Bundle branch block as a predictor of long-term survival after acute myocardial infarction

Using a community-based population of patients with acute myocardial infarction (AMI), we sought to: (1) determine the prevalence of bundle branch block (BBB) on the presenting electrocardiogram (ECG), (2) compare the clinical characteristics and the treatment administered to patients with and without BBB, and (3) determine the association of BBB with mortality. We analyzed the admission ECGs of 894 consecutive patients with AMI from Olmsted County, Minnesota, seen at our institution from January 1988 to March 1998. Of these, 53 had left BBB (LBBB) (5.9%) and 60 had right BBB (RBBB) (6.7%). Patients with BBB were more likely to be older, have a history of AMI or hypertension, and to be in Killip class >I at presentation. They were less likely to receive primary reperfusion therapy, beta blockers, or heparin, but more likely to receive angiotensin-converting enzyme inhibitors. They had lower mean predischarge ejection fractions (38 +/- 16% vs 50 +/- 15%, p <0.0001). In-hospital mortality was 13.3%, 17.0%, and 9.1% for patients with RBBB, LBBB, and no BBB, respectively (p = 0.11). Respective postdischarge survival at 1, 3, and 5 years was 80%, 60%, and 50% in the RBBB group, 78%, 56%, and 51% in the LBBB group, and 92%, 85%, and 76% in the group without BBB (p <0.0001). Although BBB was not an independent predictor of mortality on multivariate analysis, the presence of transient or persistent BBB with AMI is an easily recognized clinical marker of increased mortality. Our conclusion from this study is that in a community-based population, patients who had LBBB or RBBB at the time of AMI had lower predischarge ejection fractions and higher in-hospital and long-term unadjusted mortality.

Characteristics of presenting electrocardiograms of acute myocardial infarction from a community-based population predict short- and long-term mortality

To investigate the relevance of presenting electrocardiographic (ECG) patterns to short- and long-term mortality in nonreferral patients with acute myocardial infarction (AMI), 6 ECG patterns were analyzed. A consecutive series of 907 patients from Olmsted County, Minnesota, admitted to the Mayo Clinic Cardiac Care Unit from January 1, 1988 to March 31, 1998 for acute myocardial infarction comprised the study population. ECG patterns and distribution in the population were: (1) ST elevation alone (20.8%), (2) ST elevation with ST depression (35.2%), (3) normal or nondiagnostic electrocardiograms (18.5%), (4) ST depression alone (11.8%), (5) T-wave inversion only (10.7%), and (6) new left bundle branch block (LBBB) (3.0%). Seven- and 28-day mortalities varied significantly (p <0.01) among the 6 ECG groups. Respective mortalities were 3.0% and 6.0% for patients with normal or nondiagnostic electrocardiograms, 3.1% and 5.2% for T-wave inversion only, 7.4% and 10.6% for ST elevation alone, 9.4% and 13.1% for ST depression alone, 10.3% and 13.8% for ST elevation with ST depression, and 18.5% and 22.2% for new LBBB. Length of hospital stay (LOS) also varied among the ECG pattern groups (p <0.001) with the longest average LOS being in the new LBBB group (12.5 days). Long-term survival was similar among 5 ECG pattern groups (45% to 55% at 8 years from discharge) with the exception of LBBB (20% at 8 years). Among non-LBBB groups, ST-segment depression with or without ST elevation was associated with increased short-term mortality. Also, in this community-based population, 18.5% of patients had normal or nondiagnostic electrocardiograms.

Two-factor theory has strong empirical evidence of validity

Traditional two-factor theory is supported by parallels in the clinical literature. Theoretical problems with two-factor theory are obviated by the role of safety signals, which serve as positive conditioned reinforcers and retard the extinction of conditioned fear.

Effect of concomitant or very early statin administration on in-hospital mortality and reinfarction in patients with acute myocardial infarction

In a retrospective analysis, 66 patients identified as having received a statin drug within 24 hours of admission for acute myocardial infarction were matched 3:1 with a control group of 198 patients not treated with a statin agent. End points of in-hospital mortality and in-hospital reinfarction were significantly lower in the statin-treated group, pointing to a benefit from very early statin treatment in acute myocardial infarction.

