Analysis of molecular markers for metamorphic competency and their response to starvation or feeding in the mosquito, Aedes aegypti (Diptera: Culicidae).

The nutritional condition of fourth instar larvae of the yellow fever mosquito, Aedes aegypti, governs female longevity and egg production, both are key determinants of pathogen transmission. As well, nutrition provisions larval growth and development and attains its greatest pace in the last larval instar in preparation for metamorphosis to an adult. These developmental processes are regulated by a complex endocrine interplay of juvenile hormone, neuropeptides, and ecdysteroids that is nutrition sensitive. We previously determined that feeding for only 24h post-ecdysis was sufficient for fourth instar Ae. aegypti larvae to reach critical weight and accumulate sufficient nutritional stores to commit to metamorphosis. To understand the genetic basis of metamorphic commitment in Ae. aegypti, we profiled the expression of 16 genes known to be involved in the endocrine and nutritional regulation of insect metamorphosis in two ways. The first set is a developmental profile from the beginning of the fourth instar to early pupae, and the second set is for fourth instars starved or fed for up to 36 h. By comparing the two sets, we found that seven of the genes (AaegCYP302, AaegJHE43357, AaegBrCZ4, AaegCPF1-2, AaegCPR-7, AaegPpl, and AaegSlif) were expressed during metamorphic commitment in fourth instars and in fed but not starved larvae. Based on these results, the seven genes alone or in combination may serve as molecular indicators of nutritional and metamorphic status of fourth instar Ae. aegypti larvae and possibly other mosquito species in field and laboratory studies to gauge sub-lethal effects of novel and traditional cultural or chemical controls.

Mechanism for site-dependent differences in the shape of the resetting response curve in fixed barrier reentry.

INTRODUCTION: We investigated whether the site of stimulation within a reentrant circuit affects the resetting response curve (RRC). RRCs are used to characterize the excitable gap of reentrant circuits, including the duration of the fully excitable gap and the presence of partially excitable tissue. METHODS AND RESULTS: We reset proximal and distal to a site of interval-dependent conduction (IDC) in canine in vitro atrial tricuspid rings. Adjustable reentry allowed changes in the cycle length and direction of reentry. In nine preparations we reset 26 tachycardias. In the 16 tachycardias with one site of IDC, RRCs were significantly different when stimulating distal and proximal to a site of interval-dependent conduction. For the distal curves, the duration of the flat portion was 42 +/- 26 msec greater (P < 0.001), the slope of the increasing portion was 0.20 +/- 0.17 less (P < 0.02), and the increase in the return cycle was 14 +/- 9 msec less (P < 0.001). These differences resulted from early activation of the site of IDC by the antidromic premature impulse when stimulating from distal sites. As a result, the coupling interval of the orthodromic impulse at the site of IDC was the same or greater than at the stimulation site. In 10 tachycardias with multiple sites of IDC, significant differences in the resetting responses did not occur even when the antidromic impulse penetrated one site of IDC. CONCLUSION: In a fixed anatomic barrier reentrant circuit with one site of IDC, resetting distal to this site misrepresents the properties of the excitable gap of the entire circuit.

A simple colorimetric assay for phenotyping the major human thermostable phenol sulfotransferase (SULT1A1) using platelet cytosols.

