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domain and an intracellular catalytic domain that
synthetizes cGMP; the other is cytoplasmatic (soluble
guanylate ciclase) and is the isoform that is the form
activated by NO. In the nervous system, NO acts directly
to open cyclic nucleotid-gated channels, and it activates
the cGMP-dependent protein kinase (36). NO, after it is
released by the vascular endothelium, is free to diffuse
into adjacent smooth muscle cells of the vascular wall,
and in these cells directly activates soluble guanylate
cyclase. This enzyme induces the formation of cGMP,
which in turn activates cGMP-specific protein kinases
that affect ion channels, calcium hemeostasis, or
phosphatases, or all of these, causing relaxation of
vascular smooth muscle (36).
As loss of NO bioavailability indicates a broadly
dysfunctional phenotype across many properties of the
endothelium, the assessment of its vasodilator properties
resulting from NO and other molecules may provide
information on the integrity and function of the
endothelium. Interestingly, most, if not all,
cardiovascular risk factors are associated with endothelial
dysfunction and risk factor modification leads to
improvement in vascular function (37, 54). Over the past
35 years, many methodological approaches have been
developed to measure the (patho) physiological function
of the endothelium in humans (55), but evaluation of
endothelial function as a clinical tool in daily practice is sensitivity of guanylate cyclase in vascular musculature
not established yet.
From the beginning, endothelial function has been
regarded as an important factor in the regulation of the
coronary circulation and it may be of pathophysiological
significance in several coronary diseases (1, 15-18).
Endothelium-dependent vasodilatation has been found in
coronary vessels from dogs (56) and man (57). Then it
was reported that human coronary vessels in vitro release
NO (58), that NO plays a significant role in modulating
basal vasomotion and endothelial-dependent dilatation in
the coronary circulation of dogs (59), and that the
coronary vascular relaxation by NO, as well as by
nitroprusside is associated with an increase in cGMP
(60).
In our laboratory we have carried out a series of
experiments to examine the functional role of NO in the
coronary circulation under normal conditions, and they
were performed using three laboratory models
(anesthetized goats, isolated, perfused hearts from rats,
and isolated coronary arteries from pigs). In these three
models we found that in the coronary circulation, NO
may be released under basal conditions, it produces a
basal vasodilator tone, it mediates the vasodilation in
response to Ach, and it reduces the response to
vasoconstrictors such as ET-1. In anesthetized goats, L-
NAME, injected by i. v. route, decreased resting coronary
blood flow and increased systemic arterial pressure.
These hemodynamic effects of L-NAME were partially
20 @Real Academia Nacional de Farmacia. Spain
Godofredo Diéguez Castrillo
reversed by L-arginine injected intravenously. We also
found that during the effects of L-NAME, the coronary
vasodilatation to Ach was attenuated, to sodium
nitroprusside was increased, and to diazoxide was
unaffected. Graded coronary hyperemic responses after 5,
10 or 20 s of coronary occlusion were increased during
treatment with L-NAME. These results suggest: a)
endogenous NO is involved in regulation of coronary
circulation by producing a basal vasodilator tone; b) Ach-
induced coronary vasodilatation is mediated, in part, by
NO; and c) inhibition of basal endogenous NO
production induces supersensitivity of coronary vessels
to nitrovasodilators and enhances hyperemic responses
after short periods of ischemia of the myocardium (61).
This agrees with observations by others in the coronary
circulation, and supports the idea that a vasodilator tone
mediated by NO is present in the coronary circulation.
The effects of sodium nitroprusside were increased
during the action of L-NAME. Sodium nitroprusside has
been used as a donor of exogenous NO and produces
vasodilatation in a similar way to NO (62). Enhanced
vasorelaxant responses to sodium nitroprusside and other
nitrous compounds have been found in the absence of a
functional endothelium or in the presence of L-arginine
analogues, and this has been related to an increased
to exogenous NO when production of endogenous NO is
reduced (63, 64). Therefore, it appears that the removal
of endogenous NO in the vasculature could lead to
increases in sensitivity to vasodilators that act by
stimulating soluble guanylate cyclase. Reactive
hyperemia after short ischemias of the myocardium is
known to occur in the coronary circulation, and
metabolic and hemodynamic factors that contribute to the
hyperemic response have been explored. However, little
is known about the role of the endothelium in the
coronary hyperemic response (56, 59, 65).
Also in anesthetized goats, when L-NAME was
intracoronarily injected, it reduced resting coronary blood
flow without changing systemic arterial pressure. These
effects of L-NAME were also partially reversed by L-
arginine. Isoproterenol, adenosine and Ach, injected
intracoronarily, increased coronary blood flow, and after
treatment with L-NAME the increases in coronary blood
flow induced by isoproterenol and Ach were reduced,
whereas those induced by adenosine were increased
further. Therefore, it is confirmed that in the coronary
circulation NO produces a basal vasodilator tone under
normal conditions, and it is suggested that NO may be an
intermediate in the coronary vasodilatation in response to
beta-adrenoceptor stimulation and Ach, and that the
vasodilatation due to adenosine is potentiated during
reduction of endogenous NO production (66) (See Figure
4).