Interindividual Variation In Sensitivity Research Articles

Characterization of the genetic determinants of context-specific DNA methylation in primary monocytes

To better understand inter-individual variation in sensitivity of DNA methylation (DNAm) to immune activity, we characterized effects of inflammatory stimuli on primary monocyte DNAm (n= 190). We find that monocyte DNAm is site-dependently sensitive to lipopolysaccharide (LPS), with LPS-induced demethylation occurring following hydroxymethylation. We identify 7,359 high-confidence immune-modulated CpGs (imCpGs) that differ in genomic localization and transcription factor usage according to whether they represent a gain or loss in DNAm. Demethylated imCpGs are profoundly enriched for enhancers and colocalize to genes enriched for disease associations, especially cancer. DNAm is age associated, and we find that 24-h LPS exposure triggers approximately 6months of gain in epigenetic age, directly linking epigenetic aging with innate immune activity. By integrating LPS-induced changes in DNAm with genetic variation, we identify 234 imCpGs under local genetic control. Exploring shared causal loci between LPS-induced DNAm responses and human disease traits highlights examples of disease-associated loci that modulate imCpG formation.

Allostatic load in children: The cost of empathic concern

Early-life adversity affects long-term health outcomes but there is considerable interindividual variability in susceptibility to environmental influences. We proposed that positive psychological characteristics that reflect engagement with context, such as being concerned about people or performance on tasks (i.e., empathic concern), could moderate the interindividual variation in sensitivity to the quality of the early environment. We studied 526 children of various Asian nationalities in Singapore (46.6% female, 13.4% below the poverty line) with longitudinal data on perinatal and childhood experiences, maternal report on empathic concern of the child, and a comprehensive set of physiological measures reflecting pediatric allostatic load assessed at 6 y of age. The perinatal and childhood experiences included adversities and positive experiences. We found that cumulative adverse childhood experience was positively associated with allostatic load of children at 6 y of age at higher levels of empathic concern but not significantly associated at lower levels of empathic concern. This finding reveals evidence for the importance of empathic concern as a psychological characteristic that moderates the developmental impact of environmental influences, serving as a source for vulnerability to adversities in children.

Foraging Experiences Durably Modulate Honey Bees\u2019 Sucrose Responsiveness and Antennal Lobe Biogenic Amine Levels

Foraging exposes organisms to rewarding and aversive events, providing a selective advantage for maximizing the former while minimizing the latter. Honey bees (Apis mellifera) associate environmental stimuli with appetitive or aversive experiences, forming preferences for scents, locations, and visual cues. Preference formation is influenced by inter-individual variation in sensitivity to rewarding and aversive stimuli, which can be modulated by pharmacological manipulation of biogenic amines. We propose that foraging experiences act on biogenic amine pathways to induce enduring changes to stimulus responsiveness. To simulate varied foraging conditions, freely-moving bees were housed in cages where feeders offered combinations of sucrose solution, floral scents, and aversive electric shock. Transient effects were excluded by providing bees with neutral conditions for three days prior to all subsequent assays. Sucrose responsiveness was reduced in bees that had foraged for scented rather than unscented sucrose under benign conditions. This was not the case under aversive foraging conditions, suggesting an adaptive tuning process which maximizes preference for high quality, non-aversive floral sites. Foraging conditions also influenced antennal lobe octopamine and serotonin, neuromodulators involved in stimulus responsiveness and foraging site evaluation. Our results suggest that individuals’ foraging experiences durably modify neurochemistry and shape future foraging behaviour.

Response of puffing pattern to in vivo treatments with organomercurials in Drosophila melanogaster.

Survival rate and length of development time were used as indicators of toxicity of methoxyethylmercuric acetate, methoxyethylmercuric chloride and methylmercuric chloride in Drosophila melanogaster. The two methoxyethyl compounds exhibit toxic effects (prolongation of development time) only in the highest concentration tested (10.0 mg Hg/l), while methylmercury already has toxic effects (lethality and prolongation of development) in concentrations of ≥ 1.25 mg Hg/l. Similar concentrations were used to test the effects of mercuric compounds on genetic balance in 0-hour prepupae, as revealed by the puffing patterns of polytene salivary chromosomes. Four aspects of puffing pattern response were observed in 20% of the 300 inspected larvae: (1) regression of total gene activity, expressed in terms of decreases in puff sizes/chromosome, (2) inhibition of normal puff regression, characterized by puff activity in 2B, 74EF, 75B and 78D which, during normal development, should be regressed by this stage, (3) inhibition of normal puff activation, which is characterized by puffs 3A, 4F, 28A, 46F, 61C, 62C, 62F and 73B being inactive, though they normally should be induced by this stage, and (4) induction of a new locus (45AB) which is not normally observed in a puffed state. Changes in the puffing pattern have been observed when all three mercuric compounds were used, but mainly in concentrations ≥ 1.25 mg Hg/l. Consequently, puffing pattern response does not necessarily correlate with toxic concentrations; instead, the variability in puff response between individuals seems to indicate interindividual variations in sensitivity to mercuric compounds.

