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Biomedical subjects

M J Davis

Publications and source records attributed to M J Davis.

At least 55 records · Page 3Linked to original sources
PubMed1997

Integrin signaling transduces shear stress--dependent vasodilation of coronary arterioles.

A direct relationship exists between shear stress and endothelium-dependent NO-mediated vasodilation of blood vessels. The transduction of shear stress to the biochemical signals resulting in the production of NO is, however, unknown. We tested the hypothesis that integrin binding to Arg-Gly-Asp(RGD) peptide sequences in extracellular matrix proteins is a critical step in initiation of the signaling sequence whereby shear stress activates endothelial tyrosine kinase(s) and induces vasodilation of isolated arterioles. Isolated coronary arterioles were exposed to increasing shear stress under control conditions and in the presence of a synthetic peptide, GRGDNP, to competitively inhibit integrin binding to extracellular matrix proteins containing RGD peptide sequences. Intraluminal GRGDNP (0.1, 0.5, and 1.0 mmol/L) inhibited shear stress-induced vasodilation in a concentration-dependent manner. Application of GRGDNP had no effect on endothelium-dependent relaxation to substance P (10(-12) to 10(-8) mol/L). An inactive structural analogue, GRGESP, did not alter shear stress-induced vasodilation. To further elucidate the integrin involved in shear stress-induced vasodilation, we administered a blocking antibody to the integrin beta 3 chain (F11) and observed significant attenuation of the vasodilation. Shear stress was also associated with an increase in tyrosine kinase activity, as assessed by anti-phosphotyrosine binding. Application of GRGDNP significantly decreased anti-phosphotyrosine binding during shear stress, suggesting a link between tyrosine kinase activation and integrin signaling during this vasodilatory response. Taken together, these results indicate that integrin-matrix interactions, possibly at focal adhesions, are of cardinal importance in the signaling pathway of shear stress-induced vasodilation.

Animals↗
PubMed1997

Use of internal controls to increase quantitative capabilities of the ribonuclease protection assay.

Through the use of two internal controls, we have developed an improved method of quantitating ribonuclease protection assay (RPA) results. A truncated sense RNA fragment and an antisense RNA fragment for the gene of interest were transcribed from PCR fragments containing T7 bacterial promoters. An 18S ribosomal RNA fragment was also used. When radiolabeled antisense and 18S probes, along with sense fragment and sample RNA, were hybridized, digested with RNase A/T1 and gel-electrophoresed, three distinct bands resulted. The antisense RNA fragment bound to the sense RNA fragment confirmed the integrity of the reaction. The antisense RNA fragment bound to endogenous mRNA measured the amount of specific gene expression in the sample. The 18S RNA fragment bound to endogenous mRNA determined the actual amount of sample added to the gel. Using the specific activities of the antisense and 18S transcripts, and scintillation counts of the protected fragments, we calculated the amounts of message and total RNA on the gel, determining picogram of message per microgram of total RNA. Final results were not based on assumed original amounts of RNA placed in the assay nor were they biased by lane-to-lane variations. Through the described adaptations, we have developed a well-controlled RPA that accurately and reproducibly quantifies gene expression.

Animals↗
PubMed1996

Acid tolerance in Listeria monocytogenes: the adaptive acid tolerance response (ATR) and growth-phase-dependent acid resistance.

Listeria monocytogenes acquired increased acid tolerance during exponential growth upon exposure to sublethal acid stress, a response designated the acid tolerance response (ATR). Maximal acid resistance was seen when the organism was exposed to pH 5.0 for 1 h prior to challenge at pH 3.0, although intermediate levels of protection were afforded by exposure to pH values ranging from 4.0 to 6.0. A 60 min adaptive period was required for the development of maximal acid tolerance; during this period the level of acid tolerance increased gradually. Full expression of the ATR required de novo protein synthesis; chloramphenicol, a protein synthesis inhibitor, prevented full induction of acid tolerance. Analysis of protein expression during the adaptive period by two-dimensional gel electrophoresis revealed a change in the expression of at least 23 proteins compared to the non-adapted culture. Eleven proteins showed induced expression while 12 were repressed, implying that the ATR is a complex response involving a modulation in the expression of a large number of genes. In addition to the exponential phase ATR, L.monocytogenes also developed increased acid resistance upon entry into the stationary phase; this response appeared to be independent of the pH-dependent ATR seen during exponential growth.

