Testosterone As a Modulator of Vascular Behavior
Written by Ben Bunting: BA, PGCert. (Sport & Exercise Nutrition) // British Army Physical Training Instructor // S&C Coach.
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Testosterone is the most important male hormone and is made mainly in the testes. It also is produced in small amounts by the adrenal glands.
It is a key hormone for sexual development, sperm production and maintenance of muscle and bone mass. It is controlled by the brain's hypothalamus and pituitary glands.
Endothelium-Independent Relaxation
The endothelium is a monolayer that covers the inner surface of blood vessels and regulates the smooth muscle cells that produce relaxation or contraction. It is a receptor-efector organ that works independently to produce, both agonistically and antagonistically, substances that keep homeostasis and maintain vasomotor balance.
The most important role of the endothelium is to modulate smooth muscle tone. This is accomplished by release of nitric oxide (NO) that diffuses across the endothelial intima and directly stimulates the guanylate cyclase (GCP) of the smooth muscle cell, releasing cGMP which regulates cytosolic Ca2+ levels. This mechanism of vascular control is also regulated by the production of prostaglandins and endothelium-derived hyperpolarizing factor (EDHF) that may affect the production of NO and EDHF and, thus, the guanylate cyclase activity and the release of cGMP in smooth muscle fibers.
Various polyphenols, including PJ- and PLs-derived polyphenols, have been shown to induce NO and EDHF-mediated relaxation of the endothelium. These effects are reported mainly in conduit arteries, such as the aorta, but there have been some reports that endothelial-independent relaxation can also occur in peripheral arteries.
In one study, the effects of PJ and PLs-derived polyphenols on the relaxation of aortic rings were investigated in both intact and endothelium-free arteries. The researher's found that PJ and PLs induced vasorelaxation of aortic rings with dose-dependent responses. The maximum relaxation was induced by 100 mg/L PJ powder and 1 g/L PLs powder.
We also studied the effect of 20(S)-PPD on the PE-induced contraction in endothelium-denuded aortic rings and found that PE induced transient contraction in both the intact and endothelium-free rings when 20(S)-PPD was pre-incubated. As illustrated in Figure 7, pre-incubation with 20(S)-PPD markedly decreased the ratio of PE-induced transient contraction to that elicited by the aortic rings in the presence and absence of the polyphenol.
In addition, 20(S)-PPD inhibited the opening of voltage-dependent Ca2+ channels and the receptor-operated Ca2+ channels in the aortic rings, which is consistent with its vasorelaxant action. However, 20(S)-PPD also attenuated b-adrenergic receptor stimulation, which is not expected since b-adrenergic receptor activation increases blood pressure and aortic contraction. This effect of 20(S)-PPD is thought to be due to the inhibition of a variety of signaling pathways, including the inhibition of voltage-dependent Ca2+ channels and a receptor-operated Ca2+ channel and the activation of Ca2+-activated K+ channels.
Endothelium-Dependent Relaxation
Endothelium-dependent relaxation is a major mechanism of vascular reactivity that regulates the flow and homeostasis of blood through capillaries by two main mechanisms:
- the ability of endothelium to deform itself into a thin sheet that can stretch over a small area, and
- the presence of a negative electrostatic charge on the endothelial cells which repels the blood cells. These functions are mediated by the release of nitric oxide (NO) from NO synthase.
The endothelial NOS enzyme is activated by the increase in intracellular calcium triggered by acetylcholine. This activated NOS enzyme phosphorylates the small and intermediate calcium-activated potassium channels and leads to the opening of these channels leading to the release of NO into the vasculature.
Multiple studies have shown that impaired endothelium-dependent relaxation is also associated with vascular dysfunction and oxidative stress in patients with diabetes mellitus or polycystic kidney disease. The endothelium is the largest vascular lining, and it has a number of functions including regulating homeostasis by controlling the production of prothrombotic and antithrombotic components, fibrynolitics and antifibrynolitics, and acting as a mediator of cellular migration, inflammatory processes and leukocyte adhesion.
To test the effect of diabetes on endothelium-dependent relaxation, reseacher's isolated aortae from normal and diabetic rats. Those from diabetic rats were significantly impaired in response to ACh. Interestingly, this impairment was reversed by treatment with tocomin (Tocomin; 1 mM, i.p.). The results indicate that tocomin decreases vascular oxidant stress, improves expression of eNOS and increases NO bioavailability leading to improved endothelium-dependent relaxation in diabetic aortae.
These findings indicate that tocomin improves endothelium-dependent relaxation through an improvement in the contribution of nitric oxide to relaxant response. This could be due to an increase in NO bioavailability, or it may be because of the antioxidant activity of tocomin that decreases vascular oxidant stress.
Tocomin treatment improved NO-mediated endothelium-dependent relaxation in aortae from diabetic rats. However, it had no effect on plasma glucose levels. The impaired ACh responses were accompanied by an increased Nox2 expression and a higher level of vascular O2- generation in diabetic rat aortae, but no evidence of hyperglycaemia was found in the diabetic aortae. Consequently, tocomin improves the contribution of NO to endothelium-dependent relaxation in diabetes by increasing NO bioavailability without affecting hyperglycaemia.
Endothelium-Induced Relaxation
The endothelium is a structure that occupy a location between the blood and vascular smooth muscle. It is considered to play an important role in the regulation of vascular tone through the production of a number of relaxing molecules that induce vasorelaxation. The effects of these relaxing substances can be altered by different agonists and antagonists.