Dose–response relationship of dicentric chromosomes in human lymphocytes obtained for the fission neutron therapy facility MEDAPP at the research reactor FRM II

The biological effectiveness of neutrons from the neutron therapy facility MEDAPP (mean neutron energy 1.9 MeV) at the new research reactor FRM II at Garching, Germany, has been analyzed, at different depths in a polyethylene phantom. Whole blood samples were exposed to the MEDAPP beam in special irradiation chambers to total doses of 0.14-3.52 Gy at 2-cm depth, and 0.18-3.04 Gy at 6-cm depth of the phantom. The neutron and gamma-ray absorbed dose rates were measured to be 0.55 Gy min(-1) and 0.27 Gy min(-1) at 2-cm depth, while they were 0.28 and 0.25 Gy min(-1) at 6-cm depth. Although the irradiation conditions at the MEDAPP beam and the RENT beam of the former FRM I research reactor were not identical, neutrons from both facilities gave a similar linear-quadratic dose-response relationship for dicentric chromosomes at a depth of 2 cm. Different dose-response curves for dicentrics were obtained for the MEDAPP beam at 2 and 6 cm depth, suggesting a significantly lower biological effectiveness of the radiation with increasing depth. No obvious differences in the dose-response curves for dicentric chromosomes estimated under interactive or additive prediction between neutrons or gamma-rays and the experimentally obtained dose-response curves could be determined. Relative to (60)Co gamma-rays, the values for the relative biological effectiveness at the MEDAPP beam decrease from 5.9 at 0.14 Gy to 1.6 at 3.52 Gy at 2-cm depth, and from 4.1 at 0.18 Gy to 1.5 at 3.04 Gy at 6-cm depth. Using the best possible conditions of consistency, i.e., using blood samples from the same donor and the same measurement techniques for about two decades, avoiding the inter-individual variations in sensitivity or the differences in methodology usually associated with inter-laboratory comparisons, a linear-quadratic dose-response relationship for the mixed neutron and gamma-ray MEDAPP field as well as for its fission neutron part was obtained. Therefore, the debate on whether the fission-neutron induced yield of dicentric chromosomes increases linearly with dose remains open.

Sixty years of follow-up of Hiroshima and Nagasaki survivors: Current progress in molecular epidemiology studies

This article provides an overview of the on-going molecular epidemiology studies among atomic-bomb survivors conducted at the Radiation Effects Research Foundation in Japan. The focus is on: (a) inter-individual variations in sensitivity to radiation-induced somatic mutations (glycophorin A (GPA) mutations) and their potential relevance to differences in susceptibility to radiation-related cancers and (b) the role of specific mutations/rearrangements in radiation-induced thyroid and colorectal cancers. The glycophorin A mutant fractions showed large differences between the survivors at each of the estimated bone marrow doses. Of note is the finding at doses>or=1 Gy; that the slope of the mutant fraction was significantly higher in the 'cancer group' than in the 'non-cancer group'. This study provided the basis for validating the use of gammaH2AX and reticulocyte micronucleus assays for evaluating radiosensitivity differences and genetic instability, respectively, in our studies in the coming years. Preliminary results from our molecular oncology studies on adult-onset papillary thyroid cancer provide evidence for the induction of RET/PTC rearrangements and BRAF point mutation (both known to be early stage events in adult-onset papillary thyroid cancer) but with a difference: cases associated with the rearrangements were more frequent at high doses, and developed sooner than those with BRAF mutation. In the case of colorectal cancer, the results suggest that radiation exposure might influence microsatellite instability (MSI) status through MSI-related epigenetic and genetic alterations-processes that might occur in the early stage of colorectal carcinogenesis.