Effects of signaled versus unsignaled delay of reinforcement on choice

Pigeons chose between 5-s and 15-s delay-of-reinforcement alternatives. The first key peck to satisfy the choice schedule began a delay timer, and food was delivered at the end of the interval. Key pecks during the delay interval were measured, but had no scheduled effect. In Experiment 1, signal conditions and choice schedules were varied across conditions. During unsignaled conditions, no stimulus change signaled the beginning of a delay interval. During differential and nondifferential signal conditions, offset of the choice stimuli and onset of a delay stimulus signaled the beginning of a delay interval. During differential signal conditions, different stimuli were correlated with the 5-s and 15-s delays, whereas the same stimulus appeared during both delay durations during nondifferential signal conditions. Pigeons showed similar, extreme levels of preference for the 5-s delay alternative during unsignaled and differentially signaled conditions. Preference levels were reliably lower with nondifferential signals. Experiment 2 assessed preference with two pairs of unsignaled delays in which the ratio of delays was held constant but the absolute duration was increased fourfold. No effect of absolute duration was found. The results highlight the importance of delayed primary reinforcement effects and challenge models of choice that focus solely on conditioned reinforcement.

Effect of current tobacco use and history of an alcohol problem on health status in hospitalized patients

This prospective study assessed the relationship between current tobacco use and history of an alcohol problem to health status in hospitalized patients. Factors associated with current tobacco use and history of an alcohol problem were also evaluated. Data were collected using a self-administered survey distributed by nursing staff to adult inpatients registered on April 27, 1994 at the Mayo Clinic, Rochester, Minnesota affiliated hospitals. Respondents (N = 589, 45% female) were classified into 4 subgroups based on current tobacco use status and history of an alcohol problem: (a) current tobacco use only (n = 94, 16%); (b) history of an alcohol problem only (n = 30, 5%); (c) both (n = 27, 5%); or (d) neither (n = 438, 74%). Patients with both current tobacco use and an alcohol problem history reported markedly lower scores on health status measures of general and mental health compared to the other three subgroups. Moreover, current tobacco use and history of an alcohol problem were each associated with increased psychological distress. Current tobacco use was predictive of a history of an alcohol problem and vice versa.

Process analysis in outpatient knee surgery: effects of regional and general anesthesia on anesthesia-controlled time

BACKGROUND

The performance of anesthetic procedures before operating room entry (e.g., with either general or regional anesthesia [RA] induction rooms) should decrease anesthesia-controlled time in the operating room. The authors retrospectively studied the associations between anesthesia techniques and anesthesia-controlled time, evaluating one surgeon performing a single procedure over a 3-yr period. The authors hypothesized that, using the anesthesia care team model, RA would be associated with reduced anesthesia-controlled time compared with general anesthesia (GA) alone or combined general-regional anesthesia (GA-RA).

METHODS

The authors queried an institutional database for 369 consecutive patients undergoing the same procedure (anterior cruciate ligament reconstruction) performed by one surgeon over a 3-yr period (July 1995 through June 1998). Throughout the period of study, anesthesia staffing consisted of an attending anesthesiologist medically directing two nurse anesthetists in two operating rooms. Anesthesia-controlled time values were compared based on anesthesia techniques (GA, RA, or GA-RA) using one-way analysis of variance, general linear modeling using time-series and seasonal adjustments, and chi-square tests when appropriate. P < 0. 05 was considered significant.

RESULTS

RA was associated with the lowest anesthesia-controlled time (11.4 +/- 1.3 min, mean +/- 2 SEM). GA-RA (15.7 +/- 1.0 min) was associated with lower anesthesia-controlled time than GA used alone (20.3 +/- 1.2 min).

CONCLUSIONS

When compared with GA without an induction room for outpatients undergoing anterior cruciate ligament reconstruction, RA with an induction room was associated with the lowest anesthesia- controlled time. Managers must weigh the costs and time required for anesthesiologists and additional personnel to place nerve blocks or induce GA preoperatively in such a staffing model.