Adaptation, Physiological↗
PubMed1996

Cost effectiveness of day stay versus inpatient radiofrequency (RF) ablation for the treatment of supraventricular tachyarrhythmias.

BACKGROUND: It is well established that radiofrequency (RF) ablation is the most cost effective treatment strategy for patients with supraventricular tachycardia. Previous cost estimates assumed at least an overnight stay following RF ablation. Day stay RF ablation however appears to be a safe alternative. AIMS: The aim of this study was to compare day stay and inpatient catheter ablation in terms of cost, efficacy and safety. METHODS: This was a retrospective cost effectiveness analysis. The study population consisted of 25 consecutive patients who underwent day stay RF ablation and 25 consecutive patients who underwent inpatient RF ablation (historical controls). Economic analysis was based upon a detailed clinical costing. RESULTS: The mean overall cost per patient of inpatient RF ablation in 1994 Australian dollar values is $2354 (SD, $642) compared with $1876 (SD, $595) for day stay RF ablation (p < 0.01). Day stay RF ablation is a cost effective alternative to inpatient RF ablation.

Adult↗
PubMed1996

At least two separate gene clusters are involved in albicidin production by Xanthomonas albilineans.

Transposon mutagenesis was used to obtain mutations affecting production of the toxin albicidin in Xanthomonas albilineans, which causes leaf scald disease of sugarcane and is also pathogenic to corn. Transposon Tn5-gusA inserted randomly into genomic DNA of X. albilineans Xa23R1 at a frequency of 10(-4) to 10(-5) per recipient after conjugal transfer from Escherichia coli. Fifty prototrophic mutants defective in albicidin production were isolated from 7,100 Tn5-gusA insertional derivatives tested for toxin production by an antibiosis bioassay. EcoRI fragments containing Tn5 flanking sequences from two mutants (AM15 and AM40) were cloned and used to probe a wild-type Xa23R1 DNA library by colony hybridization. Nine cosmids showed homology to the AM15 probe, and six showed homology to the AM40 probe. Four cosmid clones hybridized to both probes. Forty-five of the 50 defective mutants were restored to albicidin production with two overlapping cosmid clones. Restriction mapping showed that these mutants span a genomic region of about 48 kb. At least one other gene cluster is also involved in albicidin production in Xa23R1. DNA fragments from the 48-kb cluster proved to be very specific to X. albilineans. Some mutants affected in albicidin production retain their ability to colonize sugarcane cultivated in vitro.

Anti-Bacterial Agents↗
PubMed1996

Calcium entry activated by store depletion in coronary endothelium is promoted by tyrosine phosphorylation.

Application of substance P (SP), a potent endothelium-dependent vasodilator, to porcine coronary artery endothelial cells (PCAECs) results in release of Ca2+ from intracellular stores followed by extracellular Ca2+ influx. We tested the hypothesis that intracellular store depletion results in tyrosine phosphorylation, which promotes Ca2+ influx. PCAECs labeled with antiphosphotyrosine antibody conjugated to fluorescein isothiocyanate showed a 3.3- to 3.4-fold increase in fluorescence in response to SP or 2,5-di-tert-butylhydroquinone (BHQ), an agent that depletes intracellular stores by inhibiting the endoplasmic reticulum Ca(2+)-adenosinetriphosphatase. In both cases, the tyrosine kinase inhibitor, genistein, reduced the fluorescence intensity to near-basal levels. Pretreatment of PCAECs with the tyrosine kinase inhibitors, genistein or tyrphostin, induced a significant reduction in the plateau phase of SP-induced Ca2+ elevation with no effect on the release of Ca2+ from stores. Neither daidzein, a structurally similar but inactive analogue of genistein, nor H-7, a serine-threonine kinase inhibitor, affected SP-induced Ca2+ influx. Voltage-clamp recordings using the perforated patch technique with simultaneous Ca2+ measurements showed that intracellular Ca2+ elevation and inward current activated by SP and BHQ were reduced by 60-70% in response to genistein. These data indicate that the link between store depletion and Ca2+ influx in endothelial cells requires tyrosine phosphorylation.