A thermostable phenol sulfotransferase, SULT1A1, has been implicated in numerous detoxification and bioactivation pathways; however, little is known regarding its endogenous function or its putative role in mediating risk for human environmental disease. A simple endpoint colorimetric assay is described that can be used for rapid phenotyping of SULT1A1 activity in human populations. The assay utilizes a microtiter-plate format and relatively small amounts of platelet cytosol-derived enzyme. The enzyme catalyzes the synthesis of 2-naphthylsulfate from 2-naphthol and 5'-phosphoadenosine 3'-phosphosulfate (PAPS), whereas addition of p-nitrophenyl sulfate to the assay contributes to an effective PAPS-regenerating system. In contrast to other sulfotransferase assay methods, 3'-phosphoadenosine 5'-phosphate (PAP) does not accumulate during the incubation to interfere with enzyme activity, but instead serves as a cofactor to cause the removal of sulfate from p-nitrophenyl sulfate to regenerate PAPS. This reaction concomitantly results in generation of p-nitrophenol that can be quantified colorimetrically at 405 nm (epsilon = 18,200 M(-1)) to give an indirect measure of sulfotransferase activity. Using platelet enzyme preparations from adult human subjects, sulfation rates of two prototypical thermostable phenol sulfotransferase substrates (2-naphthol and p-nitrophenol) and one thermolabile phenol sulfotransferase substrate (dopamine) were determined using standard radiochemical protocols. These data were then compared with results from the colorimetric assay using 2-naphthol as substrate. There was a good correlation between the phenotyping assay and radiochemical assays for both 2-naphthol sulfotransferase and p-nitrophenol sulfotransferase activity (r = 0.85 and 0.69, respectively). However, SULT1A1 activity was approximately 10 to 20 times higher with the colorimetric determination. As anticipated, there was no correlation between SULT1A1 activity and dopamine sulfotransferase activity (r = 0.07) in these human platelet preparations. This inexpensive and rapid method for phenotyping SULT1A1 activity may help investigators assess a role for this enzyme in disease susceptibility.

Understanding reentry to out-of-home care for reunified infants.

Although many children placed in out-of-home care are reunified with their families of origin, a significant portion reenter care, reflecting continued family problems and weaknesses within the child welfare system. For infants, the stability of reunification is particularly crucial, given their developmental stage. This study reviewed the case records of 88 randomly selected infants who had been reunified with their families. Thirty-two percent of those infants reentered care within four to six years of their reunification. The identification of factors predictive of reentry into care has both policy and practice implications.

Variability of human hepatic UDP-glucuronosyltransferase activity.

The availability of a unique series of liver samples from human subjects, both control patients (9) and those with liver disease (6; biliary atresia (2), retransplant, chronic tyrosinemia type I, tyrosinemia, Wilson's disease) allowed us to characterize human hepatic UDP-glucuronosyltransferases using photoaffinity labeling, immunoblotting and enzymatic assays. There was wide inter-individual variation in photoincorporation of the photoaffinity analogs, [32P]5-azido-UDP-glucuronic acid and [32P]5-azido-UDP-glucose and enzymatic glucuronidation of substrates specific to the two subfamilies of UDP-glucuronosyltransferases. However, the largest differences were between subjects with liver disease. Glucuronidation activities toward one substrate from each of the UDP-glucuronosyltransferases subfamilies, 1A and 2B, for control and liver disease, respectively, were 1.7-4.5 vs 0.4-4.7 nmol/mg x min for hyodeoxycholic acid (2B substrate) and 9.2-27.9 vs 8.1-75 nmol/mg x min for pchloro-m-xylenol (1A substrate). Microsomes from a patient with chronic tyrosinemia (HL32) photoincorporated [32P]5-azido-UDP-glucuronic acid at a level 1.5 times higher than the other samples, was intensely photolabeled by [32P]5-azido-UDP-glucose and had significantly higher enzymatic activity toward p-chloro-m-xylenol. Immunoblot analysis using anti-UDP-glucuronosyltransferase antibodies demonstrated wide inter-individual variations in UDP-glucuronosyltransferase protein with increased UDP-glucuronosyltransferase protein in HL32 microsomes, corresponding to one of the bands photolabeled by both probes. Detailed investigation of substrate specificity, using substrates representative of both the 1A (bilirubin, 4-nitrophenol) and 2B (androsterone, testosterone) families was carried out with HL32, HL38 (age and sex matched control) and HL18 (older control). Strikingly increased (5-8-fold) glucuronidation activity was seen in comparison to HL18 only with the phenolic substrates. The results indicate that one or more phenol-specific UDP-glucuronosyltransferase 1A isoforms are expressed at above normal levels in this tyrosinemic subject.

