Effect of COVID-19 on outpatient services in patients with aortovascular disease: a UK multicentre study

INTRODUCTION

The SARS-CoV-2 coronavirus disease 2019 (COVID-19) pandemic has disrupted healthcare services worldwide. Outpatient services have necessarily been restructured to accommodate COVID-19 patients and to maintain social distancing measures. The aim of our study was to investigate how the COVID-19 pandemic has affected outpatient healthcare provision for patients with aortovascular disease.

METHODS

In this prospective study, a standardised proforma was circulated to seven aortic centres in the UK. Data on outpatient encounters were collected from March to July 2020. Captured data included demographic details, disease pattern, type of encounter (face-to-face, video or telephone), clinic outcome and availability of imaging.

RESULTS

A total of 632 patients were included in the study, including 164 (25.9%) new referrals. In this cohort, clinic settings have shifted towards remote consultations, with 424 (67.1%) patients undergoing telephone appointments. Over a third of new patients (34.8%) had a delay in diagnostic tests, which might be attributable to the indirect effects of COVID-19. A total of 102 (16.1%) patients were added to a surgical waiting list following clinic.

CONCLUSIONS

To the best of our knowledge, this is the largest study of outpatient activity during the COVID-19 pandemic in patients with aortovascular disease. We demonstrate how the speciality has adapted to accommodate government-endorsed changes in healthcare provision, and question how COVID-19 may have affected access to diagnostics. Finally, we discuss how COVID-19 will affect patients added to surgical waiting lists.

Aberrant subclavian: new face of an old disease

An aneurysm of an aberrant subclavian artery is un usual prevalence of anomalies of aortic arch, with a literature reported prevalence of 2%. Timely elective intervention is of paramount. The advancement in stenting techniques has promoted this practice to be an alternative to conventional open surgical repair which is associated with high rates of perioperative complications including mortality outcomes. However, new faces of this old disease are emerging as we move to novelty and innovation era. We discuss in this review the advancements in this disease entity highlighting and collecting the world experiences.

Varying Evidence on Deep Hypothermic Circulatory Arrest in Thoracic Aortic Aneurysm Surgery

Cardiovascular surgeons have long debated the safe duration of deep hypothermic circulatory arrest during thoracic aortic aneurysm surgery. The rationale for using adjunctive cerebral perfusion (or not) is to achieve the best technical aortic repair with the lowest risk of morbidity and death. In this literature review, we highlight the debates surrounding these issues, evaluate the disparate findings on deep hypothermic circulatory arrest durations and temperatures, and consider the usefulness of adjunctive perfusion.

Low immunogenicity in non-small cell lung cancer; do new developments and novel treatments have a role?

Approximately 1.6 million new cases of lung cancer are diagnosed annually (Jemal et al. CA: A Cancer Journal for Clinicians, 61, 69-90, 2011) and it remains the leading cause of cancer-related mortality worldwide. Despite decades of bench and clinical research to attempt to improve outcome for locally advanced, good performance status patients, the 5-year survival remains less than 15 % (Molina et al. 2008). Immune checkpoint inhibitor (ICH) therapies have shown a significant promise in preclinical and clinical trails to date in the treatment of non-small cell lung cancer (NSCLC). The idea of combining these systemic immune therapies with local ablative techniques is one that is gaining momentum. Electrochemotherapy (ECT) is a unique atraumatic local therapy that has had very promising objective response rates and a number of advantages including but not limited to its immunostimulatory effects. ECT in combination with ICHs offers a novel approach for dealing with this difficult disease process.

Successful Management of an Occult Cardiac Tamponade with Prompt Surgical Intervention and a Novel, Defined Pericardial Irrigation Protocol

Purulent pericarditis is a rare entity in the postantibiotic era. It usually occurs in patients who have underlying chronic and immunosuppressing conditions and its presentation in the healthy adult population is quite rare. Infection of the pericardial space can occur via direct extension from infectious endocarditis, pneumonia, or empyema, or from a more distant source such as meningitis. Purulent pericarditis carries a very high mortality because of delay in the diagnosis and early occurrence of fatal complications. We describe a case of purulent pericarditis with impending cardiac tamponade in a previously healthy 40-year-old female patient, which was successfully treated with a combination of prompt surgical drainage and a novel irrigation protocol.

