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Biomedical subjects

D J Webb

Publications and source records attributed to D J Webb.

At least 217 records · Page 12Linked to original sources
PubMed1994

Direct and sympathetically mediated venoconstriction in essential hypertension. Enhanced responses to endothelin-1.

Endothelin-1 is a potent endothelium-derived vasoconstrictor peptide. Although circulating concentrations are not increased in essential hypertension, enhanced sensitivity to endothelin-1 has been observed in animal models of hypertension. We investigated dorsal hand vein responses to local infusion of endothelin-1 and norepinephrine in 12 patients with essential hypertension who had never received treatment and in 12 age and sex matched normotensive control subjects. The maximal venoconstriction and the geometric mean of the dose of norepinephrine that caused 50% of maximal venoconstriction were similar in hypertensive (mean +/- SE; 80 +/- 4%; 31 +/- 8 pmol/min) and normotensive subjects (87 +/- 5%, 22 +/- 9 pmol/min). In contrast, mean venoconstriction to endothelin-1 was significantly greater in hypertensive (49 +/- 5%) than in normotensive subjects (27 +/- 2%; P = 0.004). Sympathetically mediated venoconstriction elicited by deep breath was substantially potentiated by endothelin-1 in hypertensive (67 +/- 7% at 90 min) but not normotensive subjects (11 +/- 3% at 90 min; P = 0.001). Venoconstriction to endothelin-1 correlated positively with mean arterial pressure in the hypertensive subjects (r = 0.82; p = 0.001) but negatively in the normotensive subjects (r = -0.58; p = 0.047). Endothelin-1 may contribute to the reduction of venous compliance occurring in the early stages of essential hypertension and to the altered systemic hemodynamics in this condition.

Adult↗
PubMed1994

The relationship between blood pressure and left ventricular mass in essential hypertension is observed only in the presence of the angiotensin-converting enzyme gene deletion allele.

An insertion/deletion polymorphism in the angiotensin-converting enzyme (ACE) gene accounts for approximately 50% of the variance in plasma ACE concentration: deletion homozygotes (DD) have the highest, and insertion homozygotes (II) the lowest ACE concentrations. ACE is responsible for the generation of angiotensin II, which is implicated in the development of left ventricular hypertrophy, an independent risk factor for morbidity and mortality in hypertension. The aim of this study was to investigate the contribution of ACE genotype to the development of left ventricular hypertrophy in patients with essential hypertension. Eighty-five patients with essential hypertension underwent echocardiographic assessment of left ventricular mass index (LVMI) and determination of ACE genotype from leukocyte DNA by polymerase chain reaction. There was no significant difference in LVMI among the genotypes (II, ID, DD). Analysis of covariance modelled for LVMI showed a significant interaction with systolic blood pressure (p = 0.036) but not diastolic blood pressure (p = 0.453). The relationship between LVMI and systolic blood pressure was strongest in the deletion homozygotes (p = 0.002, R2 = 0.47), and also present in the heterozygotes (p = 0.013, R2 = 0.40). No relationship was seen in the insertion homozygotes (p = 0.914, R2 = 0.23). These findings suggest that the effect of blood pressure on LVMI in essential hypertension is expressed only in the presence of the ACE gene deletion allele.

Alleles↗
PubMed1993

Binding of platelet-derived growth factor-BB and transforming growth factor-beta 1 to alpha 2-macroglobulin in vitro and in vivo: comparison of receptor-recognized and non-recognized alpha 2-macroglobulin conformations.

alpha 2-Macroglobulin (alpha 2M) undergoes a major conformational change when reacting with proteinases or primary amines. This conformational change has been referred to as the 'slow' to 'fast' transformation based on the increase in alpha 2M mobility shown by non-denaturing PAGE. Previous studies demonstrated that many cytokines, including transforming growth factor beta 1 (TGF-beta 1) and interleukin-1 beta, bind preferentially or exclusively to alpha 2M which has undergone conformational change. In this study, we demonstrate that platelet-derived growth factor-BB (PDGF-BB) also binds preferentially to conformationally transformed alpha 2M (alpha 2M-methylamine, alpha 2M-trypsin) in vitro. Purified 125I-PDGF-BB-alpha 2M-methylamine complex cleared rapidly from the circulation of mice via the alpha 2M receptor/low-density-lipoprotein-receptor-related protein (alpha 2M-R/LRP). In order to determine whether PDGF-BB or TGF-beta 1 binds to native alpha 2M, we defined the native conformation by lack of interaction with alpha 2M-R/LRP instead of electrophoretic mobility. 125I-PDGF-BB was incubated with 4.3 microM native alpha 2M and 0.47 microM alpha 2M-methylamine. The 125I-PDGF-BB distributed evenly between slow-form and fast-form alpha 2M without shifting the electrophoretic mobility of either species. When the mixed preparation was injected intravenously in mice, 125I-PDGF-BB-fast-form-alpha 2M cleared rapidly and selectively from the circulation; 125I-PDGF-BB which was bound to slow-form alpha 2M was stable in the blood (apparently not recognized by alpha 2M-R/LRP). Therefore, while conformationally transformed alpha 2M binds PDGF-BB preferentially in vitro, non-alpha 2M-R/LRP-recognized alpha 2M binds PDGF-BB as well. Binding of 125I-PDGF-BB and 125I-TGF-beta 1 to alpha 2M was demonstrated in vivo by injecting the free growth factors intravenously into mice. Plasma samples which were subjected to non-denaturing PAGE and autoradiography demonstrated binding of both growth factors exclusively to the slow-form of alpha 2M. Therefore, under normal physiological conditions, native alpha 2M (non-alpha 2M-R/LRP-recognized) is the primary form of the proteinase inhibitor functioning as a carrier of PDGF-BB and TGF-beta 1 in the blood.

