No description available.

No description available.

Dilation of blood vessels in response to a large number of agents has been shown to be dependent on an intact vascular endothelium. The present studies examine some aspects of endothelium-dependent vasodilation in blood vessels of the rabbit and rat. Using the rabbit ear artery and the subtype-selective muscarinic antagonist pirenzepine, muscarinic receptors of the endothelium and smooth muscle cells were shown to be of the low affinity M₂ subtype. Inhibition of [³H](-)quinuclidinyl benzilate was used to determine affinity for the smooth muscle receptors while antagonism of methacholine induced vasodilation yielded the endothelial cell receptor affinity. The effect of increasing age (1-27 months) on endothelium-dependent relaxation was studied in aortic rings, perfused tail artery and perfused mesenteric bed of the Fisher 344 rat. Both aortic ring segments and perfused caudal arteries showed an age-related increase in sensitivity of endothelium-mediated relaxation to the cholinergic agonist methacholine. This increased sensitivity occurs between the ages of 6 and 12 months, with no further significant increase up to 27 months of age, suggesting this is a consequence of growth and development rather than old age. No difference with age in cholinergic relaxation was observed in the perfused mesenteric bed indicating either no change of sensitivity in smaller resistance vessels or an effect which is hidden in this more complex perfused system. In contrast to findings with cholinergic stimution, responses of the perfused caudal artery to the calcium ionophore A23187 were not altered with age. This suggests that the alteration with age in response to methacholine involves the muscarinic receptor or receptor coupling mechanism rather than the generation of, or response to, endothelium-derived relaxing factor (EDRF). The influence of endothelium on contractile responses was examined using the perfused caudal artery. Endothelium removal significantly increased contraction to the α-adrenergic agonists methoxamine and BH-T 920 as well as to transmural nerve stimulation. Inhibition of contraction to agents which must first cross the smooth muscle layer before reaching the endothelium suggests that a continuous or basal level of EDRF release is responsible for decreased contraction rather than an receptor stimulated release of EDRF.

Thesis (M. Phil.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 108-119).

No description available.

Thirty-four dogs were anesthetized with sodium pentothal i.v. and maintained with i.v. alpha-chloralose. Neuro-muscular blockade was accomplished with gallamine triethiodide (Flaxedil). Respiratory PCO₂ was monitored continuously while artificial ventilation at a rate of 1 5 cpm and appropriate tidal volume was adjusted to maintain expiratory P CO₂ between 38 and 40 mm Hg. Blood gas analysis (P CO₂, PC₂ and pH) allowed maintenance of blood pH between 7.35 and 7.45 by periodic administration of i.v. sodium bicarbonate. Blood volume was maintained with Dextran 75 when necessary. Body temperature was monitored continuously with an esophageal thermister and maintained automatically with heating elements in the operating table. Arterial vascular isolation of the hindlimbs was accomplished by ligating all major branches of the aorta below the renal arteries except the external iliac arteries. The dog's own blood, taken from a cannula in the abdominal aorta just distal to the renal arteries, was perfused at constant flow into cannulae in the external iliac arteries through separate pumps. Each external iliac artery pressure was monitored separately (Fig. 1). A bilateral laminectomy allowed access to the L₅, ₆ and ₇ spinal segments for electrical stimulation of their ventral roots after section of the corresponding dorsal root. In 26 dogs monophasic square wave stimulation (3 to 10V, 3 msec, 8 to 20 Hz) of the ventral root of L₅, L₆ or L₇ induced:1) a decrease in the perfusion pressure (PP) in the ipsilateral hindlimb (-41.8 - 2.7 mm Hg; mean - SE); 2) a decrease in the PP in the contralateral hindlimb (-32.2 - 2.7); 3) a fall in the aortic pressure (-15.6 - 0.7). (Fig.. 3). Similar effects were observed on stimulation of the peripheral stump of the ventral root. The above described vascular effects of ventral root stimulation were resistant to intra-arterial injections of cholinergic and beta-adrenergic blocking agents administered directly into the hindlimb perfusion lines. The effectiveness of the blockades was tested with direct intra-arterial injections of the appropriate agonists. Antihistaminics (diphenhydramine and mepyramine) similarly administered and tested did abolish the response in doses which did not suppress vascular-reactivity to acetylcholine or isoproterenol. These experiments do not provide a clear explanation of the mechanisms responsible for the contralateral vasodilatation or the fall in aortic pressure. The presence of significant anastomotic channels connecting either the two hindlimbs and/or the hindlimbs with the rest of-the body was excluded. Contralateral vasodilatation might perhaps be explained by the presence of nerve fibres crossing the midline in. the fused impar ganglion of the dog. The drop in aortic pressure was not due to the activation of afferent fibres coursing in the ventral roots, nor to the peripheral release of a vasodilator substance since the onset of the phen-omenom was too fast to be explained on these grounds. The possibility exists that the drop in aortic pressure is due to the activation by the stimulated efferent fibres of some afferent nervous pathways carrying inhibitory impulses to the vasomotor centers. The present experiments, however, do not provide data supporting or excluding this hypothesis. The experimental results strongly suggest that the described vasodilatation may be mediated by histamine released directly or indirectly by the activation of fibres coursing into the lower-ventral roots. / Medicine, Faculty of / Cellular and Physiological Sciences, Department of / Graduate

No description available.