Modulation of endocrine pathways by 4,4'-DDE in the deer mouse Peromyscus maniculatus.

4,4'-DDT and 4,4'-DDE are widespread environmental contaminants that cause eggshell thinning in birds, altered sex ratios in the American alligator, and changes in the anal-genital distance in rodents. These contaminants are known to cause some of their toxicity by altering steroid receptor-mediated mechanisms. However, chemical-specific alterations in the expression of hormone-metabolizing enzymes may also be a mechanism for endocrine disruption, by altering the half-life of hormones in critical tissues. Previously, we showed that 4,4'-DDE causes a dose-dependent increase in ethoxyresorufin-O-deethylase (EROD) activity, but not pentoxyresorufin-O-dealkylase (PROD) activity, in the deer mouse. In this study, we demonstrated that 4,4'-DDE elicited a corresponding increase in CYP1A protein expression but not CYP2B using Western blotting and immunoprecipitation. 4,4'-DDE-mediated changes in phase II conjugating enzymes; UDP-glucuronosyltransferase (UGT) and phenolsulfotransferase (ST), were also investigated for the first time. Prepubescent female deer mice were dosed with 4,4'-DDE by gavage on days 1 and 2, then euthanized on day 4. As anticipated, dose-dependent increases in hepatic EROD and MROD activities, but not PROD or BROD, were observed. UGT activity was monitored by incubating liver microsomes and 14C-UDP-GA with potential substrates and measuring incorporation of radioactivity into TLC-resolved glucuronides. Dose-dependent increases in conjugation were observed with p-nitrophenol (a general UGT substrate) but not testosterone. Interestingly, a biphasic dose-response curve was observed for ST activity, with a peak at the 3 mg/kg dose. Dose-dependent increases in CYP1A1 and UGT-specific immunoreactive proteins were observed, suggesting de novo synthesis as a consequence of 4,4'-DDE exposure. We also measured Phase I and II enzymes in deer mouse platelets. Preliminary results indicate that the 4,4'-DDE-induced changes in liver Phase I and II enzyme activity were similar, but not identical, to those found in platelets. These results indicate that environmentally-relevant levels of 4,4'-DDE modulate the activity and expression of CYP1A1 and phase II enzymes in the deer mouse and that certain changes may be measured non-lethally.

Identification of enzymes responsible for the metabolism of heme in human platelets.

The major enzymes involved in the degradation of heme were identified in human platelets. It was determined that heme oxygenase activity levels in umbilical cord blood platelets were higher, whereas biliverdin reductase activity levels were comparable with that found in platelets from adults. In membranes prepared from adenosine diphosphate-activated platelets, UDP-glucuronic acid-dependent bilirubin conjugation was detected, whereas activity was negligible in unactivated platelets and undetected in serum and heat-inactivated platelets, and in platelets prepared from umbilical cord blood. Platelet fractions were analyzed by Western blot and shown to express heme oxygenase, biliverdin reductase, and UDP-glucuronosyltransferases, and there was concordance with known developmental profiles found in other tissues. Heme oxygenase expression was higher, whereas UGT expression was lower, in neonatal compared with adult platelets. These data suggest that platelets are involved in multiple steps of heme and bilirubin metabolism and that developmental regulation of these enzymes may be similar to that in other human tissues.

Long-term effects of amphetamine neurotoxicity on tyrosine hydroxylase mRNA and protein in aged rats.

