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Departments of Internal Medicine and Cardiovascular Sciences (R.N., V.G., V.A., E.Z., C.D., M.M., U.O., L.S.) and Clinical Pathology (D.T., V.M.), University Federico II, 80131 Naples, Italy
Address all correspondence and requests for reprints to: Dr. R. Napoli, Department of Internal Medicine, Via Pansini, 80131 Napoli, Italy. E-mail: napoli{at}unina.it.
| Abstract |
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Objective: Our objective was to clarify whether acute changes in serum T3 concentration affect endothelial function.
Design, Setting, and Subjects: Ten healthy subjects (age, 24 ± 1 yr) participated in a double-blind, placebo-controlled trial at a university hospital.
Interventions: T3 (or placebo) was infused for 7 h into the brachial artery to raise local T3 to levels observed in moderate hyperthyroidism. Vascular reactivity was tested by intraarterial infusion of vasoactive agents.
Main Outcome Measures: We assessed changes in forearm blood flow (FBF) measured by plethysmography.
Results: FBF response to the endothelium-dependent vasodilator acetylcholine was enhanced by T3 (P = 0.002 for the interaction between T3 and acetylcholine). The slopes of the dose-response curves were 0.41 ± 0.06 and 0.23 ± 0.04 ml/dl·min/µg in the T3 and placebo study, respectively (P = 0.03). T3 infusion had no effect on the FBF response to sodium nitroprusside. T3 potentiated the vasoconstrictor response to norepinephrine (P = 0.006 for the interaction). Also, the slopes of the dose-response curves were affected by T3 (1.95 ± 0.77 and 3.83 ± 0.35 ml/dl·min/mg in the placebo and T3 study, respectively; P < 0.05). The increase in basal FBF induced by T3 was inhibited by NG-monomethyl-L-arginine.
Conclusions: T3 exerts direct and acute effects on the resistance vessels by enhancing endothelial function and norepinephrine-induced vasoconstriction. The data may help clarify the vascular impact of the low T3 syndrome and point to potential therapeutic strategies.
| Introduction |
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A low-T3 syndrome may occur in acute diseases and is characterized by depressed cardiac function and elevated SVR (7, 8, 9, 10, 11). It was hypothesized that acute correction of the low T3 levels might be beneficial to the cardiovascular hemodynamics (12, 13, 14, 15, 16, 17, 18). However, the precise link between T3 and the altered hemodynamics remains elusive because the effects of acute elevations in serum T3 concentration on human vascular physiology have never been explored in depth. In particular, there is no information as to whether acute changes in circulating T3 are able to exert any effect on endothelial function and the vascular sensitivity to vasoactive agents. The present study was thus designed to address this specific question.
| Subjects and Methods |
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The study was performed on 10 healthy volunteers, recruited from the medical population of the medical school (six male, four female, 23.9 ± 0.6 yr of age; body mass index, 23.6 ± 1.6 kg/m2; arterial blood pressure, 119 ± 6/74 ± 5 mm Hg). Informed consent was obtained from all subjects, and the study protocol was approved by the Ethics Committee of the Federico II University School of Medicine.
Experimental procedures
The study design was double blinded and placebo controlled, with each subject serving as his/her own control. The test (T3) and control study were performed in random order and separated by a 3- to 4-wk interval. In the test study, T3 was infused into the brachial artery and forearm hemodynamics were monitored for 240 min. Then the responses of forearm blood flow (FBF) to vasoactive test substances were measured while keeping the infusion of T3 going. In the control study, the protocol was the same except that placebo was infused instead of T3. The dose of T3 was 40 ng/min. This dose was chosen in pilot experiments as the one able to raise free T3 (fT3) concentration in the forearm circulation to moderate hyperthyroidism values. T3 vials for human use (Thyrotardin) were provided by Henning GMBH, Sanofi-Synthelabo, Berlin, Germany.
All the experiments were performed in the morning in a quiet room kept at 2224 C. A plastic cannula (20 gauge) was inserted into the brachial artery of the nondominant arm under local anesthesia and used for the infusion of the test substances and the monitoring of arterial blood pressure. Systolic and diastolic blood pressure and heart rate were recorded by a transducer connected to the arterial cannula. FBF was measured by a strain-gauge plethysmograph (Hokanson 045 EC4; PMS Instruments, Maidenhead, UK). The data were monitored continuously with McLab software. Additional details of the procedure have been previously published (3, 19). Each subject underwent the following stepwise infusions into the brachial artery in this order: 1) acetylcholine (Ach) infused at a rate of 15, 30, and 45 µg per liter of forearm per min (µg/liter·min) to assess endothelium-mediated vasodilation; 2) sodium nitroprusside (NP), a direct nitric oxide donor, infused at the rate of 1, 3, and 9 µg/liter·min to assess non-endothelium-mediated vasodilation; 3) norepinephrine infused at a rate of 140, 280, and 560 µg/liter·min to assess the vascular sensitivity to sympathetic stimulation; and 4) NG-monomethyl-L-arginine (L-NMMA), a competitive inactive analog of L-arginine, infused at the rate of 1 mg/liter·min to assess the role of endothelial NO release in the maintenance of the basal vascular tone. Test substances were infused in the same order in all the subjects and the infusion of each substance started only when the effect on FBF of the previous infusion was dissolved and baseline FBF restored. Each dose of the test substances was infused for 5.5 min, and FBF was measured during the last 1.5 min of infusion. At least 30 min of washout time was allowed between each substance. The infusion rates were adjusted according to the forearm volume of each subject, measured by water displacement. FBF was measured simultaneously in both arms to ensure that no systemic effects occurred during the experiment. Each FBF value represents the mean of six consecutive measurements performed at 10-sec intervals.
