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OOTAWA T, WU S, SEKIO R, SMITH H, ISLAM MZ, NGUYEN HTT, UNO Y, SHIRAISHI M, MIYAMOTO A. Habu snakes (Protobothrops flavoviridis) show variation in thoracic aortic vasoreactivity between adjacent Japanese islands. J Vet Med Sci 2024; 86:202-206. [PMID: 38104972 PMCID: PMC10898993 DOI: 10.1292/jvms.23-0361] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2023] [Accepted: 11/27/2023] [Indexed: 12/19/2023] Open
Abstract
Habu snakes (Protobothrops flavoviridis) are pit vipers found in the geographically adjacent but ecologically divergent islands of Tokunoshima and Amami-Oshima in southwestern Japan. Abiotic factors can cause variation in animal populations between the two islands, and Habu snakes may show such intraspecific physiological variation. We therefore evaluated the vasoreactivity in aortas isolated from the Habu of both islands. Tokunoshima Habu showed significantly greater contractile responses to angiotensin (Ang) II, acetylcholine (ACh) and noradrenaline, and significantly higher affinities (pEC50) for Ang II and ACh, than Amami-Oshima Habu. ACh caused contractions in aortas from both populations, a finding previously unreported in snakes. Our findings indicate that vasoreactivity may differ between Tokunoshima and Amami-Oshima Habu.
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Affiliation(s)
- Tomoki OOTAWA
- Department of Basic Veterinary Science, Joint Graduate
School of Veterinary Medicine, Kagoshima University, Kagoshima, Japan
- Japan Wildlife Research Center, Tokyo, Japan
| | - Siyuan WU
- Department of Basic Veterinary Science, Joint Graduate
School of Veterinary Medicine, Kagoshima University, Kagoshima, Japan
| | - Ryoya SEKIO
- Department of Veterinary Pharmacology, Joint Faculty of
Veterinary Medicine, Kagoshima University, Kagoshima, Japan
| | - Henry SMITH
- Department of Basic Veterinary Science, Joint Graduate
School of Veterinary Medicine, Kagoshima University, Kagoshima, Japan
| | - Md Zahorul ISLAM
- Department of Pharmacology, Faculty of Veterinary Science,
Bangladesh Agricultural University, Mymensingh, Bangladesh
| | - Ha Thi Thanh NGUYEN
- Department of Veterinary Pharmacology and Toxicology,
Faculty of Veterinary Medicine, Vietnam National University of Agriculture, Hanoi,
Vietnam
| | - Yasuhiro UNO
- Department of Basic Veterinary Science, Joint Graduate
School of Veterinary Medicine, Kagoshima University, Kagoshima, Japan
- Department of Veterinary Pharmacology, Joint Faculty of
Veterinary Medicine, Kagoshima University, Kagoshima, Japan
| | - Mitsuya SHIRAISHI
- Department of Basic Veterinary Science, Joint Graduate
School of Veterinary Medicine, Kagoshima University, Kagoshima, Japan
| | - Atsushi MIYAMOTO
- Department of Basic Veterinary Science, Joint Graduate
School of Veterinary Medicine, Kagoshima University, Kagoshima, Japan
- Department of Veterinary Pharmacology, Joint Faculty of
Veterinary Medicine, Kagoshima University, Kagoshima, Japan
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The vasopressor action of angiotensin II (ANG II) in ball pythons (Python regius). Comp Biochem Physiol A Mol Integr Physiol 2020; 252:110839. [PMID: 33166686 DOI: 10.1016/j.cbpa.2020.110839] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/02/2020] [Revised: 11/03/2020] [Accepted: 11/03/2020] [Indexed: 11/20/2022]
Abstract
Angiotensin II (ANG II) is part of the renin-angiotensin system (RAS) in vertebrates and exert vasoconstriction in all species studied. The present study examines the vasopressor effect of ANG II in the ball python (Python regius), and examines whether ANG II exert its effect through direct angiotensin receptors or through an activation of α-adrenergic receptors. The studies were conducted in snakes with chronic arterial catheters that had recovered from anesthesia. In addition to demonstrating a clear and pronounced dose-dependent rise in arterial blood pressure upon repeated injections of boluses with ANG II (0.001-1 μg/kg), we demonstrate that the pressor response persisted following α-adrenergic blockade using the α-adrenergic antagonist phentolamine (2.5 mg/kg). Unfortunately, it proved impossible to block the ANG receptors using losartan (1, 3 or even 10 mg/kg). The pressor response to ANG II was associated with a significant rise in heart rate at the higher dosages, pointing to a resetting of the barostatic mechanism for heart rate regulation. The responses were similar in fasting and digesting pythons despite the expected rise in baseline values for blood pressure and heart rate of the digesting snakes.