Surgical repair of an asymptomatic giant right coronary artery aneurysm

Background Coronary artery aneurysm (CAA) is a rare finding, being mostly diagnosed on angiography or at autopsies. It is defined as being a dilation of the coronary artery that exceeds the diameter of the patient's largest coronary vessel by 1.5 to 2 times.

Case Report We describe the operative correction of a giant right CAA measuring in excess of 10 cm.

Conclusion Management of giant CAAs is not standardized and surgical strategy remains controversial. In our case, the patient has a successful surgical repair with no postoperative shunts on follow-up investigations.

Intravascular cell delivery device for therapeutic VEGF-induced angiogenesis in chronic vascular occlusion

Site specific targeting remains elusive for gene and stem cell therapies in the cardiovascular field. One promising option involves use of devices that deliver larger and more sustained cell/gene payloads to specific disease sites using the versatility of percutaneous vascular access technology. Smooth muscle cells (SMCs) engineered to deliver high local concentrations of an angiogenic molecule (VEGF) were placed in an intravascular cell delivery device (ICDD) in a porcine model of chronic total occlusion (CTO) involving ameroid placement on the proximal left circumflex (LCx) artery. Implanted SMC were retained within the ICDD and were competent for VEGF production in vitro and in vivo. Following implantation, micro-CT analyses revealed that ICDD-VEGF significantly enhanced vasa vasora microvessel density with a concomitant increase in tissue VEGF protein levels and formation of endothelial cell colonies suggesting increased angiogenic potential. ICDD-VEGF markedly enhanced regional blood flow determined by microsphere and contrast CT analysis translating to a functional improvement in regional wall motion and global left ventricular (LV) systolic and diastolic function. Our data indicate robust, clinically relevant angiogenesis can be achieved in a human scale porcine chronic vascular occlusion model following ICDD-VEGF-based delivery of angiogenic cells. This may have implications for percutaneous delivery of numerous therapeutic factors promoting creation of microvascular bypass networks in chronic vaso-occlusive diseases.

Phrenic nerve block as a novel adjunct to the local treatment of bronchopleural fistula

Bronchopleural fistula (BPF) is a life threatening complication after pneumonectomy with an incidence of about 2-5% and a mortality rate of up to 50%. Topical treatment such as fibrin glue has been previously described with limited success. We present a novel case in which blocking the phrenic nerve assisted in a successful topical closure of the BPF.

Regulation of synaptic strength by sphingosine 1-phosphate in the hippocampus

Although the hippocampus is a brain region involved in short-term memory, the molecular mechanisms underlying memory formation are not completely understood. Here we show that sphingosine 1-phosphate (S1P) plays a pivotal role in the formation of memory. Addition of S1P to rat hippocampal slices increased the rate of AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) recorded from the CA3 region of the hippocampus. In addition long-term potentiation (LTP) observed in the CA3 region was potently inhibited by a sphingosine kinase (SphK) inhibitor and this inhibition was fully reversed by S1P. LTP was impaired in hippocampal slices specifically in the CA3 region obtained from SphK1-knockout mice, which correlates well with the poor performance of these animals in the Morris water maze test. These results strongly suggest that SphK/S1P receptor signaling plays an important role in excitatory synaptic transmission in the CA3 region of hippocampus and has profound effects on hippocampal function such as spatial learning.

Sphingosine kinase 1 regulates mucin production via ERK phosphorylation

Our previous report showed that inhibition of sphingosine kinase (SphK) ameliorates eosinophilic inflammation and mucin production in a mouse asthmatic model. To clarify the role of SphK in airway mucin production, we utilized the mouse asthmatic model and found that both SphK and MUC5AC expression were increased and co-localized in airway epithelium. Next we cultured normal human bronchial epithelial cells in an air-liquid interface and treated with IL-13 to induce their differentiation into goblet cells. We found that SphK1 and MUC5AC expression was increased by IL-13 treatment at both protein and mRNA levels, whereas SphK2 expression was not changed. N,N-dimethylsphingosine (DMS), a potent SphK inhibitor, decreased MUC5AC expression up-regulated by IL-13 treatment. Furthermore, DMS inhibited IL-13-induced ERK1/2 phosphorylation but neither p38 MAPK nor STAT6 phosphorylation. These results suggest that SphK1 is involved in MUC5AC production induced by IL-13 upstream of ERK1/2 phosphorylation, and independent of STAT6 phosphorylation.

