Performance of subcutaneously implanted glucose sensors for continuous monitoring

Despite a considerable amount of research attributed to the development of an implantable glucose sensor, to date there is no clinically applicable concept for continuous glucose monitoring. Investigations to validate the subcutaneous tissue for continuous glucose sensing mostly comprised short-term implantations of glucose sensors. Most implanted glucose sensors showed a significant drift in sensitivity over the implantation period. This bioinstability was not to be expected from the in vitro performance of the sensors. In this paper, the influence of possible failure mechanisms on the poor in vivo performance of subcutaneously implanted glucose sensors is reviewed.

The impact of blood pressure measurement methods on the assessment of differences in blood pressure levels between patients with normoalbuminuric type 1 diabetes and healthy controls

The aim of the study was to investigate the impact of various blood pressure (BP) measurement methods on the assessment of differences in BP levels between patients with normoalbuminuric type 1 diabetes and healthy controls. We measured intra-arterial BP (i.a.), sphygmomanometric BP (sphygmo), 24-h ambulatory blood pressure (ABPM, auscultatory, Profilomat) and oscillometric BP (Dinamap) in 51 patients with normoalbuminuric type 1 diabetes (DP) with a mean diabetes duration of 8.4 years and 42 healthy controls (C). Results are expressed as mean +/- SE. There was no significant difference in i.a. BP between DP and C (systolic/diastolic BP and mean arterial pressure (MAP) 116.2 +/- 1.2/61.7 +/- 0.8 (82.8 +/- 0.9) mm Hg in DP vs 115.6 +/- 1.2/63.2 +/- 0.9 (83.4 +/- 1.1) in C). Sphygmo BP was 117.7 +/- 1.3/69.8 +/- 1.0 mm Hg in DP vs 116.5 +/- 1.5/67.8 +/- 1.3 in C (NS). Also, ABPM was not significantly different between both groups. Daytime BP between 10.00-23.00 h was 120.9 +/- 1.2/84.4 +/- 0.9 mm Hg in DP vs 120.4 +/- 1.5/83.7 +/- 1.0 in C (NS). Night-time BP between 01.00-07.00 h was 102.4 +/- 1.2/69.3 +/- 0.9 mm Hg in DP vs 103.4 +/- 1.5/69.1 +/- 1.3 in C (NS). In contrast, systolic Dinamap BP was higher in DP (118.6 +/- 1.3 in DP vs 113.4 +/- 1.4 mm Hg in C, P = 0.01) as was MAP (85.6 +/- 0.7 in DP vs 83.3 +/- 1.0 mm Hg in C, P = 0.05). Diastolic Dinamap BP was not significantly different (66.6 +/- 0.7 in DP vs 65.0 +/- 1.0 mm Hg in C). We conclude that intra-arterial BP was similar in patients with normoalbuminuric type 1 diabetes and healthy controls. Also, when using auscultatory BP devices there were no apparent differences in blood pressure. In contrast, using the oscillometric method (Dinamap), BP especially systolic, was higher in diabetic patients. Measurements with an oscillometric device (Dinamap) might therefore overestimate BP in patients with normoalbuminuric type 1 diabetes, thus confusing conclusions on the relationship between development of hypertension and microalbuminuria in the early phase of diabetes.

Insulin-induced vasodilatation and endothelial function in obesity/insulin resistance. Effects of troglitazone

Insulin resistance is associated with a decreased vasodilator response to insulin. Because insulin's vasodilator effect is nitric oxide dependent, this impairment may reflect endothelial dysfunction. Troglitazone, an insulin-sensitiser, might thus improve insulin-dependent and/or endothelium-dependent vascular function in insulin resistant obese subjects. For 8 weeks, fifteen obese subjects were treated with either 400 mg troglitazone once daily or placebo, in a randomised, double-blind, cross-over design. At the end of each treatment period, we measured forearm vasodilator responses (plethysmography) to intra-arterial administered acetylcholine and sodium nitroprusside; insulin sensitivity and insulin-induced vascular and neurohumoral responses (clamp); vasoconstrictor responses to NC-monomethyl-L-arginine (L-NMMA) during hyperinsulinaemia; and ambulatory 24-h blood pressure (ABPM). Baseline data (placebo) of obese subjects were compared with those obtained in lean control subjects. Obese subjects were insulin resistant compared with leans (whole-body glucose uptake: 26.8+/-3.0 vs. 53.9+/-4.3 [tmol kgl min-, p < 0.001). Troglitazone improved whole-body glucose uptake (to 31.9+/-3.3 micromol x kg(-1) x min(-1) , p=0.028), and forearm glucose uptake (from 1.09+/-0.54 to 2.31+/-0.69 micromol dL(-1) x min(-1), p=0.006). Insulin-induced vasodilatation was blunted in obese subjects (percent increase in forearm blood flow (FBF) in lean 66.5+/-23.0%, vs. 10.1+/-11.3% in obese, p=0.04), but did not improve during troglitazone. Vascular responses to acetylcholine, sodium nitroprusside and L-NMMA did not differ between the obese and lean group, nor between both treatment periods in the obese individuals. In conclusion, in insulin resistant obese subjects, endothelial vascular function is normal despite impaired vasodilator responses to insulin. Troglitazone improved insulin sensitivity but it had no effects on endothelium-dependent and -independent vascular responses. These data do not support an association between insulin resistance and endothelial function.

