) made LCA than anti-D (9/49; 18%). Of the 21 alloimmunized patients, 4 made both antibodies, while 17 had selective alloimmunization. It would thus appear that alloimmunization to D and HLA are not strongly linked and may indeed be unrelated.

Anhydride modified cantharidin analogues: synthesis, inhibition of protein phosphatases 1 and 2A and anticancer activity.

Two series of anhydride modified cantharidin analogues were synthesised and screened for their phosphatase inhibition (PP1 and PP2A) and cytotoxicity in various cancer cell lines (Ovarian A2780, ADDP; Osteosarcoma 143B; and Colon HCT116 and HT29). One series was synthesised by a novel, high yielding one-pot hydrogenation-ring-opening-esterification procedure, the other by acid catalysed acetal formation. Analogues 5-7 and 9 displayed moderate PP2A selectivity (ca. 5- to 20-fold) and inhibition typically in the low microM range (comparable, in some cases to cantharidin). The anticancer activity of these analogues varied with the cell line under study; however, many of them showed selective cytotoxicity for the colon tumour cell lines.

Prostate cancer recurrence in radical surgery patients receiving autologous or homologous blood.

An evaluation of the effects of blood transfusion on recurrence and survival after radical surgery for prostate cancer was performed. Between 1982 and 1986, 315 consecutive patients underwent radical retropubic prostatectomy by a single surgeon; of 309 patients for whom transfusion data were available, 94 received homologous blood (Group I) and 215 received autologous blood or no blood (Group II). At the time of surgery, there were no differences between Group I and Group II with respect to age, preoperative cancer stage, preoperative histologic grade (Gleason grade), prostatic acid phosphatase score, and preoperative potency. At discharge, the groups were similar in the status of neurovascular bundles, capsular involvement, seminal vesicle involvement, lymph node involvement, postoperative Gleason grade, and postoperative potency. No adjuvant hormone therapy or radiation therapy was administered until tumor recurrence. The patients were followed annually by physical examinations and measurements of prostate-specific antigen. Cancer recurrence was detected in 23 (24.5%) Group I patients and 49 (22.7%) Group II patients. These proportions were not significantly different in univariate or multivariate analysis, and the time to recurrence curves overlapped. It is concluded that homologous blood transfusions are not associated with more rapid tumor recurrence or death after radical surgery for prostate cancer than is seen with autologous transfusions. These results differ from previous reports, which suggested that transfusions may cause recurrence of cancer in patients with colorectal or prostate cancer because of the immunosuppressive effects of blood transfusions.

Detection and quantitation of fetal maternal hemorrhage utilizing an enzyme-linked antiglobulin test.

Existing methods to evaluate fetal-maternal hemorrhage depend upon red blood cell agglutination or blood film elution techniques. These tests are insensitive and difficult to quantitate and reproduce. An enzyme-linked antiglobulin test was evaluated to determine its suitability for clinical testing of postpartum candidates for Rh immune globulin administration. Prepared mixtures of Rh positive fetal and Rh negative adult red blood cells approximating fetal maternal hemorrhage ratios of 0-2.0 percent were studied. In 43 assays, the enzyme-linked antiglobulin test consistently detected Rh positive fetal red blood cells in the 0.5 and 0.25 percent mixtures representing a 25 ml and a 12.5 ml hemorrhage, respectively, in a 70-kg woman. The 0.125 percent red blood cell suspension was positive in 85 percent of the assays and the 0.0625 percent suspension was positive in 56 percent of the tests. Agglutination testing by Du variant technique failed to detect 25 percent of the 0.5 percent mixtures. Only 45 percent of tests with the Rh immune globulin crossmatch detected the 0.5 percent mixture. A modified Kleihauer-Betke procedure was as sensitive, but less reproducible than the enzyme-linked antiglobulin test. Forty-seven Rh immune globulin candidates were studied to assess the quantity of fetal maternal hemorrhage. Fourteen patients (29.8%) had detectable Rh positive red blood cells by enzyme-linked antiglobulin tests but all hemorrhages were less than 12 ml; agglutination tests did not detect any fetal red blood cells. We conclude that the enzyme-linked antiglobulin test is a simple, sensitive, and objective procedure for detecting small amounts of Rh positive red blood cells in Rh negative blood and should be applicable to clinical testing of post-partum Rh immune globulin candidates.

