Vasodilator mechanisms in the dorsal aorta of the giant shovelnose ray, Rhinobatus typus (Rajiformes; Rhinobatidae)

Mass Transport: Circulatory System with Emphasis on Nonendothermic Species

Mass transport can be generally defined as movement of material matter. The circulatory system then is a biological example given its role in the movement in transporting gases, nutrients, wastes, and chemical signals. Comparative physiology has a long history of providing new insights and advancing our understanding of circulatory mass transport across a wide array of circulatory systems. Here we focus on circulatory function of nonmodel species. Invertebrates possess diverse convection systems; that at the most complex generate pressures and perform at a level comparable to vertebrates. Many invertebrates actively modulate cardiovascular function using neuronal, neurohormonal, and skeletal muscle activity. In vertebrates, our understanding of cardiac morphology, cardiomyocyte function, and contractile protein regulation by Ca2+ highlights a high degree of conservation, but differences between species exist and are coupled to variable environments and body temperatures. Key regulators of vertebrate cardiac function and systemic blood pressure include the autonomic nervous system, hormones, and ventricular filling. Further chemical factors regulating cardiovascular function include adenosine, natriuretic peptides, arginine vasotocin, endothelin 1, bradykinin, histamine, nitric oxide, and hydrogen sulfide, to name but a few. Diverse vascular morphologies and the regulation of blood flow in the coronary and cerebral circulations are also apparent in nonmammalian species. Dynamic adjustments of cardiovascular function are associated with exercise on land, flying at high altitude, prolonged dives by marine mammals, and unique morphology, such as the giraffe. Future studies should address limits of gas exchange and convective transport, the evolution of high arterial pressure across diverse taxa, and the importance of the cardiovascular system adaptations to extreme environments. © 2017 American Physiological Society. Compr Physiol 7:17-66, 2017.

PI3K/Akt-independent NOS/HO activation accounts for the facilitatory effect of nicotine on acetylcholine renal vasodilations: modulation by ovarian hormones

We investigated the effect of chronic nicotine on cholinergically-mediated renal vasodilations in female rats and its modulation by the nitric oxide synthase (NOS)/heme oxygenase (HO) pathways. Dose-vasodilatory response curves of acetylcholine (0.01–2.43 nmol) were established in isolated phenylephrine-preconstricted perfused kidneys obtained from rats treated with or without nicotine (0.5–4.0 mg/kg/day, 2 weeks). Acetylcholine vasodilations were potentiated by low nicotine doses (0.5 and 1 mg/kg/day) in contrast to no effect for higher doses (2 and 4 mg/kg/day). The facilitatory effect of nicotine was acetylcholine specific because it was not observed with other vasodilators such as 5′-N-ethylcarboxamidoadenosine (NECA, adenosine receptor agonist) or papaverine. Increases in NOS and HO-1 activities appear to mediate the nicotine-evoked enhancement of acetylcholine vasodilation because the latter was compromised after pharmacologic inhibition of NOS (L-NAME) or HO-1 (zinc protoporphyrin, ZnPP). The renal protein expression of phosphorylated Akt was not affected by nicotine. We also show that the presence of the two ovarian hormones is necessary for the nicotine augmentation of acetylcholine vasodilations to manifest because nicotine facilitation was lost in kidneys of ovariectomized (OVX) and restored after combined, but not individual, supplementation with medroxyprogesterone acetate (MPA) and estrogen (E2). Together, the data suggests that chronic nicotine potentiates acetylcholine renal vasodilation in female rats via, at least partly, Akt-independent HO-1 upregulation. The facilitatory effect of nicotine is dose dependent and requires the presence of the two ovarian hormones.

Complex innervation patterns of the conus arteriosus in the heart of the longnose gar, Lepisosteus osseus

Anatomical and functional studies of the autonomic innervation in the conus arteriosus of the garfishes are lacking. This study reveals that the conus arteriosus of the longnose gar is primarily myocardial in nature, but additionally, large numbers of smooth muscle cells are present in the subendocardium. A well-developed system of adrenergic, cholinergic, substance P (SP) and neuronal nitric oxide synthase (nNOS) positive nerve terminals are found in the wall of the conus arteriosus. Coronary blood vessels running in the adventitia receive a rich supply of nNOS positive nerve fibers, thus suggesting their importance in the nitrergic control of blood flow in the conus arteriosus. The present data show that the patterns of autonomic innervation of the garfish conus arteriosus are more complex than previously appreciated.

