Azetidines Kill Multidrug-Resistant Mycobacterium tuberculosis without Detectable Resistance by Blocking Mycolate Assembly

Tuberculosis (TB) is the leading cause of global morbidity and mortality resulting from infectious disease, with over 10.6 million new cases and 1.4 million deaths in 2021. This global emergency is exacerbated by the emergence of multidrug-resistant MDR-TB and extensively drug-resistant XDR-TB; therefore, new drugs and new drug targets are urgently required. From a whole cell phenotypic screen, a series of azetidines derivatives termed BGAz, which elicit potent bactericidal activity with MIC99 values <10 μM against drug-sensitive Mycobacterium tuberculosis and MDR-TB, were identified. These compounds demonstrate no detectable drug resistance. The mode of action and target deconvolution studies suggest that these compounds inhibit mycobacterial growth by interfering with cell envelope biogenesis, specifically late-stage mycolic acid biosynthesis. Transcriptomic analysis demonstrates that the BGAz compounds tested display a mode of action distinct from the existing mycobacterial cell wall inhibitors. In addition, the compounds tested exhibit toxicological and PK/PD profiles that pave the way for their development as antitubercular chemotherapies.

Dilemmas in anticoagulation and use of inferior vena cava filters in venous thromboembolism; a survey of Respiratory Physicians, Haematologists and Medical Oncologists and a review of the literature

Twenty percent of patients with Cancer Associated Thrombosis receive an inferior vena cava filter annually. Insertion is guided by practice guidelines, which do not specify or discuss the use of inferior vena cava filters in malignancy. Adherence to these guidelines is known to be variable. We aimed to see if there was consistent management of venous thromboembolism among Medical Oncologists/Haematologists and Respiratory Physicians, with respect to inferior vena cava filter use in the setting of suspected and confirmed malignancy. Medical Oncologists, Haematologists and Respiratory Physicians were surveyed with four theoretical cases. Case 1 concerns a patient who develops a pulmonary embolism following spinal surgery. Cases 2 and 4 explore the use of inferior vena cava filters in the setting of malignancy. Case 3 covers the role of inferior vena cava filters in recurrent thrombosis despite systemic anticoagulation. There were 56 responses, 32 (57%) Respiratory Physicians and 24 (43%) Haematologists/Oncologists. Respiratory Physicians were significantly more likely to insert an inferior vena cava filter in case 1 (p = 0.04) whilst Haematologists/Medical Oncologists were more likely to insert an inferior vena cava filter in case 3 (p = 0.03). No significant differences were found in cases 2 and 4. There were significant disparities in terms of type and timing of anticoagulation. Consistency of recommendations with guidelines was variable likely in part because guidelines are themselves inconsistent. The heterogeneity in responses highlights the variations in venous thromboembolism management, especially in Cancer Associated Thrombosis. International Societies should consider addressing inferior vena cava filter use specifically in the setting of Cancer Associated Thrombosis. Collaboration between interested specialities would assist in developing consistent, evidence-based guidelines for the use of inferior vena cava filters in the management of venous thromboembolism.

Phosphino-Triazole Ligands for Palladium-Catalyzed Cross-Coupling

Twelve 1,5-disubtituted and fourteen 5-substituted 1,2,3-triazole derivatives bearing diaryl or dialkyl phosphines at the 5-position were synthesized and used as ligands for palladium-catalyzed Suzuki-Miyaura cross-coupling reactions. Bulky substrates were tested, and lead-like product formation was demonstrated. The online tool SambVca2.0 was used to assess steric parameters of ligands and preliminary buried volume determination using XRD-obtained data in a small number of cases proved to be informative. Two modeling approaches were compared for the determination of the buried volume of ligands where XRD data was not available. An approach with imposed steric restrictions was found to be superior in leading to buried volume determinations that closely correlate with observed reaction conversions. The online tool LLAMA was used to determine lead-likeness of potential Suzuki-Miyaura cross-coupling products, from which 10 of the most lead-like were successfully synthesized. Thus, confirming these readily accessible triazole-containing phosphines as highly suitable ligands for reaction screening and optimization in drug discovery campaigns.