Roles of extracellular and intracellular sphingosine 1-phosphate in cell migration

Sphingosine 1-phosphate (S1P) is an important factor for the regulation of cell motility acting both inside and outside the cells. The precise role of S1P in the control of cell motility, however, remains unclear. Here we describe the roles of S1P in the regulation of cell motility by dissecting them into intracellular and extracellular actions using a liposomal S1P transfer technique. In a Boyden chamber assay free S1P enhanced directional cell movement, whereas liposomal S1P induced nondirectional cell movement. Furthermore, inhibition of sphingosine kinase (SphK) 1 by several inhibitors or knockdown of the enzyme expression by siRNA caused reduced wound-faced cell polarity formation as assessed by wound-healing assay. Moreover, S1P-induced cell migration was strongly inhibited by SphK inhibitors. These results indicate that extracellular S1P acting through S1P receptors facilitates the formation of cell polarity, whereas S1P generated inside the cells functions as an intracellular mediator per se to enhance nondirectional cell movement, thus S1P enhances directional cell movement in a coordinated fashion.

CtBP1/BARS is an activator of phospholipase D1 necessary for agonist-induced macropinocytosis

Vesicular trafficking such as macropinocytosis is a dynamic process that requires coordinated interactions between specialized proteins and lipids. A recent report suggests the involvement of CtBP1/BARS in epidermal growth factor (EGF)-induced macropinocytosis. Detailed mechanisms as to how lipid remodelling is regulated during macropinocytosis are still undefined. Here, we show that CtBP1/BARS is a physiological activator of PLD1 required in agonist-induced macropinocytosis. EGF-induced macropinocytosis was specifically blocked by 1-butanol but not by 2-butanol. In addition, stimulation of cells by serum or EGF resulted in the association of CtBP1/BARS with PLD1. Finally, CtBP1/BARS activated PLD1 in a synergistic manner with other PLD activators, including ADP-ribosylation factors as demonstrated by in vitro and intact cell systems. The present results shed light on the molecular basis of how the ‘fission protein' CtBP1/BARS controls vesicular trafficking events including macropinocytosis.

Sphingosine kinase/sphingosine 1-phosphate signalling in central nervous system

Sphingolipids were once regarded as inert structural components of cell membranes. Now these metabolites are generally believed to be important bioactive molecules that control a wide repertoire of cellular processes such as proliferation and survival of cells. Along with these ubiquitous cell functions observed in many peripheral tissues sphingolipid metabolites, especially sphingosine 1-phosphate, exert important neuron-specific functions such as regulation of neurotransmitter release. This review summarizes physiological and pathological roles of sphingolipid metabolites emphasizing the role of sphingosine 1-phosphate in the central nervous system.

Inhalation of sphingosine kinase inhibitor attenuates airway inflammation in asthmatic mouse model

Sphingosine 1-phosphate (S1P) produced by sphingosine kinase (SPHK) is implicated in acute immunoresponses, however, mechanisms of SPHK/S1P signaling in the pathogenesis of bronchial asthma are poorly understood. In this study, we hypothesized that SPHK inhibition could ameliorate lung inflammation in ovalbumin (OVA)-challenged mouse lungs. Six- to eight-week-old C57BL/6J mice were sensitized and exposed to OVA for 3 consecutive days. Twenty-four hours later, mice lungs and bronchoalveolar lavage (BAL) fluid were analyzed. For an inhibitory effect, either of the two different SPHK inhibitors, N,N-dimethylsphingosine (DMS) or SPHK inhibitor [SK-I; 2-(p-hydroxyanilino)-4-(p-chlorophenyl) thiazole], was nebulized for 30 min before OVA inhalation. OVA inhalation caused S1P release into BAL fluid and high expression of SPHK1 around bronchial epithelial walls and inflammatory areas. DMS or SK-I inhalation resulted in a decrease in S1P amounts in BAL fluid to basal levels, accompanied by decreased eosinophil infiltration and peroxidase activity. The extent of inhibition caused by DMS inhalation was higher than that caused by SK-I. Like T helper 2 (Th2) cytokine release, OVA inhalation-induced increase in eotaxin expression was significantly suppressed by DMS pretreatment both at protein level in BAL fluid and at mRNA level in lung homogenates. Moreover, bronchial hyperresponsiveness to inhaled methacholine and goblet cell hyperplasia were improved by SPHK inhibitors. These data suggest that the inhibition of SPHK affected acute eosinophilic inflammation induced in antigen-challenged mouse model and that targeting SPHK may provide a novel therapeutic tool to treat bronchial asthma.