Analysis of Thiopurine S-methyltransferase Polymorphism in the Population of Serbia and Montenegro and Mercaptopurine Therapy Tolerance in Childhood Acute Lymphoblastic Leukemia

Thiopurine S-methyltransferase (TPMT) is an enzyme that converts thiopurine drugs into inactive metabolites. It is now well established that interindividual variation in sensitivity to thiopurines can be the result of the presence of genetic polymorphisms in the TPMT gene. The aim of this study was to determine the frequency and type of TPMT polymorphisms in the population of Serbia and Montenegro and to assess its relevance in the management of childhood acute lymphoblastic leukemia (ALL). Blood samples from 100 healthy adults and 100 children with ALL were analyzed for common mutations in the TPMT gene using polymerase chain reaction-based assays. The results revealed that allelic frequencies were 0.2% for TPMT*2, 3.2% for TPMT*3A, and 0.5% for TPMT*3B. A rare TPMT*3B allele was detected in 2 families. No TPMT*3C allele was found. The general pattern of TPMT-variant allele distribution as well as their frequencies in the population of Serbia and Montenegro, is similar to those determined for other Slavic and Mediterranean populations. The ability to tolerate 6-mercaptopurine (6-MP) -based maintenance therapy was used as a surrogate marker of hematologic toxicity. In the study of 50 patients with childhood ALL treated according to the BFM-like protocol, it was found that even TPMT-heterozygous patients are at greater risk of thiopurine drug-related leukopenia (mean duration of period when children missed therapy as a result of leukopenia for TPMT-heterozygous patients was 11.3 weeks vs 3.4 weeks for wild-type genotype patients, P < 0.01). In another group of 50 patients, the TPMT genotype was determined prospectively. The therapy protocol was modified considering their TPMT genotype. Administering reduced 6-MP dosages in the initial phase of maintenance allowed TPMT-heterozygous patients to later receive full protocol doses of both 6-MP and nonthiopurine therapy without omitting therapy resulting from myelotoxicity. These results justify performing TPMT genotyping before initiating thiopurine therapy in all children with ALL to minimize consequent toxicity.

Delta Plots in the Study of Individual Differences: New Tools Reveal Response Inhibition Deficits in AD/HD That Are Eliminated by Methylphenidate Treatment.

The authors highlight the utility of distribution-analytical techniques in the study of individual differences and clinical disorders. Cognitive deficits associated with attention-deficit/hyperactivity disorder (AD/HD) were examined by using delta-plot analyses of performance data (reaction time and accuracy) obtained through the use of a prototypical conflict task, the Eriksen flanker task. In 20 children with AD/HD (compared with matched control participants), overall performance measures indicated a marginal performance deficit. Delta-plot analyses indicated that performance deficits associated with AD/HD involve response inhibition but not automatic response activation. In a within-subjects titration study, the response inhibition deficit was eliminated by methylphenidate treatment, but these effects were highly dose specific. The beneficial effect of methylphenidate was clarified further after correcting for inter-individual variation in sensitivity to medicine dosage.

Effects of the mGluR2/3 agonist LY354740 on computerized tasks of attention and working memory in marmoset monkeys

LY354740 is a recently developed metabotropic glutamatergic receptor 2 and 3 (mGluR2/3) agonist. A high density of mGluR2 has been reported in terminal fields of the perforant path in rodents and humans, suggesting its involvement in cognitive functions mediated by the temporal lobe, including memory. A small number of in vivo studies in rodents have assessed the effects of LY354740 on memory tasks, reporting the induction of impaired memory for spatial orientation in a water maze task and for delayed match and non-match to position in an operant version of these tasks. In the present primate study, we used radioautography to describe the distribution and intensity of (3)H-LY354740 binding in the hippocampal formation of the common marmoset monkey (Callithrix jacchus) relative to the rat. In the major, in vivo part of the study, the effects of systemic LY354740 on computerized tasks of attention and memory were investigated. Adult common marmosets were trained to perform a five-choice serial reaction time (5-CSRT) task and a concurrent delayed match-to-position (CDMP) task from the Cambridge Neuropsychological Automated test Battery (CANTAB). Filter tests of LY354740 effects on motor dexterity and motivation for reward revealed high inter-individual variation in sensitivity; therefore, on the 5-CSRT, subjects were tested at a dose range of 3--10 mg/kg, and on the CDMP, subjects were tested at 1--3 or 3--10 mg/kg. Radioautography revealed a relatively low level of (3)H-LY354740 binding in the marmoset hippocampal formation compared to the rat. Despite low binding, LY354740 reduced sustained-attention accuracy in the 5-CSRT, and reduced accuracy in two stages of the CDMP. The current study provides novel evidence for the importance of mGluR2/3 in the regulation of primate cognitive functioning.