Inferior vena cava filters (IVCFs): a review of uses and application to international guidelines at a single Australian center; implications of venous thromboembolism associated with malignancy

Venous thromboembolism (VTE) is a potentially lethal event. Anticoagulation is the cornerstone of treatment. Inferior vena cava filters (IVCFs) may be used in circumstances when anticoagulation is contraindicated or as an adjunct to anticoagulation. IVCF use is not without controversy due to concerns over their safety profile, differences in guidelines from international societies, and a limited randomized control trial evidence. We retrospectively undertook a review of IVCF use over a three-year period (2014–2016) at our center, which has a large oncology service but no trauma unit. There were 44 patients with successful IVCF insertion and one patient with an unsuccessful attempt. Indications for insertion included: a contraindication to anticoagulation (n = 28); recurrent VTE on anticoagulation (n = 10); and extensive VTE (n = 7). There were 13 retrieval attempts, of which ten were successful. There were five documented IVCF complications (tilting: n = 2, IVC thrombus: n = 3) with one episode of IVCF failure and two episodes of deep vein thrombosis during the follow-up period. Of the patients, 71% had an active malignancy (of whom 71% had metastatic disease). Seventeen patients died due to progressive malignancy during the study period. There were no life-threatening VTEs or IVCF-associated mortalities. Adherence with published international guidelines was variable. Patients with malignancy were less likely to undergo IVCF retrieval and had a reduced rate of retrieval success. None of the international guidelines comment on the use of IVCFs in patients with malignancy despite being commonly used. IVCF use may be an underappreciated tool in this group.

Use of FDG PET scanning to evaluate 5-FU myocardial toxicity as a global metabolic effect rather than vascular spasm

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Background: This is the first ever case series which presents a series of PET images that conclusively demonstrate reversible abnormal myocardial glucose utilisation in 7 patients with normal coronary arteries occurring during 5-FU infusions. Fluoropyrimidine induced myocardial toxicity is estimated to occur in 9% of cases, with some instances proving fatal. Traditionally some hypothesised coronary artery spasm as the mechanism of action behind such events and an animal study suggesting dysfunction of the Krebs cycle, with depletion of high-energy phosphate compounds, was largely ignored. Having observed abnormal myocardial FDG uptake in a patient with chest pain undergoing FDG PET scanning, we prospectively evaluated a further 6 patients presenting with cardiac symptoms whilst receiving infusional 5-FU. Methods: Over an eighteen-month period, 7 patients experienced cardiac like chest pain during 5FU infusion. All were investigated for cardiac ischaemia as per institutional protocol (serial troponin, ECGs and coronary artery imaging), as well as FDG PET scanning to assess FDG uptake in the myocardium. Results: All 7 cases showed reduced FDG uptake throughout the myocardium, with the ventricular blood pool demonstrating a higher affinity for FDG than the myocardium itself. All 7 cases showed normal physiological uptake of FDG in the myocardium on previous and subsequent PET imaging. Imaging of the myocardium and coronary arteries in all cases showed no structural vascular disease. Conclusions: All cases demonstrated a global pattern of reduced FDG myocardial uptake that could not be isolated to a single coronary territory. Angiography or myocardial perfusion scanning demonstrated no significant coronary artery disease, and there were no features consistent with coronary artery spasm found on ECG. This supports the hypothesis that 5FU inhibits physiological myocardial glucose utilisation, thus acting as a direct myocardial toxin. We believe our findings warrant further investigation into the metabolic effects of 5FU on myocardial tissue.

Fluoropyrimidine-associated myocardial toxicity as a global metabolic effect compared to vascular spasm and visibility on FDG PET scanning