Fate restriction in limb muscle precursor cells precedes high-level expression of MyoD family member genes

The mechanisms by which pluripotent embryonic cells generate unipotent tissue progenitor cells during development are unknown. Molecular/genetic experiments in cultured cells have led to the hypothesis that the product of a single member of the MyoD gene family (MDF) is necessary and sufficient to establish the positive aspects of the determined state of myogenic precursor cells: i.e., the ability to initiate and maintain the differentiated state (Weintraub, H., Davis, R., Tapscott, S., Thayer, M., Krause, M., Benezra, R., Blackwell, T. K., Turner, D., Rupp, R., Hollenberg, S. et al. (1991) Science 251, 761–766). Embryonic cell type determination also involves negative regulation, such as the restriction of developmental potential for alternative cell types, that is not directly addressed by the MDF model. In the experiments reported here, phenotypic restriction in myogenic precursor cells is assayed by an in vivo ‘notochord challenge’ to evaluate their potential to ‘choose’ between two alternative cell fate endpoints: cartilage and muscle (Williams, B. A. and Ordahl, C. P. (1997) Development 124, 4983–4997). Two separate myogenic precursor cell populations were found to be phenotypically restricted while expressing the Pax3 gene and prior to MDF gene activation. Therefore, while MDF family members act positively during myogenic differentiation, phenotypic restriction, the negative aspect of cell specification, requires cellular and molecular events and interactions that precede MDF expression in myogenic precursor cells. The qualities of muscle formed by the determined myogenic precursor cells in these experiments further indicate that their developmental potential is intermediate between that of myoblastic stem cells taken from fetal or adult tissue (which lack mitotic and morphogenetic potential when tested in vivo) and embryonic stem cells (which are multipotent). We hypothesize that such embryonic myogenic progenitor cells represent a distinct class of determined embryonic cell, one that is responsible for both tissue growth and tissue morphogenesis.

An oxygen-, acid- and anaesthetic-sensitive TASK-like background potassium channel in rat arterial chemoreceptor cells

The biophysical and pharmacological properties of an oxygen-sensitive background K+ current in rat carotid body type-I cells were investigated and compared with those of recently cloned two pore domain K+ channels. Under symmetrical K+ conditions the oxygen-sensitive whole cell K+ current had a linear dependence on voltage indicating a lack of intrinsic voltage sensitivity. Single channel recordings identified a K+ channel, open at resting membrane potentials, that was inhibited by hypoxia. This channel had a single channel conductance of 14 pS, flickery kinetics and showed little voltage sensitivity except at extreme positive potentials. Oxygen-sensitive current was inhibited by 10 mM barium (57% inhibition), 200 microM zinc (53% inhibition), 200 microM bupivacaine (55% inhibition) and 1 mM quinidine (105 % inhibition). The general anaesthetic halothane (1.5%) increased the oxygen-sensitive K+ current (by 176%). Halothane (3 mM) also stimulated single channel activity in inside-out patches (by 240%). Chloroform had no effect on background K+ channel activity. Acidosis (pH 6.4) inhibited the oxygen-sensitive background K+ current (by 56%) and depolarised type-I cells. The pharmacological and biophysical properties of the background K+ channel are, therefore, analogous to those of the cloned channel TASK-1. Using in situ hybridisation TASK-1 mRNA was found to be expressed in type-I cells. We conclude that the oxygen- and acid-sensitive background K+ channel of carotid body type-I cells is likely to be an endogenous TASK-1-like channel.

Estimating the extent of degradation of ruminant feeds from a description of their gas production profiles observed in vitro: comparison of models

An evaluation of general models that describe gas production profiles is presented. The models are derived from first principles by considering a simple three-pool scheme and permit the extent of ruminal degradation to be calculated, as described in the companion paper. The models evaluated were the generalized Mitscherlich, simple Mitscherlich, generalized Michaelis-Menten, simple Michaelis-Menten, Gompertz, and logistic. Five sets of gas production data consisting of 216 curves, obtained using a wide range of feeds (including straw, hay, silage, grain and various byproducts), were analysed to study the performance of these gas production models. Application of the non-sigmoidal models (simple Mitscherlich and Michaelis-Menten) to the data resulted in convergence problems and these models were found to be inadequate in many cases. Based on results of a pairwise comparison between models (variance ratio test), ranking of residual mean squares, lack-of-fit test, and of analyses of residuals, the generalized Mitscherlich and the generalized Michaelis-Menten models seemed particularly suited because of their flexibility to encompass sigmoidal and non-sigmoidal shapes of gas production profiles, whether symmetrical or not.