The effect of 29 kV X rays on the dose response of chromosome aberrations in human lymphocytes.

The induction of chromosome aberrations in human lymphocytes irradiated in vitro with X rays generated at a tube voltage of 29 kV was examined to assess the maximum low-dose RBE (RBE(M)) relative to higher-energy X rays or 60Co gamma rays. Since blood was taken from the same male donor whose blood had been used for previous irradiation experiments using widely varying photon energies, the greatest possible accuracy was available for such an estimation of the RBE(M), avoiding the interindividual variations in sensitivity or differences in methodology usually associated with interlaboratory comparisons. The magnitude of the linear coefficient alpha of the linear-quadratic dose-effect relationship obtained for the production of dicentric chromosomes by 29 kV X rays (alpha = 0.0655 +/- 0.0097 Gy(-1)) confirms earlier observations of a strong increase in alpha with decreasing photon energy. Relating this value to previously published values of alpha for the dose-effect curves for dicentrics obtained in our own laboratory, RBE(M) values of 1.6 +/- 0.3 in comparison with weakly filtered 220 kV X rays, 3.0 +/- 0.7 compared to heavily filtered 220 kV X rays, and 6.1 +/- 2.5 compared to 60Co gamma rays have been obtained. These data emphasize that the choice of the reference radiation is of fundamental importance for the RBE(M) obtained. A special survey of the RBE(M) values obtained by different investigators in the narrow quality range from about 30 to 350 kV X rays indicates that the present RBE is in fairly good agreement with previously published findings for the induction of chromosome aberrations or micronuclei in human lymphocytes but differs from recently published findings for neoplastic transformation in a human hybrid cell line.

Recent advances in the pharmacogenomics of thiopurine methyltransferase.

The thiopurine drugs (6-mercaptopurine, 6-thioguanine and azathioprine) are commonly used cytotoxic agents and immunosuppressants. One important route for the metabolism of these agents is methylation, mediated by thiopurine methyltransferase (TPMT). It is now well established that inter-individual variation in sensitivity to thiopurines can be due to the presence of common genetic polymorphisms affecting the TPMT gene. More recently variations in the number of tandem repeats in the 5' promoter region have been shown to influence TPMT expression in vitro. In this article, we review recent advances in the understanding of the range of inter-individual variation that may be involved in the open reading frame or promoter region of the TPMT gene. We also review the data which have been published regarding the influence such variations may have on both the clinical efficacy and toxicity of the thiopurine drugs.

Relationship between a GABAA alpha 6 Pro385Ser substitution and benzodiazepine sensitivity.

In humans, interindividual variation in sensitivity to benzodiazepine drugs may correlate with behavioral variation, including vulnerability to disease states such as alcoholism. In the rat, variation in alcohol and benzodiazepine sensitivity has been correlated with an inherited variant of the GABAA alpha 6 receptor. The authors detected a Pro385Ser [1236C > T] amino acid substitution in the human GABAA alpha 6 that may influence alcohol sensitivity. In this pilot study, they evaluated the contribution of this polymorphism to benzodiazepine sensitivity. Sensitivity to diazepam was assessed in 51 children of alcoholics by using two eye movement measures: peak saccadic velocity and average smooth pursuit gain. Association analysis was performed with saccadic velocity and smooth pursuit gain as dependent variables and comparing Pro385/Ser385 heterozygotes and Pro385/Pro385 homozygotes. The Pro385Ser genotype was associated with less diazepam-induced impairment of saccadic velocity but not with smooth pursuit gain. The Pro385Ser genotype may play a role in benzodiazepine sensitivity and conditions, such as alcoholism, that may be correlated with this trait.

Association between bleomycin genotoxicity and non-constitutional risk factors for head and neck cancer

Sensitivity of phytohaemagglutinin-stimulated lymphocytes to bleomycin clastogenicity is increased in patients with tumours in organs and tissues that are in direct contact with the external environment, such as the mucosa of the head and neck [5]. Sensitivity to bleomycin may reflect a genetically determined hypersensitivity to certain genotoxicants and therefore may be important in the carcinogenic process. In this study the applicability of bleomycin genotoxicity was investigated in cultured lymphocytes of forty individuals without a tumour history. No correlations were observed with increasing age or the well-known head and neck cancer risk factors alcohol and tobacco consumption. Since inter-individual variation in sensitivity greatly exceeded intra-individual variation, our results suggest that an elevated bleomycin clastogenicity score may identify individuals who have a constitutional hypersensitivity towards certain genotoxicants and may show an increased cancer susceptibility.