Estrogen dependence of the renal vasodilatory effect of nicotine in rats: role of α7 nicotinic cholinergic receptor/eNOS signaling

AIMS

We recently reported that acute exposure to nicotine vasodilates the renal vasculature of male rats via facilitation of endothelial nitric oxide synthase (eNOS). In this study, we investigated whether this effect of nicotine is sexually dimorphic and the role of estrogen in modulating the nicotine effect.

MAIN METHODS

Nicotine-evoked vasodilation was evaluated in phenylephrine-preconstricted perfused kidneys obtained from male, proestrus female, ovariectomized (OVX) and estrogen-replaced OVX (OVXE(2)) rats.

KEY FINDINGS

Nicotine infusion (5×10(-5), 1×10(-4), and 5×10(-4) M) produced greater concentration-dependent reductions in the renal perfusion pressure (RPP) in an isolated kidney from proestrus females than from males. Inhibition of NOS by N(G)-nitro-L-arginine abolished the nicotine-evoked reduction in RPP and abolished the gender difference in the nicotine effect. Nicotine vasodilation was also attenuated in kidneys isolated from OVX and diestrus rats, models characterized by reduced estrogen levels. Further, estrogen or L-arginine supplementation in OVX rats largely restored the renal vasodilatory response to nicotine. Estrogen receptor blockade by tamoxifen abrogated the enhanced nicotine-evoked vasodilation elicited by E(2) in OVX rats. The nitrite/nitrate levels and protein expressions of eNOS and α(7) nicotinic cholinergic receptor (α(7) nAChRs) were significantly higher in renal tissues of OVXE(2) compared with OVX rats, suggesting a facilitatory effect for E(2) on α(7) nAChRs/eNOS signaling.

SIGNIFICANCE

Estrogen-dependent facilitation of NOS signaling mediates the enhanced vasodilator capacity of nicotine in the renal vasculature of female rats. Preliminary evidence also suggests a potential role for α(7) nAChRs in this estrogen-dependent phenomenon.

Nervous control of circulation--the role of gasotransmitters, NO, CO, and H2S

The origins and actions of gaseous signaling molecules, nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H(2)S) in the mammalian cardiovascular system have received considerable attention and it is evident that these three "gasotransmitters" perform a variety of homeostatic functions. The origins, actions and disposition of these gasotransmitters in the piscine vasculature are far from resolved. In most fish examined to date, NO or NO donors are generally in vitro and in vivo vasodilators acting via soluble guanylyl cyclase, although there is evidence for NO-mediated vasoconstriction. Injection of sodium nitroprusside into trout causes hypotension that is attributed to a reduction in systemic resistance. Unlike mammals, NO does not appear to have an endothelial origin in fish blood vessels as an endothelial NO synthase has not identified. However, neural NO synthase is prevalent in perivascular nerves and is the most likely source of NO for cardiovascular control in fish. CO is a vasodilator in lamprey and trout vessels, and it, like NO, appears to exert its action, at least in part, via guanylyl cyclase and potassium channel activation. Inhibition of CO production increases resting tone in trout vessels suggestive of tonic CO activity, but little else is known about the origin or control of CO in the fish vasculature. H(2)S is synthesized by fish vessels and its constrictory, dilatory, or even multi-phasic actions, are both species- and vessel-specific. A small component of H(2)S-mediated basal activity may be endothelial in origin, but to a large extent H(2)S affects vascular smooth muscle directly and the mechanisms are unclear. H(2)S injected into the dorsal aorta of unanesthetized trout often produces oscillations in arterial blood pressure suggestive of H(2)S activity in the central nervous system as well as peripheral vasculature. Collectively, these studies hint at significant involvement of the gasotransmitters in piscine cardiovascular function and hopefully provide a variety of avenues for future research.