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Background: Myocardial toxicity from fluoropyrimidines is a rare but potentially serious side effect, estimated by some as occurring in up to 9%. Coronary spasm has been suggested as the underlying mechanism, despite a lack of supporting evidence and other toxicity mechanisms have been proposed. Matsubara described Krebs cycle dysfunction in the presence of 5FU with depletion of high energy phosphate compounds in rodent myocardial tissue with ECG changes. Following a chance discovery of abnormal myocardial FDG uptake on a PET scan shortly after presenting with presumed 5FU cardiac toxicity (angina, ST elevation, troponin rise; normal coronary vessels on imaging), we prospectively evaluated all instances of angina occurring during 5FU infusion with coronary artery imaging and FDG PET scan. Methods: We identified 5 patients who experienced angina during 5FU therapy. They were investigated for coronary ischaemia and also underwent PET scanning to assess myocardial FDG uptake. Data was collected from patient records, and subsequent cardiac investigations. Results: In all 5 cases, PET scan demonstrated markedly abnormal FDG uptake throughout the myocardium, with the ventricular blood pool demonstrating more FDG activity than myocardium. No significant underlying coronary artery disease was identified. All 5 patients had previous PET scans with normal myocardial FDG uptake. Conclusions: We identified a consistent pattern of abnormal FDG uptake throughout the myocardium for all scanned patients with chest pain following administration of 5FU. This was not restricted to a single arterial territory. There were no typical ECG changes of spasm. Obstructive coronary disease was excluded with angiographic imaging or myocardial perfusion scanning. The FDG PET scans suggest global myocardial metabolic change, supporting the notion of 5FU being a direct myocardial toxin inhibiting myocardial glucose utilization. The myocardium may then be dependent on fatty acid metabolism, posing additional risk to patients on low fat diets. Our data provides new insight into the mechanism of 5-FU myocardial toxicity and further prospective assessment using PET is warranted.

A divergent synthetic approach to diverse molecular scaffolds: assessment of lead-likeness using LLAMA, an open-access computational tool

LLAMA was used to assess the lead-likeness of scaffolds prepared via complementary cyclisations of hex-2-ene-1,6-diamine derivatives.

Diagnosis of appendicitis by bedside ultrasound in the ED

BACKGROUND

Computed tomography (CT) has largely become standard of care for diagnosing appendicitis at the expense of increased patient radiation exposure, cost, and time to surgical intervention. To date, there are very limited data on the accuracy of bedside ultrasound (BUS) for the diagnosis of appendicitis in adults.

OBJECTIVE

The objective of this study is to evaluate test characteristics of BUS for diagnosis of acute appendicitis in the emergency department.

METHODS

Data were prospectively collected on 97 cases of suspected appendicitis, which had BUS performed by trained residents with attending supervision between August 2011 and November 2013. All BUS interpretation and additional diagnostic imaging were left to the discretion of the physician or surgical consultants. A blinded ultrasound fellowship-trained physician reviewed all images after clinical treatment. Bedside ultrasound findings and patient outcomes were reported.

RESULTS

A total of 97 adult cases underwent diagnostic ultrasound scans for suspected appendicitis. Of 97 cases, 34 had acute appendicitis by surgery/pathology report. Twenty-four BUS were positive for acute appendicitis and 11 were nondiagnostic. Of 24 positive ultrasounds, 23 had appendicitis on pathology report. There was 1 false-positive result, yielding a sensitivity of 67.65% (95% confidence limits, 49.5%-82.6%) and a specificity of 98.41% (95% confidence limits, 91.4%-99.7%). Of 23 positive BUS, 12 cases went to the Operating Room without an abdominal CT yielding a 12% reduction in CT utilization. If all positive BUS went to the OR without a CT scan, this would yield a 24% reduction in CT utilization.

CONCLUSIONS

Bedside ultrasound may be an appropriate initial test to evaluate patients with suspected acute appendicitis in the emergency department.

Prospective, 10-year evaluation of the impact of Hispanic ethnicity on pain management practices in the ED

INTRODUCTION

Hispanic ethnicity has been reported as an independent risk factor for oligoanalgesia in the emergency department (ED).

OBJECTIVES

The objectives are to compare pain management practices in White and Hispanic patients in the ED to determine whether treatment differences exist.

METHODS

Prospective analysis of a convenience sample of patients presenting to an urban, academic, tertiary-care ED over the 10-year period from 2000 to 2010. We compared patients with pain-related complaints of any nature, who self-identified their race as White or Hispanic, and evaluated initial morphine administration/dosing, arrival/disposition pain scores, and overall ED satisfaction scores (0-10 scale).

