Ulrika Wall

Regulation of local availability of active tissue‐type plasminogen activator in vivo in man

Summary.  Free, biologically active tissue‐type plasminogen activator (tPA) is the main initiator of intravascular fibrinolysis, but little is known about the regulation of active tPA on the organ level. The aim was to investigate if the local availability of active tPA on the organ level depends on the local release rate of tPA or the arterial input of tPA and plasminogen activator inhibitor type 1 (PAI‐1). Also, we wanted to evaluate if plasma levels predict capacity for endothelial release of fibrinolytic proteins. Invasive perfused‐forearm studies were performed in 96 healthy subjects. Local release rates of fibrinolytic proteins were assessed at baseline and during endothelial stimulation. Stimulation by methacholine and desmopressin induced a 6‐ and 12‐fold increase in total tPA release rates, respectively. With increasing local release rates of tPA a gradually closer correlation emerged between the total tPA secretion and the forearm output of active tPA (from r = 0.102, ns to r = 0.85, P < 0.0001). Forearm availability of active tPA was not related to arterial input of either tPA or PAI‐1. Release rates and plasma levels of tPA were not correlated. Baseline release rates of active tPA increased to noon. The major determinant for the local availability of active tPA is the capacity of the endothelium to release tPA rather than the arterial input of PAI‐1 or tPA. Despite a molar excess of PAI‐1, the majority of tPA released during stimulation does not undergo local inactivation. The capacity to release tPA locally cannot be predicted from its plasma concentration.

Gene Polymorphism of t-PA is Associated With Forearm Vascular Release Rate of t-PA

We have observed marked interindividual differences in release rates of tissue-type plasminogen activator (t-PA) among healthy subjects. The objective of the current study was to test the hypothesis that there is an association between a genetic variation at the t-PA locus and the in vivo release rate of t-PA. Fifty-one healthy males were studied at rest in the morning and 27 of these were also subjected to a mental stress test. Net release rates of total t-PA across the forearm vascular bed were calculated as the product of the venoarterial concentration gradient and forearm plasma flow. Zygosity for an Alu-repeat polymorphism in intron 8 of the t-PA gene was determined by a polymerase chain reaction. Basal t-PA release rates differed markedly by genotype (ANOVA, P<0.05); subjects homozygous for the insertion had a significantly higher release rate (mean 10.9 ng. min-1. L-1, n=19) than both heterozygotes (4.5 ng. min-1. L-1, n=26) and subjects homozygous for the deletion (0.9 ng. min-1. L-1, n=6). After 2 minutes of mental stress release rates had increased approximately 2-fold in all groups. Arterial and venous plasma levels of t-PA were unrelated to genotype. In conclusion, the current results provide the first evidence of an association between a common genetic variation at the t-PA locus and interindividual differences in net release rates of t-PA in vivo. The relationship is not reflected by circulating steady-state plasma levels and can thus not be disclosed by conventional venous plasma sampling.

Endothelium-Dependent Vasodilation and Tissue-Type Plasminogen Activator Release in Borderline Hypertension

