Biomedical subjects

V Ralevic

Publications and source records attributed to V Ralevic.

At least 73 records · Page 4Linked to original sources

PubMed1992

An isolated dual-perfused rabbit liver preparation for the study of hepatic blood flow regulation.

An original, isolated dual-perfused rabbit liver preparation was developed for investigations into mechanisms that control the hepatic vascular tone. The hepatic artery (HA) and portal vein (PV) were perfused at constant flows of 0.16 +/- 0.01 and 0.64 +/- 0.05 mL/g/min (n = 5), respectively. Responses of the hepatic arterial and portal venous vascular beds to noradrenaline (NA) were measured as changes in perfusion pressure. Noradrenaline injected directly into the hepatic artery and portal vein produced dose-dependent increases in pressure in the respective vascular beds, the maximum response in the hepatic arterial bed being two to three times greater than that in the portal venous bed. A restricted transmission of vasoconstrictor stimulus between the intrahepatic portal venous and hepatic arterial vasculature was demonstrated. The results demonstrate the suitability of the dual-perfused rabbit liver model for detailed studies of the control of hepatic vascular tone.

Animals↗

PubMed1992

Nitric oxide and sensory nerves are involved in the vasodilator response to acetylcholine but not calcitonin gene-related peptide in rat skin microvasculature.

1. The contributions of sensory nerves and nitric oxide (NO) to vasodilator responses to acetylcholine (ACh) and calcitonin gene-related peptide (CGRP) were examined in rat skin microvasculature with a laser Doppler flowmeter to monitor relative blood flow. 2. Perfusion of ACh (100 microM; for 30 min) over a blister base on the rat hind footpad elicited microvascular vasodilatation and this response was not sustained. CGRP (1 microM; 10 min perfusion) also elicited vasodilatation and this response was maintained even when CGRP was no longer in contact with the blister base. 3. The vasodilator response to ACh was significantly smaller in rats pretreated as neonates with capsaicin to destroy primary sensory afferents than it was in age-matched controls. The vasodilator response to CGRP was unaffected by capsaicin pretreatment. 4. Selective inhibitors of NO synthase, NG-nitro-L-arginine (L-NOARG) and NG-monomethyl-L-arginine (L-NMMA) (both at 100 microM) attenuated the vasodilator response to ACh in control rats, but had no effect on the vasodilator response to CGRP. There was a significant L-NOARG-resistant component in control rats while in capsaicin-treated rats the vasodilator response to ACh was virtually abolished by L-NOARG. The inactive stereoisomer NG-monomethyl-D-arginine (100 microM) did not affect the vasodilator response to ACh. 5. The efficacy of L-NOARG and L-NMMA as inhibitors of endothelium-dependent responses was confirmed by use of an endothelium-dependent vasodilator, the calcium ionophore A23187 (100 microM; 10 min perfusion). Vasodilatation to A23187 was strongly attenuated by both L-NOARG and L-NMMA.6. These results suggest that sensory nerves and NO are both involved in the dilatation produced by ACh in rat skin microvasculature. A component of the vasodilator response elicited by ACh involves a direct action on the microvascular endothelium with subsequent generation of NO, while an additional component is elicited via activation of sensory nerves. The vasodilator mediator(s) released by ACh from sensory nerves acts largely independently of NO.7. The vasodilator response to CGRP is independent of a prejunctional action on sensory nerves and of NO.

Acetylcholine↗

PubMed1991

Acrylamide-induced autonomic neuropathy of rat mesenteric vessels: histological and pharmacological studies.