Performance of subcutaneously implanted glucose sensors: a review

Despite a considerable amount of research attributed to the development of an implantable glucose sensor, to date there is no clinically applicable concept for continuous glucose monitoring. Investigations to validate the subcutaneous tissue for continuous glucose sensing mostly comprise short-term implantations of glucose sensors. Most implanted glucose sensors showed a significant decay in sensitivity over the implantation period. This bioinstability was not to be expected from the in vitro performance of the sensors. In this article, the influence of possible failure mechanisms on the poor in vivo performance of subcutaneously implanted glucose sensors is reviewed.

Insulin stimulates epinephrine release under euglycemic conditions in humans

In healthy subjects, acute physiological hyperinsulinemia induces activation of the sympathetic nervous system, but in the absence of hypoglycemia, plasma epinephrine levels have not been found to increase during insulin administration. However, the venous level of epinephrine reflects the net result of release, clearance, and uptake and therefore is not a good measure of adrenomedullary epinephrine secretion. The influence of 90 minutes of euglycemic physiological hyperinsulinemia (60 mU x m(-2) x min(-1); plasma insulin concentration, approximately 700 pmol x L[-1]) on epinephrine kinetics using the 3H-epinephrine tracer method was studied in 12 healthy normotensive, non-obese subjects. After bolus injection, [3H]-epinephrine was continuously infused with arterial and venous blood sampling at regular intervals, enabling calculation of total body (systemic) and forearm epinephrine release and clearance. Studies were performed in the basal state and during sympathetic stimulation by lower-body negative pressure (LBNP) of -15 mm Hg for 15 minutes. Control experiments ("sham" clamps, but with LBNP) were performed in four of the 12 individuals. Euglycemic hyperinsulinemia (all arterial glucose samples > or = 4.2 mmol x L[-1]) induced an increase of the arterial epinephrine concentration (P = .03), and tended to increase total body epinephrine release (P = .08). Total body epinephrine clearance did not change during hyperinsulinemia. The insulin-induced increase in forearm blood flow ([FBF] by plethysmography, from 3.0 +/- 0.4 to 3.8 +/- 0.6 mL x dL(-1) x min(-1), P = .01) was strongly correlated with the increase in arterial epinephrine (r = .78, P < .01). Plasma epinephrine concentrations did not change during control experiments (sham clamp). Sympathetic stimulation alone as induced by LBNP did not stimulate epinephrine release. However, the combination of insulin and LBNP significantly increased epinephrine release (from 0.37 +/- 0.06 to 0.56 +/- 0.12 nmol x m(-2) x min(-1), P = .03). We conclude that acute physiological hyperinsulinemia under euglycemic conditions induces epinephrine release. This effect is enhanced when hyperinsulinemia is combined with sympathetic stimulation by LBNP. Due to increased forearm removal, venous epinephrine concentrations hardly change. Epinephrine release was strongly correlated with the hemodynamic effects of insulin.

Haemodynamic actions of insulin

Several lines of evidence indicate a significant association between insulin and cardiovascular disease. This association might be explained by direct (cardio) vascular effects of insulin. Two hemodynamic actions of insulin are discussed in this review; it induces direct vasodilation in skeletal muscle and stimulation of the sympathetic nervous system. These closely linked effects normally offset each other. Although more insight has been obtained into responses in insulin-resistant individuals and possible mechanisms, direct evidence to support a causative role for insulin is not yet available.

[Diabetes mellitus in connection with a hereditary disease]

Diabetes mellitus is usually subdivided into type I (insulin-dependent) and type II (relative insulin shortage and reduced sensitivity to insulin). Diabetes may also be related to pregnancy, malnutrition, pancreatic disease, pharmaceuticals, endocrine diseases and hereditary disorders. The hereditary diseases which may be associated with diabetes mellitus or impaired glucose tolerance can be subdivided into syndromes (such as maternally inherited diabetes and deafness, Down, Turner and Klinefelter syndrome), metabolic diseases (like cystic fibrosis and haemochromatosis) and endocrine diseases (like polyglandular autoimmune insufficiency syndrome and familial phaeochromocytoma). Although diabetes mellitus as part of a hereditary disorder is infrequent, the possibility should be kept in mind with a view to a correct diagnosis. In patients with diabetes mellitus a hereditary disorder may be involved, while patients with a hereditary disorder run a higher risk of developing diabetes mellitus.

