Cilostazol's Effect on the Response to Perivascular Nerve Stimulation in Isolated Dog Cerebral and Mesenteric Arteries

Nitrergic perivascular innervation in health and diseases: Focus on vascular tone regulation

For a long time, the vascular tone was considered to be regulated exclusively by tonic innervation of vasoconstrictor adrenergic nerves. However, accumulating experimental evidence has revealed the existence of nerves mediating vasodilatation, including perivascular nitrergic nerves (PNN), in a wide variety of mammalian species. Functioning of nitrergic vasodilator nerves is evidenced in several territories, including cerebral, mesenteric, pulmonary, renal, penile, uterine and cutaneous arteries. Nitric oxide (NO) is the main neurogenic vasodilator in cerebral arteries and acts as a counter-regulatory mechanism for adrenergic vasoconstriction in other vascular territories. In the penis, NO relaxes the vascular and cavernous smooth muscles leading to penile erection. Furthermore, when interacting with other perivascular nerves, NO can act as a neuromodulator. PNN dysfunction is involved in the genesis and maintenance of vascular disorders associated with arterial and portal hypertension, diabetes, ageing, obesity, cirrhosis and hormonal changes. For example defective nitrergic function contributes to enhanced sympathetic neurotransmission, vasoconstriction and blood pressure in some animal models of hypertension. In diabetic animals and humans, dysfunctional nitrergic neurotransmission in the corpus cavernosum is associated with erectile dysfunction. However, in some vascular beds of hypertensive and diabetic animals, an increased PNN function has been described as a compensatory mechanism to the increased vascular resistance. The present review summarizes current understanding on the role of PNN in control of vascular tone, its alterations under different conditions and the associated mechanisms. The knowledge of these changes can serve to better understand the mechanisms involved in these disorders and help in planning new treatments.

Recent advances in research on nitrergic nerve-mediated vasodilatation

Cerebral vascular resistance and blood flow were widely considered to be regulated solely by tonic innervation of vasoconstrictor adrenergic nerves. However, pieces of evidence suggesting that parasympathetic nitrergic nerve activation elicits vasodilatation in dog and monkey cerebral arteries were found in 1990. Nitric oxide (NO) as a neurotransmitter liberated from parasympathetic postganglionic neurons decreases cerebral vascular tone and resistance and increases cerebral blood flow, which overcome vasoconstrictor responses to norepinephrine liberated from adrenergic nerves. Functional roles of nitrergic vasodilator nerves are found also in peripheral vasculature, including pulmonary, renal, mesenteric, hepatic, ocular, uterine, nasal, skeletal muscle, and cutaneous arteries and veins; however, adrenergic nerve-induced vasoconstriction is evidently greater than nitrergic vasodilatation in these vasculatures. In coronary arteries, neurogenic NO-mediated vasodilatation is not clearly noted; however, vasodilatation is induced by norepinephrine released from adrenergic nerves that activates β1-adrenoceptors. Impaired actions of NO liberated from the endothelium and nitrergic neurons are suggested to participate in cerebral hypoperfusion, leading to brain dysfunction, like that in Alzheimer's disease. Nitrergic neural dysfunction participates in impaired circulation in peripheral organs and tissues and also in systemic blood pressure increase. NO and vasodilator peptides, as sensory neuromediators, are involved in neurogenic vasodilatation in the skin. Functioning of nitrergic vasodilator nerves is evidenced not only in a variety of mammals, including humans and monkeys, but also in non-mammals. The present review article includes recent advances in research on the functional importance of nitrergic nerves concerning the control of cerebral blood flow, as well as other regions, and vascular resistance. Although information is still insufficient, the nitrergic nerve histology and function in vasculatures of non-mammals are also summarized.