Animals↗
PubMed1996

Coronary arteriolar flow-induced vasodilation signals through tyrosine kinase.

Coronary arterioles demonstrate flow-dependent vasodilation that is mediated by endothelial release of nitric oxide. The signaling mechanisms for this response remain unknown. Because tyrosine kinases are an enzyme family linked to many signaling pathways, including some for mechanosensitive transduction, we hypothesized that tyrosine kinase activation is a critical step in flow-induced vasodilation. To test this hypothesis, coronary arterioles were isolated, cannulated with micropipettes, and perfused by two independent reservoir systems. Intraluminal pressure was set at 60 cmH2O, and flow was generated by changing the heights of the reservoirs in equal and opposite directions, thus establishing a pressure difference across the arteriole without altering intraluminal pressure. Vasodilatory responses to intraluminal flow and substance P (1 x 10(-12) to 1 x 10(-7) M) were evaluated before and after intraluminal application of the tyrosine kinase inhibitors genistein (5 microM) and piceatannol (10 microM). Exposure to these inhibitors did not alter spontaneous tone. Substance P caused dose-dependent vasodilation that was not affected by genistein or piceatannol. Increases in intraluminal flow (generated by pressure differences ranging from 4 to 60 cmH2O) elicited graded increases in diameter. Both genistein and piceatannol inhibited the vasodilatory responses to flow. Treatment with daidzein, an inactive analogue of genistein, had no effect on either the flow-induced responses or substance P-induced vasodilation. To further confirm that tyrosine kinase activation is involved in flow-induced vasodilation, vessels were exposed to flow in the absence or presence of genistein and subsequently stained with a fluorescein isothiocyanate-labeled phosphotyrosine antibody. Exposure to flow significantly increased fluorescence of endothelial cells. Genistein treatment reversed the flow-induced increase in tyrosine phosphorylation. These results indicate that endothelium-dependent, flow-induced vasodilation in isolated porcine coronary arterioles is accompanied by an increase in tyrosine kinase activity. We conclude that endothelium-dependent, nitroxidergic, flow-induced vasodilation is mediated, at least in part, by a signaling pathway involving a tyrosine kinase.

Animals↗
PubMed1996

In vivo and in vitro vasoactive reactions of coronary arteriolar microvessels to nitroglycerin.

The actions of nitroglycerin on the coronary microcirculation are controversial, with some laboratories reporting that coronary arterioles dilate to the drug and others reporting that they do not. Our goal was to reconcile these disparate observations. Specifically, we hypothesized that dilation of coronary arterioles by nitroglycerin is overwhelmed by intrinsic autoregulatory escape mechanisms. Accordingly, we projected that coronary arterioles would show transient, but not sustained, dilation to nitroglycerin in vivo. Furthermore, we hypothesized that isolated coronary arterioles would show sustained dilation to the drug, because intrinsic escape mechanisms would be absent under these conditions. To test these hypotheses, we measured diameter changes of canine coronary microvessels in vivo during continuous nitroglycerin administration (intracoronary infusion or epicardial suffusion) using intravital fluorescent microscopy (n = 17 dogs) at two time points: early (1-3 min), when coronary artery blood flow velocity was increased, and late (15-20 min), after blood flow velocity returned to control. Tb study responses of coronary arterioles in the absence of autoregulatory influences, we measured the diameter of isolated canine coronary arterioles to varying doses of nitroglycerin (n = 8 vessels, maximal diameter 81 +/- 4 microns). During the early phase of nitroglycerin infusion (1,3, and 10 micrograms.kg-1.min-1), coronary arterioles dilated by 4 +/- 1, 7 +/- 2, and 13 +/- 2% (all P < 0.05), whereas small arteries dilated by 1 +/- 2, 3 +/- 1, and 4 +/- 1%, respectively (P < 0.05 for the higher doses). Coronary artery blood velocity measured increased by 45 +/- 15% (3 micrograms.kg-1.min-1, P < 0.05). Suffusion of nitroglycerin (10(-5) M) dilated coronary arterioles, but not small arteries, by 17 +/- 5% (P < 0.05) between 1 and 3 min. After 15-20 min of nitroglycerin (3 micrograms.kg-1.min-1 by intracoronary infusion), diameters of coronary arterioles and coronary artery blood velocity returned to control, whereas dilation of small arteries remained significant at 4 +/- 1%. Coronary arteriolar dilation by epicardial suffusion of nitroglycerin also waned to control values by 15-20 min, whereas dilation of small arteries was observed: 5 +/- 2% (P < 0.05). In vitro, nitroglycerin caused dose-dependent dilation of coronary arterioles to their maximal diameter, which was sustained for 20 min. Thus nitroglycerin dilates coronary arterioles and small arteries. The dilation in vivo is transient for arterioles but sustained for arteries. In vitro, the dilation is sustained. Because microvessels in vitro are capable of sustaining dilation for 20 min, we conclude that the waning of arteriolar dilation in vivo is related to autoregulatory escape from dilation by nitroglycerin.

