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Biomedical subjects

G D Johnston

Publications and source records attributed to G D Johnston.

At least 19 recordsLinked to original sources
PubMed2001

Bradykinin infusion in chronic cardiac failure and the effects of captopril.

BACKGROUND: Patients with chronic cardiac failure (CCF) have abnormal vascular responses. Bradykinin (BK) is thought to contribute to the vasodilator effects of ACE inhibitors, but the effect of BK itself in patients with CCF has not been examined. METHODS: We studied the responses to infused BK at 10, 30 and 100 pmol min(-1) in patients with CCF (n=10) and controls (n=10). The slope of the dose-response curve was used for comparisons between the groups. Forearm blood flow (FBF) was measured by venous occlusion plethysmography. RESULTS: Following BK, vasodilatation was observed in both groups as the slopes were positive in all, but the difference between the groups was not significant (P=0.77). The study was repeated with the co-administration of 4 micromol min(-1) of N(G)-monomethyl L-arginine (L-NMMA). The vasodilator response to BK was reduced in both groups, and the effect was somewhat greater in the patient group (P=0.23). The vasodilator response to the endothelium-independent vasodilator sodium nitroprusside was slightly less in the patient group (P=0.08). The patients only then underwent repeat infusion of BK before and after a single oral dose of captopril 12.5 mg or placebo. Following captopril, the vasodilator response to BK was unchanged when compared to placebo (difference between slopes, P=0.53). CONCLUSIONS: BK produces dose-dependent vasodilatation in both patients with CCF and controls; there was no difference in the responses, which were antagonised by L-NMMA and therefore in part NO (endothelium)-dependent. The responses were also unchanged after administration of an ACE inhibitor (captopril).

Aged↗
PubMed2001

The effects of oral methionine and homocysteine on endothelial function.

BACKGROUND: Raised homocysteine is a risk factor for vascular disease. Homocysteine is formed from methionine, and dietary manipulation of homocysteine in primates and humans with oral methionine is associated with endothelial dysfunction. A cause-effect relation has not been clearly established. AIM: To study the effect of oral methionine and then oral homocysteine on endothelial function. METHODS: 22 healthy adults were recruited for two randomised crossover studies, each containing 11 subjects. Endothelial function was determined by measuring forearm blood flow in response to intra-arterial infusion of acetylcholine (endothelium dependent) and sodium nitroprusside (endothelium independent). Subjects received methionine or placebo (study 1), or homocysteine or placebo (study 2). Methionine and homocysteine were determined at baseline and t = 4 hours. Endothelial function was determined at four hours. The responses to the vasoactive substances are expressed as the area under the curve of change in forearm blood flow from baseline. RESULTS: Study 1: plasma methionine and homocysteine concentrations increased significantly versus placebo. The increases were associated with a reduction of endothelium dependent responses (mean (95% confidence interval), arbitrary units), from 48.8 (95% CI 36.4 to 61.2) to 29.9 (95% CI 18.0 to 41.1), p < 0.04; endothelium independent responses were unchanged. Study 2: homocysteine concentration increased significantly while methionine remained unchanged. Endothelium dependent responses were reduced from 34.6 (95% CI 20.6 to 48.6) to 22.8 (95% CI 12.0 to 33.6), p < 0.03. CONCLUSIONS: Homocysteine and not methionine is responsible for the changes in endothelial function. This supports the hypothesis that homocysteine promotes atherosclerosis by inducing endothelial dysfunction.

Administration, Oral↗
PubMed2001

Hyperhomocysteinaemia in young adults is not associated with impaired endothelial function.