RESULTS

Fifteen thousand sixty patients were enrolled. Eighty-one point 2 percent (n, 12 232) of the patients were White and 11.2% (n, 1680), Hispanic. White and Hispanic patients reported similar pain at presentation (6.7 vs 7.3, P < .001) and discharge/admission (4.6 vs 4.8, P = .14). Hispanic patients were not less likely to receive an analgesic during the ED visit (odds ratio, 1.06; confidence interval, 0.96-1.17; P = .62), nor less likely to receive an opioid analgesic (odds ratio, 0.97; confidence interval, 0.88-1.08; P = .70). Hispanic patients, on average, received similar initial doses of morphine (4.1 vs 4.3 mg, P = .29) and had similar wait times from arrival to initial dose of morphine (82 vs 86 minutes). Overall ED satisfaction scores were the same (8.7 vs 8.7, P = .65).

CONCLUSION

White and Hispanic patients were similar in rates of initial morphine administration for pain-related complaints. These findings contrast with previous studies reporting lower rates of initial analgesia administration among Hispanic patients in the ED.

Lead toxicity and endoscopic removal of ingested firearm cartridges

CONTEXT

Lead toxicity from the ingestion of a lead foreign body has been described in several case reports. Management of ingested live ammunition presents its own challenges due to the risk of accidental discharge. A safe and effective method of retrieving a live cartridge must be considered.

CASE DETAILS

We present two cases of lead toxicity due to intact firearm cartridge ingestion with the removal of the cartridges via endoscopy. The first case is of severe pediatric lead toxicity due to the ingestion of 30-mm rifle cartridges. The second case is an adult ingestion of .22 caliber cartridges resulting in mild lead toxicity.

DISCUSSION

These cases illustrate a diagnostic dilemma in both the diagnosis of lead toxicity and the removal of live ammunition from the stomach.

Patient anxiety may influence the efficacy of ED pain management

OBJECTIVE

The aim of this study was to evaluate the incidence of anxiety and rates of anxiety treatment in emergency department (ED) patients presenting with pain-related complaints.

METHODS

We prospectively evaluated patients in an urban academic tertiary care hospital ED from 2000 through 2010. We enrolled a convenience sample of adult patients presenting with pain and recorded patient complaint, medication administration, satisfaction, and pain and anxiety scores throughout their stay. We stratified patients into 4 different groups according to anxiety score at presentation (0, none; 1-4, mild; 5-7, moderate; 8-10, severe).

RESULTS

We enrolled 10 664 ED patients presenting with pain-related complaints. Patients reporting anxiety were as follows: 25.7%, none; 26.1%, mild; 23.7%, moderate; and 24.5%, severe. Although 48% of patients described moderate to severe anxiety at ED presentation and 60% were willing to take a medication for anxiety, only 1% received anxiety treatment. Thirty-five percent of patients still reported moderate/severe anxiety at discharge. Severe anxiety at ED presentation was associated with increased demand for pain medication (odds ratio [OR], 1.40; 95% confidence interval [CI], 1.10-1.79) and anxiety medication (OR, 4.34; 95% CI, 3.68-5.11) during the ED stay and decreased satisfaction with the treatment of pain (β coefficient = -0.328; P < .001). After adjusting for age, sex, and presentation pain scores, patients who reported severe anxiety were more likely to receive an analgesic (OR, 1.33; 95% CI, 1.19-1.50) and an opioid (OR, 1.25; 95% CI, 1.11-1.41) during the ED stay.

CONCLUSION

Anxiety may be underrecognized and undertreated in patients presenting with pain-related complaints. Patients reporting severe anxiety were less likely to report satisfaction with the treatment of their pain, despite higher rates of analgesic administration.

Aspects of the ecology of the eastern pygmy-possum Cercartetus nanus at Jervis Bay, New South Wales