No description available.

published_or_final_version / Physiology / Master / Master of Philosophy

The natriuretic peptides, atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) are members of a family of hormones that play an important role in mammalian fluid and electrolyte balance. In the periphery, natriuretic peptides reduce blood volume and subsequently blood pressure by increasing renal natriuresis and diuresis and relaxation of vascular smooth muscle. The actions of natriuretic peptides are mediated via two membrane-linked guanylate cyclase receptors (NPR-GC); natriuretic peptide receptor-A (NPR-A) which has a high affinity for ANP and BNP; and natriuretic peptide receptor-B (NPR-B)which has the greatest affinity for CNP. A third receptor not linked to guanylate cyclase, natriuretic peptide receptor-C (NPR-C) also exists, which binds to ANP, BNP and CNP with a relatively equal affinity, and is involved with clearance of the peptides from the circulation and tissues. The natriuretic peptides are present in the brain and are particularly predominant in cardiovascular and fluid and electrolyte regulating areas such as the anteroventral third ventricle (AV3V) region. This distribution has led to the suggestion natriuretic peptides play a neuromodulatory role in the central control of fluid homeostasis. Natriuretic peptides in the brain have been observed to inhibit the release of other fluid and electrolyte regulating hormones such as arginine vasopressin (AVP) and angiotensin II (AII). Natriuretic peptides have also been identified in the non-mammalian vertebrates although information regarding the distribution of the peptides and their receptors in the non-mammalian brain is limited. In amphibians, immunohistochemical studies have shown that natriuretic peptides are highly concentrated in the preoptic region of the brain, an area believed to be analogous to the A\T3\ region in mammals, which suggests that natriuretic peptides may also be involved in central fluid and electrolyte regulation in amphibians. To date, CNP is the only natriuretic peptide that has been isolated and cloned from the lower vertebrate brain, although studies on the distribution of CNP binding sites in the brain have only been performed in one fish species. Studies on the distribution of ANP binding sites in the lower vertebrate brain are similarly limited and have only been performed in one fish and two amphibian species. Moreover, the nature and distribution of the natriuretic peptide receptors has not been characterised. The current study therefore, used several approaches to investigate the distribution of natriuretic peptides and their receptors in the brain of the amphibian Bufo marinus. The topographical relationship of natriuretic peptides and the fluid and electrolyte regulating hormone arginine vasotocin was also investigated, in order to gain a greater understanding of the role of the natriuretic peptide system in the lower vertebrate brain. Immunohistochemical studies showed natriuretic peptides were distributed throughout the brain and were highly concentrated in the preoptic region and interpeduncular nucleus. No natriuretic peptide-like immunoreactivity (NP-IR) was observed in the pituitary gland. Arginine vasotocin-like immunoreactivity (AvT-IR) was confined to distinct regions, particularly in the preoptic/hypothalamic region and pituitary gland. Double labelling studies of NP-JR and AvT-IR showed the peptides are not colocalised in the same neural pathways. The distribution of natriuretic peptide binding sites using the ligands 125I-rat ANP (125I-rANP) and 125I-porcine CNP (125I-pCNP) showed different distributions in the brain of B. marinus. The specificity of binding was determined by displacement with unlabelled rat ANP, porcine CNP and C-ANF, an NPR-C specific ligand. 125I-rANP binding sites were broadly distributed throughout the brain with the highest concentration in pituitary gland, habenular, medial pallium and olfactory region. Minimal 125I-rANP binding was observed in the preoptic region. Residual 125I-rANP binding in the presence of C-ANF was observed in the olfactory region, habenular and pituitary gland indicating the presence of both NPR-GC and NPR-C in these regions. 125I-pCNP binding was limited to the olfactory region, pallium and posterior pituitary gland. All 125I-pCNP binding was displaced by C-ANF which suggests that CNP in the brain of B. marinus binds only to NPR-C. Affinity cross-linking and SDS-PAGB demonstrated two binding sites at 136 kDa and 65 kDa under reducing conditions. Guanylate cyclase assays showed 0.1 µM ANP increased cGMP levels 50% above basal whilst a 10-fold higher concentration of CNP was required to produce the same result. Molecular cloning studies revealed a 669 base pair fragment showing 91% homology with human and rat NPR-A and 89% homology with human, rat and eel NPR-B. A 432 base pair fragment showing 67% homology to the mammalian NPR-C and 58% homology with eel NPR-D was also obtained. The results show natriuretic peptides and their receptors are distributed throughout the brain of B. marinus which indicates that natriuretic peptides may participate in a range of regulatory functions throughout the brain. The potential for natriuretic peptides to regulate the release of the fluid and electrolyte regulating hormone AVT also exists due to the high number of natriuretic peptide binding sites in the posterior pituitary gland. At least two populations of natriuretic peptide receptors are present in the brain of B. marinus, one linked to guanylate cyclase and one resembling the mammalian clearance receptor. Furthermore, autoradiography and guanylate cyclase studies suggest ANP may be the major ligand in the brain of B. marinus, even though CNP is the only natriuretic peptide that has been isolated from the lower vertebrate brain to date.