Requirement of phospholipase D for ilimaquinone-induced Golgi membrane fragmentation

Although organelles such as the endoplasmic reticulum and Golgi apparatus are highly compartmentalized, these organelles are interconnected through a network of vesicular trafficking. The marine sponge metabolite ilimaquinone (IQ) is known to induce Golgi membrane fragmentation and is widely used to study the mechanism of vesicular trafficking. Although IQ treatment causes protein kinase D (PKD) activation, the detailed mechanism of IQ-induced Golgi membrane fragmentation remains unclear. In this work, we found that IQ treatment of cells caused a robust activation of phospholipase D (PLD). In the presence of 1-butanol but not 2-butanol, IQ-induced Golgi membrane fragmentation was completely blocked. In addition, IQ failed to induce Golgi membrane fragmentation in PLD knock-out DT40 cells. Furthermore, IQ-induced PKD activation was completely blocked by treatment with either 1-butanol or propranolol. Notably, IQ-induced Golgi membrane fragmentation was also blocked by propranolol treatment. These results indicate that PLD-catalyzed formation of phosphatidic acid is a prerequisite for IQ-induced Golgi membrane fragmentation and that enzymatic conversion of phosphatidic acid to diacylglycerol is necessary for subsequent activation of PKD and IQ-induced Golgi membrane fragmentation.

Protein kinase D-mediated phosphorylation and nuclear export of sphingosine kinase 2

Sphingosine kinase (SPHK) is a key enzyme producing important messenger sphingosine 1-phosphate and is implicated in cell proliferation and suppression of apoptosis. Because the extent of agonist-induced activation of SPHK is modest, signaling via SPHK may be regulated through its localization at specific intracellular sites. Although the SPHK1 isoform has been extensively studied and characterized, the regulation of expression and function of the other isoform, SPHK2, remain largely unexplored. Here we describe an important post-translational modification, namely, phosphorylation of SPHK2 catalyzed by protein kinase D (PKD), which regulates its localization. Upon stimulation of HeLa cells by tumor promoter phorbol 12-myristate 13-acetate, a serine residue in a novel and putative nuclear export signal, identified for the first time, in SPHK2 was phosphorylated followed by SPHK2 export from the nucleus. Constitutively active PKD phosphorylated this serine residue in the nuclear export signal both in vivo and in vitro. Moreover, down-regulation of PKDs through RNA interference resulted in the attenuation of both basal and phorbol 12-myristate 13-acetate-induced phosphorylation, which was followed by the accumulation of SPHK2 in the nucleus in a manner rescued by PKD over-expression. These results indicate that PKD is a physiologically relevant enzyme for SPHK2 phosphorylation, which leads to its nuclear export for subsequent cellular signaling.

Involvement of sphingosine-1-phosphate in glutamate secretion in hippocampal neurons

Neuronal activity greatly influences the formation and stabilization of synapses. Although receptors for sphingosine-1-phosphate (S1P), a lipid mediator regulating diverse cellular processes, are abundant in the central nervous system, neuron-specific functions of S1P remain largely undefined. Here, we report two novel actions of S1P using primary hippocampal neurons as a model system: (i) as a secretagogue where S1P triggers glutamate secretion and (ii) as an enhancer where S1P potentiates depolarization-evoked glutamate secretion. Sphingosine kinase 1 (SK1), a key enzyme for S1P production, was enriched in functional puncta of hippocampal neurons. Silencing SK1 expression by small interfering RNA as well as SK1 inhibition by dimethylsphingosine resulted in a strong inhibition of depolarization-evoked glutamate secretion. Fluorescence recovery after photobleaching analysis showed translocation of SK1 from cytosol to membranes at the puncta during depolarization, which resulted in subsequent accumulation of S1P within cells. Fluorescent resonance energy transfer analysis demonstrated that the S1P(1) receptor at the puncta was activated during depolarization and that depolarization-induced S1P(1) receptor activation was inhibited in SK1-knock-down cells. Importantly, exogenously added S1P at a nanomolar concentration by itself elicited glutamate secretion from hippocampal cells even when the Na(+)-channel was blocked by tetrodotoxin, suggesting that S1P acts on presynaptic membranes. Furthermore, exogenous S1P at a picomolar level potentiated depolarization-evoked secretion in the neurons. These findings indicate that S1P, through its autocrine action, facilitates glutamate secretion in hippocampal neurons both by secretagogue and enhancer actions and may be involved in mechanisms underlying regulation of synaptic transmission.