Four injections (intraperitoneal) of 3 mg/kg amphetamine (2 hr apart) produced pronounced hyperthermia and sustained decreases in dopamine levels and tyrosine hydroxylase (TH) protein levels in the striatum of 15-month-old male rats. A partial recovery of striatal dopamine levels was observed at 4 months after amphetamine. In contrast, TH mRNA and TH protein levels in the midbrain were unaffected at all time points tested up to 4 months after amphetamine treatment. The number of TH-immunopositive cells in the midbrain was also unchanged at 4 months after amphetamine, even though the number of TH-positive axons in the striatum remained dramatically decreased at this time point. Interestingly, TH-immunopositive cell bodies were observed 4 months after amphetamine in the lateral caudate/putamen, defined anteriorly by the genu of the corpus collosum and posteriorly by the junction of the anterior commissures; these striatal TH-positive cells were not observed in saline- or amphetamine-treated rats that did not become hyperthermic. In addition, low levels (orders of magnitude lower than that present in the midbrain) of TH mRNA were detected using reverse transcription-polymerase chain reaction in the striatum of these amphetamine-treated rats. Our results suggest that even though there is a partial recovery of striatal dopamine levels, which occurs within 4 months after amphetamine treatment, this recovery is not associated with increased TH gene expression in the midbrain. Furthermore, new TH-positive cells are generated in the striatum at this 4-month time point.

Caloric restriction as a mechanism mediating resistance to environmental disease.

It has been observed that susceptibility to many degenerative diseases increases concurrently with industrialization and rising living standards. Although epidemiologic studies suggest that specific environmental and dietary factors may be important, caloric intake alone (as reflected in body size) may account for much of the differential risk observed among diverse human populations. It has been suggested from animal studies that caloric intake may be the primary effector for many hormonal, metabolic, physiologic, and behavioral responses that coordinate reproductive strategy to apparent availability of food. When caloric intake is excessive, particularly at critical developmental stages, physiologic priorities are set for body growth and fecundity rather than for endurance and longevity. The converse occurs during periods of famine, thus increasing the probability that sufficient individuals survive to restore the population when conditions improve. Calorically restricted rodents have significantly longer reproductive and total life spans than their ad libitum-fed controls and exhibit a spectrum of biochemical and physiologic alterations that characterize their adaptation to reduced intake. These include reduced stature, hypercorticism in the absence of elevated adrenocorticotropic hormone levels, increased metabolic efficiency, decreased mitogenic response coupled with increased rates of apoptosis, reduced inflammatory response, induction of stress proteins and DNA repair enzymes, altered drug-metabolizing enzyme expression, and modified cell-mediated immune function. The overall profile of these changes is one of improved defense against environmental stress. This has been suggested as the mechanistic basis for the protective effects of low body weight on radiation and chemically induced cancers in experimental animals. It may also explain the significantly higher thresholds of acute toxicity observed when calorically restricted rodents are exposed to certain test compounds.

Termination of reentry by lidocaine in the tricuspid ring in vitro. Role of cycle-length oscillation, fast use-dependent kinetics, and fixed block.

We hypothesized that drugs with rapid recovery kinetics from use-dependent sodium channel block could promote oscillatory termination of reentry by enhancing interval-dependent conduction. Mechanisms of termination were related to properties of the reentrant circuit. Nine adjustable reentrant preparations were used in which the canine atrial tricuspid ring was cut and then reconnected electronically by sensing activation on one side of the cut and pacing the other after an adjustable delay. The cycle length and diastolic interval during reentry were manipulated by changing this delay. Lidocaine (1.28 x 10(-5) mol/L) significantly increased refractoriness (94 +/- 39 ms) and the slope of the conduction curve (-0.12 +/- 0.07) at the site of block during pacing. Lidocaine terminated sustained reentry by two mechanisms. Early termination resulted from increased cycle length oscillation and refractoriness (reproducible in each experiment) but only at short delays with short initial diastolic intervals. The range of delays showing this mechanism of termination was 100 +/- 48 ms. Increased cycle-length oscillation resulted from an increased slope of the conduction curve. In eight experiments, lidocaine terminated reentry by causing fixed block after 50 minutes of drug superfusion, which prevented reentry at all delays. Fixed block occurred at one of two vulnerable sites and was transiently reversed by acetylcholine. Termination due to refractory block occurred only when the initial diastolic interval was short, and termination due to fixed block developed when there was a susceptible region with a low safety factor for propagation. Fast recovery from sodium channel block promotes oscillatory termination by increasing the slope of the conduction curve.