Associative competition in operant conditioning: blocking the response-reinforcer association

Naive rats were trained to leverpress with a 30-sec delay-of-reinforcement contingency from the start of training. In Experiment 1, the delay interval for different groups of subjects included a signal in the first 5 sec, a signal in the last 5 sec, or no signal at any time. Rats with the signal at the start of the delay interval learned most rapidly. Rats with the signal at the end of the delay failed to learn. In Experiment 2, a choice procedure was used, in which each of two levers was associated with its own 30-sec delay of reinforcement. The delay for one lever included a 5-sec signal at the end of the delay. The delay for the second lever had no signal in its 30-sec delay. Preference was in favor of the lever without the signal in the delay interval. The results demonstrate that the acquisition of new response can be blocked in a manner analogous to the blocking of Pavlovian conditioning.

Value transmission in discrimination learning involving stimulus chains

Rats learned a series of reversals of a positional discrimination in which responses to one lever led to delayed food and responses to a second lever led to no food. Interpolated within the delays leading to the different outcomes were two-link stimulus chains. The pairing of each stimulus element with the delayed outcome of food or no food varied across reversals. Either stimulus element could have the same correlation with outcome as occurred on the preceding reversal or the opposite correlation as on the preceding reversal. New reversals were acquired more quickly when both stimulus elements had the same status as during the preceding reversal, and were acquired most slowly when both stimulus elements had the opposite status as that of the preceding reversal. The rate of learning was intermediate when only one of the stimulus elements had the same status as that during the preceding reversal. All of the data are compatible with an interpretation in terms of backward chaining of stimulus value.

Kinetics of rate-dependent shortening of action potential duration in guinea-pig ventricle; effects of IK1 and IKr blockade

1. The kinetics of shortening of action potential duration (APD) following an increase in pacing rate, from 2 to 3.3 Hz, was characterized in guinea-pig ventricular preparations. Terikalant (RP62719), an inhibitor of the inwardly rectifying K+ current (IK1), and dofetilide, a specific inhibitor of the rapidly activating delayed-rectifier current (IKr), were applied to determine the effect of inhibition of these ion currents on slow APD shortening. 2. Action potentials were recorded from isolated guinea-pig ventricular myocytes using the perforated-patch patch-clamp technique, and monophasic action potentials were recorded from Langendorff-perfused guinea-pig ventricles using a contact epicardial probe. 3. Under control conditions, after an increase in pacing rate, APD immediately decreased, and then shortened slowly with an exponential time course. In ventricular myocytes, the time constant of this exponential shortening was 28+/-4 s and the amount of slow shortening was 21.9+/-0.9 ms (n=8) for an increase in rate from 2 to 3.3 Hz. Similar values were observed in Langendorff-perfused ventricles. 4. Terikalant dose-dependently increased APD and the increase was enhanced by rapid pacing ('positive' rate-dependence). The drug dose-dependently decreased the time constant of shortening and amount of slow APD shortening. In contrast, dofetilide, an inhibitor of IKr, which shows 'reverse' rate-dependent APD widening, had no significant effect on the time constant or amount of slow shortening. 5. These observations suggest that IK1 plays a role in rate-dependent shortening of APD. The results appear to support the hypothesis that 'reverse' rate-dependent effects of IKr blockers are due to these drugs not affecting the ion current(s) mediating intrinsic rate-dependent slow shortening of APD.