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Distribution and properties of cardiac and pulmonary β-adrenergic receptors in corn snakes (Pantherophis guttatus) and Boa constrictor (Boa constrictor). Comp Biochem Physiol A Mol Integr Physiol 2019; 233:17-23. [PMID: 30930202 DOI: 10.1016/j.cbpa.2019.03.023] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2018] [Revised: 03/22/2019] [Accepted: 03/25/2019] [Indexed: 11/23/2022]
Abstract
The aim of the present study was to characterize β-adrenergic receptors in the snake heart and lung of corn and Boa constrictor snakes. The β-adrenergic receptor binding sites were studied in purified heart and lung membranes using the specific β-adrenergic receptor antagonist [125J]-iodocyanopindolol (ICYP) and subtypes using selective β1-adrenergic receptor antagonist CGP-20712A and selective β2-adrenergic receptor antagonist ICI-118.551. A saturable and specific β-adrenergic receptor binding site was detected in cardiac membranes with maximal receptor density (Bmax) of 43.99 ± 3.86 fmol/mg protein (corn snake) and 58.07 ± 2.88 fmol/mg protein (Boa constrictor) as well as KD of 24.21 ± 7.38 pM (corn snake) and 21.48 ± 3.85 pM (Boa constrictor) and in lung membranes (Bmax fmol/mg protein: 55.95 ± 16.28 (corn snake) and 107.00 ± 14.21 (Boa constrictor); KD pM: 71.25 ± 21.92 (corn snake) and 55.04 ± 18.68 (Boa constrictor)). Competition-binding studies showed β-adrenergic receptors with low affinities to the β2-selective adrenergic receptor antagonist and high affinity binding to β1-selective adrenergic receptor antagonist in both heart and lung tissues of both snake species, suggesting the presence of high population of the post-synaptic β1-adrenergic receptor subtype. It seems that the presence of the predominant β1-subtype also in lung tissues may indicate the importance of the vascular system in the snake lung.
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Affiliation(s)
- James E. Bogan
- The Critter Fixer of Central Florida, LLC, 83 Geneva Drive, 621679, Oviedo, FL 32765, USA
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Esteves CA, Burckhardt PL, Breno MC. Presence of functional angiotensin II receptor and angiotensin converting enzyme in the aorta of the snake Bothrops jararaca. Life Sci 2012; 91:944-50. [PMID: 23000029 DOI: 10.1016/j.lfs.2012.09.006] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2012] [Revised: 08/29/2012] [Accepted: 09/07/2012] [Indexed: 11/24/2022]
Abstract
AIM Angiotensin II (Ang II) interacts with AT(1) and AT(2) receptors and, in some vertebrates, with an Ang II binding site showing low affinity for AT(1) and AT(2) receptor antagonists. This study was carried out to characterize the Ang II receptor, and the presence of an angiotensin-converting enzyme (ACE) in the aorta of the Bothrops jararaca snake. MAIN METHOD Contraction induced by Ang I or II in aortic ring from the snake was evaluated in the absence or in the presence of ACE-blocker or Ang II antagonists. KEY FINDINGS Ang II analogs, modified at positions 1 and 5, induced vasoconstriction with differences in their potencies. The relative rank order was: [Asp(1), Val(5)] Ang II=[Asp(1), Ile(5)] Ang II>>>[Asn(1), Val(5)] Ang II. ACE-like activity was detected, as well as an Ang II receptor with low affinity for AT(1) and AT(2) selective receptor antagonists (pK(B) values of 5.62±0.23 and 5.08±0.25). A disulfide reducing agent almost abolished the Ang II effect, while an alpha adrenoceptor antagonist, or removing the endothelium, did not modify the Ang II effect. These results indicate that the B. jararaca aorta has an Ang II receptor pharmacologically distinct from AT(1) and AT(2) receptors, and the vasoconstrictor effect observed is independent of catecholamine or endothelium modulation. ACE and the AT receptor in the aorta of B. jararaca may be part of a tissue renin-angiotensin system. SIGNIFICANCE The data contribute to the knowledge of the renin-angiotensin system in vertebrate species, and provide insight into the understanding of snake Ang II receptor characteristics and diversity.