The synthesis of these molecules is controlled by the endothelial cell itself, which possesses many different receptors that are activated by different physical and chemical stimulus. The endothelium can then respond to these stimuli by either modifying the shape or releasing molecules that counteract their action and maintain homeostasis in the vascular system. The endothelium is therefore capable of producing a great variety of molecules, including agonists and antagonists, as well as procoagulants and anticoagulants, and inflammatory and anti-inflammatory mediators.
Researcher's have also shown that a particular nonprostanoid substance, called EDRF (endothelium-derived relaxant factor), is responsible for the relaxation induced by acetylcholine and a number of other agonists. EDRF is released from endothelial cells and acts on vascular smooth muscle cells by stimulating a mechanism similar to NO.
EDRF is sensitive to membrane hyperpolarization, causing activation of K+ channels and the cGMP pathway through a mechanism that involves NO. In 2K arteries, EDRF is insensitive to indomethacin, whereas in 2K-1C arteries it is required for the relaxation induced by acetylcholine.
Another important role of EDRF is in regulating the synthesis of NO by the endothelial cells, thereby maintaining the basal vasomotor tone and inhibiting thrombosis. The presence of risk factors that reduce NO synthesis, such as diabetes, hypertension and smoking, may lead to reduced endothelium-dependent relaxation.
As such, NO plays a key role in the maintenance of basal vasomotor tone and antithrombotic properties through its ability to bind to guanyl cyclase and inhibit platelet adhesion activation. Hence, it is essential that the endothelium has sufficient NO synthesis capacity to allow normal nitric oxide to be released.
This process of NO synthesis is affected by age, as a decrease in the synthesis is seen in elderly subjects. In addition, the synthesis of NO is reduced in patients with atherosclerotic lesions, in postmenopausal women who have a decreased production of estrogen due to aging, and in patients with hypertension, hypercholesterolemia, or diabetes.
Single-Cell Studies
Different sex hormones have been shown to modulate vascular behavior in various tissues. For instance, testosterone has been shown to affect nitric oxide (NO) synthesis and bioavailability as well as endothelial function in vivo. In addition, testosterone has been shown to influence the expression of inflammatory cytokines that are known to be associated with cardiovascular disease and atherosclerosis.
Testosterone has also been shown to enhance the vasodilatory response of endothelium-derived NO, which is crucial for vascular relaxation. It has been also found that testosterone can reduce oxidative stress, which is associated with the progression of atherosclerosis and heart failure.
Furthermore, testosterone can modify the expression of an ETA receptor, which is associated with the production of nitric oxide (NO). It has been reported that the level of NO produced by endothelial cells is greater in old men than young people and that aging itself leads to dysfunction of endothelium-dependent NO synthesis.
The ability to examine individual cells is critical for identifying biological signals at a micro-scale. This is especially important for systems biology, where a detailed analysis of a single cell can reveal new information that is not available at the population level.
However, there are several challenges with this approach. One is that the RNA capture and conversion rate in scRNA-seq is not very efficient, which can result in an excessive number of zero entries in a gene expression matrix. This leads to distortions that are difficult to correct.
Another limitation is the high dimensionality of the single-cell transcriptome, which makes it challenging to analyze. This is particularly true when combining single-cell data from different species, conditions, technologies, and protocols.
Fortunately, different approaches have been developed that allow for the analysis of scRNA-seq data across multiple data sets. These methods include computational amplification of data to correct for batch effects, matching mutual nearest neighbors (MNNs), and integrating single-cell transcriptomic data across different conditions, technologies, and species. These techniques can help separate technical from biological variations and facilitate the transfer of knowledge between established bulk datasets and single-cell data.
Conclusion
The principal mammalian androgen, testosterone, exhibits an array of vascular properties at physiologic concentrations. These include both endothelium-dependent and endothelium-independent effects, depending on dose, duration of exposure, underlying vascular disease, and biological sex.
In vitro and in vivo studies suggest that adrenergic receptor-mediated vascular function is modulated by testosterone through mechanisms involving NADPH oxidase-dependent ROS generation, and NO synthase activation (Chignalia et al., 2012). These mechanisms are also responsible for the anti-inflammatory activity of testosterone and suppressed expression of IL-1b and TNF-a in skeletal muscle (Menini et al., 2005).
Testosterone-induced vasodilation is mediated by an opening of large conductance Ca2+-activated K+ channels in vascular smooth muscle cells via PLC/IP3-dependent mechanism. This is accompanied by stimulation of thromboxane synthase, COX-1 and COX-2 pathways.
However, aging decreases the activity of BKCa channels, which can account for reduced testosterone-induced vasodilation in elderly individuals (Cheetham et al., 2006). Consequently, the relaxation induced by testosterone may be less effective in older men.
Aging-associated decreases in circulating free testosterone are associated with increased incidence of CVD (Hyde et al., 2012). It has also been shown that low free testosterone is an independent predictor of cardiovascular mortality in elderly men (Hyde et al., 2015).
As testosterone is largely eliminated through renal excretion in the urine, the onset of CVD is delayed in men with low serum testosterone levels. Moreover, low free testosterone has been linked with an atherogenic lipid milieu in the serum of CVD patients (Hyde et al., 2011).