Metabolic differences and their impact on human disease Sulfotransferase and colorectal cancer.

Gene-environment interaction is an important aspect of human cancer risk. Genetic polymorphisms in acetylation and N-oxidation have previously been described regarding their impact on the heterocyclic amine-induced risk for colon cancer. Here, we report that another enzyme involved in the metabolism of food-borne carcinogens, sulfotransferase (ST1A3 measured by 2-naphthol activity), may function as a potential protective factor for colon cancer in humans. Initially characterized in human liver and colon (Chou et al., 1995), TS-PST activity can also be measured in platelets. A simple microtiter-based colorimetric technique was developed for use in this case-control study. African-Americans had a higher mean ST activity than Caucasians (2.32±0.24 versus 1.77±0.09 nmols/min per mg cytosolic protein, P=0.036). Furthermore, the slow ST phenotype (ST≤1.53) was more frequently associated with colon cancer than controls (57 versus 40%, P=0.026). These data suggest that the ST1A3 isoform may play a role in the differential risk for colorectal cancer.

Lessons from animal models of atrial arrhythmias.

This article presents selected lessons from experimental studies of atrial fibrillation and atrial flutter that pertain to the mechanisms and predisposing factors for flutter and fibrillation and approaches to treatment by antiarrhythmic drugs. Experimental studies also provide lessons for the effects of ablation and surgical lesions on prevention or facilitation of atrial fibrillation and flutter.

Assessing the excitable gap in reentry by resetting. Implications for tachycardia termination by premature stimuli and antiarrhythmic drugs.

BACKGROUND: The shortest excitable gap during reentry may determine responses to pacing and antiarrhythmic drugs. The resetting response has been used clinically to assess the excitable gap, but it cannot directly indicate the shortest excitable gap. METHODS AND RESULTS: We studied resetting in the in vitro canine atrial tricuspid ring using an adjustable reentry preparation in which the ring was cut and reconnected electronically with an adjustable delay to vary the cycle length and excitable gap. We reset the tachycardias using 31 delays in 12 experiments. Tachycardias were terminated by premature stimuli in 16 delays. The reset window overestimated the shortest excitable gap by 25 +/- 14 ms, and the maximum degree of advancement of tachycardia underestimated the shortest excitable gap by 22 +/- 11 ms. The slope of the increasing portion of the resetting response curve was steeper in tachycardias terminated by premature stimuli than in those not terminated (-0.69 +/- 0.2 versus -0.37 +/- 0.2, P < .01). The effective refractory period difference between the sites of pacing and of block correlated with the slope of the resetting response curve. Damped cycle length oscillation after a long return cycle during resetting was always present when there was a partially excitable gap. CONCLUSIONS: The reset window during pacing within the circuit and the maximum degree of advancement provided equally good estimates bracketing the shortest excitable gap. The slope of the resetting response curve predicted the likelihood of termination by premature stimuli. Damped cycle length oscillation after resetting detected a partially excitable gap.

Reversal of reentry and acceleration due to double-wave reentry: two mechanisms for failure to terminate tachycardias by rapid pacing.

OBJECTIVES: We sought to demonstrate mechanisms by which rapid pacing can cause conduction block without terminating reentry. BACKGROUND: Rapid pacing can fail to terminate or can accelerate tachycardias in patients. Mechanisms for these responses are poorly understood. METHODS: We studied reentry in the canine atrial tricuspid ring and a left ventricular ring in vitro in 12 preparations. Activations were recorded from 10 sites around the ring, and monophasic action potentials were recorded from critical sites of block. Rapid pacing at cycle lengths that intermittently caused conduction block was performed at multiple sites. RESULTS: Action potential alternans contributed to block of an orthodromic impulse during rapid pacing. When pacing continued for two stimuli after orthodromic block, a second episode of block could reverse the direction of tachycardia. Continued pacing at this site was likely to produce block of an antidromic impulse, which may initiate double-wave reentry. Double-wave reentry could be sustained or nonsustained. Its cycle length was 56% to 77% of the single-wave cycle length. The ratio of double-wave cycle length to single-wave cycle length was inversely correlated with the relative excitable gap (p < 0.01). Double-wave reentry can be a mechanism for persistent cycle length alternation during tachycardia. CONCLUSIONS: Successful termination of reentry by rapid pacing required block of an othrodromic impulse and stopping pacing within one stimulus after orthodromic block. Reversal of reentry makes the circuit resistant to termination from this site of pacing. Antidromic block can cause acceleration due to double-wave reentry when there is a substantial excitable gap.