A mild to moderate elevation of the total homocysteine concentration (tHcy) is now recognized as a risk factor for vascular disease. It is also associated with endothelial dysfunction in middle-aged and elderly individuals without overt atherosclerotic vascular disease. This is important, as endothelial dysfunction is a well recognized early and potentially reversible marker of the atherosclerotic process. We investigated whether mild hyperhomocysteinaemia was associated with endothelial dysfunction in otherwise healthy young males. We compared endothelial function, by measuring forearm blood flow, in 17 males with mild hyperhomocysteinaemia (defined as tHcy >10 micromol/l) and 14 controls with low tHcy (defined as <5 micromol/l). Forearm blood flow was measured in response to the intra-arterial infusion of acetylcholine (endothelial-dependent response) or sodium nitroprusside (endothelial-independent response). Responses to the vasoactive substances were expressed as the area under the curve of the change in forearm blood flow from baseline. Data are given as mean (95% confidence interval). The two groups were well matched for age, body mass index, pulse rate and blood pressure. tHcy was significantly different between the groups [12.3 (10.4-14.2) micromol/l compared with 4.9 (4.6-5.1) micromol/l; P<0.001]. Concentrations of vitamin B(12) and folate were significantly higher in the control group. There was no difference in basal forearm blood flow between the group with mild hyperhomocysteinaemia and the controls, and both the endothelial-dependent [37.5 (26.2-38.8) and 35.3 (26.1-44.4) arbitrary units respectively] and -independent [26.1 (22.2-29.9) and 25.9 (21.0-30.8) units respectively] responses were not significantly different between the groups. Thus the present study demonstrates that, in healthy adults, mild elevation of tHcy was not associated with impaired endothelial-dependent vasodilation. These data suggest an age effect with regard to homocysteine and endothelial dysfunction. The development of vascular disease in individuals with hyperhomocysteinaemia may only result with higher concentrations or after prolonged exposure.

Acetylcholine↗
PubMed2000

Vasoconstriction to endothelin-1 is blunted in non-insulin-dependent diabetes: a dose-response study.

The haemodynamic hypothesis for the pathogenesis of diabetic microangiopathy argues that an initial increase in microvascular blood flow leads to microvascular sclerosis and disturbed autoregulation. Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor peptide that contributes to basal vascular tone. Impairment of the vasoconstrictor response to ET-1 could result in hyperperfusion and subsequent microvascular damage. The purpose of this study was to determine whether vascular responses to ET-1 are impaired in patients with non-insulin-dependent diabetes mellitus (type 2 diabetes). Ten patients with type 2 diabetes and nine control subjects underwent brachial artery cannulation. Forearm blood flow was measured using strain-gauge venous occlusion plethysmography. ET-1 in three doses of 5, 10, and 20 pmol/min and 0.9% saline placebo was infused in a balanced double-blind randomised manner. Vascular smooth muscle function also was assessed using sodium nitroprusside. Control subjects showed vasoconstriction to ET-1 of 5 (p < 0.05), 10 (p < 0.05), and 20 pmol/min (p < 0.01). In the diabetic group, there was no significant response to ET-1 at 5 pmol/min (p > 0.05); however, significant vasoconstriction developed at 10 and 20 pmol/min (p < 0.01). There was a significant difference in response to ET-1 at 5 pmol/min between the diabetic and control groups (p < 0.05). Responses to sodium nitroprusside were similar in both groups (p > 0.05). Patients with type 2 diabetes have a blunted vasoconstrictor response to ET-1 despite preserved vascular smooth muscle function.

Adult↗
PubMed2000

Differential pharmacokinetics of digoxin in elderly patients.

Digoxin remains one of the most commonly prescribed of all cardiac medications. The main indications for digoxin usage include atrial fibrillation and heart failure; both these conditions are more prevalent in older patients. Given the aging population and the increasing incidence of heart failure we would expect prescribing of digoxin to remain as frequent or to even increase in older patients. Older patients are also more likely to develop toxicity and diagnosis of digoxin toxicity can be difficult in this group. Numerous components contribute to the development of toxicity in older patients, ranging from aging-related changes in renal function or body mass to polypharmacy and possible interactions with digoxin. It is therefore important to understand how the pharmacokinetics of digoxin may be altered in the older population. Application of basic pharmacological principles may be helpful in anticipating these problems. This review describes the pharmacokinetics of digoxin, the changes in pharmacokinetics with increasing age and how concomitant disease states or drug interactions may affect the pharmacokinetics of digoxin. Greater knowledge about the causes and prevention of digoxin toxicity should further reduce the morbidity and mortality arising from digoxin toxicity, especially in the elderly population.

Absorption↗
PubMed2000

Vasoconstriction to endogenous endothelin-1 is impaired in patients with type II diabetes mellitus.