Effect of monodesethyl amodiaquine on human polymorphonuclear neutrophil functions in vitro.

We have previously observed that the antimalarial drug amodiaquine impairs the human polymorphonuclear neutrophil (PMN) oxidative burst in vitro. However, the drug acted at a concentration of 100 micrograms/ml, far higher than that which is achievable therapeutically. Since amodiaquine is extensively metabolized into monodesethyl amodiaquine, we investigated whether the metabolite modified PMN functions at lower concentrations than amodiaquine does. Monodesethyl amodiaquine strongly depressed PMN chemotaxis and phagocytosis at concentrations as low as 10 micrograms/ml. This inhibition was reversed by washing out the drug. The PMN oxidative burst was markedly depressed by monodesethyl amodiaquine, whatever the assay technique (luminol-amplified chemiluminescence, lucigenin-amplified chemiluminescence, myeloperoxidase activity) or stimulus used (opsonized zymosan, phorbol myristate acetate, formylmethionyl leucyl phenylalanine). There were extreme interindividual variations in sensitivity to the depressive effect of monodesethyl amodiaquine when the PMN oxidative burst was assayed in terms of luminol-amplified chemiluminescence or lucigenin-amplified chemiluminescence. PMN samples were divided into two groups on the basis of the MIC of the drug: 60% of the samples were "highly sensitive," being strongly inhibited at concentrations as low as 0.1 micrograms/ml (obtained during therapy), whereas the "moderately sensitive" samples were inhibited at concentrations of 10 micrograms/ml and above. The difference between the two groups was highly significant. This PMN sensitivity to the inhibitory effect of the drug was not related to intrinsic oxidative metabolism. Our data indicate that monodesethyl amodiaquine, the main metabolite of amodiaquine, has a far stronger inhibitory effect on various PMN functions in vitro than the parent drug, warranting relevant in vivo studies.

NSAIDs and the Gastrointestinal Tract—Case Closed?

The clinical efficacy of non-steroidal, anti-inflammatory drugs (NSAIDs) is undisputed, as is their propensity to induce gastrointestinal mucosal lesions. Although several new substances and formulations have been developed during the past decades in the search for drugs with a more favourable side effect profile, the difference between the individual drugs remains small and is overshadowed by the interindividual variation in sensitivity.

Applications of Human Peripheral Blood Lymphocytes in Genotoxicity and Cytotoxicity

A number of features make peripheral blood lymphocytes an excellent system for studying both genotoxicity and cytotoxicity in humans. They are an abundant and readily accessible source of somatic cells, mostly in a non-proliferative state, but able to be stimulated by mitogens to enter the cell cycle. The blastocyte transformation of lymphocytes is a useful model for investigating the mechanisms which regulate cell-cycle progression in mammalian cells. By stimulating lymphocytes in vitro, it is possible to detect the genetic damages they have sustained in vivo, which become manifest as chromosomal aberrations, sister-chromatid exchanges or gene mutations. The metabolic properties of lymphocytes have been extensively studied, especially with reference to their characteristic collection of enzymes involved in nucleotide turnover, which makes them exquisitely sensitive to changes in intracellular levels of DNA precursors. The data collected on the ability of lymphocytes to metabolise xenobiotics show a marked quantitative difference between resting and proliferating lymphocytes, and minor qualitative differences between lymphocytes and other cell types, e.g. hepatocytes. An indirect approach to detect the metabolism of genotoxic xenobiotics by lymphocytes is the analysis of DNA adducts in their chromatin after in vivo or in vitro exposure. Lymphocytes can be employed to identify the (cyto)genetic consequences of in vivo genotoxic exposure and inter-individual variation in sensitivity to genotoxic agents. The analysis of mutations at the hgprt locus in lymphocytes is a promising approach for the study of somatic-cell mutations in humans and of the possible mechanisms of in vivo selection against mutants.In the field of cytotoxicity, the applications of lymphocytes are, as yet, still few: the main effect measured is the impairment of the proliferative response to mitogens. But lymphocytes can be employed as primary human cells to be treated in vitro with mutagenic or toxic chemicals in standard genotoxicity and cytotoxicity assays, and offer the advantage of avoiding the problems of inter-species extrapolation of results by testing in a human system. Moreover, the (geno)toxic effects detected in lymphocytes after treatments in vitro may give information on the spontaneous or environmentally-determined susceptibility of the individual donors to xenobiotics.