Effects of carbon monoxide on trout and lamprey vessels

Carbon monoxide (CO) is endogenously produced by heme oxygenase (HO) and is involved in vascular, neural, and inflammatory responses in mammals. However, the biological activities of CO in nonmammalian vertebrates is unknown. To this extent, we used smooth muscle myography to investigate the effects of exogenously applied CO (delivered via a water-soluble CO-releasing molecule, CORM-3) on isolated lamprey ( Petromyzon marinus) dorsal aortas and examined its mechanisms of action on trout ( Oncorhynchus mykiss) efferent branchial (EBA) and celiacomesenteric (CMA) arteries. CORM-3 dose-dependently relaxed all vessels examined. Trout EBA were twofold more sensitive to CORM-3 when precontracted with norepinephrine (NE) than KCl and CORM-3 relaxed five-fold more of the NE- than KCl-induced tension. Glybenclamide (10 μM), an ATP-sensitive potassium channel inhibitor, inhibited NE-induced contraction, but did not affect CORM-3-induced relaxation. NS-2028 (10 μM), a soluble guanylyl cyclase inhibitor, had no effect on a NE-contraction, but inhibited a subsequent CORM-3-induced relaxation. Zinc protopophyrin-IX (ZnPP-IX, 0.3–30 μM), a HO inhibitor, elicited a small, yet dose-dependent and significant, increase in baseline tension but did not have any effect on subsequent NE-induced contractions or a nitric oxide-induced relaxation (via sodium nitroprusside). [ZnPP-IX] greater than 3 μM, however, significantly reduced the predominant vasodilatory response of trout EBA to hydrogen sulfide. These results implicate an active HO/CO pathway in trout vessels having an impact on resting vessel tone and CO-induced vasoactivity that is at least partially mediated by soluble guanylyl cyclase.

Chondrichthyans have a bulbus arteriosus at the arterial pole of the heart: morphological and evolutionary implications

It has been generally assumed that the outflow tract of the chondrichthyan heart consists of the conus arteriosus, characterized by cardiac muscle in its walls. However, classical observations, neglected for many years, indicated that the distal component of the cardiac outflow tract of several elasmobranch species was composed of tissue resembling that of the ventral aorta. The present study was outlined to test the hypothesis that this intrapericardial, non-myocardial component might be homologous to the actinopterygian bulbus arteriosus. The material consisted of Atlantic catshark adults and embryos, which were examined by means of histochemical and immunohistochemical techniques for light and fluorescence microscopy. In this species, the distal component of the outflow tract differs histomorphologically from both the ventral aorta and the conus arteriosus; it is devoid of myocardium, is covered by epicardium and is crossed by the coronary arterial trunks. In the embryonic hearts examined, this distal component showed positive reactivity for 4,5-diaminofluorescein 2-diacetate (DAF-2DA), a fluorescent nitric oxide indicator. These findings, together with other observations in holocephals and several elasmobranch species, confirm that chondrichthyans possess a bulbus arteriosus interposed between the conus arteriosus and the ventral aorta. Therefore, the primitive heart of gnathostomates consists of five intrapericardial components, sinus venosus, atrium, ventricle, conus arteriosus and bulbus arteriosus, indicating that the bulbus arteriosus can no longer be regarded as an actinopterygian apomorphy. The DAF-2DA-positive reactivity of the chondrichthyan embryonic bulbus suggests that this structure is homologous to the base of the great arterial trunks of birds and mammals, which derives from the embryonic secondary heart field.

Mechanisms of vasodilation in the dorsal aorta of the elephant fish, Callorhinchus milii (Chimaeriformes: Holocephali)

This study investigated vasodilator mechanisms in the dorsal aorta of the elephant fish, Callorhinchus milii, using anatomical and physiological approaches. Nitric oxide synthase could only be located in the perivascular nerve fibres and not the endothelium of the dorsal aorta, using NADPH histochemistry and immunohistochemistry. In vitro organ bath experiments demonstrated that a NO/soluble guanylyl cyclase (GC) system appeared to be absent in the vascular smooth muscle, since the NO donors SNP (10(-4) mol l(-1)) and SIN-1 (10(-5) mol l(-1)) were without effect. Nicotine (3 x 10(-4) mol l(-1)) mediated a vasodilation that was not affected by ODQ (10(-5) mol l(-1)), L-NNA (10(-4) mol l(-1)), indomethacin (10(-5) mol l(-1)), or removal of the endothelium. In contrast, the voltage-gated sodium channel inhibitor, tetrodotoxin (10(-5) mol l(-1)), significantly decreased the dilation induced by nicotine, suggesting that it contained a neural component. Pre-incubation of the dorsal aorta with the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP(8-37) (10(-6) mol l(-1)) also caused a significant decrease in the nicotine-induced dilation. We propose that nicotine is mediating a neurally-derived vasodilation in the dorsal aorta that is independent of NO, prostaglandins and the endothelium, and partly mediated by CGRP.