Endothelin-1 has potent vasoconstrictor and vasopressor actions contributing to basal vascular tone and maintenance of blood pressure acting predominantly through endothelin-A receptors. Endothelin antagonists may be of value in the treatment of hypertension and heart failure. However, the role of endothelin-1 in the regulation of vascular tone and the potential benefits of endothelin antagonists in non-insulin-dependent diabetes mellitus (Type II diabetes) are less clear. Vasoconstriction to exogenous endothelin-1 is impaired in Type II diabetes. The purpose of this study was to determine whether vasoconstriction to endogenous endothelin-1 acting through the endothelin-A receptor is impaired in Type II diabetes. In ten patients with Type II diabetes and nine controls the endothelin-A receptor antagonist BQ123 was infused intra-arterially at 100 nmol/min for 60 min followed by normal saline for 30 min. Forearm blood flow was measured using venous occlusion plethysmography. Control subjects showed gradual onset of vasodilation in response to BQ123 (P < 0.001). Diabetic subjects, however, showed no significant response (P > 0.05). There was a significant difference between the diabetic and control groups (P < 0.05). Blockade of the endothelin-A receptor is associated with impaired vasodilation in Type II diabetes indicating vasoconstriction to endogenous endothelin-1 mediated by the endothelin-A receptor is impaired.

Adult↗
PubMed1999

Forearm reactive hyperaemia is not mediated by nitric oxide in healthy volunteers.

AIMS: To determine the role of nitric oxide (NO) in forearm reactive hyperaemia in healthy human subjects. METHODS: Ten healthy subjects aged 19-34 years underwent brachial artery cannulation. Forearm circulatory arrest was achieved by means of an upper arm cuff inflated to 200 mmHg for 5 min. The blood flow responses during reactive hyperaemia were measured using venous occlusion plethysmography following a 10 min intra-arterial infusion of 8 micromol min-1 N-monomethyl L-arginine (L-NMMA) and following matching placebo administered in random order. Results were analysed by repeated measures anova and t-tests. RESULTS: L-NMMA resulted in a significant reduction of basal forearm blood flow indicating inhibition of basal NO release (P=0.005). There was no significant difference between the blood flow responses during reactive hyperaemia following L-NMMA and placebo (P=0.97). CONCLUSIONS: Nitric oxide production does not make a significant contribution to the vasodilatation associated with reactive hyperaemia in the human forearm.

Adult↗
PubMed1999

Endothelial function in offspring of Type 1 diabetic patients with and without diabetic nephropathy.

AIMS: Familial aggregation of diabetes and nephropathy has been observed in several populations. Various interpretations have included underlying genetic mechanisms possibly mediated by a predisposition to hypertension. Endothelial dysfunction is now recognized as central to these conditions and offers a unifying mechanism to explain the disease aggregation. To test this hypothesis, we have examined endothelial function in healthy subjects at differing risks for diabetic nephropathy. METHODS: Endothelial function was assessed in 12 healthy offspring (nine male (M)/three female (F); age 25.8+/-1.2 years, mean +/- SEM) of parents with Type 1 diabetes mellitus (DM) and end-stage renal disease compared with 12 control offspring (9 M/3 F; age 26.3+/-1.3 years) of parents with long duration (>20 years) Type 1 DM but without evidence of nephropathy. Forearm blood flow responses to brachial artery infusion of acetylcholine (endothelium-dependent), sodium nitroprusside (endothelium-independent), noradrenaline and the competitive inhibitor of basal nitric oxide release N(G)-monomethyl-L-arginine were assessed using venous occlusion plethysmography. RESULTS: There were no significant differences between the groups in von Willebrand factor, fasting plasma glucose (4.2+/-0.1 vs. 4.0+/-0.2 mmol/l) or in 24-h ambulatory blood pressure. Similarly, the groups did not differ in vasodilation to acetylcholine (P = 0.75) and sodium nitroprusside (P = 0.79) or in vasoconstriction to noradrenaline (P = 0.45) and N(G)-monomethyl-L-arginine (P = 0.30). CONCLUSION: These data do not support a role for endothelial dysfunction in the familial aggregation of diabetic nephropathy.

Adult↗
PubMed1999

Effect of methionine supplementation on endothelial function, plasma homocysteine, and lipid peroxidation.