We recently showed that muscarinic receptor stimulation causes a marked increase in the net release of tissue-type plasminogen activator (TPA) antigen and activity across the human forearm in vivo, in conjunction with endothelium-dependent vasodilation. Because hypertension has been associated with endothelial dysfunction, the aim of the study was to compare forearm TPA release and vasodilation in response to muscarinic stimulation in normotensive (NC) and borderline hypertensive (BH) subjects. The study was performed in 10 apparently healthy young men with BH and 10 male NC subjects. Methacholine (MCh: 0.1, 0.8, and 4.0 micrograms/min) and sodium mitroprusside (SNP: 0.5, 2.5, and 10 micrograms/min) were administered in randomized order as double-blind, stepwise, intrabrachial artery infusions. Forearm blood flow was assessed by plethysmography. Net release/uptake was calculated as the product of the arteriovenous concentration gradient and forearm plasma flow. Vasodilator responses to MCh were similar in both groups (P = NS), whereas the decrease in forearm vascular resistance in response to SNP was somewhat less in BH subjects (P = .005). At rest, both groups showed a significant arteriovenous gradient and net release of TPA antigen across the forearm (P < .05 throughout). However, in contrast to the significant net increment in TPA activity across the forearm in the NC group (P < .018), BH subjects had no basal forearm increment in TPA activity (NC vs BH, P = .006). Arterial and venous plasma levels of plasminogen activator inhibitor 1 (PAI-1) antigen and activity were higher in BH subjects (P < or = .05 throughout), who in contrast to NC subjects, also had a significant forearm net release of PAI-1 antigen (P = .006). Across the whole group, there was a significant inverse relation between arterial PAI-1 antigen levels and increment in TPA activity across the forearm (r = -.57, P = .008) but no relation to TPA antigen release. In response to MCh infusion, both the net release of TPA antigen and increment in TPA activity increased markedly and to similar extents in both groups (P < .01 throughout). SNP infusion had no effect on either TPA antigen release or increment in TPA activity in the NC group but elicited a significant net release of TPA antigen and increase in TPA activity in the BH group (P < .05). Both circulating levels and local release of PAI-1 antigen were significantly correlated to fasting plasma insulin. Endothelium-dependent vasodilation and endothelial TPA release in response to muscarinic receptor stimulation were preserved in BH subjects. At rest, BH subjects had higher circulating PAI-1 antigen levels and a corresponding decrease in circulating levels and local increment of TPA activity. In contrast to NC subjects, BH subjects responded with a TPA release also in response to increased flow, which may indicate an enhanced endothelial cell responsiveness to fluid shear stress.

Enhanced Levels of Tissue-Type Plasminogen Activator in Borderline Hypertension

Despite effective antihypertensive therapy, essential hypertension is still associated with considerable residual risk of cardiovascular complications. The aim of the present study was to investigate the state of the endogenous fibrinolytic system in young subjects with borderline hypertension. Thirty-nine young (age, 24 to 34 years) male subjects with borderline hypertension (systolic BP [SBP] 140 to 160 mm Hg and/or diastolic BP [DBP] 85 to 95 mm Hg) and 17 normotensive control subjects (age, 22 to 31 years; SBP 110 to 130 and DBP 60 to 80 mm Hg) were recruited from a population screening. Plasma levels of tissue-type plasminogen activator (t-PA) antigen and activity and plasminogen activator inhibitor 1 (PAI-1) antigen were determined at rest and in response to a venous occlusion test. Borderline-hypertensive subjects had metabolic and anthropometric characteristics similar to normotensive individuals. In comparison with normotensive subjects, borderline-hypertensive subjects had higher plasma concentration of t-PA antigen both at rest and after venous occlusion but similar levels of t-PA activity or PAI-1 antigen. The increase in t-PA antigen and activity in response to venous occlusion was significantly greater in borderline-hypertensive subjects than in normotensive control subjects (P < .0001 and P = .003, respectively). In stepwise regression analyses, 24-hour mean arterial pressure emerged as the single most powerful predictor of t-PA antigen levels, but body mass index was the most important determinant of t-PA activity and PAI-1 antigen. However, PAI-1 was explained by both body mass index (partial r = .48, P < .001) and 24-hour mean arterial pressure (partial r = .29, P < .05). Thus, early hypertension may be associated with significant alterations in endogenous fibrinolysis.

High capacity for tissue-type plasminogen activator release from vascular endothelium in vivo.

Background Tissue-type plasminogen activator (t-PA) is the key mediator of intravascular fibrinolysis. We showed recently that local administrations both of methacholine (MCH) and of desmopressin (1-desamino-8-D-arginine vasopressin, DDAVP) induced a massive local release of t-PA in the human forearm vasculature. Objective To determine whether the human vascular endothelium could respond to repeated stimulation with the same agonist, and, if so, to further evaluate the releasable endothelial pool of t-PA. Method Seven young, healthy men participated. MCH and DDAVP were administered as local infusions into the brachial artery. In protocol 1 (n = 3) 2 μg/min MCH was infused for 2 (30 min with a free interval of 20 min. In protocol 2 (n = 4) 70 ng/min DDAVP was infused for 2 (20 min with an interval of 75 min. Dosages and time intervals were based on the different release profiles of the two drugs observed in previous studies. Brachial arterial and venous blood samples were obtained at baseline and throughout infusions. Net release of t-PA was calculated as the product of the arteriovenous concentration gradient and forearm plasma flow. Results Forearm blood flow was increased 3–4-fold by MCH and 2–3-fold by DDAVP infusions. During the first and second infusions of MCH, the average amounts of t-PA released were 1600 and 1000 ng/l forearm tissue, respectively. In contrast, DDAVP induced similar t-PA responses during both infusions; the average total releases of t-PA were 2300 and 2400 ng/l tissue, respectively. Conclusion The results show that the vascular endothelium is responsive to repeated stimulation both with MCH and with DDAVP. The diminished t-PA response to the second MCH infusion can not be explained in terms of depletion of intracellular pools, in view of the large amount of t-PA released by a single infusion of DDAVP. The dynamic pool of t-PA available for release is very large, but further studies are required in order to quantify the releasable endothelial stores.

