“Adrenomedullin (AM)” is a novel hypotensive peptide discovered in human pheochromocytoma by monitoring the elevating activity of platelet cAMP. It has potent and long-lasting vasodilator effects in several vascular systems. In addition, a novel 20-residue hypotensive peptide, termed “proadrenomedullin N-terminal 20 peptide” (PAMP), is processed from proadrenomedullin. Although initially isolated from human pheochromocytoma tissue and porcine adrenal medullae, AM mRNA is highly expressed in several organs including cardiovascular tissues. Taken together with its widespread distribution and its ability to influence the bioactivity of cells in situ, AM may function as a paracrine or autocrine hormone rather than a classical endocrine system. Furthermore, ubiquitous expression of AM mRNA may indicate its various biological functions as well as the existence of a novel circulation control system. Plasma AM as well as PAMP concentrations significantly increased in various cardiovascular diseases including hypertension, chronic renal failure and congestive heart failure. The present review summarizes the recent advances in AM research and showed that AM and PAMP are important vasoactive peptides, such investigations should enable the elucidation of the basic physiologic mechanisms of novel circulatory homeostasis.
The natriuretic peptide (NP) system is one of the most important systems regulating blood pressure and body-fluid homeostasis. The biological activities of the system are determined by the NPs and the receptors, which are comprised of three subtypes: NP-AR and NP-BR related to biological activities and NP-CR related to the clearance of NP. We focused our studies on the receptor subtypes. In hypertensive rats (SHR-SP/Izm, DOCA/salt), NP-AR was upregulated and NP-CR was downregulated. The ACE inhibitor derapril, but not the Ca2+ blocker manidipine, normalized the upregulated NP-AR, but the effect was completely abolished by the bradykinin β2- receptor antagonist, suggesting that bradykinin regulates the vascular NP-AR. The AT1 antagonist TCV-116, but not manidipine, reversed the downregulated NP-CR. Ang II decreased NP-CR in cultured aortic smooth muscle cells. These results suggest that upregulation of NP-AR and downregulation of NP-CR with the increased plasma NPs counteract hypertension by enhancing the action of NP. A β-blocker (carvedilol) potentiated the hypotensive action of NPs by increasing plasma NPs and enhancing the vascular response to NPs via downregulation of the vascular and lung NP-CR. The newly found mode of actions could be related to its anti-heart failure effect. In genetically hyperglycemic Wistar fatty rats, vascular NP-BR and NP-AR were upregulated. Since plasma ANP and vascular CNP were significantly increased, the local CNP/NP-BR system as well as the systemic ANP/NP-AR system may play an important role in counteracting vascular remodeling in diabetes mellitus. All these observations provide in vivo evidence for the pathophysiological significance of the receptor subtype of the NPs.
Nitric oxide (NO) is produced by nitric oxide synthases (cNOS and iNOS) in endothelial cells upon stimulation by various agents like Ca2+-calmodulin, cytokines and TNF. It acts as a paracrine on adjacent cells to activate soluble guanylyl cyclase in the production of cGMP, a second messenger in signal transduction cascades, leading to various cellular responses. The circulating blood contains certain steady-state concentrations of NO in the plasma in order to maintain normal vascular tone and other appropriate conditions for the systemic and pulmonary circulation. This homeostasis of NO in the rapidly moving blood must be maintained by a delicate balance between its production by NOSs and its instant scavenging by hemoglobin (Hb) in the erythrocytes. Under physiological conditions ([NO] ?? [Hb]), NO is sequestered by deoxy Hb to form α-nitrosyl Hb, α(Fe-NO)2β(Fe)2, where NO is tightly (KD ?? 10-12 M) bound to the α-subunits. Upon binding NO to the α-subunits, Hb shifts its conformation to a T-(low-affinity extreme) state and its β-subunits become an efficient O2 carrier. The same molecular mechanism of NO-induced conformation change operates in both Hb and soluble guanylyl cyclase. This is caused by the NO-induced trans-axial cleavage of the heme Fe-proximal His bonds in these hemoproteins. This bond cleavage mechanism allows Hb to survive as an effective O2 carrier even after sequestration of NO. The NO sequestered in Hb is eventually oxidized aerobically to NO3- in the reaction of Fe-NO + O2 → Fe+ + NO3-. Met Hb (Fe+) so formed is cycled back to deoxy Hb (Fe) by intra-erythrocyte Hb reductase to complete the NO scavenging. Thus, the NO in the blood acts on soluble guanylyl cyclase in vascular smooth muscles to dilate the blood vessels to increase blood delivery, whereas excess NO in the blood, which is sequestered by Hb, could help Hb to deliver O2 more efficiently in peripheral tissues.