Assessment of immunosuppression by serum inhibition of alloreaction and measurement of cyclosporin A (CyA) serum levels in kidney graft recipients under CyA

Abstract. The immunosuppressive effect of kidney graft recipient sera was studied on T-lymphocyte alloreactive line (4H) proliferation and compared to native cyclosporin A (CyA) and CyA metabolite concentrations determined by radioimmunoassay (RIA) using specific or nonspecific monoclonal antibodies. Three clinical groups were studied: (1) patients experiencing acute renal rejection episodes (CyA-R), (2) patients experiencing CyA-dependent nephrotoxicity episodes (CyA-TOX) and (3) patients in a clinically steady state (CyA-ST), according to their therapeutic regimen i. e., monotherapy (CyA alone) or polytherapy (CyA associated with prednisolone and/or azathioprine). Regardless of the clinical state, sera of patients in polytherapy displayed more inhibitory activity than those of monotherapy patients (24% and 40% inhibition of 4H proliferation, respectively, at sera dilution of 1:2), something which was no doubt due to the inhibitory activity of prednisolone on T-lymphocyte growth. In the two therapeutic regimens, CyA-ST patient sera exhibited the lowest inhibitory activity on the 4H line (45% and 65% inhibition of 4H proliferation in mono-and polytherapy, respectively, at sera dilution of 1:2). Sera from CyA-TOX patients were highly inhibitory (74% and 86% inhibition of 4H proliferation in mono-and polytherapy, respectively, at sera dilution of 1:2), in agreement with RIA assays showing increased native circulating CyA and CyA metabolites and daily CyA intake in this group as compared to CyA-ST. Surprisingly, CyA-R patient sera were no less inhibitory than those of CyA-ST patients on 4H-line, antigen-induced proliferation. This clinical group did not differ from others for CyA intake or level of circulating immunosuppressive molecules, suggesting that rejection could be associated with a state of interin-dividual variation in sensitivity to CyA. In addition, a polytherapeutic regimen seemed to modify CyA bioavailability in CyA-ST group patients, with a decreased CyA metabolite level as compared to their monotherapy counterparts (native CyA plus metabolite/native CyA ratio being 2. 73 and 3. 73, respectively). In contrast, in the CyA-R patient group, polytherapy appeared to be associated with an increase in CyA metabolite circulating levels (ratio 4. 79). In view of the low inhibitory activity of CyA metabolites, this profile might lead to rejection.

The interpretation of the differential threshold in the central visual field.

Computer assisted perimetry has revolutionised the investigation of the visual field. Experience of central field assessment with the Octopus Automated Perimeter shows that sensitivity recorded with target size 3 across all age groups can frequently be greater than the published normative values. Use of the latter values can therefore provide a serious underestimation of field loss. Inter-individual variation in sensitivity is found within and between age groups. The limitations associated with the use of the measurement error to define abnormality and the additional problems of hypernormal thresholds and resolution of the blind spot are discussed. It is suggested that methods should be developed to evaluate sensitivity on an intra-individual basis.

The Pharmacology of Verapamil. IV. Kinetic and Dynamic Effects after Single Intravenous and Oral Doses

Kinetics and plasma level-effect correlates for verapamil were studied in 20 normal young men (mean age, 25.2 +/- 3.6 yr). In a randomized four-way crossover design, each subject received 10 mg verapamil intravenously and 80, 120, and 160 mg in single oral doses. Changes in heart rate, blood pressure, and PR interval were evaluated serially after each dose; plasma concentrations of verapamil were measured by high-performance liquid chromatography. Levels of the active metabolite norverapamil were determined in five subjects. Verapamil kinetics were the same after intravenous and oral doses: elimination half-life (t 1/2 beta) ranged from 3.7 to 4.8 hr, apparent volume of distribution varied between 4.2 and 5.5 l/kg, and total clearance was 0.71 to 0.86 l/hr/kg. Verapamil bioavailability was not dose dependent and averaged 19.4%. Norverapamil, found only after oral doses, had a t 2/2 beta and maximum concentration much the same as the parent drug. There were only minor effects on heart rate and blood pressure after single doses. Hysteresis analysis showed that plasma verapamil concentrations after intravenous doses correlated with PR interval prolongation only after a 30-min lag time; there was no lag after oral doses. There was considerable interindividual variation in sensitivity to verapamil's effect on atrioventricular conduction; subjects with longer control PR interval values tended to have greater prolongation of effect than those with shorter intervals. Verapamil was well tolerated in both dosage forms by all subjects.