Involvement of N-terminal-extended Form of Sphingosine Kinase 2 in Serum-dependent Regulation of Cell Proliferation and Apoptosis

Sphingosine kinase (SPHK) 1 is implicated in the regulation of cell proliferation and anti-apoptotic processes by catalyzing the formation of an important bioactive messenger, sphingosine 1-phosphate. Unlike the proliferative action of SPHK1, another isozyme, SPHK2, has been shown to possess anti-proliferative or pro-apoptotic action. Molecular mechanisms of SPHK2 action, however, are largely unknown. The present studies were undertaken to characterize the N-terminal-extended form of SPHK2 (SPHK2-L) by comparing it with the originally reported form, SPHK2-S. Real-time quantitative PCR analysis revealed that SPHK2-L mRNA is the major form in several human cell lines and tissues. From sequence analyses it was concluded that SPHK2-L is a species-specific isoform that is expressed in human but not in mouse. At the protein level it has been demonstrated by immunoprecipitation studies that SPHK2-L is the major isoform in human hepatoma HepG2 cells. SPHK2-L, when expressed in human embryonic kidney (HEK) 293 cells, did not show any inhibition of DNA synthesis in the presence of serum, whereas it showed marked inhibition in the absence of serum. Moreover, serum deprivation resulted in the translocation of SPHK2-L into the nuclei. In addition, serum deprivation induced SPHK2-L expression in HEK293 cells. Furthermore, suppression of SPHK2 by small interfering RNA treatment prevented serum deprivation- or drug-induced apoptosis in HEK293 cells. Taken together, these results indicate that a major form of SPHK2 splice variant, SPHK2-L, in human cells does not inhibit DNA synthesis under normal conditions and that SPHK2-L accumulation in the nucleus induced by serum deprivation may be involved in the cessation of cell proliferation or apoptosis depending on the cell type.

Delta-catenin/NPRAP (neural plakophilin-related armadillo repeat protein) interacts with and activates sphingosine kinase 1

Sphingosine kinase (SPHK) is a key enzyme catalysing the formation of sphingosine 1-phosphate (SPP), a lipid messenger that is implicated in the regulation of a wide variety of important cellular events acting through intracellular, as well as extracellular, mechanisms. However, the molecular mechanism of intracellular actions of SPP remains unclear. Here, we have identified delta-catenin/NPRAP (neural plakophilin-related armadillo repeat protein) as a potential binding partner for SPHK1 by yeast two-hybrid screening. From co-immunoprecipitation analyses, the C-terminal portion of delta-catenin/NPRAP containing the seventh to tenth armadillo repeats was found to be required for interaction with SPHK1. Endogenous delta-catenin/NPRAP was co-localized with endogenous SPHK1 and transfected delta-catenin/NPRAP was co-localized with transfected SPHK1 in dissociated rat hippocampal neurons. MDCK (Madin-Darby canine kidney) cells stably expressing delta-catenin/NPRAP contained elevated levels of intracellular SPP. In a purified system delta-catenin/NPRAP stimulated SPHK1 in a dose-dependent manner. Furthermore, delta-catenin/NPRAP-induced increased cell motility in MDCK cells was completely inhibited by dimethylsphingosine, a specific inhibitor of SPHK1. These results strongly suggest that at least some of delta-catenin/NPRAP functions, including increased cell motility, are mediated by an SPHK-SPP signalling pathway.