S.T.E.P.S.: a comprehensive program for controlling and monitoring access to thalidomide

In July 1998, the US Food and Drug Administration approved the marketing of thalidomide for the treatment of cutaneous manifestations of erythema nodosum leprosum. To ensure that fetal exposure to this teratogenic agent does not occur, the manufacturer has instituted a comprehensive program to control prescribing, dispensing, and use of the drug. This program, known as the System for Thalidomide Education and Prescribing Safety (S.T.E.P.S. [Celgene Corporation, Warren, New Jersey]), is based in part on experience gained with other drugs--specifically isotretinoin and clozapine--that offer important clinical benefits but carry the potential for serious harm. To achieve its goal of the lowest possible incidence of drug-associated teratogenicity, the S.T.E.P.S. program uses a three-pronged approach: (1) controlling access to the drug; (2) educating prescribers, pharmacists, and patients; and (3) monitoring compliance. Clinicians who wish to prescribe thalidomide must be registered in the S.T.E.P.S. Prescriber Registry and agree to prescribe the drug in accordance with S.T.E.P.S. patient eligibility criteria and monitoring procedures. Pharmacies must also register and agree to comply with patient identification and monitoring criteria. Finally, patients receive visual aids, including a videotape, written material, and verbal counseling about the benefits and risks of thalidomide therapy, the importance of not becoming pregnant during therapy, and the types of contraception required (including emergency contraception) and their availability. Women of childbearing potential must agree to undergo pregnancy testing before starting therapy and on a regular schedule during therapy. All patients must agree to complete a confidential survey about their compliance with contraception, testing, and drug therapy. The manufacturer is monitoring survey results and outcome data and is prepared to make whatever modifications to the S.T.E.P.S. program are necessary to ensure its effectiveness. In addition to minimizing the potential risk for fetal harm associated with thalidomide therapy, the S.T.E.P.S. program may provide a model for future cases in which a drug offers compelling benefits but poses profound risks unless its distribution is carefully controlled.

Benchmarking the perioperative process: II. Introducing anesthesia clinical pathways to improve processes and outcomes and to reduce nursing labor intensity in ambulatory orthopedic surgery

STUDY OBJECTIVES

(1) To introduce anesthesia clinical pathways as a management tool to improve the quality of care; (2) to use the Procedural Times Glossary published by the Association of Anesthesia Clinical Directors (AACD) as a template for data collection and analysis; and (3) to determine the effects of anesthesia clinical pathways on surgical processes, outcomes, and costs in common ambulatory orthopedic surgery.

DESIGN

Hospital database and patient chart review of consecutive patients undergoing anterior cruciate ligament reconstruction (ACLR) during academic years (AY) 1995-1996 and 1996-1997. Patient data from AY 1995-1996, during which no intraoperative anesthesia clinical pathways existed, served as historical controls. Data from AY 1996-1997, during which intraoperative anesthesia clinical pathways were used, served as the treatment group. Regional anesthesia options were routinely offered to patients in the clinical pathway.

SETTING

Ambulatory surgery center in a teaching hospital.

MEASUREMENTS AND MAIN RESULTS

The records of 503 ASA physical status I and II patients were reviewed. 1996-1997 patients underwent clinical pathway anesthesia care in which the intraoperative and postoperative anesthesia process was standardized with respect to symptom management, drugs, and equipment used. 1995-1996 patients did not have a standardized intraoperative and postoperative anesthetic course with respect to the management of common symptoms or to specific drugs and supplies used. Intervals described in the AACD Procedural Times Glossary, anesthesia drug and supply costs, and patient outcome variables (postoperative nursing interventions required and unexpected admissions), as influenced by the use of the anesthesia clinical pathway, were measured. Clinical pathway anesthesia care of ACLR in 1996-1997, which actively incorporated regional anesthesia options, reduced pharmacy and materials cost variability; slightly increased turnover time; improved intraoperative anesthesia and surgical efficiency, recovery times, and unexpected admission rates; and decreased the number of required nursing interventions for common postoperative symptoms.

CONCLUSIONS

Clinical pathway patient management systems in anesthesia care are likely to produce useful outcome data of current practice patterns when compared with historical controls. This management tool may be useful in simultaneously containing costs and improving process efficiency and patient outcomes.

Benchmarking the perioperative process: III. Effects of regional anesthesia clinical pathway techniques on process efficiency and recovery profiles in ambulatory orthopedic surgery

STUDY OBJECTIVES

(1) To incorporate regional anesthesia options for common outpatient orthopedic surgery into clinical pathways; (2) to use the clinical pathway format and the Procedural Times Glossary published by the Association of Anesthesia Clinical Directors (AACD) as management tools to measure postoperative same-day surgery processes and discharge outcomes; and (3) to determine the effects of general, regional, and combined general-regional anesthesia on these processes and outcomes.

