In vivo studies on local release of tissue-type plasminogen activator from vascular endothelium

Abstract

Tissue-type plasminogen activator (t-PA) is synthesized and released from the vascular endothelium. The regulation of the secretion of t-PA in man is not fully elucidated. The aim of the present studies was to, by use of a perfused-forearm model, characterize basal and stimulated release of t-PA in man.Catheters were introduced in the brachial artery and a deep antecubital vein of the non-dominant arm. Blood samples were obtained simultaneously from artery and vein under basal and stimulated conditions. Forearm blood flow was measured by occlusion plethysmography. Net release of endothelial proteins were defined as the product of the arterio-venous concentration gradient of the respective protein and local plasma flow. The endothelium was stimulated by mental stress and intraarterial infusions of methacholine (Mch) or desmopressin (DDAVP). Since these stimuli also are endothelium-dependent vasodilators, sodium nitroprusside (SNP) was used for reference.During basal conditions there was a net release of t-PA of 5-15 ng per min and L tissue from the forearm vascular bed. General sympathetic stimulation by mental stress elicited a 2-fold increase in local release rates of t-PA, whereas a 10-fold increase in forearm t-PA release was induced by intraarterial infusion of Mch. The release pattern was biphasic in response to both mental stress and Mch. Repeat stimulation by Mch induced a tachyphylactoid t-PA response. DDAVP induced a dramatic (25-fold) increase in t-PA release rates, but had no effect on the release of von Willebrand Factor. The response to DDAVP was monophasic and not subjected to tachyphylaxis on repeat stimulation. The massive amount of t-PA that was released by two DDAVP infusions show that the endothelial pool of t-PA is very large. Inter-individual variations in basal release rates were associated with an insertion/deletion polymorphism in the t-PA gene locus. In borderline hypertensive subjects, there was an enhanced basal release of t-PA as demonstrated by forearm-perfusion experiments and a venous occlusion test. In addition, borderline hypertensives showed a basal forearm net release of PAI-1, that was not observed in the normotensive control group. Plasma t-PA was directly correlated to blood pressure, whereas PAI-1 was determined both by blood pressure and metabolic factors. There were no differences in Mch-induced t-PA release or endothelium-dependent vasodilation between the groups. Endothelium-independent vasodilation had no effect on t-PA in healthy subjects, whereas borderline hypertensives responded with t-PA releaseIt is concluded that the human vascular endothelium has a high capacity for regulated release of t-PA in vivo. DDAVP is a very powerful local stimulator of t-PA release. Since the demonstrated release patterns are not uniform, more than one intracellular pathway may be involved in the regulated release of t-PA. The basal secretion rate of t-PA from the endothelium is influenced by a variation in the t-PA gene. The perfused-forearm model is well suited for further studies on the regulation of endothelial t-PA release in vivo.