Sphingosine kinase 2 is a nuclear protein and inhibits DNA synthesis

Sphingosine kinase-1 (SPHK1) is a key enzyme catalyzing the formation of an important bioactive lipid messenger, sphingosine 1-phosphate, and is implicated in the regulation of cell proliferation and antiapoptotic processes. Biological features of another isozyme SPHK2, however, remain unclear. The present studies were undertaken to characterize SPHK2 by comparison with SPHK1. When SPHK2 was transiently expressed in various cell lines, it was localized in the nuclei as well as in the cytosol, whereas SPHK1 was distributed in the cytosol but not in the nucleus. We have mapped a functional nuclear localization signal (NLS) to the N-terminal region of SPHK2. We have observed that the expression of SPHK2 in various cell types causes inhibition of DNA synthesis, resulting in the cell cycle arrest at G1/S phase. We have also demonstrated that an NLS mutant of SPHK2, SPHK2R93E/R94E, failed to enter the nucleus and to inhibit DNA synthesis. Moreover, a fusion protein, NLS-SPHK1, where SPHK1 was fused to the NLS sequence of SPHK2 acquired the ability to enter nuclei and inhibited DNA synthesis. These results indicate that SPHK2 localizes in the nuclei and causes inhibition of DNA synthesis, and this may affect subsequent cellular events.

Identification and characterization of RPK118, a novel sphingosine kinase-1-binding protein

Sphingosine kinase (SPHK) is a key enzyme catalyzing the formation of sphingosine 1 phosphate (SPP), a lipid messenger that is implicated in the regulation of a wide variety of important cellular events through intracellular as well as extracellular mechanisms. However, the molecular mechanism of the intracellular actions of SPP remains unclear. Here we have cloned a novel sphingosine kinase-1 (SPHK1)-binding protein, RPK118, by yeast two-hybrid screening. RPK118 contains several functional domains whose sequences are homologous to other known proteins including the phox homology domain and pseudokinase 1 and 2 domains and is shown to be a member of an evolutionarily highly conserved gene family. The pseudokinase 2 domain of RPK118 is responsible for SPHK1 binding as judged by yeast two-hybrid screening and immunoprecipitation studies. RPK118 is also shown to co-localize with SPHK1 on early endosomes in COS7 cells expressing both recombinant proteins. Furthermore, RPK118 specifically binds to phosphatidylinositol 3-phosphate. These results strongly suggest that RPK118 is a novel SPHK1-binding protein that may be involved in transmitting SPP-mediated signaling into the cell.

Regulation of mammalian phospholipase D2: interaction with and stimulation by G(M2) activator

We have previously reported that a heat-stable activator for ganglioside metabolism, G(M2) activator, potently stimulates ADP-ribosylation factor (ARF)-dependent phospholipase D (PLD) activity (presumably PLD1) in an in vitro system [Nakamura, Akisue, Jinnai, Hitomi, Sarkar, Miwa, Okada, Yoshida, Kuroda, Kikkawa and Nishizuka (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 12249-12253]. However, little is known about the regulation of PLD2. In the present studies we have investigated the regulation of PLD2 by G(M2) activator and various other regulators including ARF. PLD2 was potently stimulated in vitro by G(M2) activator in a time- and dose-dependent manner. Neither ARF nor protein kinase C caused any significant changes in PLD2 activity. Importantly, PLD2 responsiveness to ARF was greatly enhanced by G(M2) activator, suggesting a possible role for G(M2) activator as a coupling factor. G(M2) activator was also demonstrated to physically associate with PLD2 in a stoichiometric manner. Further, PMA stimulation of COS-7 cells overexpressing both G(M2) activator and PLD2 resulted in a marked increase in the association of the two molecules. Interestingly, ARF association with PLD2 was greatly increased by G(M2) activator. Moreover, G(M2) activator enhanced PMA-induced PLD activity in a synergistic manner with ARF in streptolysin-O-permeabilized, cytosol-depleted HL-60 cells, suggesting that G(M2) activator may regulate PLD in a concerted manner with other factors, including ARF, inside the cells.

