Thyroid hormone is a major regulator of thermogenesis, acting both in peripheral organs and on central autonomic pathways. Mice heterozygous for a point mutation in thyroid hormone receptor α1 display increased thermogenesis as a consequence of high sympathetic brown fat stimulation. Surprisingly, despite the hypermetabolism, their body temperature is not elevated. Here we show, using isolated tail arteries, that defective thyroid hormone receptor α1 signaling impairs acetylcholine-mediated vascular relaxation as well as phenylephrine-induced vasoconstriction. Using infrared thermography on conscious animals, we demonstrate that these defects severely interfere with appropriate peripheral heat conservation and dissipation, which in turn leads to compensatory alterations in brown fat activity. Consequently, when the vasoconstrictive defect in mice heterozygous for a point mutation in thyroid hormone receptor α1 was reversed with the selective α1-adrenergic agonist midodrine, the inappropriate heat loss over their tail surface was reduced, normalizing brown fat activity and energy expenditure. Our analyses demonstrate that thyroid hormone plays a key role in vascular heat conservation and dissipation processes, adding a unique aspect to its well-documented functions in thermoregulation. The data thus facilitate understanding of temperature hypersensitivity in patients with thyroid disorders. Moreover, the previously unrecognized connection between cardiovascular regulation and metabolic activity revealed in this study challenges the interpretation of several experimental paradigms and questions some of the currently derived hypotheses on the role of thyroid hormone in thermogenesis.adipose tissue | tail temperature T hyroid hormone affects energy metabolism, body temperature, and cardiovascular function (1, 2). This is evident in hypo-and hyperthyroid patients, who display metabolic and cardiovascular problems as well as inadequacies in heat and cold tolerance (3, 4). The latter effects are thought to be a consequence of a shift in obligatory thermogenesis, because thyroid hormone affects basal metabolic rate through the regulation of genes controlling cellular metabolism and mitochondrial function. In addition, facultative thermogenesis, brought about by shivering and brown adipose tissue (BAT) activation (5, 6), is also modulated by thyroid hormone. The recent discovery of BAT in adult humans, where previously thought to exist only in rodents and neonates (7), suggests that the role of BAT in thermoregulation and energy expenditure may be underestimated. Recently, a new role of thyroid hormone in facultative thermogenesis has emerged, controlling BAT via the central nervous system (8, 9). The importance of central thyroid hormone receptor α1 (TRα1) signaling for BAT activity was further supported by findings in mice heterozygous for a R384C mutation in TRα1 (TRα1+m), which display a strong hypermetabolism as the result of a central overactivation of BAT (10). However, the finding was unexpected, as the specific ...

Heptanol, 18α‐glycyrrhetinic acid (18αGA) and 18β‐glycyrrhetinic acid (18βGA) are known blockers of gap junctions, and are often used in vascular studies. However, actions unrelated to gap junction block have been repeatedly suggested in the literature for these compounds. We report here the findings from a comprehensive study of these compounds in the arterial wall. Rat isolated mesenteric small arteries were studied with respect to isometric tension (myography), [Ca2+]i (Ca2+‐sensitive dyes), membrane potential and – as a measure of intercellular coupling – input resistance (sharp intracellular glass electrodes). Also, membrane currents (patch‐clamp) were measured in isolated smooth muscle cells (SMCs). Confocal imaging was used for visualisation of [Ca2+]i events in single SMCs in the arterial wall. Heptanol (150 μM) activated potassium currents, hyperpolarised the membrane, inhibited the Ca2+ current, and reduced [Ca2+]i and tension, but had little effect on input resistance. Only at concentrations above 200 μM did heptanol elevate input resistance, desynchronise SMCs and abolish vasomotion. 18βGA (30 μM) not only increased input resistance and desynchronised SMCs but also had nonjunctional effects on membrane currents. 18αGA (100 μM) had no significant effects on tension, [Ca2+]i, total membrane current and synchronisation in vascular smooth muscle. We conclude that in mesenteric small arteries, heptanol and 18βGA have important nonjunctional effects at concentrations where they have little or no effect on intercellular communication. Thus, the effects of heptanol and 18βGA on vascular function cannot be interpreted as being caused only by effects on gap junctions. 18αGA apparently does not block communication between SMCs in these arteries, although an effect on myoendothelial gap junctions cannot be excluded. British Journal of Pharmacology (2004) 142, 961–972. doi:

Synthetic peptides homologous to the extracellular loops of the major vascular connexins represent a novel class of gap junction blockers that have been used to assess the role of direct cellular communication in arteries and veins. However, the specificity of action of such peptides on the coupling between smooth muscle cells (SMCs) has not yet been fully characterized. Isolated third-order rat mesenteric arteries were therefore studied with respect to isometric tension (myography), intracellular Ca2+ concentration ([Ca2+]i) (Ca2+ -sensitive dyes), membrane potential, and input resistance (sharp intracellular glass electrodes). Confocal imaging was used for visualization of [Ca2+]i events in individual SMCs in the arterial wall and membrane currents (patch clamp) measured in individual SMCs isolated from the same arteries. A triple peptide combination (37,43Gap 27 + 40Gap 27 + 43Gap 26) increased intercellular resistance (measured as input resistance) in intact arterial segments without affecting the membrane conductance of individual cells and also interrupted electrical coupling between pairs of rat aortic A7r5 myocytes. In intact arterial segments, the peptides desynchronized [Ca2+]i transients in individual SMCs and abolished vasomotion without suppressing Ca2+ transients in individual cells. They also depolarized SMCs, increased [Ca2+]i, and attenuated acetylcholine-induced, endothelium-dependent smooth muscle hyperpolarization. Experiments with endothelium-denuded arteries suggested that the depolarization produced by the peptides under basal conditions was in part secondary to electrical uncoupling of the endothelium from SMCs with loss of a tonic hyperpolarizing effect of the endothelium. Taken together, the results indicate that connexin-mimetic peptides block electrical signaling in rat mesenteric small arteries without exerting major nonjunctional effects.

Objective: We tested the hypothesis that cGMP can induce a state of only partial coordination of vascular smooth muscle cells (VSMC). Methods: This was done by studying the concentration-dependent effect of 8Br-cGMP on isometric and isobaric force development of noradrenaline-activated segments of rat mesenteric small arteries in which the endothelium was removed. We further measured the concentration-dependent effect of 8Br-cGMP on VSMC membrane potential, spatially resolved [Ca²⁺]_i and VSMC membrane conductance. Results: With 300 µM 8Br-cGMP, coordinated [Ca²⁺]_i activity and vasomotion were seen as previously reported. At 10–30 µM 8Br-cGMP, beating isometric tension oscillations were seen. Isobaric recordings revealed oscillations with different frequencies in different parts of the arteries. At these (10–30 µM) 8Br-cGMP concentrations, membrane potential oscillations did not always concur with isometric tension oscillations, and [Ca²⁺]_i oscillations were only synchronized locally within groups of cells. 8Br-cGMP concentration-dependently decreased the frequency of vasomotion and, in unsynchronized hyperpolarized VSMC, the frequency of [Ca²⁺]_i waves. Conclusion: Our results demonstrated that cGMP can cause a partial coordination of the VSMC in the vascular wall (and at high concentrations near complete coordination). Furthermore, the cGMP concentration-dependent decrease of Ca²⁺ wave frequency and of vasomotion frequency suggests that cGMP modifies oscillatory Ca²⁺ release from the sarcoplasmic reticulum and supports the suggestion that this oscillatory release paces vasomotion.

These findings suggest that NO contributes significantly to the "EDHF-like" response seen in rat small arteries and that the source of this NO may be preformed vascular stores.