Animals↗
PubMed1993

Characterization of the antiplasmin activity of human thrombospondin-1 in solution.

These studies demonstrate relatively rapid association of plasmin with thrombospondin and the effects of this interaction on plasmin activity towards D-Val-L-Leu-L-Lys p-nitroanilide hydrochloride (S-2251) and the proteinase inhibitors alpha 2-antiplasmin (alpha 2AP) and alpha 2-macroglobulin (alpha 2M). Binding of plasmin to thrombospondin reached an apparent reversible equilibrium within 3 min at 22 degrees C. The amidase activity of bound plasmin was inhibited. An analysis of S-2251 hydrolysis indicated that thrombospondin is a linear mixed-type plasmin inhibitor. The dissociation constant (KD) for the binding of plasmin to thrombospondin was 0.5 microM, assuming one plasmin binding site per thrombospondin homotrimer. Plasmin and miniplasmin slowly cleaved thrombospondin, yielding products which were comparable with those generated by other proteinases. Tranexamic acid inhibited the digestion of thrombospondin by plasmin and miniplasmin, suggesting an important role for the kringle-5 domain in this process. When plasmin was incubated first with thrombospondin and then with alpha 2AP, plasmin that was apparently bound to thrombospondin reacted with alpha 2AP at a decreased rate; however, within 20 min, all of the plasmin was recovered in complex with alpha 2AP. Similar results were obtained with alpha 2M. Transfer of plasmin from thrombospondin to alpha 2AP or alpha 2M probably required plasmin-thrombospondin-complex dissociation. A low level of reaction of alpha 2AP with thrombospondin-associated plasmin could not be ruled out. These results demonstrate that the activity of plasmin, when bound to thrombospondin, is greatly diminished or eliminated. The plasmin-thrombospondin complex, which is formed within 3 min, is fully reversible and the associated plasmin is in a latent form protected from proteinase inhibitors. Therefore, thrombospondin may regulate plasmin activity in a manner which is distinct from conventional proteinase inhibitors and other extracellular-matrix proteins.

Amidohydrolases↗
PubMed1993

Endothelium-dependent modulation of responses to endothelin-I in human veins.

1. We have investigated whether local vascular production of nitric oxide or prostacyclin regulates venoconstriction induced by the endothelium-derived peptide, endothelin-1, in vivo in man. 2. Six healthy subjects received local dorsal hand vein infusion of endothelin-1 for 60 min alone or, on two separate occasions, co-infused with the donator of nitric oxide, glyceryl trinitrate, or the vasodilator prostaglandin, prostacyclin. In further studies, endothelin-1 was co-infused with an inhibitor of nitric oxide production, NG-monomethyl-L-arginine, or after oral administration of the irreversible inhibitor of prostaglandin production, acetylsalicylic acid (aspirin). 3. At a low dose (5 pmol/min), endothelin-1 alone caused slowly developing and long-lasting venoconstriction (maximal constriction: 66 +/- 4%). Although glyceryl trinitrate partially prevented endothelin-1-induced venoconstriction (maximum: 33 +/- 5%), inhibition of nitric oxide production did not affect endothelin-1-induced venoconstriction (maximum: 55 +/- 4%). 4. Prostacyclin was more effective at blocking the venoconstriction in response to endothelin-1 than glyceryl trinitrate (maximum: 12 +/- 3%), and there was substantial potentiation of endothelin-1-induced venoconstriction after pretreatment with aspirin (maximum: 90 +/- 3%). 5. Despite the capacity of nitric oxide to attenuate responses to endothelin-1, NG-monomethyl-L-arginine did not potentiate endothelin-1-induced venoconstriction, suggesting little or no stimulated production of nitric oxide in human veins. However, the potentiation of responses to endothelin-1 by aspirin indicates that endothelial production of prostacyclin attenuates responses to endothelin-1 in human veins in vivo.