Animals↗
PubMed1996

Calcium dependence of indolactam-mediated contractions in resistance vessels.

The protein kinase C activator, (-)-indolactam, has been shown to enhance reactivity of arterioles by a mechanism not requiring an increase in intracellular Ca++ (Ca++i). The aim of this study was to characterize the Ca++ requirement for indolactam-induced contraction of resistance vessels. Studies were performed in small mesenteric arteries (diameter, 260 +/- microm), using intact segments or after permeabilization with alpha-toxin (500 U/ml). Additional studies were preformed using isolated smooth muscle cells to allow electrophysiological assessment of the effect of indolactam on voltage-gated Ca++ entry. Intact and permeabilized vessel segments showed dose-dependent constriction to indolactam. Studies of Fura 2-loaded vessels and permeabilized segments maintained at low Ca++i, showed that the constriction occurred without an overt increase in Ca++i. That Ca++ was required was evident by near maximal relaxation after the removal of Ca++. Patch clamp studies indicated that indolactam potentiated voltage-gated Ca++ entry; however, nifedipine (0.5 microM) and La (0.2 or 1 mM) were relatively ineffective in reversing the contraction, indicating that voltage-gated Ca++ entry was not an absolute requirement. In intact vessel segments, the myosin light chain (MLC) kinase inhibitors, ML-7 and ML-9, reversed the indolactam contraction, suggesting the requirement of the MLC chain phosphorylation pathway. Furthermore, indolactam caused by a significant increase in MLC phosphorylation in permeabilized vessels, despite clamping of Ca++i at pCa 7.0. The data are consistent with the suggestion that the protein kinase C activator, indolactam, acts to modulate the Ca++ sensitivity of the smooth muscle contractile process such that higher than expected levels of MLC phosphorylation exist for a given level of Ca++i.

Animals↗
PubMed1996

Role of antimalarials in rheumatoid arthritis--the British experience.

Antimalarials have been used to treat rheumatoid arthritis (RA) for over 40 years, the first report of suggestive efficacy being published in 1951. Over the years they have become part of the established treatment of RA being one of a category of drugs referred to as disease modifying anti-rheumatic drugs (DMARDs). The onset of action with antimalarials is slow. Most patients use these drugs in combination with non-steroidal anti-inflammatory drugs (NSAIDs) and analgesics. This article reviews the evidence for the efficacy of antimalarials, their place in comparison to other DMARDs and comments on the current use in RA as perceived in British rheumatology.

Antimalarials↗
PubMed1996

Local anesthetic safety in pediatric patients.

Local anesthetic, when administered properly and in correct dosages, is an essential part of pediatric oral health care. Recent adjustments to maximum safe dosage and careful administration techniques make local anesthetic use with children even safer and more effective. Safe dosage levels, technique recommendations and early clinical signs of possible overdose are presented here.