DESIGN

Hospital database and patient chart review of consecutive patients undergoing anterior cruciate ligament reconstruction (ACLR) during academic years (AY) 1995-1996 and 1996-1997. Patient data from AY 1995-1996, during which no intraoperative anesthesia clinical pathway existed, served as historical controls. Data from AY 1996-1997, during which intraoperative anesthesia clinical pathways were used, served as the treatment group.

SETTING

Ambulatory surgery center in a teaching hospital.

MEASUREMENTS AND MAIN RESULTS

The records of 503 ASA physical status I and II patients were reviewed. 1996-1997 patients selected general anesthesia (+/- femoral nerve block) or epidural anesthesia, after which the remainder of the perioperative anesthesia process was standardized with respect to the drugs and equipment used. 1995-1996 patients did not necessarily have a choice in anesthesia technique and did not have a standardized perioperative anesthetic course with respect to specific drugs and supplies. Intervals described in the AACD Procedural Times Glossary, anesthesia drug and supply costs, and patient outcome variables (postoperative nursing interventions required and unexpected admissions), as influenced by anesthesia technique used, were measured. Combined general-regional anesthesia care for ACLR in 1996-1997, when compared with general anesthesia alone, led to increased pharmacy and materials costs and increased turnover time. However, patients with the combined technique showed improved recovery profiles and lower unexpected admission rates, and they required fewer nursing interventions for common postoperative symptoms. Patients receiving epidural anesthesia showed discharge outcomes similar to those patients receiving general anesthesia with femoral nerve block. Postanesthesia care unit bypass (fast-tracking) was more likely in clinical pathway regional anesthesia patients, when compared with the clinical pathway general anesthesia used.

CONCLUSIONS

Clinical pathway regional anesthesia care for outpatient orthopedics may have a significant role in simultaneously containing costs and improving both process efficiency and patient outcomes.

Z/I and A-band lattice spacings in frog skeletal muscle: effects of contraction and osmolarity

A-band and Z-line/I-band lattice spacings were measured by small-angle X-ray diffraction from relaxed and isometrically-contracting whole frog sartorius muscles with lattice spacings reduced or swollen by changing the osmolarity of the bathing solution. A-band spacing increased by approximately 3% upon isometric contraction at reduced lattice spacings (245-356 mOsm) and decreased by approximately 1% at swollen spacings (172 mOsm), similarly to the behaviour of skinned muscles upon changing from the relaxed state to rigor. The Z/I lattice underwent a significant lattice expansion (3-8%) upon isometric contraction at all osmolarities, in qualitative agreement (but quantitative disagreement) with results from electron microscopy on mammalian skeletal muscle. Lattice areas calculated for the Z/I and A-band lattices indicate a barrel-shaped sarcomere in the resting state, which may provide a partial explanation for how longitudinal forces produced in the A-band can produce a radial expansive force in the Z-line during contraction. The radial component of cross-bridge stiffness was calculated from the A-band data for contracting muscle, using a lattice stability model incorporating structural, osmotic and electrostatic forces. The calculations gave a radial cross-bridge stiffness during contraction of about 9 x 10(5) N m-2, and outward radial force per thick filament in normal Ringer's solution of 6 x 10(-9) N, corresponding to a radial force per cross-bridge of 10(-11) N.

The use of cumulative gas and volatile fatty acid production to predict in vitro fermentation kinetics of Italian ryegrass leaf cell walls and contents at various time intervals