Nitric oxide is an exceptionally stable molecule as a radical species. The smallest signal molecule is formed from oxygen and L-arginine by well-defined enzymes. Nitric oxide plays roles not only in physiological regulation such as blood pressure, platelet aggregation and neuronal function, but also in pathophysiological states. The reactions of nitric oxide with target molecules involve electron transfer. This mechanism differs essentially from those of neurotransmitters and hormones identified so far. It is reasonable to as sume that distinct signal transduction pathways are involved in this redox signal molecule. In this review, we briefly summarize nitration, nitrosation and nitrosylation of target molecules in vivo by nitric oxide.
Over the past several years, a large body of evidence has shown the injurious action of superoxide. However, with the discovery that most types of cells can generate nitric oxide (NO), the pathological conditions previously attributed to superoxide must to be re-examined considering the reaction of superoxide and NO. This is particularly important if there are conditions where the production of peroxynitrite is relevant, since peroxynitrite is a very potent oxidant and yielded by the reaction of superoxide and NO with a diffusion controlled rate. In this article, we reviewed various pathological conditions, focusing on the generation of these three reactive substances and demonstrated actual examples of peroxynitrite-related injuries.
Glutamate is postulated to play an important role in the pathogenesis of the neuronal cell loss that is associated with neurological disorders in the CNS. Nitric oxide (NO) mediates glutamate neurotoxicity observed in the primary culture derived from the cerebral cortex, substantia nigra and retina. In search of endogenous protective factors that inhibit the neurotoxic action of glutamate and NO, we found that certain neurotransmitters such as nicotinic acetylcholine, dopamine and cholecystokinin prevent glutamate neurotoxicity by suppressing the activity of NO synthase. Neurotrophins such as brain-derived neurotrophic factor (BDNF) also prevented glutamate neurotoxicity via TrkB neurotrophin receptors although prolonged pretreatment was necessary to induce marked neuroprotection. BDNF prevented the neurotoxic actions of NO donors. This evidence indicates that BDNF protects neurons against glutamate cytotoxicity by reducing the radical chain reaction triggered by NO. We also found that the conditioned medium of the striatal cultures reduced glutamate neurotoxicity. Moreover, ether extract of fetal calf serum (FCS), contained in the conditioned medium, also reduced glutamate neurotoxicity. Concomitant application of ether extract of FCS with S-nitrosocysteine (SNOC), a NO donor, markedly reduced SNOC-induced neurotoxicity. These substances may have roles as neuroprotective factors that promote neuron survival under neurological disease states in the CNS.
NO is believed to be involved in neurotoxicity after various neuronal stresses. NO donors are toxic and cause changes in cellular morphology such as condensed and fragmented chromatin, shriveled nuclei, apoptotic bodies and membrane blebbing. These observations are consistent with the overall description of apoptosis. The crucial mechanism of NO-induced cytotoxicity is still unclear. Several mechanisms for NO-induced cytotoxicity in neurons have been proposed. It has been reported that NO enhances ADP-ribosylation or S-nitrosylation of an increasing number of proteins, and two of these proteins were identified as NO-target proteins. One is glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a key enzyme of glycolytic conversion, which is S-nitrosylated by NO inhibiting the enzyme activity. Hence, inhibition of GAPDH activity by NO would decrease the amount of ATP. NO also activates poly (ADP-ribose) polymerase (PARP) in the presence of DNA damage. The activation of PARP results in depletion of NAD and ATP. The energy depletion by NO could cause cell death. Recently, several factors such as Fas, the caspases (interleukin-1β-converting enzyme (ICE)-like proteases), Bcl-2 and the tumor suppressor gene product p53 have been shown to be involved in apoptotic cell death. We here discuss the crucial mechanisms of NO-induced cytotoxicity and also discuss recent findings about the protective effect of NO on cell death.