The effects of chronic acrylamide treatment on the autonomic nervous system were investigated by histochemical and pharmacological studies. Histochemical studies showed that acrylamide caused different degrees of damage to different nerve fibre types: calcitonin gene-related peptide (CGRP)-immunoreactive (IR) nerves showed the greatest reduction in intensity and number; noradrenaline (NA)-containing nerves were somewhat less affected; substance P (SP)-IR nerves were reduced in number, but this was not significant. The profiles of SP- and particularly of CGRP-IR nerves from treated animals were noticeably different to those from the control group, being flattened and irregular. Periarterial nerve stimulation (4-32 Hz) of the isolated rat mesenteric arterial bed preparation at basal tone elicited frequency-dependent vasoconstrictor responses. The magnitude of these responses was significantly reduced at higher frequencies in acrylamide-treated animals. In preparations with tone raised by the addition of methoxamine (10(-5) M), and in the presence of guanethidine (5 x 10(-6) M), periarterial nerve stimulation elicited vasodilator responses. These responses, which result from stimulation of sensory nerves, were greatly reduced in acrylamide-treated animals. There was a tendency for mesenteric beds from acrylamide-treated animals to show increased vasoconstrictor responses to doses of exogenous NA, although this was not significant. Responses to exogenous adenosine 5'-triphosphate (a cotransmitter with NA from sympathetic nerves) were not affected. In the raised-tone preparation, vasodilator responses to exogenous CGRP (the principal vasodilator sensory transmitter of rat mesenteric arteries) were not affected by acrylamide treatment. Hence, it is unlikely that the reduced responses to nerve stimulation were due to defects in the postjunctional receptors for the principal transmitters of sympathetic and sensory-motor nerves. There was no difference in the ability of mesenteric beds from control and treated animals to vasodilate in response to acetylcholine or sodium nitroprusside, or to vasoconstrict in response to potassium chloride, indicating normal smooth muscle and endothelial responses. These results suggest that chronic acrylamide treatment produces peripheral autonomic neuropathy of rat mesenteric vessels, manifested as a dysfunction of sympathetic and sensory-motor nerves. Furthermore, the graded destruction of nerve types, such that damage occurred in the order: CGRP-IR greater than NA greater than SP-IR, indicated a differential sensitivity of different nerves to this toxin.

Acrylamide↗

PubMed1991

Nitric oxide is the mediator of ATP-induced dilatation of the rabbit hepatic arterial vascular bed.

1. Livers of 10 New Zealand White rabbits were perfused in vitro with Krebs-Bülbring buffer via the hepatic artery (HA) and portal vein (PV) at constant flows of 23 +/- 1 and 77 +/- 1 ml min-1 100 g-1 respectively. The tone of the preparation was raised with noradrenaline (concentration: 10 microM). 2. Dose-response curves for the vasodilatation produced by adenosine 5'-triphosphate (ATP), acetylcholine (ACh), adenosine, and sodium nitroprusside (SNP) were obtained following injection into the HA supply. Injections were then repeated in the presence of the L-arginine to nitric oxide pathway inhibitors N-monomethyl-L-arginine (L-NMMA, n = 6) and N-nitro-L-arginine methyl ester (L-NAME, n = 4) at concentrations of 30 microM and 100 microM for each inhibitor. 3. Both L-NMMA and L-NAME antagonized the responses to ATP and ACh; L-NAME was 2-3 times more potent than L-NMMA as an inhibitor of these endothelium-dependent vasodilatations. Neither L-NMMA nor L-NAME attenuated responses of the endothelium-independent vasodilators, adenosine and SNP. 4. These results indicate that nitric oxide is the mediator of ATP-induced vasodilatation in the HA vascular bed of the rabbit and that the receptor responsible for the release of nitric oxide, the P2y-purinoceptor, is located predominantly on the endothelium.

Acetylcholine↗

PubMed1991

Adenosine-induced dilatation of the rabbit hepatic arterial bed is mediated by A2-purinoceptors.