The disaggregation theory of signal transduction revisited: further evidence that G proteins are multimeric and disaggregate to monomers when activated

We have compared the sedimentation rates on sucrose gradients of the heterotrimeric GTP-binding regulatory (G) proteins Gs, G(o), Gi, and Gq extracted from rat brain synaptoneurosomes with Lubrol and digitonin. The individual alpha and beta subunits were monitored with specific antisera. In all cases, both subunits cosedimented, indicating that the subunits are likely complexed as heterotrimers. When extracted with Lubrol all of the G proteins sedimented with rates of about 4.5 S (consistent with heterotrimers) whereas digitonin extracted 60% of the G proteins with peaks at 11 S; 40% pelleted as larger structures. Digitonin-extracted Gi was cross-linked by p-phenylenedimaleimide, yielding structures too large to enter polyacrylamide gels. No cross-linking of Lubrol-extracted Gi occurred. Treatment of the membranes with guanosine 5'-[gamma-thio]triphosphate and Mg2+ yielded digitonin-extracted structures with peak sedimentation values of 8.5 S--i.e., comparable to that of purified G(o) in digitonin and considerably larger than the Lubrol-extracted 2S structures representing the separated alpha and beta gamma subunits formed by the actions of guanosine 5'-[gamma-thio]triphosphate. It is concluded that the multimeric structures of G proteins in brain membranes are at least partially preserved in digitonin and that activation of these structures in membranes yields monomers of G proteins rather than the disaggregated products (alpha and beta gamma complexes) observed in Lubrol. It is proposed that hormones and GTP affect the dynamic interplay between multimeric G proteins and receptors in a fashion analogous to the actions of ATP on the dynamic interactions between myosin and actin filaments. Signal transduction is mediated by activated monomers released from the multimers during the activation process.

Fecal bile acid excretion and composition in response to changes in dietary wheat bran, fat and calcium in the rat

The effect and possible interactive influence of different dietary amounts of wheat bran, fat and calcium on the fecal excretion, concentration and composition of bile acids was studied in Fischer-344 rats. The fecal bile acids were analyzed using gas-liquid chromatography. Dietary wheat bran increased both total bile acid excretion and fecal weight without changes in fecal bile acid concentration. The proportion of fecal hyodeoxycholic acid decreased with increasing dietary fiber, whereas that of lithocholic and deoxycholic acids increased significantly with fiber intake. The percent content of fecal chenodeoxycholic acid did not change. Increasing dietary fat led to an increase in bile acid excretion without changes in either fecal weight or bile acid concentration. In contrast, the level of dietary calcium did not affect the total excretion of bile acids. However, since calcium increased the fecal weight, it consequently diluted bile acids and decreased their fecal concentration. Dietary fat and calcium had no influence on fecal bile acid composition. There were no interactive effects of wheat bran, fat and calcium on fecal bile acids. The finding in this study that dietary fiber, fat and calcium induce significant changes in fecal bile acids may be of relevance to the potential of bile acids to promote carcinogenesis.

Magnesium and calcium absorption in Fischer-344 rats influenced by changes in dietary fibre (wheat bran), fat and calcium

Magnesium and calcium absorption were affected by changes in dietary wheat bran fibre and calcium, but not fat, in Fischer-344 rats when studied in a full factorial study which was a portion of a larger study of diet and colon carcinogenesis. For four weeks, nine-week-old rats were fed experimental purified diets to which had been added: wheat bran 0, 2.5 10, or 20%; fat 1, 5 or 10%; and calcium 0.18, 0.52, or 1.04% of diet weight. From day 26 to 29 all faeces were collected in metabolic cages, and food consumption noted. Dietary magnesium intake and net magnesium absorption increased in direct relation to the quantity of wheat bran in the diet. Calcium supplementation inhibited magnesium absorption on fibre-free diet, but had little effect on magnesium absorption when fibre was present. Fat had no measurable effect on magnesium absorption. A low dietary fibre content enhanced Ca absorption compared to that on a fibre-free diet. However, further increases in fibre content slightly inhibited calcium absorption. We conclude that the magnesium content of dietary wheat bran fibre is available for absorption to rats. Calcium supplementation inhibits magnesium absorption in a fibre-free diet, but presence of dietary fibre protects magnesium absorption from the calcium inhibition observed on a fibre-free diet. Absorption of calcium is increased by including some fibre in the diet. However, calcium absorption may be diminished slightly by increasing wheat bran content of the diet to a high level, probably through calcium binding and excretion with undigested fibre.