Properties of endothelium-derived hyperpolarizing factor (EDHF) have been reviewed briefly. The production of EDHF requires an increase in endothelial [Ca2+]i, the properties being similar to those of nitric oxide (NO). EDHF activates K+-channels and hyperpolarizes vascular smooth muscle. The EDHF-induced hyperpolarization is greatly inhibited by charybdotoxin (ChTX) and partially inhibited by apamin, but not by K+-channel inhibitors such as Ba2+, glibenclamide, 4-aminopyridine, suggesting that the K+-channels involved are mainly the Ca2+-sensitive type. Membrane hyperpolarization induces vasodilatation by unidentified mechanisms. Experiments using K+-channel openers and electrophysiology suggest that hyperpolarization may reduce (i) influx of Ca2+ through voltage-sensitive Ca2+-channels, (ii) production of InsP3 in the case of agonist-induced contraction, (iii) Ca2+-sensitivity of contractile elements and (iv) agonist stimulated ion channel activities. In the endothelium-dependent vasodilation, the EDHF/EDRF ratio is larger in peripheral vessels than in the proximal ones, indicating significant importance of EDHF mainly in peripheral arteries. The chemical nature of EDHF remains undetermined, although some candidates such as arachidonic acid metabolites or endogenous cannabinoids are proposed. As the inhibition of gap junctions in artrerial tissues reduces the amplitude of EDHF-induced relaxation, the possible involvement of electrical communication between endothelial and smooth muscle cells has also been considered.
Angiotensin-converting enzyme (ACE) inhibitors attenuated the contractile responses to angiotensin (Ang) I of arterial strips of humans, monkeys, and dogs, as can be expected. Unexpectedly, however, the response was not abolished by sufficient doses of ACE inhibitors, the facts suggesting the Ang I conversion by a non-ACE enzyme(s). HPLC analysis of the incubation product of Ang I with vascular tissues revealed that Ang II was yet formed despite complete ACE inhibition, and the ACE inhibitor-insensitive Ang II formation was blocked by chymostatin. The disclosed Ang II-forming enzyme was identified as chymase, which was later found in abundance in the human heart. Another notable discovery by us is the species difference in chymase processing of Ang I: chymases of primates, dog, and hamster convert Ang I to Ang II, while chymases of rat, rabbit, and probably mouse do not. Accumulating evidence indicating that Ang II is not merely a vasopressor agent but also a growth-promoting factor, which leads to tissue hypertrophy and fibrosis, together with the results our studies lead us to propose the tissue-remodeling roles of chymaseformed Ang II in various cardiovascular diseases: dog neointimal proliferation after angioplasty, hamster cardiomyopathy, etc., in which chymase mRNA is increased concordantly with tissue remodeling. The fact that Ang II receptor antagonists, not ACE inhibitors, suppress the tissue remodeling supports our argument that Ang II is formed predominantly by chymase in diseased tissues. Orally active chymase inhibitors, evolving in our study, should help explore the actual roles of chymase as well as the rational treatment of tissue-remodeling disorders.
The renin-angiotensin system, composed of enzymatic and signal-transduction cascades, plays a key role in the regulation of arterial blood pressure and in the development of certain forms of experimental and human hypertension. The products of this system, angiotensin peptides, exert a wide range of physiologically important effects on many tissues, including those of the cardiovascular systems, through their actions on angiotensin receptors. Molecular genetic and transgenic studies have begun to implicate some of the genes encoding components of the renin-angiotensin system in the development of cardiovascular diseases. Here, I review new developments related to the functional role of angiotensin II, an important product of the renin-angiotensin system, by focusing on transgenic approaches, including gene targeting.