Assays and calculations
T3 plasma level was measured by RIA. Forearm vascular resistance (FVR) was measured as the ratio of mean arterial blood pressure to FBF. Comparison between placebo and T3 was performed by a two-way ANOVA for repeated measures (version 12.0; SPSS Inc., Chicago, IL). Comparison between the slopes of the dose-response curves was performed by paired t test. Results are expressed as the mean ± SEM.
| Results |
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FBF was very similar in the T3 and placebo studies in the basal state (Table 1
). During intraarterial T3 infusion, FBF increased progressively and reached values that were significantly higher than those in the placebo study at 120 min and thereafter. This was due to a significant fall in SVR, because arterial blood pressure remained totally unchanged.
Figure 1
depicts the dose-response curve to the endothelium-dependent vasodilator Ach. The increment of FBF observed in the T3 study was much more pronounced as compared with placebo (P = 0.014 for the treatment effect by ANOVA for repeated measures). The values reached at the highest Ach level were 17.5 ± 2.7 and 10.2 ± 1.7 ml/100 m·min in the test and placebo study, respectively (P = 0.04). The dynamics of the response was also profoundly affected by T3, as evidenced by the significant interaction between T3 and Ach (P = 0.002) and the markedly steeper slope of the T3 dose response compared with that in the placebo study (0.41 ± 0.06 and 0.23 ± 0.04 ml/dl·min/µg, respectively, P = 0.03).
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The intrabrachial T3 infusion did not affect systemic hemodynamics, as demonstrated by the unchanged heart rate and arterial blood pressure. In addition, FBF and FVR in the contralateral arm did not change from their basal values throughout the study period.
| Discussion |
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Previous experiments using mesenteric resistance vessels showed that the vasorelaxing effect of TH was impaired by pretreatment with an arginine analog that prevented endothelial nitric oxide production (20). Similarly, in another study on renal artery rings, endothelium-dependent relaxation to Ach was enhanced 36 h after T3 treatment (21). The present data are consistent with those in vitro observations regarding the role played by the endothelium in mediating the vascular effects of TH. At variance with these data, other studies have supported a role for VSMCs in mediating the vasodilating effect of T3. For instance, the vasorelaxing effect of T3 in isolated resistance vessels was attenuated but not abolished by endothelial denudation of the arteries (22), suggesting activation by T3 of both the endothelial and the nonendothelial component of vasodilation. In another study performed in isolated VSMCs, it was reported that TH was able to cause vasorelaxation (23). The reasons for the difference between our findings and some of the previous studies indicating also a role of VSMC is not clearly apparent and may entail differences among species and in vitro vs. in vivo data. However, taking the data altogether, one thing that emerges clearly is that whenever the vascular effects of TH were tested in human models (normal subjects, hyperthyroidism, overt hypothyroidism, or subclinical hypothyroidism), the changes in vascular reactivity always involved exclusively the endothelial component (3, 4, 6, 24). The current study documents the ability of acute T3 increments to enhance the activity of endothelial NO synthase, as evidenced by the attenuation of FBF after exposure to the arginine antagonistic agent L-NMMA. This finding is consistent with previous studies in chronically hyperthyroid patients as well as experimental studies in various animal models (3, 25).
T3 mainly acts by binding to specific nuclear receptors, so regulating transcription of target genes. In addition, T3 may also activate extranuclear, nongenomic mechanisms, by which it regulates the activity of plasma membrane ion channels. Examples are provided by the well-known T3 effects to enhance myocardial contractility, to regulate the contractile state of VSMCs, and to stimulate glucose uptake (26, 27, 28). The current study was not intended to clarify whether T3 affects vascular function through genomic or extranuclear mechanisms. However, the examination of the time course of FBF response to T3 infusion suggests that T3 acted through the activation of genomic mechanisms. This interpretation is also supported by the fact that by using the local infusion approach, the desired T3 concentration in the forearm is reached almost immediately, whereas the first demonstrable increment in FBF occurred after 2 h.
In a previous study in normal volunteers, T3 was able to reduce SVR as early as 3 min after the start of the infusion (29). This clearly speaks in favor of T3 ability to activate very rapidly nongenomic mechanisms. The data, however, are not in contrast with the present study, because we infused relatively low doses of T3, whereas in previous studies, T3 was used in big doses that raised the hormone concentration to more than 40 pg/ml (four times the values achieved in our study).
In conclusion, the present data demonstrate that T3 is a strong modulator of the resistance vessel response to vasoactive agents, as a consequence of an acute and direct interaction.
| Acknowledgments |
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| Footnotes |
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Disclosure Summary: All the authors have nothing to declare.
First Published Online October 17, 2006
Abbreviations: Ach, Acetylcholine; FBF, forearm blood flow; fT3, free T3; FVR, forearm vascular resistance; L-NMMA, NG-monomethyl-L-arginine; NP, nitroprusside; SVR, systemic vascular resistance; TH, thyroid hormone; VSMC, vascular smooth muscle cell.
Received July 17, 2006.
Accepted October 10, 2006.
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