1. This study was carried out in order to identify the receptor responsible for adenosine-induced dilatation of the hepatic arterial vascular bed. 2. Livers of 10 New Zealand White rabbits were perfused in vitro with Krebs-Bülbring buffer via the hepatic artery and the portal vein at constant flows of 26 and 77 ml min-1 100 g-1 liver respectively. The tone of the preparation was raised by the presence of noradrenaline in the perfusate (concentration: 10(-5) M). 3. Dose-response curves for adenosine and its analogues 5'-N-ethyl-carboxamido-adenosine (NECA), the 2-substituted NECA analogue CGS 21680C, and R- and S-N6-phenyl-isopropyl-adenosine (R- and S-PIA) were obtained after their injection into the hepatic arterial supply. 4. The order of vasodilator potency of these agents was: NECA greater than CGS 21680C greater than adenosine greater than R-PIA greater than S-PIA. Their potency, expressed relative to that of adenosine, was in the approximate ratio 10:3:1:0.3:0.1, consistent with that resulting from activation of P1-purinoceptors of the A2 sub-type (which mediate vasodilatation due to adenosine). 5. The P1-purinoceptor antagonist 8-phenyltheophylline (10(-5) M) caused significant attenuation of the vasodilatation to adenosine and analogues. 6. It is concluded that adenosine-induced dilatation of the hepatic arterial vascular bed is mediated by P1-purinoceptors of the A2 sub-type.

Acetylcholine↗

PubMed1991

Characterization of P2X- and P2Y-purinoceptors in the rabbit hepatic arterial vasculature.

1. Responses to adenosine 5'-triphosphate (ATP) and its agonists were studied in the isolated liver of the rabbit dually perfused through the hepatic artery and the portal vein. 2. In the hepatic arterial vascular bed at basal tone, ATP and its agonists elicited vasoconstrictor responses with the rank order of potency alpha,beta-methylene ATP greater than 2-methylthio ATP greater than ATP, consistent with their action at the P2X-purinoceptor. 3. When tone was raised with noradrenaline (10(-5) M), vasodilator responses were produced with ATP and 2-methylthio ATP; alpha,beta-methylene ATP produced only further constriction. The rank order of vasodilator potency was 2-methylthio ATP greater than ATP much greater than alpha,beta-methylene ATP, consistent with their action at the P2Y-purinoceptor. 4. Methylene blue (10(-5) M) antagonized vasodilator responses to acetylcholine and ATP, but not those to adenosine or sodium nitroprusside. Addition of 8-phenyltheophylline (10(-5) M) antagonized responses to adenosine but not those to sodium nitroprusside. Responses to ATP remaining after antagonism with methylene blue were not further antagonized by 8-phenyltheophylline. 5. These results present evidence for discrete P2X- and P2Y-purinoceptors in the rabbit hepatic arterial bed which mediate vasoconstrictor and vasodilator responses respectively. 6. Vasodilatation produced by ATP was entirely due to direct action at the P2Y-purinoceptor, and not at a P1-purinoceptor following breakdown to adenosine. The antagonism of these responses by methylene blue is consistent with the view that vasodilatation by ATP takes place largely via endothelial P2Y-purinoceptors that lead to release of endothelium-derived relaxing factor. However, we cannot exclude the possibility that P2y-purinoceptors located on the vascular smooth muscle play a contributory role in ATP-induced vasodilatation.

Acetylcholine↗

PubMed1991

NG-nitro-L-arginine methyl ester attenuates vasodilator responses to acetylcholine but enhances those to sodium nitroprusside.

The effects of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of the synthesis of the endothelium-derived relaxing factor nitric oxide, were studied in two isolated perfused vascular beds: the rat mesenteric arterial bed and the hepatic arterial bed of the rabbit liver. The tone of both preparations was raised with noradrenaline (10 and 30 microM for rabbit and rat preparations, respectively). In both preparations, L-NAME (30 microM) significantly attenuated vasodilator responses to the endothelium-dependent vasodilator acetylcholine, but enhanced responses to sodium nitroprusside (a direct smooth muscle dilator). The evidence supports the view, previously established from work carried out in isolated vessels, that in addition to acting as an inhibitor of nitric oxide, L-NAME enhances the responsiveness of smooth muscle to direct relaxation by nitrovasodilators.

Acetylcholine↗

PubMed1991

Roles of P2-purinoceptors in the cardiovascular system.