Long-term stability of blood pressure and pressor reactivity to mental stress in borderline hypertension

Borderline hypertension is characterized by pressor hyperreactivity to mental stress. However, it has not been shown whether blood pressure hyperresponsiveness is a temporary phenomenon due to situational anxiety or a stable feature of the borderline hypertensive state. We therefore evaluated the long-term stability of invasively assessed blood pressure and central hemodynamic responses to mental stress in 10 young male subjects with borderline hypertension recruited from a population screening. Two identical 10-min mental arithmetic stress tests were performed 50 to 62 months apart (median, 4 years 8 months). Intraarterial blood pressure was monitored continuously before, during, and after stress. Cardiac output was measured by the indocyanine green dye dilution technique and indexed for body surface area. Total peripheral resistance index was computed from cardiac index and mean arterial pressure. During the 4-year follow-up period, none of the central hemodynamic parameters had changed significantly, either with respect to rest or stress levels. Test-retest variability of blood pressure measures was low, and errors of measurement ranged between 4.8 and 8.2 mm Hg for blood pressure levels at rest and during stress. Mental arithmetic induced highly significant blood pressure increments on both occasions (ANOVA, P < .0001 throughout). Pressor responses were somewhat but not significantly lower during the second test. Errors of measurement for absolute blood pressure reactivity ranged between 3.9 and 7.1 mm Hg. Intersession correlation coefficients for blood pressure levels attained during stress were above r = 0.75 throughout (P < or = .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired Glucose Tolerance at Five-year Follow-up of Young Men with Borderline Hypertension

BACKGROUND Recent studies suggest that patients with essential hypertension have impaired glucose tolerance and are hyperinsulinemic compared with normotensive subjects. The aims of the study were (1) to follow blood pressures of 56 young men with borderline hypertension for 5 years, (2) to investigate glucose tolerance in these subjects, and (3) to determine the relation of insulin/glucose metabolism to structural vascular changes and hemodynamic patterns in borderline hypertension. METHODS Thirty-nine young (age 22-34 years) male subjects with borderline hypertension (SBP 140-160 and or DBP 85-95 mmHg initially) and 17 normotensive control subjects (SBP 110-130 and DBP 60-80 mmHg) participated in the study. Blood pressure was measured, a standard oral glucose tolerance test (OGTT) was performed, and glucose, insulin and C-peptide were determined before and 30, 60, 90 and 120 minutes after a standard 75-g glucose load. Post-ischemic forearm vasodilatory responses were examined by plethysmography. RESULTS At follow-up, the borderline hypertensives had maintained significantly higher blood pressures than control subjects. Borderline hypertensives also had significantly impaired glucose tolerance compared to control subjects. The insulin response had a somewhat more sluggish descent, but did not differ significantly from the response of normotensives. The C-peptide response pattern resembled that of insulin, but C-peptide was significantly elevated after 120 min. On the whole group level, there were only weak relations of insulin to blood pressure. By contrast, fasting insulin and post-load insulin levels were strongly correlated with body mass index, the waist-hip circumference ratio, triglyceride, and both total and LDL cholesterol. Across the whole group, there were significant correlations between forearm minimal vascular resistance and fasting insulin (r = +0.37 p = 0.007) and insulin area-under-the-curve (r = +0.28 p = 0.044). However, Rmin was even more strongly correlated with body mass index, suggesting that this relationship was related to degree of obesity. CONCLUSION Borderline hypertension in young men is a persistent condition which is associated with impaired glucose tolerance without hyperinsulinemia. This finding suggests that impaired glucose tolerance might be a more primary phenomenon in early hypertension devoid of lipid metabolic aberrations.