High frequencies of short frameshifts in poly-CA/TG tandem repeats borne by bacteriophage M13 in Escherichia coli K-12

Slipped-strand mispairing (SSM) may play an major role in repetitive DNA sequence evolution by generating large numbers of short frameshift mutations within simple tandem repeats. Here we examine the frequency and size spectrum of frameshifts generated within poly-CA/TG sequences inserted into bacteriophage M13 in Escherichia coli hosts. The frequency of detectable frameshifts within a 40 bp tract of poly-CA/TG is greater than one percent and increases more than linearly with length, being lower by a factor of four in a 22 bp target sequence. The frequency increases more than 13-fold in mutL and mutS host cells, suggesting that a high proportion of frameshift events are normally repaired by methyl-directed mismatch repair. Of the 87 sequenced frameshifts in this study, 96% result from deletion or insertion of only or two 2 bp repeat units. The most frequent events are 2 bp deletions, 2 bp insertions, and 4 bp deletions, the relative frequencies of these events being about 18:6:1.

Slipped-strand mispairing: a major mechanism for DNA sequence evolution

Simple repetitive DNA sequences are a widespread and abundant feature of genomic DNA. The following several features characterize such sequences: (1) they typically consist of a variety of repeated motifs of 1-10 bases--but may include much larger repeats as well; (2) larger repeat units often include shorter ones within them; (3) long polypyrimidine and poly-CA tracts are often found; and (4) tandem arrangements of closely related motifs are often found. We propose that slipped-strand mispairing events, in concert with unequal crossing-over, can readily account for all of these features. The frequent occurrence of long tandem repeats of particular motifs (polypyrimidine and poly-CA tracts) appears to result from nonrandom patterns of nucleotide substitution. We argue that the intrahelical process of slipped-strand mispairing is much more likely to be the major factor in the initial expansion of short repeated motifs and that, after initial expansion, simple tandem repeats may be predisposed to further expansion by unequal crossing-over or other interhelical events because of their propensity to mispair. Evidence is presented that single-base repeats (the shortest possible motifs) are represented by longer runs in mammalian introns than would be expected on a random basis, supporting the idea that SSM may be a ubiquitous force in the evolution of the eukaryotic genome. Simple repetitive sequences may therefore represent a natural ground state of DNA unselected for coding functions.

Cross-hybridizing snake satellite, Drosophila, and mouse DNA sequences may have arisen independently

Previous reports have interpreted hybridization between snake satellite DNA and DNA clones from a variety of distant taxonomic groups as evidence for evolutionary conservation, which implies common ancestry (homology) and/or convergence (analogy) to produce the cross-hybridizing sequences. We have isolated 11 clones from a genomic library of Drosophila melanogaster, using a cloned 2.5-kb snake satellite probe of known nucleotide sequence. We have also analysed published sequence data from snakes, mice, and Drosophila. These data show that (1) all of the cross-hybridization between the snake, fly, and mouse clones can be accounted for by the presence of either of two tandem repeats, [GATA]n and [GACA]n and (2) these tandem repeats are organized differently among the different species. We find no evidence that these sequences are homologous apart from the existence of the simple repeat itself, although their divergence from a common ancestral sequence cannot be ruled out. The sequences contain a variety of homogeneous clusters of tandem repeats of CATA, GA, TA, and CA, as well as GATA and GACA. We suggest that these motifs may have arisen by a self-accelerating process involving slipped-strand mispairing of DNA. Homogeneity of the clusters might simply be the result of a rate of accumulation of tandem repeats that exceeds that of other mutations.