Blockade of vascular ATP-sensitive potassium channels reduces the vasodilator response to ischaemia in humans

Experimental data show that ATP-sensitive potassium (KATP) channels not only occur in pancreatic beta cells, but also in the cardiovascular system, where they mediate important cardioprotective mechanisms. Sulphonylurea derivatives can block the cardiovascular KATP channels and may therefore interfere with these cardioprotective mechanisms. Therefore, it is of clinical importance to investigate whether sulphonylurea derivatives interact with vascular KATP channels in humans. Using venous-occlusion strain-gauge plethysmography, we investigated whether ischaemia-induced reactive hyperaemia is reduced by the sulphonylurea derivative glibenclamide in 12 healthy male non-smoking volunteers. Forearm vasodilator responses to three periods of arterial occlusion (2, 5 and 13 min) during concomitant infusion of placebo into the brachial artery were compared with responses during concomitant intra-arterial infusion of glibenclamide (0.33 microgram.min-1.dl-1). A control study (n = 6) showed that time itself did not change the vasodilator response to ischaemia. Glibenclamide significantly increased minimal vascular resistance (from 2.1 +/- 0.1 to 2.3 +/- 0.2 arbitrary units, Student's t-test: p = 0.01), and reduced mean forearm blood flow (from 37.5 +/- 2.0 to 35.4 +/- 2.0 ml min-1.dl-1 after 13 min occlusion, ANOVA with repeated measures: p = 0.006) and flow debt repayment during the first reperfusion minute (ANOVA with repeated measures: p = 0.04). In contrast, total flow debt repayment was not affected. Infusion of glibenclamide into the brachial artery resulted in local concentrations in the clinically relevant range, whereas the systemic concentration remained too low to elicit hypoglycaemic effects. Our results suggest that therapeutic concentrations of glibenclamide induce a slight but significant reduction in the early and peak vasodilation during reactive hyperaemia.

Skin microcirculation of the foot in diabetic neuropathy

1. In the feet of patients with diabetic neuropathy, total skin blood flow is increased due to an increased shunt flow. The question is, does this increased anastomotic shunt flow lead to either under- or overperfused nutritive capillaries. 2. To solve this question, skin microcirculation tests of the left big toe were performed in 20 healthy control subjects and in 40 insulin-dependent diabetic patients without macroangiopathy, 20 without and 20 with neuropathy. Skin temperature measurements and laser Doppler fluxmetry were performed to record mainly shunt flow and capillaroscopy to study nailfold capillary blood flow. 3. The insulin-dependent diabetic patients with neuropathy had a higher baseline skin temperature (mean +/- SEM; 30.0 +/- 0.6 degrees C) and laser Doppler fluxmetry [26.2 +/- 2.2 perfusion units (pu)] than patients without neuropathy (27.2 +/- 0.8 degrees C, P < 0.01; 16.1 +/- 2.0 pu, P < 0.01) and healthy control subjects (27.9 +/- 0.7 degrees C, P < 0.05; 18.6 +/- 2.8 pu, P < 0.05). Sympathetic stimulation (inspiratory gasp) resulted in a smaller laser Doppler fluxmetry decrease in the neuropathic patients (31.4 +/- 4.6%) compared with non-neuropathic patients (48.2 +/- 5.1%, P < 0.05) and control subjects (49.0 +/- 3.8%, P < 0.05), while no difference between the three groups was seen in the laser Doppler fluxmetry decrease during a postural vasoconstriction test. The number of visible capillaries was highest in the neuropathic patients (10.2 +/- 0.6/0.5 mm2), when compared with non-neuropathic patients (8.7 +/- 1.2/0.5 mm2, P < 0.05) and control subjects (8.3 +/- 0.3/0.5 mm2, P < 0.001). Capillary blood-cell velocity was significantly higher in the neuropathic patients (0.32 +/- 0.05 mm/s) compared with non-neuropathic patients (0.23 +/- 0.03 mm/s, P < 0.05) and control subjects (0.23 +/- 0.02 mm/s, P < 0.01). 4. We conclude that there is an overperfused nutritive capillary circulation in the feet of patients with diabetic neuropathy. This is in contradiction to the capillary steal phenomenon and favours the hyperdynamic hypothesis to explain the decreased healing potential in diabetic neuropathic foot ulceration.