Characterization of P2-purinoceptor subtypes has facilitated understanding of the many diverse effects produced by purine nucleotides. P2X-Purinoceptors are located on vascular smooth muscle where they mediate vasoconstriction resulting from ATP released as a cotransmitter with noradrenaline from sympathetic nerves. P2Y-Purinoceptors are usually located on the vascular endothelium where they have a role as mediators of vascular relaxation by locally produced ATP. In some vessels, P2Y-purinoceptors are also located on the smooth muscle, perhaps in association with purinergic or sensory nerves, where they can elicit direct relaxation to neuronally released ATP. The net effect of ATP and its analogues on isolated vessels or on vascular beds will be the results of actions mediated by P2X- and P2Y-purinoceptor subtypes, although changes in vascular tone and in integrity of nerves and endothelial cells may alter the balance of the response. Such changes have been observed in diseased states (e.g., atherosclerosis) and may have important implications for the involvement of P2-purinoceptors in, for example, vasospasm. The development of selective and potent antagonists to P2X- and P2Y-purinoceptors has so far remained elusive, and their therapeutic potential can only be guessed.

Adenosine Triphosphate↗

PubMed1991

Effects of purines and pyrimidines on the rat mesenteric arterial bed.

The effects of the purines, adenosine 5'-triphosphate (ATP) and guanosine 5'-triphosphate (GTP), and the pyrimidines, uridine 5'-triphosphate (UTP), cytidine 5'-triphosphate (CTP), and thymidine 5'-triphosphate (TTP), on vascular resistance were investigated in the rat mesenteric arterial bed. In preparations at basal tone, these agents produced dose-related vasoconstriction with a potency order of ATP greater than CTP greater than UTP much greater than TTP = GTP. When tone was raised with norepinephrine (30 microM), these agents caused dose-related vasodilatation with the potency order of UTP = ATP greater than TTP = GTP. CTP did not elicit vasodilatation. Removal of the endothelium with sodium deoxycholate resulted in an increased responsiveness of the mesenteric bed preparation to the vasoconstrictor effects of each of the purines and pyrimidines tested. The selective P2 X-purinoceptor-desensitizing agent alpha,beta-methylene ATP inhibited vasoconstrictor responses to ATP and to CTP but had no effect on vasoconstrictor responses elicited by UTP, TTP, and GTP. In raised-tone preparations, vasodilator responses to ATP, UTP, TTP, and GTP were abolished after removal of the endothelium with sodium deoxycholate. Responses to acetylcholine were also abolished; those to sodium nitroprusside were unimpaired. An inhibitor of the formation of nitric oxide from L-arginine, N omega-nitro-L-arginine methyl ester (30 microM), which antagonizes responses mediated by endothelium-derived relaxing factor (nitric oxide), attenuated vasodilatation to ATP, UTP, and acetylcholine but not to sodium nitroprusside.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Endothelial cells cultured from human umbilical vein release ATP, substance P and acetylcholine in response to increased flow.

Cultured human umbilical vein endothelial cells superfused with Krebs' solution were used to investigate release of ATP, substance P and acetycholine with shear stress. ATP was consistently released when the cells were exposed to increased flow rate; release was rapid, had declined by 1 min and occurred upon a second exposure. Release of substance P and acetylcholine was more varied; increased shear stress led to release of substance P from 4 out of 16 endothelial-cell columns, whereas acetylcholine was released in 4 out of 12 columns. This is the first time that unequivocal evidence has been presented for release of these neurotransmitter substances from vascular endothelial cells. These findings have important implications about the mechanisms of local regulation of vascular tone.

Acetylcholine↗

PubMed1990

Postjunctional synergism of noradrenaline and adenosine 5'-triphosphate in the mesenteric arterial bed of the rat.

