Development of a highly sensitive radioimmunoassay for digoxin and its application in pediatric practice

The sensitivity of two established routine digoxin radioimmunoassay methods has been increased to enable the provision of a rapid and relatively atraumatic inpatient and outpatient service for neonates and small children, using capillary blood samples obtained by heel-prick. The methods employ 125I- or 3H-labelled digoxin, a rabbit antiserum raised against a digoxin: bovine serum albumin conjugate and only 10 or 25 microliter of plasma as the sample. The results obtained using these highly sensitive assays correlate closely with those found using conventional assays, requiring larger sample volumes. An apparent difference in sensitivity to digoxin has been demonstrated between infants and children more than 1 yr old. Thus infants appear to tolerate plateau phase plasma levels (mean value for non toxic infants 2.6 +/- 1.8 ng/ml) that in older children or adults would be associated with digoxin toxicity.

Antigen-Initiated B Lymphocyte Differentiation

The characteristics of memory B cell antibody-forming cell (AFC) progenitors from long-term hapten-primed CBA mice were investigated by using sedimentation velocity and buoyant density separation to isolate physically distinct B cell sub-sets. The isolated fractions were assayed by the adoptive immune response to NIP-POL antigen, under conditions where neither T cells nor other accessory cells were limiting the IgM or IgG AFC responses. The results were compared to previous studies on the IgM AFC-progenitors of unprimed adult mice.

Splenic IgM and IgG memory AFC-progenitor activity was largely found among the typical B cells of slow to medium sedimentation rate, in contrast to the faster sedimenting IgM AFC-progenitor activity of unprimed animals. Splenic IgM and IgG memory AFC-progenitor activity was found among the medium to light density cells, and so resembled by this parameter the IgM AFC-progenitor activity in unprimed animals. Thoracic duct lymphocytes from hapten-primed mice also exhibited memory IgM and IgG AFC-progenitor activity in the slow-medium sedimentation range. However, in contrast to spleen, the IgM and IgG memory AFC-progenitor activity in lymph was found among very dense B cells.

Two physically distinct sub-populations of memory B cells have thus been identified, namely: i) small, medium-light density, presumably tissue-resident B lymphocytes found in spleen; and ii) small, dense, presumably recirculating B lymphocytes found in lymph. Both physical forms include IgM and IgG progenitors. Both forms are distinct from the larger, medium-light density “virgin” AFC-progenitors in the spleen of unprimed adult mice.

Antigen-initiated B lymphocyte differentiation. IX. Characterization of memory AFC progenitors by buoyant density and sedimentation velocity separation

The characteristics of memory B cell antibody-forming cell (AFC) progenitors from long-term hapten-primed CBA mice were investigated by using sedimentation velocity and buoyant density separation to isolate physically distinct B cell sub-sets. The isolated fractions were assayed by the adoptive immune response to NIP-POL antigen, under conditions where neither T cells nor other accessory cells were limiting the IgM or IgG AFC responses. The results were compared to previous studies on the IgM AFC-progenitors of unprimed adult mice. Splenic IgM and IgG memory AFC-progenitor activity was largely found among the typical B cells of slow to medium sedimentation rate, in contrast to the fastre sedimenting IgM AFC-progenitor activity of unprimed animals. Splenic IgM and IgG memory AFC-progenitor activity was found among the medium to light density cells, and so resembled by this parameter the IgM AFC-progenitor activity in unprimed animals. Thoracic duct lymphocytes from hapten-primed mice also exhibited memory IgM and IgG AFC-progenitor activity in the slow-medium sedimentation range. However, in contrast to spleen, the IgM and IgG memory AFC-progenitor activity in lymph was found among very dense B cells. Two physically distinct sub-populations of memory B cells have thus been identified, namely: i) small, medium-light density, presumably tissue-resident B lymphocytes found in spleen; and ii) small, dense, presumably recirculating B lymphocytes found in lymph. Both physical forms include IgM and IgG progenitors. Both forms are distinct from the larger, medium-light density "virgin" AFC-progenitors in the spleen of unprimed adult mice.