Anesthesia, Dental↗
PubMed1996

Integrated regulation of pressure and flow in the coronary microcirculation.

There are many mechanisms that contribute to the regulation of coronary blood flow including vasodilatory metabolites, myogenic regulation, flow- or shear stress-mediated vasodilation, and neurohumoral influences. It is interesting to note that coronary arterioles of varying sizes appear to possess different sensitivities to these regulatory factors, but each appears to dominate the control of a particular segment of the microvasculature. For example, during metabolic hyperemia the smallest arterioles seem to be most sensitive to the effects of metabolites, but metabolic and myogenic mechanisms that dictate the tone of upstream microvessels likely act in concert to facilitate the overall adjustments in coronary vasomotor tone. Neurohumoral mechanisms, seem to modulate the robustness of these intrinisic adjustments, perhaps by restraining the extent of vasodilation. The purpose of this review is to discuss these many regulatory mechanisms and also present a framework by which the vasoactive reactions elicited by these different mechanisms are integrated into coordinated responses of the entire coronary vascular network.

Animals↗
PubMed1996

Outcomes and accountability. Can dentistry prove the benefits of care?

As managed care increases in influence, dentists must be prepared to document the efficacy of all treatment modalities in order to maintain control over the care their patients receive. We, rather than third parties, are ultimately accountable for the results of the care we provide. The outcomes assessment process can yield essential, clear evidence of the efficacy, cost-effectiveness and benefits of the care we provide to patients.

Clinical Competence↗
PubMed1995

Longitudinal gradients for endothelium-dependent and -independent vascular responses in the coronary microcirculation.

BACKGROUND: Coronary microvessels (< 300 microns in diameter) have been demonstrated to be important in the regulation of local resistance and flow. Recent studies also suggest that these microvessels are more responsive to physiological and pharmacological stimuli than conduit vessels. However, little is known regarding the relative sensitivity of different microvascular segments in response to flow (shear stress) and agonists. The goal of this study was to test the hypothesis that a longitudinal gradient for shear stress- and agonist-induced dilation exists in the coronary microcirculation. METHODS AND RESULTS: Experiments were performed in four different sizes of porcine subepicardial coronary arterial microvessels: small arterioles (40 +/- 1-micron ID with resting tone); intermediate arterioles (60 +/- 1 micron); large arterioles (106 +/- 4 micron); and small arteries (179 +/- 9 microns). Vessels were isolated and cannulated to allow luminal pressure and flow to be independently controlled. All vessels developed active tone (to approximately 65% to 75% of maximum diameter) at their control luminal pressures and showed graded dilations to stepwise increases in shear stress (0 to 10 dynes/cm2). For arterioles, the magnitude of the dilations increased as vessel size increased. The highest shear stress produced 21 +/- 3%, 32 +/- 2%, and 52 +/- 5% increases in diameter in small, intermediate, and large arterioles, respectively. Small arteries dilated only 22 +/- 6%. The endothelium-dependent vasodilator substance P (SP) produced dose-dependent dilation of all vessels with a threshold at 10(-16) mol/L. Arterioles were maximally dilated at 10(-9) mol/L SP. However, this dose produced only 80% dilation in small arteries. The ED50 for SP was shifted to the right by two orders of magnitude in small arteries compared with the arterioles. Adenosine preferentially dilated small arterioles, and the dose-response curves shifted to the right for larger vessels. The thresholds for adenosine-induced dilation were 10(-12), 10(-11), and 10(-9) mol/L for small, intermediate, and large arterioles, respectively. The endothelium-independent vasodilator nitroprusside produced identical dose-dependent dilations in all vessel segments. CONCLUSIONS: The results indicate that the pig coronary circulation exhibits a heterogeneity in physiological and pharmacological responses along the microvascular network. Small arterioles are more sensitive to adenosine, but large arterioles are more responsive to shear-stress stimulation. We speculate that site-specific preferential responses may play a crucial role in coordinating overall vascular function in the coronary microvascular network.