BACKGROUND: Acetaminophen (paracetamol) poisoning is a major source of morbidity and mortality. It has been proposed that methionine be incorporated into acetaminophen tablets routinely as a protective mechanism. Methionine has been shown to be effective in the treatment of acetaminophen toxicity and a combination preparation of acetaminophen and methionine may prevent toxicity. However, there has been some concern that chronic methionine supplementation may be associated with vascular disease. The aim of the study was to investigate if methionine supplementation causes changes in endothelial function, plasma homocysteine, or lipid peroxidation which may be associated with atherosclerosis. METHODS: Sixteen healthy volunteers were studied. Forearm blood flow in response to local intra-arterial infusion of acetylcholine to assess endothelium-dependent vasodilatation and sodium nitroprusside to assess endothelium-independent vasodilatation was measured by venous occlusion plethysmography. Plasma homocysteine and lipid peroxidation, measured as thiobarbituric acid reactive substances, were measured using high-performance liquid chromatography. Forearm vascular responses, plasma homocysteine concentrations, and thiobarbituric acid reactive substances were measured at baseline and following methionine supplementation. RESULTS: There was no significant difference in endothelial-dependent vascular responses after acute (methionine 250 mg orally, p > 0.05), 1 month of low-dose (methionine 250 mg daily, p > 0.05), or 1 week of high-dose (methionine 100 mg/kg daily, p > 0.05) methionine administration. There was no significant difference in plasma homocysteine concentrations after acute (p > 0.05) or 1 month of low-dose (p > 0.05) methionine administration. However, 1 week of high-dose methionine (100 mg/kg) administration daily significantly increased homocysteine concentrations (p < 0.0015). Thiobarbituric acid reactive substances were unchanged during the period of study (p > 0.05). CONCLUSIONS: Methionine supplementation does not impair endothelial-dependent vascular responses in healthy volunteers. Although high-dose methionine administration causes elevation of plasma homocysteine concentrations, doses similar to those used in combination preparations with acetaminophen do not affect plasma homocysteine concentrations.

Acetylcholine↗
PubMed1999

A comparison of enalapril 20 mg once daily versus 10 mg twice daily in terms of blood pressure lowering and patient compliance.

OBJECTIVE: To compare enalapril 20 mg once daily with 10 mg twice daily in terms of blood pressure reduction and patient compliance. DESIGN: Cross-over study of patients randomly assigned to a sequence of enalapril 20 mg once daily or 10 mg twice daily in three 4-week periods following a 4-week placebo run-in. SETTING: General practices in the greater Belfast and Lisburn area in Northern Ireland. PATIENTS: Twenty-five hypertensive patients who had a mean diastolic blood pressure of between 90 and 110 mm Hg after receiving placebo for 4 weeks. MAIN OUTCOME MEASURES: Reduction in blood pressure and estimation of patient compliance. RESULTS: Patient compliance was superior on the once daily regimen. However, the twice daily regimen was associated with a greater blood pressure reduction which almost reached statistical significance at the 5% level. CONCLUSIONS: Enalapril 20 mg should be prescribed as 10 mg twice daily and measures taken to improve patient compliance.

Angiotensin-Converting Enzyme Inhibitors↗
PubMed1999

Relationship of endothelial function to birth weight in humans.

OBJECTIVE: Low birth weight has been associated with hyperlipidemia, hypertension, diabetes, and coronary heart disease in adult life, but the precise mechanism is debated. The endothelium is thought to play a pivotal role in each of the above conditions with abnormalities being detectable before the development of overt disease. To investigate the possibility that endothelium has a role in mediating the excessive risk of adult vascular disease associated with low birth weight, endothelial function was assessed in young healthy adults who were either of low or normal birth weight at term. RESEARCH DESIGN AND METHODS: Twelve low birth weight (2.2 +/- 0.05 kg, mean +/- SEM) subjects (six men/six women; age 28 +/- 0.2 years) and twelve age- and sex-matched normal birth weight (3.3 +/- 0.07 kg) control subjects were studied. The L-arginine-nitric oxide pathway was assessed in the forearm vascular bed by using venous occlusion plethysmography during intra-arterial brachial infusion of acetylcholine, sodium nitroprusside, norepinephrine, and NG-monomethyl-L-arginine (L-NMMA). Von Willebrand factor, a noninvasive marker of endothelial dysfunction, was also measured. Comparisons were made using Student's t test. RESULTS: Von Willebrand factor was significantly elevated in low birth weight compared with normal birth weight subjects (136.9 +/- 12.7 vs. 95.6 +/- 9.5%; P = 0.016). The groups did not differ in the responses to acetylcholine (P = 0.76), sodium nitroprusside (P = 0.84), norepinephrine (P = 0.21), or L-NMMA (P = 0.35). CONCLUSIONS: The finding of elevated von Willebrand factor in low birth weight subjects is suggestive of endothelial cell injury but does not appear to be associated with dysfunction of the L-arginine-nitric oxide pathway.