d-Sotalol terminates reentry by two mechanisms with different dependence on the duration of the excitable gap.

We used eight adjustable preparations in which the canine atrial tricuspid rings were cut and reconnected electronically by sensing activation on one side of the cut and pacing the other after an adjustable delay. A long delay resulted in a long cycle length (CL) and excitable gap (EG) during reentry. Decreasing delay decreased CL and EG. d-Sotalol (4 mg/l) significantly increased effective refractory period (ERP) and action potential duration with no effects on conduction time during constant 400-msec pacing. During reentry, d-sotalol increased action potential durations more than CLs, so it decreased diastolic intervals. It decreased EG by increasing ERP more than CL. Although d-sotalol increased action potential duration more at longer delays with longer CLs, showing reverse use-dependence, it terminated sustained tachycardias by increasing ERP only for the short delays when the initial EG was short. In 5 of 8 experiments, longer equilibration with d-sotalol produced fixed block at a vulnerable site, so reentry could not be induced at any delays. Fixed block could be transiently reversed by ACh and resolved after washout of d-sotalol. We conclude that d-sotalol terminated reentry by two mechanisms: 1) It terminated sustained reentry by increasing ERP when the initial EG was sufficiently short. 2) In some preparations, it caused fixed block at a vulnerable site, which prevented reentry regardless of the initial EG.

Effects of caloric restriction on rodent drug and carcinogen metabolizing enzymes: implications for mutagenesis and cancer.

Caloric restriction in rodents results in increased longevity and a decreased rate of spontaneous and chemically induced neoplasia. The low rates of spontaneous neoplasia and other pathologies have made calorically restricted rodents attractive for use in chronic bioassays. However, caloric restriction also alters hepatic drug metabolizing enzyme (DME) expression and so may also alter the biotransformation rates of test chemicals. These alterations in DME expression may be divided into two types: (1) those that are the direct result of caloric restriction itself and are detectable from shortly after the restriction is initiated; (2) those which are the result of pathological conditions that are delayed by caloric restriction. These latter alterations do not usually become apparent until late in the life of the organism. In rats, the largest direct effect of caloric restriction on liver DMEs is an apparent de-differentiation of sex-specific enzyme expression. This includes a 40-70% decrease in cytochrome P450 2C11 (CYP2C11) expression in males and a 20-30% reduction of corticosterone sulfotransferase activity in females. Changes in DME activities that occur late in life in calorically restricted rats include a stimulation of CYP2E1-dependent 4-nitrophenol hydroxylase activity and a delay in the disappearance of male-specific enzyme activities in senescent males. It is probable that altered DME expression is associated with altered metabolic activation of chemical carcinogens. For example the relative expression of hepatic CYP2C11 in ad libitum-fed or calorically restricted rats of different ages is closely correlated with the amount of genetic damage in 2-acetylaminofluorene- or aflatoxin B1-pretreated hepatocytes isolated from rats of the same age and caloric intake. This suggests that altered hepatic drug and carcinogen metabolism in calorically restricted rats can influence the carcinogenicity of test chemicals.

A further evaluation of the effects of K+ depolarization on glutamate-evoked [3H]dopamine release from striatal slices.