Dual mechanisms for nitric oxide control of large arteries in the estuarine crocodile Crocodylus porosus

In reptiles, accumulating evidence suggests that nitric oxide (NO) induces a potent relaxation in the systemic vasculature. However, very few studies have examined the source from which NO is derived. Therefore, the present study used both anatomical and physiological approaches to establish whether NO-mediated vasodilation is via an endothelial or neural NO pathway in the large arteries of the estuarine crocodile Crocodylus porosus. Specific endothelial nitric oxide synthase (NOS) staining was observed in aortic endothelial cells following nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and endothelial NOS immunohistochemistry (IHC), suggesting that an endothelial NO pathway is involved in vascular control. This finding was supported by in vitroorgan bath physiology, which demonstrated that the relaxation induced by acetylcholine (10-5 mol l-1) was abolished in the presence of the NOS inhibitor, N-omega-nitro-l-arginine(l-NNA; 10-4 mol l-1), the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ;10-5 mol l-1), or when the endothelium was removed. Interestingly, evidence for a neural NO pathway was also identified in large arteries of the crocodile. Neural NOS was located in perivascular nerves of the major blood vessels following NADPH-d histochemistry and neural NOS IHC and in isolated aortic rings, l-NNA and ODQ, but not the removal of the endothelium, abolished the relaxation effect of the neural NOS agonist,nicotine (3×10-4 mol l-1). Thus, we conclude that the large arteries of C. porosus are potentially regulated by NO-derived from both endothelial and neural NOS.

Neuronal nitric oxide synthase in the gill of the killifish, Fundulus heteroclitus

Neuronal NOS (nNOS) is a constitutively expressed enzyme that catalyzes the oxidation of L-arginine and water to L-citrulline and the gas nitric oxide (NO). Nitric oxide is involved in regulation of a variety of processes, including: vascular tone, neurotransmission, and ion balance in mammals and fishes. In this study, we have cloned and characterized a putative NOS homologue from the brain of the euryhaline killifish, Fundulus heteroclitus. Killifish NOS has 75% amino acid identity to human nNOS, and phylogenetic analysis groups the killifish sequence with the mammalian nNOS, suggesting that it is a mammalian orthologue. Relative quantitative reverse transcriptase-PCR demonstrated that killifish nNOS mRNA is highly expressed in the brain and gill followed by the stomach, kidney, opercular epithelium, intestine and heart. Immunohistochemistry localized nNOS to nerve fibers and epithelial cells adjacent to mitochondrion-rich cells (ion transporting cell) in the gill, suggesting that nNOS production of NO may contribute to regulation of vascular tone and/or MRC function in the teleost gill.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

NO as a mediator during the early development of the cardiovascular system in the zebrafish

As a general pattern innervation of the cardiovascular system appears late during development in vertebrate embryos, and cardiovascular control may be achieved by hormonal activity in early stages. However, very little is known about the onset of NO-responsiveness during development, which in adult vertebrates is known to play a key function in many physiological processes such as control of vascular tone, neurotransmission, macrophage activity, and angiogenesis. Analysis of the effect of NO on the cardiovascular system in zebrafish (Danio rerio) embryos and larvae revealed almost no effect on cardiac activity during chronic exposure to NO-producing chemicals, whereas vascular reactivity was observed in veins and arteries of the zebrafish in early developmental stages (5-6 days post fertilization). Chronic exposure also modified the development of the vascular system. The presence of an NO donor (sodium nitroprusside) did not change the patterning of the vascular bed, but it induced an earlier appearance of some blood vessels in the trunk region of the zebrafish larvae. The data reveal that NO plays an important role in the development of the cardiovascular system and in the ontogeny of the cardiovascular control system in fish.