Bupivacaine-induced cardiotoxicity in hypoxic and acidotic sheep

Awake, unanesthetized, and paralyzed sheep made hypoxic and acidotic were given equivalent low and high intravenous doses of lidocaine and bupivacaine over 10 sec. Within 30 sec of injections, all animals had electroencephalographic evidence of convulsions. After administration of low-dose lidocaine, arrhythmias associated with significant hemodynamic changes did not occur; after administration of high-dose lidocaine, half of the animals became hypotensive but had no arrhythmias other than sinus tachycardia. However, after administration of low-dose bupivacaine, all sheep had evidence of serious electrocardiographic changes or arrhythmias, and one animal died. After administration of high-dose bupivacaine, serious electrocardiographic changes occurred in all animals, and despite resuscitative efforts, all died. The most common abnormality after bupivacaine administration was a wide-QRS-complex bradycardia, occurring in most animals regardless of dose. Two-thirds of the animals given high-dose bupivacaine had electromechanical dissociation and subsequent refractory asystole. Although the mechanism of action is not known, bupivacaine appears to be more cardiotoxic than lidocaine. This toxicity is enhanced in animals by the presence of hypercarbia, acidosis, and hypoxia.

Comparative maternal and neonatal effects of halothane and enflurane for cesarean section

The effects of placental transfer of enflurane and halothane were studied in 81 women undergoing cesarean sections. All patients had rapid sequence induction using thiopental, succinylcholine, and endotracheal intubation. They were then randomly assigned to one of five groups: Group I (n = 16) received N2O and oxygen, Group II (n = 16) N2O, oxygen, and 0.25% halothane, Group III (n = 18) N2O, oxygen, and 0.5% halothane, Group IV (n = 18) N2O, oxygen, and 0.5% enflurane, Group V (n = 13) N2O, oxygen, and 1% enflurane. At delivery, blood was drawn from the maternal artery, umbilical vein and artery for measurement of the halogenated agents using gas chromatography. The neonates were evaluated by Apgar scores, umbilical artery and vein acid base status and the Early Neonatal Neurobehavioral Scores (ENNS) at 2 and 24 h of age. Blood loss and the incidence of maternal awareness were also determined. The umbilical vein to maternal vein ratio was approximately 0.5 and 0.6 for enflurane and halothane, respectively. The umbilical artery to umbilical vein ratio was 0.5 with both agents; higher inspired anesthetic concentrations produced higher blood levels. All neonates had Apgar scores of 8 or more at 5 min with the exception of one neonate in the N2O group. Maternal and neonatal acid base status, blood loss, and ENNS were not affected by the addition of the halogenated agents. Of the patients who had N2O alone, 12% had awareness versus none in the other groups. These data demonstrate that low dose halothane or enflurane decreases the incidence of maternal awareness and does not adversely affect the neonate.

Efficacy of clear antacid prophylaxis in obstetrics

We studied 33 women scheduled to undergo general anesthesia for cesarean section. Prior to induction of anesthesia, each patient received in a random fashion one of two prophylactic antacids. Group I (n = 17) received 30 ml Gelusil orally and Group II (n = 16) received 30 ml of 0.3 mol/l sodium citrate. After induction of general anesthesia an oro-gastric tube was passed. The amount and pH of the gastric aspirate were measured at the time of delivery and 45 min after delivery "before extubation." Both antacids elevated gastric pH above 3.5 in all patients but one in the Gelusil group. The pH remained elevated 45 min later. It is concluded that 0.3 mol/l sodium citrate is effective as an antacid when given orally prior to induction of anesthesia.

Bupivacaine-induced cardiac arrhythmias in sheep

Controversy persists about the cardiac toxicity of bupivacaine if accidentally administered intravenously during regional anesthesia. Using awake, unanesthetized sheep, we evaluated the cardiac effects of low and high equivalent doses of lidocaine and bupivacaine given intravenously over 10 s. All animals convulsed within 30 s of injections. Although both drugs significantly increased heart rate and systemic and pulmonary arterial blood pressure for up to 10 min, cardiac output was affected variably. The magnitude of hemodynamic changes that each drug produced did not differ significantly from each other at either dose level. However, of the sheep receiving intravenous lidocaine, none developed arrhythmias other than mild sinus tachycardia and minimal ST-T wave changes (which occurred in 25% of the animals). After intravenous bupivacaine injection, all sheep had transient changes on the EKG and/or arrhythmias (e.g., supraventricular tachycardia; atrioventricular condition blocks; ventricular tachycardia; multiform premature ventricular contractions; wide QRS complexes; ST-T wave changes; and in one animal, fatal ventricular fibrillation). Normal sinus rhythm usually returned within 8-10 min. Arterial blood gas and acid-base values stayed within the normal range during the studies, and serum potassium did not change significantly from control. In conclusion, in conscious adult sheep, equivalent doses of lidocaine or bupivacaine produced similar central nervous system (CNS) toxicity when rapidly injected intravenously. In the absence of marked hypoxia, respiratory or metabolic acidosis, hyperkalemia, or hypotension, serious cardiac arrhythmias occurred after bupivacaine but not lidocaine.