Microcirculation in the footsole as a function of mechanical pressure

OBJECTIVE: In this study an experimental set-up for measuring skin microvascular responses of the footsole to changes in externally applied pressure was analysed. DESIGN: A clinical study. Skin microvascular blood flow was measured in healthy volunteers, during and after external mechanical pressure of different magnitudes. BACKGROUND: During standing and walking the footsole is commonly exposed to high static and dynamic mechanical pressure, resulting in changes in the microcirculation of the footsole. In diabetic patients a disturbed interaction between externally applied pressure and skin microvascular response seems to be involved in the development of a foot ulcer. METHODS: Eleven volunteers participated in the study. Static loads were applied to the heel part of the footsole with the person in a supine position. Contact pressure and skin blood flux, based on the laser Doppler technique, were simultaneously monitored. The pressure used was varied in five discrete steps between 10 and 160 kPa and applied during a period of 5 min each. The microcirculation was measured during as well as after pressure loading. RESULTS: Pressures of 40 kPa and higher do stop the blood flow in the skin microcirculation. Releasing the applied pressure resulted in a hyperaemic response. This response appears to increase in amplitude at increasing pressures up to 800% of the baseline laser Doppler fluxmetry level. Beyond a pressure level of 80 kPa the hyperaemic response seems not to be influenced by the pressure level. The time needed to achieve the maximal laser Doppler fluxmetry level decreased when the pressure was raised from 10 to 80 kPa, but increased again when higher pressures were applied (P = 0.051). An intraindividual variation of 11-50% was observed for the parameters describing the blood flux before, during, and after pressure application. CONCLUSION: Simultaneously measuring changes in contact pressure and laser Doppler flux of the footsole is a useful method to study the interaction of external mechanical pressure and skin microvascular reactions. Pressures above 40 kPa stop skin microvascular blood flow. Releasing the applied pressure results in a hyperaemic response, which increases when the applied pressure increases from 40 to 80 kPa. Higher pressures do not influence the amplitude in skin microvascular response, but result in a longer delay to maximal hyperaemia.

Interaction of sulphonylurea derivatives with vascular ATP-sensitive potassium channels in humans

Cardiovascular adenosine-5'-triphosphate-sensitive potassium (KATP) channels have been reported to play an important role in endogenous cardioprotective mechanisms. Sulphonylurea derivatives can inhibit these cardioprotective mechanisms in animal models. We investigated whether therapeutic concentrations of sulphonylurea derivatives can block vascular KATP channels in humans. The forearm vasodilator responses to administration of the specific KATP channel opener diazoxide into the brachial artery of healthy male volunteers were recorded by venous occlusion plethysmography. This procedure was repeated with concomitant intraarterial infusion of:1) the sulphonylurea derivative glibenclamide (0.33 or 3.3 micrograms. min-1. dl-1, both n = 12), 2) the new sulphonylurea derivative glimepiride (2.5 micrograms.min-1. dl-1, n = 12) or 3) placebo (n = 12). The effects of glibenclamide on the vasodilator responses to sodium nitroprusside were also studied (n = 12). Glibenclamide significantly inhibited the diazoxide-induced increase in forearm blood flow ratio (ANOVA with repeated measures: p < 0.01). During the highest diazoxide dose this ratio (mean +/- SEM) was lowered from 892 +/- 165 to 449 +/- 105%, and from 1044 +/- 248 to 663 +/- 114% by low- and high-dose glibenclamide, respectively. In contrast, neither glimepiride nor placebo attenuate diazoxide-induced vasodilation. Furthermore, glibenclamide did not affect nitroprusside-induced vasodilation. We conclude that therapeutic concentrations of the classical sulphonylurea derivative glibenclamide result in significant blockade of vascular KATP channels in humans. The newly developed glimepiride seems to be devoid of these properties.

Effects of tolbutamide on vascular ATP-sensitive potassium channels in humans. Comparison with literature data on glibenclamide and glimepiride

Sulfonylurea (SU) derivatives exert their hypoglycemic effect by blockade of adenosine-5'-triphosphate-sensitive potassium (KATP) channels in the beta-cell of the pancreas. Interestingly, KATP channels also occur in the cardiovascular system, where they are thought to play an important role in cardioprotective mechanisms against ischemia. We have recently shown that the classical second generation SU-derivative glibenclamide is able to block vascular KATP channels in man, whereas the newly developed second generation derivative glimepiride was devoid of this property. The aim of this study was to determine whether the first generation SU derivative tolbutamide has KATP channel blocking properties in humans. In a group of 12 healthy male non-smoking volunteers, we investigated whether therapeutic concentrations of tolbutamide were able to inhibit the forearm vasodilation in response to the infusion of the KATP channel opening drug diazoxide into the brachial artery. Changes in forearm blood flow were recorded by venous occlusion mercury-in-silastic strain-gauge plethysmography. Diazoxide alone increased the forearm blood flow ratio dose-dependently by ultimately 691 +/- 198%. A second diazoxide infusion in the presence of tolbutamide revealed a comparable vasodilator response with a percentage increase in forearm blood flow ratio of ultimately 542 +/- 111%. This response did not differ from the vasodilator response to diazoxide alone. The present study shows that therapeutic concentrations of tolbutamide are not able to attenuate the vasodilation caused by the KATP channel opener diazoxide in man. When compared with published data on second generation SU derivatives, tolbutamide shows an intermediate position between glibenclamide (with significant blockade of vascular KATP channels) versus glimepiride (with no blockade at all). It remains to be determined whether these acute effects of SU derivatives on pharmacological opening of forearm vascular KATP channels can be extrapolated to the chronic effects of these drugs on ischemia-mediated opening of myocardial KATP channels during treatment of NIDDM patients.