Exogenous glutamate will evoke dopamine (DA) release from striatal slices in vitro. To further characterize glutamate-evoked DA release from striatal slices, experiments were designed to: 1) determine if sufficient endogenous glutamate can be released in vitro to presynaptically mediate [3H]DA release in the absence of Mg++ and 2) reevaluate how K+ depolarization affects glutamate-evoked [3H]DA release. Removal of Mg++ to potentiate N-methyl-D-aspartate (NMDA) receptor-mediated DA release increased 15 mM K(+)-evoked [3H]DA release to about 200% of control. The potentiation of this release was probably not mediated by NMDA receptors because it was not blocked by the glutamate receptor antagonists MK-801, 6,7-dinitroquinoxalinedione (DNQX) or kynurenate. Furthermore, the removal of Mg++ increased DA release substantially (200%) in the presence of 5 microM sulpiride and 10 microM nomifensine, indicating that DA reuptake and DA D2 autoreceptors are not primarily responsible for increased DA release. In the absence of Mg++, depolarization produced by 20 mM or greater [K+] inhibited DA released by exogenous glutamate, whereas a much higher [K+] was necessary to evoke endogenous glutamate release. In the presence of 1.5 mM Mg++, a reduction of the "Mg++ blockade" of NMDA receptors by 15 mM K+ depolarization during glutamate-evoked DA release was evaluated with and without the DA reuptake inhibitor nomifensine and the DA D2 antagonist sulpiride. DA released by K+ depolarization (Mg++ present) was markedly increased by 1 mM glutamate, but this effect was only partially reversed by kynurenate or high concentrations of either MK-801 (25 microM) or DNQX (100 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Proarrhythmic and antiarrhythmic effects of flecainide on nonsustained reentry around the canine atrial tricuspid ring in vitro.

The effect of flecainide, 0.3 mg/L and 1.0 mg/L, on inducible nonsustained reentry was studied, in vitro, in the canine tricuspid ring. Nonsustained reentry was engineered by cutting the ring and reconnecting it with an adjustable electronic delay. Delays were used that produced reentry lasting 1-3 beats (group A), 4-10 beats (group B), and 11-25 beats (group C). Reentry was initiated multiple times at each selected delay. A proarrhythmic effect, defined as a significant increase in the duration of reentry, was observed in all 14 trials at the low dose and in two of 15 trials at the high dose in seven experiments. In four more trials a transient proarrhythmic response was seen initially during exposure to the high dose. In five of seven experiments, reentry became sustained after at least one dose of flecainide. Proarrhythmic responses resulted when flecainide increased the tachycardia cycle length more than the effective refractory period and there was less cycle length oscillation after initiation. Antiarrhythmic responses resulted either from a marked increase in effective refractory period at the site of block or production of fixed block.

Spontaneous termination of reentry after one cycle or short nonsustained runs. Role of oscillations and excess dispersion of refractoriness.

This study describes factors that contribute to spontaneous termination of reentry lasting one to 10 cycles after induction by a single premature stimulus. Reentry was studied in vitro in rings of canine atrial tissue from around the tricuspid valve orifice. Activation was recorded from a circular array of 10 extracellular bipolar electrodes equally spaced around the ring. In some experiments, transmembrane or monophasic action potential recordings were made near critical sites. Termination of reentry within one cycle after induction was recorded 110 times in 11 of 35 experiments. Important factors contributing to termination were 1) an obligatory reversal of the activation sequence that resulted in a long coupling interval in the critical region beyond the site of unidirectional block after the premature stimulus and 2) much longer refractory periods limited to this critical region, which facilitated unidirectional block but contributed to termination when this region was first activated with a short coupling interval at the end of the first reentrant cycle. Termination of nonsustained reentry lasting longer than one cycle resulted from oscillations of conduction and refractoriness initiated by the abrupt shortening of cycle length after initiation of reentry. Oscillations of conduction resulted from interval-dependent conduction of reentrant impulses that encountered partially refractory tissue. For reentry to become sustained, the oscillations after induction of reentry must dampen. Thus, damped cycle length oscillations after induction may identify clinical tachycardias caused by reentry with a partially excitable gap.