Nitric oxide control of lower vertebrate blood vessels by vasomotor nerves

In mammals, much is understood about the endothelial and neural NO control mechanisms in the vasculature. In contrast, NO control of blood vessels in lower vertebrates is poorly understood, with the majority of research focusing on the presence of an endothelial NO system; however, its presence remains controversial. This study examined the mechanisms by which NO regulates the large blood vessels of non-mammalian vertebrates. In all species examined, the arteries and veins contained a plexus of NOS-positive perivascular nerves that included nerve bundles and fine, varicose nerve terminals. However, in the large arteries and veins of various species of fishes and amphibians, no anatomical evidence was found for endothelial NOS using both NADPH-diaphorase and eNOS immunohistochemistry. In contrast, perinuclear NOS staining was readily apparent in blue-tongue lizard, pigeon and rat, which suggested that eNOS first appeared in reptiles. Physiological analysis of NO signalling in the vascular smooth muscle of short-finned eel and cane toad could not find any evidence for endothelial NO signalling. In contrast, it appears that activation of the nitrergic vasomotor nerves is responsible for NO control of the blood vessels.

Nitric oxide control of large veins in the toad Bufo marinus

This study examined the nitric oxide (NO) control of the vascular smooth muscle of the ventral abdominal vein and vena cava of the toad, Bufo marinus, by using anatomical and physiological approaches. Nicotinamide adenine di-nucleotide phosphate-diaphorase histochemistry and immunohistochemistry using endothelial nitric oxide synthase (NOS) and neural NOS antibodies produced no evidence for endothelial NOS in the veins, but, neural NOS-immunoreactive perivascular nerves were present. Acetylcholine (10(-5) M) caused a vasodilation in both veins that was endothelium-independent, and which was blocked by the soluble guanylyl cyclase inhibitor, ODQ (10(-5) M). The NOS inhibitors, L-NNA (10(-4) M) and L-NAME (10(-4) M), did not significantly reduce the vasodilatory effect of acetylcholine in the veins; this suggested that the vasodilation was not due to NO. However, in the presence of phenoxybenzamine (10(-7)-10(-8) M), L-NNA significantly reduced the vasodilatory effect of acetylcholine in the veins. This unusual response is due to phenoxybenzamine partially inactivating the muscarinic receptor pool in the veins. In addition, the neural NOS inhibitor, vinyl-L-NIO (10(-5) M), significantly reduced the acetylcholine-mediated vasodilation in the presence of phenoxybenzamine. The results show that in toad veins, nitrergic nerves rather than an endothelial NO system are involved in NO-mediated vasodilation.

Nitric Oxide (NO) in normal and hypoxic vascular regulation of the spiny dogfish,Squalus acanthias

Nitric oxide (NO) is a potent vasodilator in terrestrial vertebrates, but whether vascular endothelial-derived NO plays a role in vascular regulation in fish remains controversial. To explore this issue, a study was made of spiny dogfish sharks (Squalus acanthias) in normoxia and acute hypoxia (60 min exposure to seawater equilibrated with 3% oxygen) with various agents known to alter NO metabolism or availability. In normoxia, nitroprusside (a NO donor) reduced blood pressure by 20%, establishing that vascular smooth muscle responds to NO. L-arginine, the substrate for NO synthase, had no hemodynamic effect. Acetylcholine, which stimulates endothelial NO and prostaglandin production in mammals, reduced blood pressure, but also caused marked bradycardia. L-NAME, an inhibitor of all NO synthases, caused a small 10% rise in blood pressure, but cell-free hemoglobin (a potent NO scavenger and hypertensive agent in mammals) had no effect. Acute hypoxia caused a 15% fall in blood pressure, which was blocked by L-NAME and cell-free hemoglobin. Serum nitrite, a marker of NO production, rose with hypoxia, but not with L-NAME. Results suggest that NO is not an endothelial-derived vasodilator in the normoxic elasmobranch. The hypertensive effect of L-NAME may represent inhibition of NO production in the CNS and nerves regulating blood pressure. In acute hypoxia, there is a rapid up-regulation of vascular NO production that appears to be responsible for hypoxic vasodilation.