Direct vasodilator effects of physiological hyperinsulin-aemia in human skeletal muscle

Systemic hyperinsulinaemia induces vasodilatation in human skeletal muscle. This effect is gradual in onset, and at low insulin levels not maximal until at least 3 h. To investigate whether the vasodilator response to insulin results from a direct vascular effect, we infused insulin directly into the cannulated brachial artery (perfused forearm technique) in a total of 30 experiments in 20 healthy, lean, normotensive volunteers. Local, intra-arterial, infusion of insulin (180 min, 0.3 mU dL-1 forearm volume min-1, n = 15, forearm venous insulin concentration approximately 540 pmol L-1) induced a gradual increase in forearm blood flow (FBF; venous occlusion plethysmography) from 1.86 +/- 0.17 to 3.64 +/- 0.64 mL dL-1 min-1 after 180 min (ANOVA P < 0.001). Percentage increases in FBF after 60, 120 and 180 min averaged 14.4 +/- 5.9, 59.4 +/- 25.5 and 124.6 +/- 51.2% respectively. Forearm glucose uptake increased from 0.24 +/- 0.05 to a maximum of 1.98 +/- 0.28 micromol dL-1 min (P < 0.001). Furthermore, insulin infusion increased forearm lactate release and potassium uptake. In 10 out of these 15 individuals, the forearm glucose uptake was further increased in a second, separate, repeat experiment with concomitant intra-arterial infusion of glucose 5% (0.2 mL dL-1 min-1), resulting in forearm venous glucose concentrations of approximately 15 mmol L-1. This combined infusion achieved a similar vasodilator response to the infusion of insulin alone. The individual vascular responses of the two paired experiments showed a strong correlation (r = 0.87, P < 0.01). In five subjects time and vehicle control experiments were performed, showing no changes in FBF or metabolism during the 180 min. We conclude that the slow vasodilator response to insulin (as observed during systemic infusion) can, at least partly, be explained by a direct vascular effect of insulin. Insulin-mediated skeletal muscle glucose uptake precedes this effect, but seems not to be an important determinant of the vasodilator response to insulin.

Activation of the sodium-potassium pump contributes to insulin-induced vasodilation in humans

Systemic hyperinsulinemia induces vasodilation in human skeletal muscle. This vasodilation contributes to insulin-stimulated glucose uptake and has been found to be reduced in various insulin-resistant states. The mechanism of the effect of insulin on vascular tone is not completely understood. We hypothesized that activation of the sodium-potassium pump (Na+, K(+)-ATPase) located in endothelial or smooth muscle cells would be involved in the insulin-mediated vasodilation. Therefore, in 24 healthy, nonsmoking, nonobese, normotensive volunteers, we infused ouabain, a specific inhibitor of Na+, K(+)-ATPase, into the brachial artery before and during euglycemic hyperinsulinemia. As expected, insulin (systemic concentrations, approximately 700 [low] and 1400 [high] pmol.L-1) induced vasodilation in the control arm (forearm blood flow [FBF, plethysmography] from 1.6 +/- 0.2 to 2.1 +/- 0.4 mL.dL-1.min-1 [low insulin] and from 1.6 +/- 0.2 to 2.1 +/- 0.2 [high insulin], P < .05 for both), but the increase in FBF was abolished in the ouabain-infused forearm (from 1.3 +/- 0.1 to 1.4 +/- 0.2 mL.dL-1.min-1 [low] and from 1.3 +/- 0.1 to 1.3 +/- 0.1 [high], P = NS). Ouabain-induced increases in forearm potassium release were partly reversed by insulin. To investigate whether the mechanism of action could be at the endothelial level, we infused NG-monomethyl-I-arginine (L-NMMA), an inhibitor of endothelial nitric oxide synthase (0.05, 0.1, and 0.2 mg.dL-1.min-1) intra-arterially in 12 subjects and induced a clear dose-dependent decrease of FBF from 1.7 +/- 0.2 to 1.2 +/- 0.1 mL.dL-1.min-1 (P < .01). In contrast, after ouabain (and continued insulin) infusion, L-NMMA had no effect on FBF (from 1.6 +/- 0.4 to 1.5 +/- 0.3 mL.dL-1.min-1, n = 6, P = .66). These results demonstrate that insulin induces vasodilation by stimulation of Na+, K(+)-ATPase. This activation of Na+, K(+)-ATPase could occur at the level of the endothelium rather than that of vascular smooth muscle and contributes to the endothelium-dependent vasodilator response to insulin.