Adult↗
PubMed1993

Inhibition of nitric oxide synthesis increases blood pressure in healthy humans.

OBJECTIVE: To examine whether endogenous production of the endothelium-derived vasodilator nitric oxide influences blood pressure in healthy humans. METHODS: After preliminary pilot dose-ranging studies, 3 mg/kg NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide synthase, and saline placebo were infused intravenously over 5 min to eight healthy subjects in a two-phase, randomized, single-blind crossover study. Blood pressure and cardiac and renal function were measured. RESULTS: Compared with placebo, L-NMMA increased mean arterial pressure by 10%, decreased heart rate by 19%, decreased cardiac index by 25% and increased calculated total peripheral resistance by 46%. Effects were maximal 10-15 min after starting L-NMMA infusion. Urinary sodium and fractional sodium excretions were increased by L-NMMA, but creatinine clearance was unchanged. CONCLUSIONS: Basal generation of nitric oxide influences total peripheral resistance and blood pressure in healthy humans. The natriuresis induced by L-NMMA may be related to the increase in blood pressure, or arise from inhibition of the intrarenal actions of nitric oxide. Any decrease in nitric oxide generation, as has been postulated to occur in essential hypertension, could have substantial effects on blood pressure and tissue blood flow.

Adult↗
PubMed1993

Modulators of calcium and potassium channels: their effects on endothelin-1 binding to cardiac membranes.

Endothelin-1 (ET-1) causes long-lasting vasoconstriction associated with a prolonged elevation of intracellular free Ca2+. Because this may be mediated through an effect on membrane ion channels, we investigated the effects of the dihydropyridine calcium channel antagonist nifedipine; two structurally distinct K+ channel openers, pinacidil and levcromakalim; and the inactive stereoisomer of levcromakalim (D-cromakalim), as well as ET-1 and ET-3, on binding of 125I-labeled endothelin-1 to rat cardiac membranes. Specific binding of 125I-ET-1 was inhibited in a concentration-dependent manner by unlabeled ET-1 (IC50 = 1.56 +/- 0.78 nM; slope = -0.49 +/- 0.10) and ET-3 (IC50 = 314 +/- 54 nM; slope = -0.34 +/- 0.11). Nifedipine, in concentrations < or = 10(-5) M, did not affect 125I-ET-1 binding. However, levcromakalim significantly inhibited 125I-ET-1 binding (maximum binding 49 4/- 9%; p = 0.04), whereas the inactive isomer, D-cromakalim, had no effect. Pinacidil also inhibited 125I-ET-1 binding, although to a lesser extent than levcromakalim (maximum binding 63 +/- 7%). These findings provide evidence for a stereospecific interaction between K(+)-channel openers and ET-1 binding in rat cardiac membranes. Because the slope of the logistic fit was substantially less than unity, and the effects of pinacidil and levcromakalim were incomplete, there may be two or more receptors for ET-1 in rat heart, only one of which is sensitive to K(+)-channel openers.

Animals↗
PubMed1993

Endothelin-1 and aggregation of human platelets in vitro.

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide produced by endothelial cells. We investigated whether ET-1, like other potent endothelium-derived vasoactive agents, interacts directly with human platelets in vitro. Platelet-rich plasma was obtained from healthy male volunteers and incubated with ET-1 (1 microM) or vehicle (sodium chloride 154 mM) for 10 min at 37 degrees C. Platelet aggregation was measured by the Born method, using light transmittance through the plasma sample as an index of activation. Although a significant increase in light transmittance was observed when plasma was incubated with ET-1 compared with vehicle, (3.8 +/- 0.4% versus 2.7 +/- 0.2%; n = 24; p = 0.038), this effect was small and is unlikely to be of biologic significance. To investigate the possibility that ET-1-stimulated platelet nitric oxide (NO) synthesis might be masking a direct aggregatory effect of ET-1, in a second study in six subjects NG-monomethyl-L-arginine (L-NMMA, 10 and 100 microM), an inhibitor of NO synthase, was preincubated with the plasma before the addition of ET-1 (1 nM and 1 microM). No significant difference was observed whether samples were incubated with L-NMMA alone or with L-NMMA and ET-1. The results of this study suggest that ET-1 does not have a major direct effect as a platelet aggregating agent.

Adult↗
PubMed1993

Venoconstriction to endothelin-1 in humans is attenuated by local generation of prostacyclin but not nitric oxide.