Antigen-initiated B-lymphocyte differentiation. VII. Quantification of AFC progenitor levels in adoptive and culture responses to NIP-POL antigen

Quantitative studies on B cells require a direct assay for antibody-forming cell (AFC) progenitor function, in which the number of AFC produced bears a simple, linear arithmetic relationship to the number of progenitors present. This might be expected under conditions where helper T-cell and accessory cell requirements are by-passed, or provided in excess. This possibility has been tested using as antigen the hapten NIP (4-hydroxy-3-iodo-5-nitrophenylacetic acid) on the carrier POL (polymerized bacterial flagellin), in adoptive transfer of normal and nude mouse spleen cells to irradiated recipients, and in cell culture. Primary and secondary IgM responses to this antigen are "T cell-independent'. The secondard IgG response is T cell-dependent but this function can be provided by 'carrier-primed' irradiated recipients. However in no case did the cell dose response curve show a linear, arithmetic relationship between cells transferred or cultured, and AFC produced. If less than 10 X 10(6) cells were adoptively transferred or cultured, a sigmoid curve was obtained, approximately linear with a slope of around 1-6 on a log-log scale. In adoptive transfer, a plateau was then seen above 10 X 10(6) cells, followed by a second sharp rise beginning around 15 X 10(6) cells. Addition of irradiated spleen cells as 'fillers' to maintain cell numbers constant produced a linear (arithmetic scale) dose response curve for the primary IgM responses, both adoptive and in culture. Lipopolysaccharide injection of recipients also produced linear regions in the adoptive transfer system. These techniques provide more direct, quantitative assay systems for the primary IgM responses to this antigen. However, arithmetic linear cell dose response curves were still not obtained for the secondary IgG responses, using irradiated filler cells.

A rapid, simple assay for digoxin

An extremely rapid radioimmunoassay for digoxin is described which is precise over the range of concentrations required to determine whether, or not, a patient has digoxin toxicity. The assay is based on the use of (125)-iodine-labelled digoxin and of a gel equilibration technique for the separation of antibody-bound and free digoxin. The results obtained compare closely with those by a conventional radioimmunoassay and the technique is sufficiently simple to enable its performance by relatively inexperienced laboratory staff.

Myocardial and skeletal muscle concentrations of digoxin in patients on long-term therapy

The digoxin content was measured in samples of left ventricular papillary muscle, skeletal muscle, and plasma obtained during mitral valve replacement from eight patients on maintenance treatment with the drug. The content in papillary muscle ranged from 15.5 to 132 ng/g (mean 77.7) and in skeletal muscle from 7.5 to 23 ng/g (mean 11.3). The ratio of myocardial digoxin concentration to plasma concentration varied between patients from 39:1 to 155:1. No simple relationship exists between plasma levels of digoxin and its concentration in the heart muscle, but total myocardial concentration may not accurately reflect therapeutic activity.

Plasma concentrations of digoxin after oral administration in the fasting and postprandial ste

After the oral administration of 0.5 mg of digoxin in tablet form to fasting subjects peak plasma levels were reached in 30 to 60 minutes. Levels then fell to reach a plateau at six to eight hours. When the same dose was given after food the peak plasma concentrations were significantly lower, but the concentrations reached in samples obtained from two to eight hours after the dose did not differ appreciably from corresponding samples obtained in the fasting experiments.In a four-week cross-over study of 21 patients on maintenance therapy, digoxin taken regularly in the fasting state produced plasma concentrations similar to those obtained when the drug was taken after meals.The rapid appearance of digoxin in the blood suggests that the oral route of administration is adequate for most patients who require rapid digitalization, and the timing of maintenance dosage in relation to meals is unimportant.

Rapid, sensitive assay for staphylococcal enterotoxin and a comparison of serological methods

Reversed passive hemagglutination was used to assay enterotoxin in culture filtrates and in food samples. With cells tanned and then sensitized with antitoxin globulin and preserved with either formaldehyde or pyruvic aldehyde, as little as 0.0007 mug of enterotoxin was detectable. The results of hemagglutination tests compared well with those obtained by quantitative precipitin tests or by immunodiffusion, but hemagglutination was 50 to 100 times more sensitive than the immunodiffusion technique. In addition, results of the hemagglutination test were available within a few hours, and neither elimination of interfering proteins from food extracts nor concentration of the sample, both of which are necessary for immunodiffusion, was required for this procedure.