A variety of ecological data were collected on the eastern pygmy-possum Cercartetus nanus at Jervis Bay, in south-eastern New South Wales between March 2006 and January 2007. Elliott traps, pitfall traps, nest-boxes and spotlighting were used to survey for the species. Data on habitat suitability including abundance of food plants (flowering trees and shrubs) and potential nest sites were also collected. Home range data were gathered via radio telemetry. Three individuals were caught in 2150 trap-nights and one animal was re-trapped once. Radio-collars were attached to one animal of each sex and tracked for 11 days during March 2006. These possums used areas (using minimum convex polygons) of 0.85 ha (male) and 0.19 ha (female). The average overnight distance moved was 44 m for the male (range = 4-81 m) and 19 m for the female (range = 0-56 m). Nest-sites included hollows in the proteaceous shrubs Banksia serrata and B. ericifolia, and in the myrtaceous trees Corymbia gummifera, Eucalyptus sclerophylla, and Syncarpia glomulifera. Cercartetus nanus captures were confined to two sites that had the most prolific flowering of potential food plants and the highest availability of potential nest-sites. A review of literature and previous surveys of the surrounding area was a necessary precursor to field study and produced 57 records. Greater understanding of the impacts of development and fire are needed for conservation and management of this species.

AIDIT and IMPACT: building research collaborations in targeted prostate cancer screening

AIDIT (Advancing International Co-operation and Developing Infrastructure for Targeted Screening of Prostate Cancer in Men with Genetic Predisposition) is a project funded by the Sixth Framework Programme of the European Community which is endeavouring to facilitate co-operation between European countries in the field of cancer research. The project also aims to raise awareness of familial prostate cancer among health professionals and the public within the associated candidate countries (ACCs) and new member states of the European Union (EU). AIDIT will focus on linking clinical and research teams in the ACCs and new member states with the IMPACT Consortium (Identification of Men with a genetic predisposition to ProstAte Cancer: Targeted screening in BRCA1/2 mutation carriers and controls), an international team investigating screening and diagnosis for men with a genetic risk of prostate cancer predisposition genes BRCA1 or BRCA2). Cancer research has been targeted as a high priority for the European Community; however, research is most successful when centralised and well coordinated, avoiding the duplication and fragmentation associated with smaller, isolated studies. AIDIT will consolidate the current IMPACT consortium and allow research partners from across the world to benefit from shared knowledge and experience. To date, the AIDIT team has established a website to facilitate communication between project collaborators (www.impact-study.co.uk), has been represented at several international meetings and has facilitated a conference for the IMPACT study to bring together international research teams, clinicians and policy makers.

Patterns of plant diversity and endemism in Namibia

Species richness, endemism and areas that are rich in both species and endemic species were assessed and mapped for Namibia. High species diversity corresponds with zones where species overlap. These are particularly obvious where there are altitudinal variations and in high-lying areas. The endemic flora of Namibia is rich and diverse. An estimated 16% of the total plant species in Namibia are endemic to the country. Endemics are in a wide variety of families and sixteen genera are endemic. Factors that increase the likelihood of endemism are mountains, hot deserts, diversity of substrates and microclimates. The distribution of plants endemic to Namibia was arranged in three different ways. Firstly, based on a grid count with the phytogeographic value o f the species being equal, overall endemism was mapped. Secondly, range restricted plant species were mapped individually and those with congruent distribution patterns were combined. Thirdly, localities that are important for very range-restricted species were identified. The resulting maps o f endemism and diversity were compared and found to correspond in many localities. When overall endemism is compared with overall diversity, rich localities may consist of endemic species with wide ranges. The other methods identify important localities with their own distinctive complement of species.

Transesophageal echocardiography during removal of central venous catheter associated with thrombus in superior vena cava

Thrombosis of upper extremity veins and superior vena cava (SVC) can occur in patients with indwelling central venous catheters. Contrary to earlier reports, pulmonary embolism (PE) can result from these thrombi, especially when they are attached to catheters (sleeve thrombi) in contrast to venous wall (mural thrombi). Removal of catheters may be required when sepsis occurs or to reduce risk of sepsis when lines have been left in for several days. We describe two patients with thrombi in SVC related to central venous catheters in whom transesophageal echocardiography (TEE) was performed during catheter removal to monitor for thrombus dislodgement. TEE may have a role in showing thrombus dislodgement and embolization during removal of venous catheters complicated by SVC thrombi. Direct visualization of thrombus dislodgement may aid in early diagnosis of PE because signs and symptoms of PE are often missed or mistaken for underlying cardiopulmonary disease. TEE may also play a role in implementing appropriate treatment in patients with PE who show right ventricular strain.