Endothelin-1 (ET-1) is known to be a potent and long-lasting constrictor of arteries and veins. We investigated whether local endothelial production of nitric oxide (NO) or prostacyclin modulates venoconstriction induced by the endothelium-derived peptide ET-1 in vivo in humans. Six healthy volunteers each received local dorsal hand vein infusion of ET-1 (5 pmol/min) for 60 min in five separate studies: once given alone; on three occasions co-infused with either the NO donor glyceryl trinitrate (GTN), the vasodilator prostaglandin, prostacyclin, or the inhibitor of nitric oxide synthase (NOS) NG-monomethyl-arginine (L-NMMA); and once given 30 min after oral administration of the irreversible inhibitor of cyclooxygenase, acetylsalicylic acid (aspirin). ET-1 alone caused slowly developing and long-lasting venoconstriction (maximal constriction 66 +/- 4%). Although GTN partially prevented ET-1-induced constriction (maximum 33 +/- 5%, p = 0.004 versus ET-1), inhibition of NOS did not affect ET-1-induced venoconstriction (maximum 55 +/- 4%). Prostacyclin was more effective at blocking the venoconstriction to ET-1 than GTN (maximum 12 +/- 3%, p = 0.0001) and there was substantial potentiation of ET-1-induced venoconstriction after pretreatment with aspirin (maximum 90 +/- 3%, p = 0.001). Despite the capacity of NO to attenuate responses to ET-1, L-NMMA did not potentiate ET-1-induced venoconstriction, suggesting little or no stimulated production of NO by ET-1 in human hand veins. However, substantial potentiation of ET-1-induced venoconstriction by aspirin indicates that endothelial production of prostacyclin modulates responses to ET-1 in human veins in vivo.

Adult↗
PubMed1993

Screening for human immunodeficiency virus: a survey of British clinical pharmacology units.

1. A survey of screening practices used to detect infection with the human immunodeficiency virus (HIV), covering the 12 months from June 1990 to May 1991, was carried out in 74 clinical pharmacology units performing phase I and II studies. Forty-five units were identified from the Technomark commercial register, and 29 units were identified from the clinical academic membership of the British Pharmacological Society. 2. An overall response rate of 92% was obtained: 41 replies from commercial units; 27 replies from academic units. Seventeen commercial units and 26 academic units reported involvement with healthy volunteer studies; these were the 43 questionnaires analysed. 3. The majority of clinical pharmacology units did not believe that it was necessary to perform HIV screening by HIV antibody testing. Six commercial units (35%) and three academic units (12%) did perform HIV antibody testing in healthy volunteers. 4. Reasons frequently given for testing were protection of clinical and laboratory staff, and the advantages to the volunteer of prophylaxis and treatment at an early stage of HIV infection. Reasons for not testing included the perceived low risk of a positive test, a lack of benefit from treatment, and the adverse implications in relation to health insurance. Advice from an Ethics Review Committee was not generally a reason for testing. 5. In future, screening patterns may change, depending on prevalence of HIV positivity in the community, the benefits of diagnosis and treatment, and the perceived or real disadvantages of having an HIV antibody test. For the present, we believe that indirect methods, such as an HIV risks questionnaire, are more appropriate than direct antibody testing.

Adolescent↗
PubMed1993

Venoconstriction to endothelin-1 in humans: role of calcium and potassium channels.

Recent studies in vitro have suggested that there may be an interaction between endothelin-1 and ATP-sensitive K+ channels in vascular smooth muscle. Here we have investigated whether agents acting on membrane Ca2+ and K+ channels modulate endothelin-1-induced venoconstriction in vivo in human subjects. In a series of studies, six healthy subjects received, on separate occasions, local infusions into dorsal hand veins of endothelin-1 coinfused with 1) the ATP-sensitive K+ channel opener, cromakalim; 2) the dihydropyridine Ca2+ antagonist, nicardipine; 3) a control vasodilator, hydralazine; and 4) saline placebo. Endothelin-1 caused local venoconstriction with a maximum reduction in vein size of 66 +/- 4% at 60 min (P = 0.0001 vs. basal). Cromakalim prevented endothelin-1-induced venoconstriction (9 +/- 10% maximum constriction; P = 0.68 vs. basal). By contrast, nicardipine, in a dose sufficient to block depolarization-induced constriction caused by K+ infusion, had only a partial effect on endothelin-1-induced venoconstriction (35 +/- 8% maximum constriction; P = 0.001 vs. basal; P = 0.02 vs. endothelin-1), whereas a 10-fold higher dose of nicardipine had no additional effect and hydralazine had no effect. In further studies, cromakalim, but not nicardipine, reversed endothelin-1-induced venoconstriction. Cromakalim did not prevent constriction induced by norepinephrine. Although calcium entry through dihydropyridine-sensitive Ca2+ channels may account in part for the vasoconstrictor action of endothelin-1 in humans, the abolition of endothelin-1 responses by a K+ channel opener suggests additional mechanisms of action for endothelin-1.

Adult↗