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Amiel JJ, Chua B, Wassersug RJ, Jones DR. Temperature-dependent regulation of blood distribution in snakes. ACTA ACUST UNITED AC 2011; 214:1458-62. [PMID: 21490254 DOI: 10.1242/jeb.053934] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Regional control of blood flow is often suggested as a mechanism for fine thermoregulatory adjustments in snakes. However, the flow of blood to different body regions at various temperatures has never been visualized to confirm this mechanism. We used (99m)technetium-labelled macroaggregated albumin ((99m)Tc-MAA), a radioactive tracer, to follow the flow of blood through the bodies of garter snakes (Thamnophis sirtalis) near their thermal maxima and minima. We injected snakes with(99m)Tc-MAA at cold (6-8°C) and hot (27-32°C) temperatures and imaged them using a gamma scanner. At cold ambient temperatures, snakes significantly reduced the blood flow to their tails and significantly increased the blood flow to their heads. Conversely, at hot ambient temperatures, snakes significantly increased the blood flow to their tails and significantly reduced the blood flow to their heads. This confirms that snakes are able to use differential blood distribution to regulate temperature. Our images confirm that snakes use regional control of blood flow as a means of thermoregulation and that vasomotor control of vascular beds is likely to be the mechanism of control.
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Affiliation(s)
- Joshua J Amiel
- School of Biological Sciences, The University of Sydney, Heydon-Laurence Building (A08), Science Road, Sydney, NSW 2006, Australia.
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Yoshinaga N, Okuno T, Watanabe Y, Matsumoto T, Shiraishi M, Obi T, Yabuki A, Miyamoto A. Vasomotor effects of noradrenaline, acetylcholine, histamine, 5-hydroxytryptamine and bradykinin on snake (Trimeresurus flavoviridis) basilar arteries. Comp Biochem Physiol C Toxicol Pharmacol 2007; 146:478-83. [PMID: 17604230 DOI: 10.1016/j.cbpc.2007.05.007] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/07/2007] [Revised: 05/30/2007] [Accepted: 05/31/2007] [Indexed: 10/23/2022]
Abstract
We investigated the responsiveness of basilar arterial rings isolated from snakes to noradrenaline (NA), acetylcholine (ACh), histamine (His), 5-hydroxytryptamine (5-HT), mammalian bradykinin (BK) and rattlesnake BK. We also examined whether endothelial cells were involved in the responsiveness to ACh, BK, rattlesnake BK and in their resting vascular tone. NA and 5-HT induced concentration-dependent contractions. The cumulative concentration response curves of NA and 5-HT were shifted to the right in parallel by phentolamine (an alpha antagonist) and methiothepin (a 5-HT(1) and 5-HT(2) antagonist), respectively. However, ketanserin (a 5-HT(2) antagonist) had no effect on the cumulative concentration response curve of 5-HT. His, ACh, BK and rattlesnake BK had no effect on resting vascular tone; however, rattlesnake BK and sodium nitroprusside relaxed arteries precontracted by 5-HT. The rattlesnake BK-induced relaxations were almost abolished by L-nitro arginine (L-NA, a nitric oxide synthase inhibitor). L-NA and indomethacin (a cyclooxygenase inhibitor) had no effect on resting vascular tone or on precontracted arteries. These results suggest that alpha and 5-HT(1) receptor subtypes might be important in arterial contraction. Endothelial cells might play an important role in the responsiveness of snake basilar arteries to rattlesnake BK, but they might not be involved in the responsiveness to ACh, BK and in resting vascular tone.
