Vasculature

MR have been located in freshly isolated vascular tissue and in both cultured   vas-
cular smooth muscle cell s ( VSMC s) and the vascular endothelium  [107] .  11  β HSD2
is also present in human VSMC, the adventitial fi broblasts and   endothelial cell s
( EC s)  [77, 108, 109] . In the mouse thoracic aorta, however, mRNA for 11 β HSD2
is confi ned to the endothelium  [46] , as it is in cultured rat aortic cells  [110] .
Whether this is a species difference or refl ects the sensitivity of enzyme expression
to conditions of culture remains uncertain and resolution awaits the development
of reliable antibodies.
Physiologically adrenal steroids      –     both  aldosterone  and  glucocorticoids     –   
potentiate the action of vasoconstrictors. This effect was fi rst described in the
1950s for catecholamines but is also true for other vasoactive agents, notably Ang
II  [111] . There is some evidence to suggest that the potentiating effect of cortico-
steroids differs throughout the vasculature. For example, in the   deoxycorticoste-
rone acetate  ( DOCA ) - salt - hypertensive rat model, the pressor effects of Ang II are
exacerbated, indicating an increased sensitivity of the resistance vasculature to
vasoconstrictors  [112] . The conduit vasculature, in contrast, was not sensitized.
However, the opposite has been reported for catecholamines: the conduit vascu-
lature being sensitized and the resistance vessels desensitized to phenylephrine
[113] .
The mechanisms of potentiation have focused on increased receptor density, in
part due to the actions of corticosteroids as transcription factors, but also because
the effects are seen   ex vivo   and are therefore a property intrinsic to the vessel. For
Ang II this appears to hold true, since both aldosterone and glucocorticoids greatly
enhance receptor density  [111] . Moreover, the increase in receptor number is
transduced to a downstream effect, there being a more robust activation by Ang
II of intracellular signaling cascades following exposure to mineralocorticoids
[114] . These effects are exclusive to the AT
1
 receptor   [115] , consistent with the fact
that the promotor region for the gene contains several steroid response elements
[116] . In addition to the effects on receptor density, mineralocorticoids also lead
to activation of a localized RAS, with increased angiotensinogen formation   [117] 
and ACE activity being found in both ECs and VSMCs   [118] .
Mineralocorticoid increases the expression of   α  - adrenergic  receptors    [119] ,
whereas adrenalectomy reduces receptor density  [120] . However, binding studies
indicate that receptor affi  nity moves in the opposite direction to number, thereby
offsetting greatly the theoretical  “ stimulatory ”   effects  of  mineralocorticoid.  There
are several confl icting reports in the literature, but overall convincing data to
suggest a receptor number - based response to corticosteroids is lacking  [111] .
Neither does altered release or uptake of catecholamines at nerve terminals con-
tribute to potentiation by aldosterone  [121] . A clue to the underlying mechanism
came from the observation that endothelium  - dependent vasodilation was impaired
in DOCA - salt rats  [122] . This was initially attributed to damage secondary to
chronic hypertension but other studies demonstrated that the heightened pressor
responses to noradrenalin were, in fact, due to a reduced synthesis of the vasodila-
tor prostaglandin E2 from the endothelium   [123] . It is now clear that attenuation
of endothelium  - derived vasodilation contributes to the potentiation by corticoste-
roids of the response to catecholamines. This is NO - dependent in conduit vascu-
lature, but not in resistance vessels. More recently, experiments have described
direct effects of aldosterone on VSMCs. By improving the coupling of  α 
1
- 
adrenoceptors to downstream signaling pathways, mineralocorticoids improve
vascular tone  [124] .
The molecular mechanism of aldosterone  ’ s action in the vasculature involves
genomic effects but these are observed more than 2    h after exposure. That vasoac-
tive responses can be observed almost immediately after infusion of aldosterone
(i.e. prior to the onset of  de novo  protein synthesis) suggests nongenomic action
in the vasculature   [104] .  Although    in vivo   systemic infusions of aldosterone almost
always promote an immediate pressor response  [125] , vasoconstriction is not a
universal fi nding  ex vivo.  Indeed, local infusion of aldosterone into the forearm
increases local blood fl  ow  [125] . This vasodilation results from stimulation of NO
production by the endothelium  [125] . There is, however, also an effect on the
VSMCs to promote vasoconstriction and the net effect on vascular function
depends on the balance between the two opposing forces. Thus, inhibition of
endothelial NO synthase with  N  - monomethyl -   L  - arginine leads to a powerful and
sustained vasoconstriction  [126] . These observations explain why the effect of
aldosterone on vascular tone may vary in different vascular beds. Moreover, local
vasodilation may be offset by blunting of the baroreceptor refl  ex  [127]  and indirect
activation of the sympathetic nervous system.
In addition to these well  - documented responses, both mineralocorticoids and
glucocorticoids will stimulate the local release of endothelin from the vasculature
[128] . This is a potent vasoconstrictor (covered in Chapter   6   )  and  could  mediate
aldosterone - stimulated increases in total peripheral resistance.