Differences between the fermentation characteristics of cell contents (CC) and protease-treated cell walls (CW) of young leaves of Italian ryegrass (Lolium multiflorum Lam.) cultivar Multimo (tetraploid), were studied in vitro. Gas and volatile fatty acid (VFA) production rates were measured at regular intervals, as was the degradability of organic matter (OM) of CW. The measured VFA were used to predict the gas production and fermentable OM using stoichiometric calculations. For CW the volume and kinetics of measured gas production were the same as those predicted from the VFA formed. In contrast, the measured gas production for CC was consistently less than predicted, indicating that the stoichiometric equations were not valid for rapidly fermenting substrates. For both CC and CW, the relative rate of acetic acid production levelled off more slowly than for other VFA, resulting in an increasing gas yield (in ml/g fermentable OM) after 12 (CW)-24 (CC) h incubation. Consequently, the fermentation of OM was not linearly related to gas production kinetics. For CW, the kinetics of decline of degradable OM and fermentable OM were the same, after correction for a constant 'lost fraction' of degradable OM of 205 g/kg OM. This work indicates the value of detailed studies of fermentation processes to evaluate herbage quality. In particular, the role of CC and the difference between degradation and fermentation require further attention.

Knowing chops from chuck: roasting myoD redundancy

The myf5 and myoD genes are implicated in the specification of vertebrate skeletal muscle. These genes have been thought to be functionally redundant because neonatal mice bearing homozygous null mutations in either gene show grossly normal muscle development. By analyzing the early embryonic development of the mutants, Michael Rudnicki and coworkers show that trunk muscle development is retarded in embryos bearing myf5 null mutations, while early limb and branchial arch muscle development is retarded by myoD null mutations. These results indicate that the myoD and myf5 genes are not redundant but that each controls the early specification of distinct muscle cell lineages.

Emergence of determined myotome precursor cells in the somite

Myotome and sclerotome precursor cells are derived, respectively, from cells in the dorsomedial and ventromedial regions of the somite. To assay changes in the specification of myotomal precursor cells during somite maturation, we implanted dorsomedial quadrant fragments, from staged quail somites, next to the notochords of host chick embryos, and superimposed two additional notochords on these implants. In this notochord signalling environment, dorsomedial quadrant cells that are developmentally plastic are expected to differentiate as cartilage, while cells determined to a myogenic fate are expected to differentiate as skeletal muscle. Large numbers of differentiated chondrocytes developed from dorsomedial quadrant grafts of all stages of paraxial mesoderm development tested, indicating that persistent chondrogenic potential in cells fated to form muscle and dermis can be elicited by notochord signals. Differentiated myocytes, however, appeared in two somite-stage-dependent phases. In the first phase, dorsomedial quadrants from segmental plate and early stage somites (II and IV) form small, disorganized clusters of individual myocytes. The frequency of first-phase myocluster formation increases as myogenic factor expression begins in the dorsomedial quadrant, indicating that myogenic determination assayed by this method is closely linked to the expression of myogenic factors in the dorsomedial quadrant. In the second phase, dorsomedial quadrants from somite stages XI-XIII consistently form morphologically organized muscle tissue containing large numbers of parallel-oriented, multinucleated myotubes. Mitotic labelling demonstrated that muscle precursors were determined to the muscle phenotype prior to withdrawal from the cell cycle. Thus, myogenic determination in cells of the dorsomedial quadrant is acquired at earlier stages of somite maturation than the ability to proliferate and form muscle tissue. These results are consistent with the hypothesis that successive lineages of myotome precursor cells with different mitotic and morphogenetic properties arise in the dorsomedial quadrant during somite maturation.

Treatment of the P815 murine mastocytoma with cisplatin or etoposide up-regulates cell-surface Fas (CD95) expression and increases sensitivity to anti-Fas antibody-mediated cytotoxicity and to lysis by anti-CD3-activated killer-T cells