Adenosine↗
PubMed1995

Role of nitric oxide in the coronary microvascular responses to adenosine and increased metabolic demand.

BACKGROUND: The purpose of this study was to test the hypothesis that endothelium-derived nitric oxide (NO) participates in coronary microvascular responses to adenosine and pacing-induced increases in metabolic demand by maintaining an optimal distribution of coronary resistance. METHODS AND RESULTS: Coronary microvascular diameters were measured by stroboscopic epi-illumination and intravital microscopy in open-chest dogs (n = 20). Epicardial coronary blood velocity (CBV) was measured by Doppler flowmetry. Responses to adenosine (1 and 10 micrograms.kg-1.min-1 IC) and left atrial pacing (180 beats per minute) were recorded before and after inhibition of NO synthesis by NG-nitro-L-arginine methyl ester (L-NAME, 30 micrograms.kg-1.min-1 IC). At baseline, adenosine dilated arterioles (< 100 microns) (11 +/- 4% and 25 +/- 3% diameter changes, P < .05) more than small arteries (> 100 microns) (-4 +/- 6% and 7 +/- 3%, P < .05 for the higher dose) and increased CBV (43 +/- 31% and 118 +/- 25%, P < .05). Left atrial pacing dilated arterioles (12 +/- 2%, P < .05) and small arteries (8 +/- 3%, P < .05) and also increased CBV (68 +/- 9%, P < .05). L-NAME abolished CBV increases caused by acetylcholine (10 and 100 ng.kg-1.min-1 IC; 53 +/- 33% and 168 +/- 82% versus -12 +/- 15% and -1 +/- 14%, P < .05) but not papaverine. Small arteries were constricted by L-NAME (-8 +/- 2%, P < .05), arterioles were dilated (10 +/- 4%, P < .05), and CBV was unchanged. After L-NAME, adenosine failed to dilate arterioles further (3 +/- 3% and 2 +/- 2%; P < .05 versus prior responses), and CBV changes were attenuated (14 +/- 16% and 8 +/- 13%; P < .05 versus prior responses). Pacing also failed to dilate arterioles (-4 +/- 2%, P < .05 versus prior response), resulting in an attenuated CBV change (34 +/- 13%, P < .05 versus prior response). The possibility that adenosine stimulates NO release in canine coronary arterioles was investigated in isolated arterioles (diameters, 81 +/- 4 microns; n = 8). Adenosine caused dose-dependent dilation to maximal diameter, which was unaffected by inhibition of NO synthesis by L-NAME. CONCLUSIONS: Inhibition of NO synthesis attenuates coronary dilation during adenosine infusions and during pacing-induced increases in metabolic demand. Inhibition of NO synthesis may shift the major site of coronary resistance into small arteries through autoregulatory adjustments in arterioles. These data therefore suggest that NO, by dilating predominantly small coronary arteries, promotes metabolic coronary dilation by preserving the tone and vasodilator reserve of arterioles.

Adenosine↗
PubMed1995

Day stay transcatheter radiofrequency ablation for supraventricular tachyarrhythmias.

OBJECTIVE: To describe our initial experience with transcatheter radiofrequency ablation as a day stay procedure for patients with supraventricular tachyarrhythmias resulting from aberrant atrioventricular pathways. DESIGN: Prospective study of the first 50 patients at the Royal Perth Hospital whom we intended to treat and discharge on the same day. Patients underwent a combined electrophysiological study and radiofrequency ablation. RESULTS: Accessory pathways were identified in 32 patients (successfully ablated in 30) and dual atrioventricular nodal pathways in 18 patients (with successful ablation of the slow pathway in 17). Thirty-eight patients were discharged on the same day (24 treated for accessory pathways, and 14 treated for dual atrioventricular nodal pathways). There were no late complications or readmissions in patients discharged on the same day. CONCLUSIONS: In patients with supraventricular tachyarrhythmia, day stay transcatheter ablation can be planned without a definitive prior diagnosis of the physiological cause. Complications or other reasons for overnight admission are apparent at completion of the procedure.

Adolescent↗