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Affiliation(s)
- Narihiro Yoshinaga
- Department of Veterinary Pharmacology, Faculty of Agriculture, Kagoshima University, 1-21-24 Korimoto, Kagoshima 890-0065, Japan
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Breno MC, Prezoto BC, Borgheresi RAMB, Lazari MFM, Yamanouye N. Characteristics of neural and humoral systems involved in the regulation of blood pressure in snakes. Comp Biochem Physiol A Mol Integr Physiol 2007; 147:766-778. [PMID: 17046304 DOI: 10.1016/j.cbpa.2006.09.002] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2006] [Revised: 09/04/2006] [Accepted: 09/05/2006] [Indexed: 12/22/2022]
Abstract
Cardiovascular function is affected by many mechanisms, including the autonomic system, the kallikrein-kinin system (KKS), the renin-angiotensin system (RAS) and the endothelin system. The function of these systems seems to be fairly well preserved throughout the vertebrate scale, but evolution required several adaptations. Snakes are particularly interesting for studies related to the cardiovascular function because of their elongated shape, their wide variation in size and length, and because they had to adapt to extremely different habitats and gravitational influences. To keep the normal cardiovascular control the snakes developed anatomical and functional adaptations and interesting structural peculiarities are found in their autonomic, KKS, RAS and endothelin systems. Our laboratory has characterized some biochemical, pharmacological and physiological properties of these systems in South American snakes. This review compares the components and function of these systems in snakes and other vertebrates, and focuses on differences found in snakes, related with receptor or ligand structure and/or function in autonomic system, RAS and KKS, absence of components in KKS and the intriguing identity between a venom and a plasma component in the endothelin system.
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Affiliation(s)
- Maria Cristina Breno
- Laboratório de Farmacologia, Instituto Butantan, Av. Vital Brazil, 1500, 05503-900, São Paulo, SP, Brazil.
| | - Benedito Carlos Prezoto
- Laboratório de Farmacologia, Instituto Butantan, Av. Vital Brazil, 1500, 05503-900, São Paulo, SP, Brazil
| | - Rosa A M B Borgheresi
- Laboratório de Farmacologia, Instituto Butantan, Av. Vital Brazil, 1500, 05503-900, São Paulo, SP, Brazil
| | - Maria Fátima M Lazari
- Departamento de Farmacologia, Setor Endocrinologia Experimental, Universidade Federal de São Paulo, Rua 3 de maio,100, 04044-020, São Paulo, SP, Brazil
| | - Norma Yamanouye
- Laboratório de Farmacologia, Instituto Butantan, Av. Vital Brazil, 1500, 05503-900, São Paulo, SP, Brazil
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Nishimura H. Angiotensin receptors--evolutionary overview and perspectives. Comp Biochem Physiol A Mol Integr Physiol 2001; 128:11-30. [PMID: 11137436 DOI: 10.1016/s1095-6433(00)00294-4] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The structure of the angiotensin molecule has been well preserved throughout the vertebrate scale with some amino acid variations. Specific angiotensin receptors (AT receptors) that mediate important physiological functions have been noted in a variety of tissues and species. Physiological and pharmacological characterization of AT receptors and, more recently, molecular cloning studies have elucidated the presence of AT receptor subtypes. Comparative studies suggest that an AT receptor subtype homologous to the mammalian type 1 receptor subtype (AT(1)), though pharmacologically distinct, is present in amphibians and birds, whereas AT receptors cloned from teleosts show low homology to both AT(1) and AT(2) receptor subtypes. Furthermore, receptors differing from both the AT(1)-homologue receptor and AT(2) receptor exist in some non-mammalian species. This may suggest that the prototype AT receptor evolved in primitive vertebrates and diverged to more than one type of AT receptor subtype during phylogeny. Furthermore, phenotypic modulation of AT receptors appears to occur during individual development/maturation.