Removal of insect basal laminae using elastase

We have used the enzyme elastase to remove the basal lamina of epithelia from two insects: the upper Malpighian tubules of Rhodnius prolixus and imaginal discs of Drosophila melanogaster. Removal of the basal lamina was confirmed using scanning and transmission electron microscopy. Use of the technique on the Malphighian tubules of Rhodnius reveals for the first time the three-dimensional organization of the circumferential folds of the basal plasma membrane. Elastase is much more effective in removing the basal lamina than are the enzymes hyaluronidase, collagenase, and chymotrypsin, either alone or in combination. Following elastase treatment, cells of the Malpighian tubules dissociate with only mild mechanical agitation into single, viable cells. Treatment with elastase removes the basal laminae of imaginal discs of Drosophila and accelerates evagination as has been previously described for trypsin. To obtain single cell preparations from elastase-treated imaginal discs, mechanical stirring in Ringer low in Ca2+ was required. In addition to its usefulness in cell isolation, elastase treatment allows examination of the effect of removal of basal laminae on the physiology and development of insect epithelia.

Maternal catecholamines decrease during labor after lumbar epidural anesthesia

To determine whether epidural anesthesia during labor affects maternal circulating catecholamines, blood samples were obtained from 15 patients at the peak of and immediately after two consecutive painful contractions. A lumbar epidural local anesthetic without epinephrine was then administered. After the onset of analgesia, four blood samples were again drawn. All samples were analyzed by a radioenzymatic assay for epinephrine and norepinephrine concentrations. Before anesthesia, the mean (+/-SEM) plasma epinephrine level was 280 +/- 49 pg/ml, and the mean norepinephrine level was 866 +/- 122 pg/ml. After anesthesia, epinephrine levels decreased 56% (p less than 0.01). Although norepinephrine levels decreased approximately 19%, this reduction was not statistically significant. At the height of a contraction, catecholamine levels did not differ significantly from those occurring between contractions. Lumbar epidural anesthesia during labor reduces maternal epinephrine levels, probably by eliminating the psychological and physical stress associated with painful uterine contractions or by denervating the adrenal medulla. Whatever the mechanism, reducing pain and activity of the sympathetic nervous system should increase uterine blood flow.

A new neurologic and adaptive capacity scoring system for evaluating obstetric medications in full-term newborns

A variety of examinations are currently available for evaluating the neurobehavior of the newborn. These exams are often difficult and time-consuming to perform, require extensive training of the examiners, and produce results that may be difficult to interpret. The authors describe a new Neurologic and Adaptive Capacity Score (NACS) for full-term neonates and compare it with the Scanlon Early Neonatal Neurobehavioral Scale (ENNS), the most widely used test for evaluating effects of obstetric medication on the neonate. The NACS was designed as a screening test to detect central nervous system depression from drugs and also to differentiate these effects from those found after birth trauma and perinatal asphyxia. The NACS is based on 20 criteria, each of which is given a score of 0, 1, or 2. These criteria assess five general areas: 1) adaptive capacity; 2) passive tone; 3) active tone; 4) primary reflexes; and 5) alertness, crying, and motor activity (general observations). In contrast to the ENNS, the NACS places more emphasis on motor tone, avoids the use of noxious stimuli (pinprick, repeated Moro examinations), takes half the time to perform, and provides for any given baby a single number that immediately identifies a depressed or vigorous neonate.