Regional hyperinsulinemia induces vasodilation but does not modulate adrenergic responsiveness in humans

The relation between insulin resistance/ hyperinsulinemia and cardiovascular disease may be related to one of the cardiovascular effects of insulin. In acute experiments in humans, systemic euglycemic hyperinsulinemia induced vasodilation in skeletal muscle. Furthermore, the sympathetic nervous system is activated, although this does not lead to increase in blood pressure (BP). We hypothesized that insulin could induce vasodilation either by reduction of alpha- or by augmentation of beta-adrenergic responsiveness. The effect of insulin infusion into the brachial artery (regional forearm hyperinsulinemia; venous insulin concentration approximately 500 pM) on forearm blood flow (FBF: plethysmography) was studied. Responses to the alpha-adrenoceptor-mediated vasoconstrictor norepinephrine (NE: once with and once without the beta-adrenoceptor antagonist propranolol, 2 x n = 12; 9 participated in both), and to the beta-adrenoceptor-mediated vasodilator isoproterenol (n = 12) were measured before and during local hyperinsulinemia. Time/control studies (n = 6) were performed. Insulin alone induced vasodilation, as indicated by an increase in FBF-ratio (infused/ control arm) from 1.2 +/- 0.1 to 1.6 +/- 0.2, p = 0.009. Increasing dosages of NE (1.25 to 240 ng.dl-1.ml-1) induced vasoconstriction that was more pronounced during concomitant propranolol infusion (p < 0.001), indicating a dose-dependent vasodilatory component in the effect of NE. Isoproterenol (ISO 0.03 to 10 ng.dl-1.ml-1), a pure beta-adrenoceptor agonist, induced vasodilation. The percentage changes of FBF-ratio during NE+propranolol were similar and not significantly different before and during hyperinsulinemia. The same was true of the response to NE alone and the response to ISO. Neither was the intrinsic beta-agonist component of NE influenced by insulin. Repeated NE infusion showed no time- or vehicle effect. We conclude that regional hyperinsulinemia in the physiological range induces local vasodilation in the skeletal muscle vascular bed, but this vasodilation is not mediated through modulation of alpha- or beta-adrenergic responsiveness.

Cardiovascular effects of sulphonylurea derivatives

The classical sulphonylurea derivatives like glibenclamide and tolbutamide are widely prescribed in non-insulin dependent diabetes mellitus in order to stimulate insulin secretion. The insulinotropic effect of these agents is based on the closure of adenosine-5'-triphosphate (ATP)-sensitive potassium channels (KATP-channels) in the beta-cells of the pancreas. Interestingly, the cardiovascular system also shares these KATP-channels. The open state probability of these channels is regulated by the intracellular concentration of ATP. During ischaemia, the KATP-channels are thought to open by a fall in the cytosolic ATP concentration. The increase in the extracellular adenosine concentration, and the release of endothelium-derived hyperpolarizing factor (EDHF) during ischaemia may further contribute to the opening of cardiovascular KATP-channels. Sulphonylurea derivatives like glibenclamide and tolbutamide have been reported to block the opening of KATP-channels in several types of tissues including myocardial and vascular smooth muscle cells. Since the opening of KATP-channels is regarded as an endogenous cardioprotective mechanism, the blocking effect of sulphonylurea derivatives in the cardiovascular system may have deleterious effects. Human studies on this issue have just been initiated, and preliminary results point towards a significant interaction between glibenclamide and cardiovascular KATP-channels at clinically relevant concentrations. In this regard, the introduction of more pancreas specific sulphonylurea derivatives like glimepiride, which do not interact with cardiovascular KATP-channels, is a promising development.