Serum gastrin increases with increasing dietary calcium but not with increasing dietary fat or fiber in Fischer-344 rats

We studied the effects of dietary calcium, fat and fiber on serum gastrin in Fischer-344 rats in a full factorial experiment as part of a larger study of diet and colon cancer risk factors. Nine- to 10-wk-old male rats were fed standard or experimental diets for 4 wk. Wheat bran was the sole source of fiber. Wheat bran levels were 0, 2.5, 10 and 20%; fat levels were 1, 5 and 10%; calcium levels were 0.18, 0.52 and 1.04% of diet weight. On d 29 serum was collected and stored at -80 degrees C until analyzed. There was a significant (P less than 0.0001) dose-dependent increase in serum gastrin from 102 to 173 ng/L, with increasing calcium. No other significant changes in serum gastrin were noted with the dietary changes. A long-term change in the level of serum gastrin, caused by dietary modification, will influence the trophic effect that gastrin has on colonic mucosa as well as on colon carcinomas. We speculate that calcium supplementation, although slowing colonic proliferation, might have an undesirable effect on the growth of early undetected colonic tumors.

Influence of microenvironmental pH on adriamycin resistance

Resistance to Adriamycin (ADR) is frequently dependent upon enhanced efflux associated with the expression of the MDR1-encoded P membrane glycoprotein. Since enhanced expression of the MDR1 gene in ADR-resistant cells may be the result of spontaneous genetic mutation or amplification, it is presumed to be relatively stable and unalterable. Yet, reducing ADR efflux could increase sensitivity, and has been attempted using calcium channel blockers and other drugs. However, since the tumor cell microenvironment varies with respect to pH because of differences in vascularization, oxygenation, and metabolite clearance, the possibility exists that these factors could influence drug transport and the critical biochemical pathways which determine cytotoxicity, even in resistant cells. Using flow cytometric analysis of ADR fluorescence, the influx and efflux of 10 microM ADR dissolved in MES buffer (pH 6.5) and 4-(2-hydroxyethylene)-1-piperazineethanesulfonic acid buffer (pH 7.5 and 8.5) was measured in sensitive P388 and resistant P388/R84 cells in vitro. Substantially enhanced uptake of ADR was detected at alkaline pH in both cell populations, while the proportion of ADR-positive cells and the level of ADR uptake was decreased at lower pH. Acidification reduced ADR efflux, whereas alkalinization increased efflux when the uptake pH was 6.5 or 7.5. At uptake pH 8.5, the pH of the external buffer had little effect, even in resistant cells. In resistant cells in an alkaline microenvironment, ADR transport and retention were superior to that observed in sensitive cells in an acidic microenvironment. No differences were observed in ADR transport when the transmembrane pH gradient was equilibrated. These observations are especially relevant to the effect of ADR on tumor cell subpopulations that are acidic, and in which drug diffusion is inefficient. Efforts to alkalinize tumor cells prior to ADR therapy might reduce ADR resistance, even of genetic origin.

Transport of beta-adrenergic antagonists in the absence of beta-adrenergic receptors in rat pituitary tumor cells

We have demonstrated that the rat pituitary tumor cell line GH3 has a carrier-mediated active transport system for the beta-adrenergic antagonist dihydroalprenolol (DHA). Transport of DHA in GH3 was saturable, with an apparent Km of 1.4 microM, was temperature and pH dependent, and was inhibited by the ionophore monensin and the amine transport inhibitor reserpine. Propranolol competed for DHA transport, but not in a stereoselective fashion. The tricyclic antidepressant imipramine also competed for DHA transport, but catecholamines or serotonin did not. This amine transport system in GH3 cells appeared to be identical to the one we recently described in several other cell types; however, analysis in those cells was complicated by the fact that they contain beta-adrenergic receptors which bind beta-adrenergic ligands. In this report we show that GH3 cells do not possess detectable beta-adrenergic receptors, based on their inability to bind the partial agonist CGP-12177, their inability to bind nanomolar concentrations of DHA in a saturable, stereospecific manner, and their failure to produce cAMP in response to stimulation by beta-adrenergic agonists. Characterization of the amine transport system in GH3 cells clearly distinguishes it from receptor-mediated phenomena and should facilitate our efforts to fully understand its mechanism and significance.