Neonatal neurobehavioral effects of inhalation analgesia for vaginal delivery

The authors studied the neonatal neurobehavioral effects of nitrous oxide:oxygen and enflurane:oxygen inhalation analgesia for vaginal delivery. Parturients were assigned randomly to receive no inhalation agent (Group 1, n = 21); enflurane, 0.3 to 0.8 per cent, and oxygen (Group 2, n = 22); or nitrous oxide, 30 to 50 per cent, and oxygen (Group 3, n = 18). Infants were tested at 15 min, 2 h, and 24 h of age using the Neurologic and Adaptive Capacity Score (NACS); and at 2 and 24 h using the Early Neonatal Neurobehavioral Scale (ENNS). No significant differences in neurobehavioral status occurred. For all groups, scores tended to be lowest at two hours of age. We conclude that neither enflurane nor nitrous oxide analgesia adversely affects neonatal neurobehavioral status at 15 min, 2 h, or 24 h of age.

The effect of maternal administration of ephedrine on fetal heart rate and variability

Ephedrine is the most popular vasopressor used in obstetrics to prevent or treat spinal or epidural hypotension. Fetal heart rate (FHR) changes during epidural anesthesia were studied in 71 patients; it was found that ephedrine administration (57 patients) was associated with significant increases in FHR and beat-to-beat variability. These changes were dose related and were not associated with fetal asphyxia as judged by measurement of fetal scalp blood pH or Apgar scores.

Enflurane analgesia in obstetrics

The effects of enflurane analgesia (approximately 0.5%) were studied in 55 patients during the second stage of normal vaginal delivery and were compared with effects of nitrous oxide (approximately 40%) in 50 similar patients. The enflurane and oxygen mixture was rated satisfactory by 89% of the mothers and 80% of the anesthesiologists. These ratings did not differ significantly from those for nitrous oxide. Obstetricians, however, rated the enflurane and oxygen mixture superior. The newborns of mothers receiving both agents wee vigorous and comparable when assessed by Apgar scores and cord blood gas tensions. The estimate of blood loss was similar in both groups. Serum inorganic fluoride concentrations in the mother after anesthesia were not significantly increased from preanesthetic levels with either agent. There was no biochemical evidence of renal toxicity. In neonates of mothers given enflurane, the mean umbilical cord concentration of serum inorganic fluoride ions was 2.4 +/- 0.2 micromoles/L, a value well below that associated with nephrotoxicity.

Circadian rhythms in teleost retinomotor movement. A comparison of the effects of circadian rhythm and light condition on cone length

The long, slender cones of the teleost retina elongate at night and contract during the day. In the Midas cichlid, Cichlasoma citrinellum, this cone excursion is elicited both by changes in light conditions and by strong endogenous circadian rhythms. In a normal day/night cycle. C. citrinellum cones change length by 69 micron. We have found that in this species an endogenous circadian rhythm induces substantial cone excursion in contrast light as well as in constant darkness. Total excursion in constant light is 34% of that seen in a normal cycle. Total excursion in constant darkness is 58% of that seen in a normal cycle. Similar excursions are observed on the second and third days of constant darkness. A change from light to darkness at a time in the cycle other than dusk induces elongation averaging 49% of the total excursion observed in the normal cycle. A change from darkness to light at a time other than dawn induces cone contraction averaging 30% of the total excursion observed in the normal cycle. The response of retinal cones to either of the above changes in light conditions is a relatively constant magnitude at all sampling times over a 24 hr period. We conclude that both the endogenous circadian rhythm and the responses to changes in light conditions of the cones and required to produces the full excursion observed in the normal day/night cycle. Full elongation of cones can occur only at night in darkness, and full contraction of the cones can occur only in the day in the light. Changing light conditions at inappropriate times produces intermediate cone lengths.