In vivo survival of K:18 red cells in a recipient with anti-K18.

Red cell survival, IgG subclass, and mononuclear phagocyte assay studies were performed on a patient with an anti-K + K18 described previously. The 51Cr survival study with Kell negative K:18 red cells showed 76.6 percent survival at 75 minutes with 30.7 percent survival at 24 hours. The anti-K18 was characterized as IgG4 + IgG1. The mononuclear phagocyte assay was 16 percent when the serum was tested with K:18 cells. Among 54,450 ABO compatible donor units tested with this serum, no K:18- unit was found. These studies were undertaken to evaluate the clinical significance of the antibody when red cell support for a chemotherapy course was considered. Our data suggest that transfusion with K:18+ blood would be ineffective and could be used only to provide red cell support in an emergency should compatible units be unavailable.

A clinically significant erythrocyte antibody detectable only by 51Cr survival studies.

Hemolytic transfusion reactions typically are explained by red cell serologic incompatibilities. We describe a patient in whom a clinically significant red cell alloantibody could not be demonstrated, despite the occurrence of several clinically severe hemolytic reactions. Serologic studies using multiple techniques demonstrated only an anti-Bga; these studies included standard procedures as well as more sensitive experimental techniques. A 51Cr survival study using red cells from a random unit, compatible in vitro with conventional techniques, showed 72 percent survival at 1 hour and 7 percent survival at 24 hours. R2R2 (hr" (e) negative) red cells in a second 51Cr survival study showed 90 percent survival at 1 hour and 92 percent survival at 6 hours. The patient was transfused with R2R2 units which were tolerated well and survived normally. Extensive serologic testing still demonstrated only an anti-Bga. A third 51Cr survival study, 10 months after the first study, with an R1R1 (hr" (e) positive) sample showed 90 percent survival at 1 hour and 42 percent survival at 6 hours. A fourth study using a larger aliquot of R2R2 (hr"(e)negative) 51Cr-labeled red cells, examined over 2 weeks showed a near normal 21-day survival of 50 percent. These 51Cr survival studies, along with normal survival of hr" (e) negative units, suggest that this patient destroys hr" (e) positive red cells despite negative serologic testing.

Na+ transport in human proximal colonic apical membrane vesicles.

BACKGROUND/AIMS: The mechanisms of Na+ movement across colonocyte plasma membranes in the human colon are not well understood. Current studies were undertaken to investigate Na+ transport pathways in apical membranes of proximal organ donor colons. METHODS: Purified apical membrane vesicles and rapid filtration 22Na-uptake techniques were used. RESULTS: An outwardly directed H(+)-gradient (pH 5.5 in/7.5 out) increased 22Na uptake into these vesicles. H+ gradient-driven 22Na uptake was significantly reduced by voltage clamping with K+/valinomycin, but was significantly stimulated by creation of an inside-negative potential. Potential sensitive 22Na uptake was inhibited by Na+ channel inhibitors phenamil and benzamil. Electroneutral 22Na uptake was insensitive to phenamil and benzamil, but was inhibited by amiloride, 5-(N,N-dimethyl)amiloride, 5-(N,N-hexamethylene)amiloride, and 5-(N-ethyl-N-isopropyl)amiloride. Electroneutral 22Na uptake showed saturation kinetics with an apparent Michaelis constant for Na+ of 11.8 +/- 2.4 mmol/L and a maximal velocity of 2.5 +/- 0.6 nmol.mg protein-1 x 5 s-1. The mechanism of amiloride inhibition was noncompetitive with an inhibitor constant for amiloride of 325 mumol/L. Acetazolamide, furosemide, bumetanide, 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stibene, and 4,4'-di-isothiocyanatostilbene-2,2'-disulfonic acid (1 mmol/L each) failed to inhibit 22Na uptake. Li+ and NH4+ (but not Cs+, K+, or choline+) inhibited H(+)-gradient driven 22Na uptake. CONCLUSIONS: Na+ transport in human proximal colonic apical membrane vesicles involves both conductive Na+ transport and an electroneutral Na(+)-H+ exchange.