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Affiliation(s)
- H Nishimura
- Department of Physiology, University of Tennessee Health Science Center, 894 Union Avenue, Memphis, TN 38163, USA.
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Conklin DJ, Lillywhite HB, Olson KR, Ballard RE, Hargens AR. Blood vessel adaptation to gravity in a semi-arboreal snake. J Comp Physiol B 1996; 165:518-26. [PMID: 8617890 DOI: 10.1007/bf00387512] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The effects of vasoactive agonists on systemic blood vessels were examined with respect to anatomical location and gravity acclimation in the semi-arboreal snake, Elaphe Obsoleta. Major blood vessels were reactive to putative neurotransmitters, hormones or local factors in vessel specific patterns. Catecholamines, adenosine triphosphate, histamine and high potassium (80 mM) stimulated significantly greater tension per unit vessel mass in posterior than anterior arteries. Anterior vessels were significantly more sensitive to catecholamines than midbody and posterior vessels. Angiotensin II stimulated significantly greater tension in carotid artery than in midbody and posterior dorsal aorta. Arginine vasotocin strongly contracted the left and right aortic arches and anterior dorsal aorta. Veins were strongly contracted by catecholamines, high potassium and angiotensin II, but less so by adenosine triphosphate, arginine vasotocin and histamine. Precontracted vessel were relaxed by acetylcholine and sodium nitroprusside, but not by atrial natriuretic peptide or bradykinin. Chronic exposure of snakes to intermittent hypergravity stress ( + 1.5 Gz at tail) did not affect the majority of vessel responses. These data demonstrate that in vitro tension correlates with that catecholamines, as well as other agonists, are important in mediating vascular responses to gravitational stresses in snakes.
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Affiliation(s)
- D J Conklin
- Life Sciences Division, NASA-Ames Research Center, Moffett Field, CA 94035-1000, USA
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Breno MC, Picarelli ZP. The vasopressor action of angiotensin in the snake Bothrops jararaca. ACTA ACUST UNITED AC 1992; 101:819-25. [PMID: 1351456 DOI: 10.1016/0300-9629(92)90364-v] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
1. Carotid blood pressure from anesthetized B. jararaca snakes was recorded in order to study angiotensin action in this reptile. 2. Whereas [Asn1,Val5] AII and AIII were less potent than [Asp1,Ile5] AII and [Asp1,Val5] AII, [Sar1,Ile5] AII was slightly more potent. 3. Captopril abolished the responses to AI (0.01-3 micrograms/kg). 4. [Sar1,Ala8] AII was uneffective but [Sar1,Leu8] AII or phenoxybenzamine were able to reduce AII vasopressor responses. 5. These results led to the conclusion that the vasopressor response of AII in B. jararaca is due to an interaction with its own receptor but, part of the AII receptor population seems to be coupled to the sympatho-adrenal system. Moreover, structural requirements seem to be necessary for the AII response in B. jararaca.
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Affiliation(s)
- M C Breno
- Serviço de Farmacologia, Instituto Butantan, São Paulo, Brazil
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Yamanouye N, Salomão LC, Picarelli ZP. Effects of catecholamines on the isolated aorta of the snake Bothrops jararaca. ACTA ACUST UNITED AC 1992; 101:197-201. [PMID: 1354090 DOI: 10.1016/0742-8413(92)90260-e] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
1. Effects of catecholamines in snakes have been examined using an aorta preparation isolated from Bothrops jararaca. Adrenaline, noradrenaline and isoprenaline produced dose-dependent contractions on this preparation. The relative potency was adrenaline greater than noradrenaline greater than isoprenaline. 2. Phentolamine displaced, to the right, the concentration-response curve of the three catecholamines tested, showing the presence of alpha-adrenoceptors in this preparation. 3. Isoprenaline has never produced a relaxation, even when the aorta was first contracted by BaCl2 and pretreated with phentolamine, indicating that beta-adrenoceptors are absent in this preparation. 4. In this Bothrops jararaca preparation, exclusively neuronal uptake was found, thus demonstrating that its existence was preserved during evolution.