Aldosterone also exerts profound pathological effects in the vasculature. The
accumulative and slowly developing disease atherosclerosis is a major cause of
heart disease. Disruption of vascular homeostasis predisposes the endothelial
vessel wall to vasoconstriction, infl ammation and atherosclerosis, all of which
can be contributors towards cardiac disease onset. The development and pro-
gression of atherosclerosis is largely associated with endothelial dysfunction
(for review, see   [129] ), and it has been suggested that the positive clinical
outcomes of the RALES and EPHESUS studies may be in part due alleviation
of vascular endothelial aldosterone and/or MR antagonism. Using cultured
human umbilical vein ECs, Oberleithner  et al.  demonstrated that aldosterone
promotes remodeling of the endothelium  in vitro    [130] . They observed that aldo-
sterone administration caused the cells to increase in both size and stiffness,
which would   in vivo   lead to endothelial dysfunction and associated pathogenesis.
Endothelial dysfunction can be rescued in the stroke - prone  spontaneously  hyper-
tensive rat by administration of eplerenone   [177] . Aldosterone also plays a
major role in Ang II - induced vascular infl ammation in the setting of high salt
intake   [98] .
It is not easy to present a unifying view of the effect of aldosterone on vascular
function since the literature is often divergent. It would appear, however, that the
net effect of systemic aldosterone is to increase blood pressure by potentiating the
action of vasoconstrictors,  “ activating ”  VSMCs and increasing sympathetic drive
(either directly or indirectly). The action on the endothelium is less clear. Physio-
logically and in healthy individuals, it would appear that aldosterone promotes
vasodilation, both acutely and chronically  [131] . In the setting of hypertension or
mineralocorticoid excess coupled with high salt, aldosterone might promote endo-
thelial dysfunction  [107] . Although controversial, some evidence suggests that the
vasculature can synthesize aldosterone locally  [132] , adding a further level of com-
plexity to the fi eld. Indeed, locally activated RAS is proinfl ammatory and promotes
detrimental remodeling of the vasculature in hypertension  [133] .  These  fi  ndings,
together with the positive outcomes of clinical trials, would advocate the use of
MR antagonists in hypertension and cardiovascular disease. Indeed, the Framing-
ham Heart Study reports a complex but positive correlation between serum aldo-
sterone in the physiological range and cardiovascular risk   [134] .
The use of MR antagonists to treat cardiovascular disease is limited, as they tend
to promote hyperkalemia due to actions in the kidney. Furthermore, targeted dis-
ruption in mice of the gene encoding MR has not been particularly informative
in terms of the role of aldosterone in cardiovascular function. MR null mice die
within 8 days of birth due to uncorrected salt wasting   [135] .  Despite  signifi  cant
activation of the RAS, MR null mice were unable to activate the epithelial sodium
channel, modeling well the autosomal dominant form of pseudohypoaldosteron-
ism type 1 (OMIM #177735), in which inactivating mutations in MR are reported.
These experiments not only indicate the critical role for renal MR in the long - term
regulation of blood pressure, but also demonstrate that activation of the GR does
not compensate for loss of MR. In order to circumvent the problem of early post -
natal death associated with global MR defi  ciency, the gene has been   “ fl   oxed ” 
allowing targeted deletion through use of the Cre  – loxP  system   [136] .  Surprisingly,
when MR was deleted in the distal nephron, mice were able to thrive, albeit with
much - elevated  aldosterone   [137] . This would suggest that MR in other systems
could compensate for the loss of renal MR and this was indeed found in the colon.
Nevertheless, the principal cell mutant mice were able to maintain near perfect
salt balance, even on a low - sodium diet, via upregulation of the epithelial sodium
channel and it was found that in a small percentage of principal cells in the early
connecting tubule MR had not been deleted. Although this highlights a pitfall of
the Cre – loxP system, it is expected that future experiments using the  “ fl  oxed ”   MR
will be informative.  
Glucocorticoids are responsible for a wide range of physiological effects (Figure
1.8 ), the majority of which are united under the common subheading of stress

responses. The release of glucocorticoids following stress - induced  stimulation
of the HPA axis promotes coordination of endocrine, immune and nervous
system responses to the initial stimuli. Examples of this include inducing the
mobilization of energy resources in response to physical stresses such as starva-
tion and the   “ fi  ght or fl  ight ”  response by stimulating gluconeogenesis and lipoly-
sis, and inhibiting glucose uptake by peripheral tissues. Glucocorticoids also act
to suppress infl  ammatory responses, cellular proliferation and tissue repair, sug-
gesting a regulatory role to prevent these responses becoming undisciplined and
destructive. 
Several  clinical  disorders  associated  with  cortisol  deregulation     –     whether  a
consequence of synthesis, HPA axis or GR - mediated  effects     –     have  been
associated with an increased rate of morbidity and mortality, which in turn is
possibly corollary to an increased risk of cardiovascular events (for review, see
[138] ).
It is diffi  cult to separate direct primary effects of glucocorticoids on the heart
and vasculature from secondary changes arising from activation of GR in other
systems (Figure  1.8 ). However, evidence from human patients and transgenic
mice have helped to establish the nature of these primary responses. It appears
that glucocorticoids, at physiological concentrations, may be benefi cial to cardiac
function, potentially by antagonizing the MR as described above. Furthermore,
glucocorticoids potentiate the action of vasoactive substances and so can clearly
infl uence vascular tone. That glucocorticoids are important cardiovascular hor-
mones is illustrated through the extremes of altered glucocorticoid secretion:
Addison ’ s disease presents with life - threatening hypotension and vascular
collapse while high blood pressure is a common feature of Cushing ’ s  syndrome