Acetylcholine↗
PubMed1999

Basal nitric oxide production is impaired in offspring of patients with essential hypertension.

There is considerable evidence that endothelium-dependent nitric oxide (NO)-mediated vasodilatation in response to acetylcholine is impaired in essential hypertension, whereas the endothelium-independent response to sodium nitroprusside is normal. More limited data have suggested that there is also reduced vasoconstriction in response to N(G)-monomethyl-L-arginine (L-NMMA), a competitive inhibitor of basal NO release. As it is not known whether endothelial dysfunction in hypertension, if indeed present, is a cause or consequence of the condition, we have studied the normotensive offspring of parents with essential hypertension. Both basal and stimulated vascular responses were examined in 12 normotensive offspring [mean age (+/-S.E.M.) 26.1+/-1.4 years] of parents with essential hypertension and compared with those in 12 age-matched offspring (mean age 25.6+/-1.1 years) of normotensive subjects. Forearm blood flow was measured simultaneously in both arms by venous occlusion plethysmography, both at baseline and during intra-arterial brachial infusion of increasing doses of acetylcholine, sodium nitroprusside, noradrenaline and L-NMMA. There were no significant differences between the groups in the responses to acetylcholine, sodium nitroprusside or noradrenaline. In contrast, the vasoconstrictor response to L-NMMA was significantly blunted in the offspring of hypertensive parents compared with that in the offspring of normotensive parents (P=0.005). Thus endothelial dysfunction, as demonstrated by impaired basal production of NO, is present in subjects at high risk of essential hypertension, and does not occur simply as a consequence of the condition.

Acetylcholine↗
PubMed1998

Effects of combination therapy with an angiotensin converting enzyme inhibitor and thiazide diuretic on insulin action in essential hypertension.

OBJECTIVE: To determine whether combination of an angiotensin converting enzyme inhibitor with a high dose of thiazide diuretic avoids adverse metabolic consequences of thiazide diuretics. DESIGN: Double-blind randomized crossover study of two 12-week treatment periods with captopril (up to 100 mg/day) either alone or in combination with 5 mg bendrofluazide given after a 6-week placebo run-in period. Treatment periods were separated by a 6-week placebo washout period. SETTING: Outpatient clinics in greater Belfast. PATIENTS: Fifteen white non-diabetic essential hypertensives (seven male) aged < 65 years recruited from general practices in greater Belfast. MAIN OUTCOME MEASURES: Systolic and diastolic blood pressures and peripheral and hepatic insulin action. RESULTS: Two patients failed to complete the study. Blood pressure was lowered (139/89+/-18/7 mmHg combination versus 160/97+/-21/7 mmHg captopril; P < 0.001). Fasting insulin level was raised (7.9+/-3.6 mU/l combination versus 6.2+/-3.2 mU/l baseline; P < 0.001). There were no differences between treatments for glucose, urate, cholesterol and triglyceride levels. Serum potassium level was lowered (3.8+/-0.4 mmol/l combination versus 4.2+/-0.4 mmol/l captopril, P < 0.05). Postabsorptive endogenous glucose production was raised (10.8+/-1.7 micromol/kg per min combination versus 10.0+/-1.5 micromol/kg per min captopril; P < 0.01) and was greater than baseline (9.7+/-2.1 micromol/kg per min, P < 0.05). Suppression of glucose production by insulin was similar with both treatments. Exogenous glucose infusion rates required to maintain euglycaemia did not differ (32.4+/-7.6 micromol/kg per min captopril, 32.7+/-6.2 micromol/kg per min combination, 31.5+/-7.2 micromol/kg per min baseline). CONCLUSIONS: Combination therapy increased glucose production (compared with captopril alone), indicating hepatic insulin resistance. It cannot be assumed that combined preparations with angiotensin converting enzyme inhibitors will ameliorate adverse effects of high doses of thiazide diuretics on insulin action.

Angiotensin-Converting Enzyme Inhibitors↗