The ability of purines to modify responses to exogenous noradrenaline (NA) was investigated using the isolated perfused rat mesenteric bed. ATP, at subthreshold doses and above-threshold doses, produced a potentiation of vasoconstrictor responses to NA while adenosine was without effect. The stable analogue of ATP, alpha,beta-methylene ATP alpha,beta-meATP, at subthreshold and above-threshold doses also enhanced pressor responses to NA (to a greater extent than ATP). This potentiation caused a shift to the left of the dose-response curve, with no increase in the maximum response. Pressor responses to 5-hydroxytryptamine (5-HT) and to potassium chloride (KCl), however, were not affected by alpha,beta-meATP. Conversely, suprathreshold doses of NA potentiated contractions evoked by alpha,beta-meATP, but no potentiation was observed using subthreshold doses of NA. These results demonstrate a postjunctional synergistic action between NA and ATP which appeared to be specific for the alpha 1-adrenoceptors and the P2x-purinergic receptors since: (i) the potentiation of the contractile response to NA by ATP was mimicked by alpha,beta-meATP but not by adenosine and (ii) pressor responses to 5-HT or to KCl were not affected by alpha,beta-meATP. Possible mechanisms for this postjunctional synergism are discussed.

Adenosine Triphosphate↗

PubMed1990

Effects of long-term laxative treatment on rat mesenteric resistance vessel responses in vitro.

The effects of long-term treatment with the laxatives senna and 1,8-dihydroxyanthraquinone (danthron) were investigated in isolated mesenteric vascular beds of rats. The senna was administrated as ground senna pods mixed with milk chocolate. Danthron was also administered in this way. Chocolate-fed, senna-fed, and danthron-fed rats were supplied with usual feed, supplemented with chocolate, chocolate adulterated with ground senna pods, and chocolate adulterated with danthron, respectively. A group of control rats had no supplement. Perivascular nerve stimulation elicited frequency-dependent vasoconstriction of the mesenteric bed. There were no significant differences in responsiveness to perivascular nerve stimulation among mesenteric beds from the four groups. During two separate consecutive applications of capsaicin, a sensory neurotoxin, pressor responses to nerve stimulation of vascular beds from the control and chocolate-fed rats were inhibited on both occasions. However, in mesenteric beds from the senna-fed and danthron-fed groups, inhibition of pressor responses was the same on the first application of capsaicin as in the control and chocolate-fed groups, but the effect of the second application of capsaicin was greatly reduced. Calcitonin gene-related peptide, adenosine 5'-triphosphate, and adenosine mimicked the inhibitory action of capsaicin on nerve stimulation in all groups, while substance P was without effect. There was no significant difference in responsiveness to these agents among the four groups. These results suggest that senna or its metabolites may cause a sensory neuropathy of mesenteric resistance vessels and that calcitonin gene-related peptide, adenosine 5'-triphosphate, and adenosine, but not substance P, are possible candidates as mediators of the inhibitory effects induced by capsaicin.

Adenosine↗

PubMed1990

Peptides and vasomotor mechanisms.

The multiple and diverse roles played by neuropeptide Y, vasoactive intestinal polypeptide, substance P, calcitonin gene-related peptide and other biologically active peptides in the cardiovascular system are considered. A model of the vascular neuroeffector junction is described, which illustrates the interactions of peptidergic and nonpeptidergic transmitters that are possible at pre- and postjunctional sites. The effects of peptides on specific endothelial receptors are also described, which highlights the ability of these agents to act as dual regulators of vascular tone at both adventitial and intimal surfaces, following local release from nerves, or from endothelial cells themselves. Changes in expression of vascular neuropeptides that occur during development and aging in some disease situations and following nerve lesion are discussed.

Animals↗

PubMed1990

Substance P is released from the endothelium of normal and capsaicin-treated rat hind-limb vasculature, in vivo, by increased flow.

The rat hind-limb vasculature releases substance P when subjected to a rapid increase in flow through the vascular bed. This release also occurs during high flow after rats have been capsaicinized, when loss of substance P-containing nerve fibers was verified by immunohistochemistry. Air treatment, a procedure shown by transmission electron microscopy to have removed endothelial cells from the arteries but not arterioles or capillaries of the hind-limb preparations, eliminated this release. Thus, the substance P released is unlikely to arise from perivascular nerves but rather from arterial endothelial cells.