We have investigated the effect of pre-treatment with the anti-cancer drugs cisplatin and etoposide on the susceptibility of P815 murine mastocytoma cells to lysis by murine spleen-derived anti-CD3-activated killer-T (AK-T) cells. A 20 hr pre-treatment with cisplatin (0.2-2 microg/ml) or etoposide (0.01-1 microg/ml) rendered P815 cells significantly more sensitive to AK-T cell-mediated lysis in a 4 hr 51Cr-release assay than untreated control tumor cells. At lower concentrations, pre-treatment with cisplatin or etoposide had no direct cytotoxic effects on P815 tumor cells, as measured by the MTT assay. AK-T cell-mediated killing of P815 tumor cells pre-treated with 2 microg/ml cisplatin or 1 microg/ml etoposide was only partially inhibitable by the Ca2+ chelator EGTA, suggesting that the Ca2+-independent Fas (CD95)/Fas ligand cytolytic pathway of AK-T cells contributes to cytotoxicity. In comparison to untreated control P815 cells, 2 microg/ml cisplatin- or 1 microg/ml etoposide-treated P815 cells exhibited increased expression of Fas mRNA and cell-surface Fas, which correlated with increased sensitivity to lysis by AK-T cells. In addition, pre-treatment with cisplatin or etoposide caused P815 tumor cells to become sensitive to the cytotoxic effects of anti-Fas antibody in a 4 hr 51Cr-release assay. Taken together, our results demonstrate that short-term exposure to concentrations of cisplatin and etoposide in the low cytotoxic range and below up-regulates Fas expression by P815 tumor cells, thereby facilitating cytotoxicity mediated through the Fas/Fas ligand cytolytic pathway.

2-chloroadenosine stimulates granule exocytosis from mouse natural killer cells: evidence for signal transduction through a novel extracellular receptor

The effect of 2-chloroadenosine (2CA), an adenosine receptor agonist, on the activation status of mouse natural killer (NK) cells was determined. Splenic lymphocytes incubated with 2CA exocytosed an NK cell-associated granzyme with N alpha-CBZ-L-lysine thiobenzyl ester (BLT) esterase activity in a dose- and time-dependent manner. Selective depletion of NK cells by anti-asialoGM1 antibody plus complement pretreatment confirmed that NK cells were the source of the BLT esterase activity. 2CA-induced granule exocytosis was not reduced in the presence of the nucleoside uptake blockers NBTI, dilazep, or dipyridamole, indicating the involvement of an extracellular receptor. However, adenosine or other A1, A2, or A3 cell-surface adenosine receptor agonists failed to trigger the exocytotic process. Furthermore, the nonselective adenosine receptor antagonist theophylline, as well as the selective A1 receptor antagonist DPCPX and the selective A2 receptor antagonist DMPX, did not interfere with 2CA-induced BLT esterase secretion. These data suggest that 2CA acts on NK cells via a novel (non-A1/A2/A3) cell-surface receptor. Genistein, a protein tyrosine kinase inhibitor, and calphostin C, a protein kinase C inhibitor, both interfered with 2CA-induced granule exocytosis. Pertussis toxin, an ADP-ribosylating toxin to which certain GTP-binding proteins are sensitive, also inhibited 2CA-stimulated BLT esterase release. In addition, 2CA-induced granule exocytosis was reduced in the presence of cyclosporin A, an inhibitor of Ca(2+)-dependent signaling pathways, and the Ca(2+)-chelating agent EGTA. We conclude that 2CA, acting through a novel extracellular receptor on mouse NK cells, triggers granule exocytosis via a Ca(2+)-dependent signal transduction pathway that is coupled to GTP-binding proteins and involves protein tyrosine kinase and protein kinase C activation.

Magnesium shifts voltage dependence of activation of delayed rectifier I(K) in guinea pig ventricular myocytes

The sensitivity of the delayed rectifier K+ current (I(K)) to intracellular Mg2+ was investigated in guinea pig ventricular myocytes using the whole cell patch-clamp technique. An increase in free intracellular Mg2+ concentration ([Mg2+]i) led to a dose-dependent decrease in I(K) with a half-maximal effect of approximately 20 nM. Activation of I(K) was shifted toward more positive voltages on increasing [Mg2+]i, but little effect was observed on activation and deactivation kinetics. Isoproterenol increased I(K) and was partially reversible in both control and 100 nM [Mg2+]i. The antiarrhythmic drug dofetilide was used to separate I(K) into its two components, rapidly activating (I(Kr)) and slowly activating (I(Ks)). The magnitude of both components decreased to a similar extent with an increase in [Mg2+]i. As [Mg2+]i was reduced, however, the number of experiments in which the dofetilide-sensitive current I(Kr) displayed inward rectification was reduced. In contrast to results previously reported for frog myocytes, it is unlikely that Mg2+ effects on guinea pig I(K) are mediated by a protein phosphatase.