Preserved vasodilator response to adenosine in insulin-dependent diabetes mellitus

Experimental data derived from animal models suggest that the endogenous nucleoside adenosine has important cardioprotective properties. The potent vasodilator effects of adenosine may contribute to this cardioprotection as ischaemia-induced release of endogenous adenosine has been suggested to adjust local blood flow to the metabolic demands of the tissue. Interestingly, the vascular effects of adenosine appeared to be impaired in animal models for diabetes mellitus. This observation may be of importance with respect to the increased cardiovascular mortality in diabetes. Therefore, the authors investigated the in vivo vasodilator effects of adenosine in insulin-dependent diabetic patients. In 12 uncomplicated insulin-dependent male diabetic patients and 12 healthy male age-matched subjects, the brachial artery was cannulated for infusion of adenosine (0 center dot 15, 0 center dot 5, 5, 15 and 50 mu g 100(-1) mL min-1) and for measurement of mean arterial pressure (MAP). Forearm blood flow (FBF) was measured by venous occlusion mercury-in-silastic strain gauge plethysmography. Maximal vasodilatation was assessed by standardized post occlusive reactive hyperaemia (PORH). Baseline forearm blood flow was 2 center dot 7 +/- 0 center dot 4 and 1 center dot 8 +/- 0 center dot 2 mL 100(-1) mL min-1 for the diabetic patients and control group respectively. In the diabetic patients, adenosine infusion raised forearm blood flow to 2 center dot 4 +/- 0 center dot 4, 2 center dot 6 +/- 0 center dot 4, 4 center dot 4 +/- 0 center dot 7, 6 center dot 3 +/- 1 center dot 0, 9 center dot 8 +/- 1 center dot 5 and 14 center dot 2 +/- 2 center dot mL 100(-1) mL min-1 for the respective dosages. In the control group these values were 1 center dot 7 +/- 0 center dot 2, 1 center dot 9 +/- 0 center dot 3, 3 center dot 2 +/- 0 center dot 8, 6 center dot 0 +/- 1 center dot 2, 10 center dot 9 +/- 2 center dot 1 and 17 center dot +/- 3 center dot 4 mL 100(-1) mL min-1 respectively (P > 0 center dot 1 for between group comparison). Forearm blood flow at the contralateral side was not significantly affected by the placebo and adenosine infusions. Similar results were obtained when results were expressed as changes in forearm vascular resistance or forearm blood flow ration (FBF infused arm/FBF control arm). Maximal vasodilatation did not differ between the two groups. The authors conclude that the forearm vasodilator response to adenosine is preserved in uncomplicated insulin-dependent diabetic patients. This observation argues against a primary role of a reduced adenosine responsiveness in the cardiovascular sequelae of diabetes.

Effects of insulin on vascular tone and sympathetic nervous system in NIDDM

Chronic activation of the sympathetic nervous system may be a pathogenetic mechanism by which hyperinsulinemia induces cardiovascular damage in insulin-resistant NIDDM patients. The influence of physiological hyperinsulinemia (approximately 700 pmol/l) on basal and stimulated sympathetic outflow was studied in 12 lean normotensive subjects with well-controlled NIDDM without complications and in 13 matched control subjects. Forearm blood flow (FBF) was measured with forearm plethysmography; sympathetic nervous system activity was assessed by the [3H]norepinephrine (NE) tracer method. NIDDM patients were insulin resistant (glucose infusion rates 31.8 +/- 3.8 vs. 48.7 +/- 2.0 mumol.kg-1.min-1 in control subjects, P < 0.01). After a mixed meal, NIDDM patients showed a hyperinsulinemic response (2-h insulin levels: NIDDM patients 324 +/- 34 pmol/l, control subjects 165 +/- 19 pmol/l, P < 0.001). Insulin infusion induced a vasodilator response (not significantly different between the groups). Arterial plasma NE levels and total-body NE spillover increased significantly (total spillover in NIDDM patients from 0.77 +/- 0.09 to 1.18 +/- 0.16 nmol.m-2.min-1, in control subjects from 0.98 +/- 0.14 to 1.23 +/- 0.18 nmol.m-2.min-1, P < 0.01 for all, not different between groups). Total-body NE clearance did not change. Sympathetic stimulation (lower-body negative pressure [LBNP] 15 mmHg) induced forearm vasoconstriction and increased arterial and venous plasma NE and total NE spillover. Responses of FBF and NE kinetics to LBNP were not significantly different between groups and were not altered by hyperinsulinemia. Although these nonobese subjects with uncomplicated NIDDM showed postprandial hyperinsulinemia and resistance to the effect of insulin on glucose metabolism, this group was not resistant to the vasodilator and sympathetic stimulant effects of insulin. Responses to sympathetic stimuli (LBNP) were normal and unaffected by physiological hyperinsulinemia. Therefore, because of daily life hyperinsulinemia, chronic sympathetic stimulation could be operative in these patients and may explain the increased incidence of hypertension and/or cardiovascular complications.