Animals↗

PubMed1989

Ultrastructural localisation of substance P and choline acetyltransferase in endothelial cells of rat coronary artery and release of substance P and acetylcholine during hypoxia.

Substance P and choline acetyltransferase have been localised in a small proportion of endothelial cells of rat coronary arteries using electron microscopic immunocytochemistry. During a hypoxic period of 1 min, coronary vasodilatation was produced in the Langendorff heart preparation and increased levels of substance P and acetylcholine were released into the perfusate. The possibility that these substances are released from endothelial cells during hypoxia and contribute to the hyperaemic response is discussed.

Acetylcholine↗

PubMed1989

A new protocol for removal of the endothelium from the perfused rat hind-limb preparation.

A new protocol has been developed for selective removal of the endothelium from the arteries of the perfused rat hind-limb preparation. The hind limb was perfused with oxygenated Krebs-Ringer bicarbonate solution with a combination of high flow and timed air bubbles (2 minutes at high flow followed by 5 minutes of air bubbles, followed by a further 2 minutes at high flow). Representative vessels at different diameters of the arterial tree--the femoral artery, the artery supplying the extensor hallucis proprius muscle, and the arteries, arterioles, and capillaries within the hallucis proprius muscle--were taken for examination by transmission and scanning electron microscopy. A graded degree of damage occurred along the length of the hind-limb vasculature, with endothelial cells having been completely removed from the femoral artery, removed from or severely damaged in the artery supplying the hallucis proprius muscle, and partially damaged in arteries within the hallucis proprius muscle. However, no damage occurred to the endothelial cells of the arterioles and capillaries within the hallucis proprius muscle. The integrity of the vascular bed after endothelial removal according to the above protocol was confirmed by the demonstration of no diminution, but in fact an increase, in contractile responses to bolus injections of alpha,beta-methylene ATP, resulting in a shift to the left of the dose-response curve. The viability was further confirmed by the fact that the conductance (flow/perfusion pressure) of the preparation during the periods of high flow was no different before and after removal of the endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗

PubMed1988

Serotonin is localized in endothelial cells of coronary arteries and released during hypoxia: a possible new mechanism for hypoxia-induced vasodilatation of the rat heart.

In this report we demonstrate the immunocytochemical localization of serotonin in endothelial cells of rat coronary vessels and a significant increase in the release of serotonin into the perfusate of Langendorff rat heart preparations during hypoxia. It is suggested that serotonin, localized in endothelial cells, is released during hypoxia and could provide part of a pathophysiological mechanism for vasodilatation to protect the heart from damage due to hypoxia.

Animals↗

PubMed1988

Actions mediated by P2-purinoceptor subtypes in the isolated perfused mesenteric bed of the rat.

1. The effects of adenosine 5'-triphosphate (ATP) and its analogues on the perfusion pressure of the isolated mesenteric bed of the rat were examined in preparations at resting tone, and with tone raised by noradrenaline. 2. In the preparations at resting tone, the effect of the analogues was to produce vasoconstriction, their rank order of potency being alpha,beta-methylene ATP greater than 2-methylthio ATP greater than ATP. 3. In raised tone preparations, dose-dependent vasodilatations were produced by ATP and 2-methylthio ATP although, at the highest doses tested, responses decreased in magnitude. The rank order of potency of the analogues in eliciting this vasodilator response was 2-methylthio ATP greater than ATP, while alpha,beta-methylene ATP was without effect. 4. Following desensitization of contractile responses to alpha,beta-methylene ATP, contractile responses to ATP and 2-methylthio ATP were abolished while their relaxant responses were potentiated. 5. Removal of the endothelium with sodium deoxycholate totally abolished the vasodilator responses and enhanced the contractile responses. 6. It is concluded that, in the rat mesentery, ATP and its analogues cause vasoconstriction via P2x-purinoceptors and vasodilatation via P2y-purinoceptors and that these are located on the smooth muscle and on the endothelium, respectively.

Adenosine Triphosphate↗