Dopamine treatment of spinal hypotension decreases uterine blood flow in the pregnant ewe

In seven pregnant ewes, 3--5 min of hypotension resulting from spinal anesthesia decreased uterine blood flow 17 per cent. Dopamine, in doses sufficient to maintain blood pressure at control values (20--40 micrgram/kg/min) for 3--5 min, further decreased uterine blood flow to 56 per cent less than control and increased uterine vascular resistance to 50 per cent more than control. Following 30 min of hypotension, 30 min of dopamine administration in doses sufficient to restore blood pressure to control values (5--20 microgram/kg/min) similarly decreased uterine blood flow to 29 per cent less than control and increased uterine vascular resistance to 35 per cent more than control.

Uterine blood flow and plasma norepinephrine changes during maternal stress in the pregnant ewe

Because maternal stress may adversely affect the fetus, the authors tested the effects of brief episodes (15-60 sec) of maternal stress in 18 awake pregnant ewes. Maternal agitation and stuggling occurred either following non-painful stimuli such as loud noises or sudden movements of personnel (ten animals) or following the brief application of the ewe's skin of a uniform electrical stimulus of 30 volts with a frequency of 167 Hz for 30-60 sec (eight animals). Stimulation of either type produced a 45-50 per cent increase in mean maternal arterial blood pressure and a concomitant 32-52 per cent decrease in uterine blood flow (P less than 0.05). The decreases in uterine blood flow were brief, lasting less than 3 min, and were not associated with fetal asphyxia. Maternal plasma norepinephrine levels were measured following electrically induced maternal stress and were increased 25 per cent. The authors conclude that maternal stress may decrease uterine blood flow secondary to release of endogenous norepinephrine.

Anesthetic considerations for myasthenia gravis and pregnancy

Three pregnant patients with myasthenia gravis are presented and anesthetic considerations discussed. The course of myasthenia gravis is highly variable and unpredictable during pregnancy. Anticholinesterase therapy should be maintained during labor, and the IM route of administration is preferred. Exacerbations very often occur in the immediate postpartum period. Regional anesthesia is preferred for vaginal delivery. In the case of cesarean section, general anesthesia may be preferable. Neonatal myasthenia gravis is a transient syndrome that appears in 20 to 30% of the newborns of myasthenic mothers.

Placental transfer of lidocaine: effects of fetal acidosis

To investigate whether fetal acidosis increases the placental transfer of lidocaine, resulting in higher fetal blood levels of the drug, lidocaine was infused intravenously into ten pregnant ewes to maintain plasma levels of 2-4 microgram/ml. After maternal-fetal equilibrium was reached, the fetus was made acidotic by infusing lactic acid intravenously. Fetal blood pH decreased from 7.35 to 7.10. With fetal acidemia, fetal blood lidocaine levels increased significantly from 1.60 +/- 0.11 microgram/ml to 2.72 +/- 0.26 microgram/ml. The fetal-maternal lidocaine ratio increased from 0.76 to 1.21. Correction of the acidosis by bicarbonate infusion returned the fetal-maternal ratios to control values. It is concluded that acidosis in the fetus may result in trapping of ionized lidocaine in the fetal circulation and increase the transfer of lidocaine across the placenta.

Dopamine administration in the normotensive pregnant ewe

Dopamine was administered to pregnant ewes at randomly selected rates of 1, 3, 5, 10, 20, 30, or 40 microgram/kg/min. Doses less than 10 microgram/kg/min produced no significant change in maternal areterial blood pressure, cardiac output, or renal or uterine blood flow. Higher doses increased maternal cardiac output and blood pressure, did not change renal blood flow, but decreased uterine blood flow. Dopamine consistently caused mild maternal and fetal respiratory acidosis.

Anaesthetic considerations in caesarean section for quadruplets

A case of a caesarean delivery with epidural analgesia of a term parturient with quadruplets is presented. Maternal considerations of hypotension, respiratory embarrassment and aspiration of gastric content and foetal considerations of prematurity and impaired placental function are discussed relative to the use of general anaesthesia or epidural analgesia.