Unusual presentation of late regional cardiac tamponade after aortic surgery

Localized pericardial effusion leading to cardiac tamponade is seen occasionally in patients after cardiac surgery. This condition may be difficult to diagnose clinically because of unusual presenting symptoms and absence of conventional signs of cardiac tamponade. A case of localized pericardial effusion with presenting symptoms of fever and increasing fatigue is described in this study. The definitive diagnosis was made using transesophageal echocardiography. Surgical drainage of localized effusion resulted in prompt hemodynamic and symptomatic improvement.

Using stainless steel tape and artery clips to hold miniplates

A technique for holding Champy miniplates is described and illustrated.

Detection of a specific bacterial antigen in urine of rats with Bacteroides fragilis infection

Human intra-abdominal infections frequently yield Bacteroides fragilis and require specific antimicrobial and surgical therapy. Noninvasive immunologic assessment of this organism might allow more optimum therapy. Therefore we raised antisera in goats to Bacteroides fragilis ATCC 23745 and allowed it to react with a solid-phase capsular polysaccharide-protein antigen extracted from the same organism. Preliminary work disclosed that 10 ng/ml antigen could be detected in competition assays in both saline and dialyzed rat urine. Results were manifest by diminution of bound antiglobulin alkaline phosphatase conjugate in an antigen-mediated antibody-inhibition enzyme-linked immunosorbent assay. Rats were then infected intra-abdominally with (1) B. fragilis ATCC 23745; (2) one of eight recent clinical isolates of B. fragilis; or (3) one of nine isolates representative of Enterobacteriaceae. Seventy-two rat urine samples obtained prior to infection disclosed essentially no assay inhibition: 98.3% +/- 10.3 (1 S.D.). Mean values of reagent antibody activity after incubation with urine aliquots from 24 hr samples collected between 24 and 72 hr were (1) strain 23745 (n = 35) 70.9% +/- 2.6 (S.E.); (2) eight isolates of B. fragilis (n = 49) 86.8% +/- 1.9; (3) nine isolates of Enterobacteriaceae (n = 47) 100.9% +/- 1.0; and (4) shams (n = 29) 95.5% +/- 1.55. Ascribing values less than or equal to 77.7% (2 S.D.) as positive, seven of the eight clinical B. fragilis isolates causing infection were detected in at least one 24 hr urine sample (sensitivity = 87% by organism); 12 of 17 infected rats were correctly identified as positive by at least one urine (sensitivity = 70.6% by rat). Specificity, as assessed in the Enterobacteriaceae group, was 89% (by organism) and 94.5% (by rat). Collectively, these results suggest the presence of a potentially specific, soluble antigen excreted in the urine of rats with B. fragilis infection.

Increased guanylate cyclase activity and guanosine 3',5'-monophosphate content in ethionine-induced hepatomas

Ethionine-induced hepatomas are characterized by high adenylate cyclase activity and cyclic adenosine 3',5'-monophosphate content relative to those of surrounding liver or liver from pair-fed control rats. The present study examined the properties of the guanylate cyclase-cyclic guanosine 3',5'-monophosphate (cGMP) system of these tissues. cGMP levels of the ethionine-induced hepatomas, determined in both specimens quick-forzen in situ and after in vitro incubation of tissue slices, were approximately 2 times higher than those of surrounding liver or controls. Higher cGMP in the tumors was associated with an increase in whole homogenate, soluble, and particulate guanylate cyclase activities, as well as an increase in soluble cGMP-phosphodiesterase activity. 3-Isobutyl-1-methylxanthine, a potent inhibitor of cGMP-phosphodiesterase activity, potentiated the differences in cGMP between slices of the hepatomas and surrounding liver or control, suggesting that the higher steady-state cGMP content of the tumors reflected enhanced basal cGMP synthesis which was partially offset by increased nucleotide degradation. In the hepatomas, a greater proportion of the total guanylate cyclase activity was located in the particulate cell fraction (31%) as compared to the subcellular distribution of enzyme activity in either surrounding liver or controls (15% of total in the particulate fraction). Carbamylcholine, which increased cGMP 3-fold in surrounding liver and controls, failed to alter cGMP levels inslices of hepatoma. Further, the relative changes in both cGMP accumulation and guanylate cyclase activity of the tumors in response to NaN3, NH2OH, and NaNO2 were blunted compared to surrounding liver or controls, although in each instance a response was clearly evident. Ethionine-induced hepatomas are thus characterized by: (a) significant increases in cGMP content and in guanylate cyclase and cGMP-phosphodiesterase activities, (b) a change in the subcellular distribution of guanylate cyclase, and (c) altered responsiveness of the guanylate cyclase-cGMP system to several agonists.