The influence of ulnar nerve blockade on skin microvascular blood flow

Microvascular research is seriously hampered by the great temporal and spatial variability of the measured skin blood flow and variation in sympathetic vasomotor reflexes within and between persons. Therefore skin vasomotor reflexes were studied before and after ulnar nerve blockade within the same person, resulting in a temporal complete denervation of the fifth finger and partial denervation of the fourth finger. Skin temperature and laser Doppler flux (LDF) were registrated to measure predominantly arteriovenous shuntflow. Measurements were performed on the palmar tip of the second and fifth finger in nine healthy volunteers, at baseline, and during a sympathetic reflex test (i.e. inspiratory gasp) and postural response test. Beat-to-beat digital blood pressure was recorded from the third and fourth finger by a Finapres device. Baseline capillary blood cell velocity (CBV) was measured at the nailfold of the second and the fifth finger. After ulnar blockade baseline skin temperature, LDF and CBV increased significantly, with respectively (mean +/- SE) 3.2 +/- 0.9 degrees C, 20.9 +/- 5.9 relative perfusion units and 0.79 +/- 0.40 mm-1 s. The percentage LDF decrease of the fifth finger during inspiratory gasp was 48.2 +/- 5.3% before and 3.1 +/- 0.9% after blockade. The postural response test showed a decrease in LDF of the fifth finger with no significant difference before and after blockade, respectively 12.3 +/- 14.7% and 8.0 +/- 2.7%, while no difference was found in the increase in digital blood pressure in the denervated fourth finger compared to both the same finger before blockade and to the third non-blocked finger.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of dexfenfluramine on eating habits in a Dutch ambulatory android overweight population with an overconsumption of snacks

OBJECTIVE

To investigate the effect of the serotonin receptor agonist dexfenfluramine on eating habits and weight loss in ambulatory, android type, moderately obese patients with an overconsumption of snacks.

DESIGN

9 week, randomized, double-blind treatment with either dexfenfluramine (30 mg/day) or placebo, without dietary intervention.

SETTING

Outpatient clinics of four University Hospitals in The Netherlands.

SUBJECTS

112 healthy obese subjects, body mass index 28-35 kg/m2, waist-to-hip ratio > or = 1.0 for men and > or = 0.8 for women, consuming more than five snacks containing in total more than 500 kcal/day and/or more than 25% of total calorie intake in the form of snacks.

MAIN OUTCOME MEASURES

Changes in macronutrient composition of the diet, food intake (total, at principal meals and in between meals in the form of snacks) and weight loss.

RESULTS

104 subjects were included for efficacy analysis. In both the dexfenfluramine group (n = 51) and the placebo group (n = 53) the total energy intake decreased. The decrease during main meals was significantly greater in the dexfenfluramine group for all parameters tested except for simple carbohydrates. In between meals the decrease in intake was significantly greater in the dexfenfluramine group for total energy intake (P < 0.05) and intake in unsaturated fat (P < 0.05). The reported reduction in total food intake in the dexfenfluramine group was similarly due to reductions in carbohydrate and fat intake. No weight loss was seen in the placebo group. The treated group lost 3.1 +/- 0.2 kg (P < 0.01).

CONCLUSION

Dexfenfluramine effectively reduces the intake of carbohydrates as well as fat in ambulatory, non diet restricted android obese subjects by reducing food intake during and in between main meals.

Effect of long-term angiotensin-converting enzyme inhibition on endothelial function in patients with the insulin-resistance syndrome

Cardiovascular risk factors such as hypertension, diabetes, and dyslipemia are associated with an impaired endothelium-dependent vasodilation. In patients with type 2 diabetes mellitus, these risk factors are frequently clustered. We investigated whether long-term treatment with the angiotensin-converting enzyme (ACE) inhibitor perindopril can improve endothelium-dependent vasodilation in this particular group of patients. We selected 10 patients with type 2 diabetes and hypertension (age 59.4 +/- 3.2 years, body mass-index 29.7 +/- 1.5 kg.m-2, blood pressure 169 +/- 6/92 +/- 1 mm Hg, total cholesterol 6.6 +/- 0.3 mM). Using venous occlusion plethysmography, we recorded the increases in forearm blood flow (FBF) in response to three vasodilator stimuli: (a) 5 min of forearm ischemia, (b) infusion of the endothelium-dependent vasodilator methacholine (Mch) into the brachial artery (0.03, 0.3, and 1.0 micrograms/min/100 ml), and (c) intraarterial infusion of the endothelium-independent vasodilator sodium nitroprusside (SNP 0.06, 0.2, 0.6 microgram/min/100 ml). This procedure was repeated after 6 months of treatment with perindopril 4-8 mg/day. Forearm vascular resistance (FVR) was calculated by the quotient of the mean arterial pressure (MAP) and the FBF. Perindopril reduced blood pressure (BP) by 19/10 mm Hg (p < 0.05) and increased baseline FVR, but improved neither the maximal percentage decrease in vascular resistance induced by Mch (from -80 +/- 2 to -82 +/- 2%) nor that induced by SNP (from -73 +/- 3 to -72 +/- 3%). Perindopril decreased the FVR reached after the ischemic stimulus from 6.5 +/- 1.2 to 4.8 +/- 0.6 U (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)