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Affiliation(s)
- N Yamanouye
- Serviço de Farmacologia, Instituto Butantan, Brazil, São Paulo
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Abstract
Three putative pressor systems, the alpha-adrenergic system (AS), the renin angiotensin system (RAS), and the arginine vasotocin system (AVT-S), were studied for their roles in blood pressure regulation and their possible interactions in the rat snake. Ptyas korros. Norepinephrine (NE), angiotensin I (ANG I), and arginine vasotocin (AVT) increased the mean arterial pressure (MAP) of the snake while administration of phentolamine, an alpha-adrenergic antagonist, and captopril, an ANG-converting enzyme inhibitor, but not KBIV24, an AVT antagonist, decreased the MAP. Treatment with any combination of two of these antagonists/inhibitor invariably decreased the MAP. Treatment with the agonist of the remaining third system invariably returned the MAP to normal or above. Phentolamine and KBIV24 attenuated the vasopressor effect of ANG I. Phentolamine and captopril enhanced the vasopressor effect of AVT. The pressor effect of NE was not altered by KBIV24 and captopril. It was concluded that there were at least two pressor systems (AS and RAS) regulating the basal MAP in the snake. There was also interaction among the three systems which could affect the MAP.
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Abstract
The hindquarter of the rat snake, Ptyas korros, was isolated for perfusion study in order to investigate the mechanism of action of angiotensin (ANG) II. Both ANG II and norepinephrine (NE) produced concentration-dependent increases in perfusion pressure. [Sar1, Ile8] ANG II significantly attenuated the response to ANG II but not that of NE. Phentolamine significantly reduced the responses to both ANG II and NE, with much greater inhibition on NE than ANG II. These findings suggest the presence of ANG II receptors and alpha-adrenergic receptors in the hindquarter. Since tyramine did not exert any increase in perfusion pressure, the release of ANG II-stimulated catecholamines was probably not involved in the increase in perfusion pressure to ANG II in the present preparation. The partial attenuation of the response to ANG II by phentolamine, therefore, suggests that ANG II may act directly on both specific ANG II receptors and alpha-adrenergic receptors.
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Chiu KW, Sham JS. Adrenergic receptors of isolated snake atria. COMPARATIVE BIOCHEMISTRY AND PHYSIOLOGY. C, COMPARATIVE PHARMACOLOGY AND TOXICOLOGY 1985; 81:445-50. [PMID: 2861969 DOI: 10.1016/0742-8413(85)90035-0] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Cardiac adrenergic receptors in snakes were examined using an isolated atria preparation of Naja naja and Ptyas korros. Treatments included an examination of the atrial responses to selective alpha- and beta-adrenergic agonists and antagonists. In both species, both phenylephrine and isoproterenol produced dose-dependent increases in the atrial beating rate and tension. Phenylephrine-induced increases were characterized with a high affinity and low affinity components. These positive chronotropic and inotropic effects produced by phenylephrine and isoproterenol were abolished with propranolol and in the phenylephrine-induced response phentolamine also attenuated the low affinity response and blocked the high affinity response. With catecholamines depletion via 6-OH dopamine or reserpine, the high affinity component in the phenylephrine-induced response was no longer observed. It is concluded that beta-adrenoceptors are the predominant post-synaptic adrenoceptors in snake atria. Stimulatory presynaptic alpha-adrenoceptors for modulating noradrenaline release may also be present.
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