Reduced sensitivity of the hepatic adenylate cyclase-cyclic AMP system to glucagon during sustained hormonal stimulation

Hormone-induced desensitization of hormonal regulation of cyclic AMP (cAMP) content has been described in a number of tissues. In the present study, we examined responses of rat liver to glucagon after periods of sustained exposure to the hormone in vivo and in vitro. In intact anesthetized rats infused with glucagon (50 ng/min) for 1 h or more and in liver slices incubated with the hormone (10 muM) for this period, hepatic cAMP responsiveness to glucagon was significantly blunted compared with that of tissue exposed to the hormone for shorter periods. The reduction in hepatic cAMP responsiveness to glucagon appeared to be fully expressed by 2 h. With the doses of hormone employed, the sequential alterations in hepatic responsiveness seemed to be limited to the cAMP system, since other parameters of glucagon action did not wane with time. Diminished hepatic cAMP responsiveness during sustained hormonal exposure could not be attributed to decreased glucagon availability, accelerated extracellular release of cAMP, hepatic ATP depletion, or enhanced phosphodiesterase activity. Studies in vitro suggested that modulation of the cAMP response occurred at the level of adenylate cyclase (AC). During sustained exposure of hepatic slices to glucagon, reductions in glucagon-responsive AC correlated temporally with those in cAMP and both changes were reversible. Alterations in glucagon-responsive AC were demonstrated over a wide range of ATP (10 muM-0.1 mM) and glucagon (10 nM-5 MM) concentrations in the cyclase reaction mixture, and appeared to be a noncompetitive phenomenon relative to glucagon. Maximal NaF-responsive AC did not fall concomitantly with time. Thus, the reduction in glucagon-responsive AC was probably not related to a reduction in the catalytic unit of the enzyme, but could have been due to an alteration in glucagon binding to its receptor sites, or in the coupling mechanism involved in transmission of the hormonal signal to the catalytic unit.

Effects of reduced ATP concent on hepatic responses to glucagon

Expression of the glycogenolytic action of glucagon in liver requires ATP for cAMP formation and for several subsequent phosphorylation reactions. To assess the extent to which ATP availability is rate-limiting to this hormonal action, responses to glucagon of intact liver and of liver with marked reductions in ATP content induced by ethionine was examined in female Wistar rats in vivo and in vitro. Compared to values in quick-frozen liver samples from control rats, basal hepatic ATP was 75% lower and cAMP, two fold higher in rats treated with ethionine. Activation of glycogen phosphorylase and inactivation of glycogen synthetase, phosphorylation reactions which require ATP and are initiated by cAMP, were also evident in basal liver samples from ethionine-treated rats. These hepatic alterations were associated with portal glucose and insulin levels which were significantly lower and portal glucagon levels which were four fold higher than values in controls. In ethionine-treated rats, glucose infusion decreased hepatic cAMP content and phosphorylase activity and increased synthetase activity. This and other observation suggested that the higher cAMP and the altered enzyme activities seen in vivo after ethionine administration were mediated by the hyperglucagonemia and/or by other endogenous glycogenolytic stimuli, and accordingly implied that liver remained responsive to such stimuli despite reduced ATP. Pharmacologic doses of exogenous glucagon clearly increased cAMP in vivo and in vitro in livers with decreased ATP. However, the lower ATP of liver exposed to ethionine was associated with a significantly blunted cAMP response to maximal glucagon stimulation. By contrast, alterations in phosphorylase and synthetase activities were not similarly blunted, suggesting that the smaller increases in cAMP seen in liver with reduced ATP content were adequate for the expression of these actions of the hormone. It is concluded that the actions of glucagon to increase cAMP and to activate phosphorylase and inactivate synthetase are not abolished by marked reductions in hepatic APT.