摘要:

Endothelium-dependent relaxations are depressed in hypertension. In this study we investigated the possible involvement of endothelium-dependent smooth muscle hyperpolarization in this phenomenon. In isolated aortic segments from control rats, acetylcholine (10−8-10−5M) elicits relaxations after precontraction with norepinephrine (10−7M), and acetylcholine or carbachol (10−5M) induce smooth muscle hyperpolarization (10.6±0.9 mV). Both effects disappear after removal of the endothelium and are depressed by tetraethylammonium (3±10−3M), a rather nonspecific blocker of K+channels, but not by glibenclamide (10−5M), a potent blocker of the ATP-regulated K+channels, which has a marked effect on the relaxation induced by BRL 38227. The relaxation effect of acetylcholine is impaired in norepinephrine-contracted preparations from hypertensive rats but is not further depressed by tetraethylammonium. In aorta from hypertensive rats, hyperpolarization induced by carbachol was significantly reduced to a mean of only 21.8% of the values obtained in preparations from normotensive rats. From the relaxation-hyperpolarization relation obtained with BRL 38227 (opening K+channels), it is derived that the endothelium-dependent hyperpolarization (∼10 mV) contributes for at least 20–30% of the maximal relaxation effect of acetylcholine on rat aorta. It is concluded that the diminished endothelium-dependent hyperpolarization may contribute to the depression of the endothelium-dependent relaxation in hypertension.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

At rest, most of left ventricular (LV) filling occurs early in diastole. This LV filling occurs in response to the pressure gradient produced as LV pressure falls below left atrial (LA) pressure. Because mitral valve flow occurs in response to an LA to LV pressure gradient, augmented diastolic mitral valve flow during exercise may be due to an increased mitral valve pressure gradient resulting from a rise in LA pressure and/or a fall in LV early diastolic pressure. Accordingly, we studied 13 conscious dogs, instrumented to measure microma-nometer LV and LA pressures, and determined LV volume from three ultrasonic dimensions during exercise. The animals ran on a treadmill for 8–15 minutes at 5–8 miles/hr. With reflexes intact, during exercise, the heart rate increased from 116±20 to 189±24 beats per minute (mean±SD,p<0.01), the maximum rate of change of LV volume (dV/dtmax) increased from 185±44 to 282±76 ml/sec (p<0.01), the ejection fraction and cardiac output increased, and the duration of diastole decreased from 296±83 to 162±71 msec (p<0.01). Mitral valve opening pressure, mean LA pressure (10.9±4.4 versus 10.2±3.9 mm Hg,p=NS), and LV end-diastolic pressure (12.8±4.8 versus 13.1±3.3 mm Hg,p=NS) were all relatively unchanged. The time constant of the fall of isovolumic LV pressure decreased from 28±3.3 to 21±4.4 msec (p<0.05). The early diastolic portion of the LV pressure-volume loop was shifted downward during exercise, with the minimum LV pressure decreasing from 3.3±2.8 to −2.8±3.4 mm Hg (p<0.05) and the maximum mitral valve pressure gradient increasing from 5.5±1.7 to 11.8±3.5 mm Hg (p<0.01). A similar downward shift of the early diastolic portion of the LV pressure-volume loop was produced by infusion of dobutaniine (6 pg/kg/min i.v.) at rest, as well as by exercise when the heart rate was held constant by right ventricular pacing at 190–210 beats per minute. The downward shift during exercise was prevented by β-blockade (metoprolol, 0.5 mg/kg i.v.). We conclude that during exercise, sympathetic stimulation and tachycardia produce a downward shift of the early diastolic portion of the LV pressure-volume loop. This fall in early diastolic LV pressure augments the early diastolic mitral valve gradient without an increase in LA pressure, producing more rapid mitral valve flow in early diastole and maintaining LV filling despite the shortening of diastole during exercise.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

This study assessed gentamicin's effects on ischemia/reperfusion renal injury to better understand when and how it worsens postischemic acute renal failure. Rats were subjected to 25 minutes of renal pedicle occlusion with and without preischemic (15-minute) or postischemic (15-minute or 8-hour) gentamicin treatment (100 mg/kg, by itself a subtoxic dose). Gentamicin's impact on hypoxia/reoxygenation injury to isolated rat proximal tubular segments was also assessed. Preischemic and postischemic gentamicin worsened the severity of acute renal failure to the same degree, suggesting that pretreatment induces its effect in the reperfusion period. Gentamicin paradoxically lessened hypoxic damage to proximal tubular segments (assessed by lactate dehydrogenase release), again implying no adverse impact on oxygen deprivation-induced tubular injury. From 0–4 hours of reperfusion, gentamicin approximately halved ATP/ADP ratios (due to increased ADP), indicating a drug-induced defect in cellular energetics. This abnormality temporally correlated with evolving morphological damage. Although antioxidants (deferoxamine and sodium benzoate) have been reported to protect against pure aminoglycoside nephrotoxicity, they did not mitigate gentamicin's adverse impact on postischemic acute renal failure. Gentamicin did not influence ischemia/immediate reperfusion deacylation/reacylation (assessed by renal free fatty acid content) despite its known antiphospholipase activity. Although in the normal kidney gentamicin preferentially accumulated in cortex, in the postischemic kidney, both cortex and outer medullary stripe developed striking (approximately threefold to fivefold) and comparable gentamicin increments. In conclusion, gentamicin appears to exacerbate postischemic acute renal failure by adversely influencing the reperfusion, not the ischemic injury, process. This may occur because increased gentamicin accumulation negatively impacts on reperfusion cellular energetics.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Although the kinetics of cardiac systolic force restitution have been well described, the restitution kinetics of left ventricular relaxation have not been examined. To define relaxation restitution behavior, we studied seven dogs chronically instrumented with left ventricular high-fidelity micromanometers and piezoelectric dimension crystals. After a priming period at a basic cycle length of 375 msec, test extrastimuli were introduced after a range of extrasystolic intervals (ESIs). Relaxation behavior of control and extrasystolic beats was characterized by the time constant of isovolumic relaxation, τ. Relaxation restitution can be described by two concatenated monoexponential curves, an early phase described by a rapid time constant and a late phase described by a slower time constant (TC1, 36.21±7.90 msec; TC2, 75.94±10.65 msec;p< 0.05). The first phase of relaxation restitution parallels systolic force restitution over the same range and displays faster recovery (TCs, 58.93±10.01 msec,p< 0.05). Postextrasystolic restitution of test pulses after beats at fixed ESIs depends on the initial ESI. Relaxation recovery of postextrasystolic beats proceeds faster with smaller initial ESIs (TC1for ESI of 300 msec, 13.27±4.05 msec; TC1for ESI of 450 msec, 72.85±21.72 msec;p<0.0001). The monoexponential pattern of restitution was seen with model-independent descriptors of relaxation as well as with τ.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The goal of this study was to test the hypotheses that endogenous ADP plays an important role in vivo in mediating platelet aggregation and cyclic coronary artery blood flow variations (CFVs) in stenosed and endothelium-injured coronary arteries in an experimental canine model. Anesthetized animals were studied and coronary blood flow velocities monitored by a pulsed Doppler flow probe positioned around the left anterior descending coronary artery. CFVs were established by an external constrictor positioned at sites with injured endothelium. Apyrase, an ADP-removing enzyme, was infused into the left anterior descending coronary artery (0.3–1.8 units/min) 30 minutes or 2 hours after the establishment of CFVs. Complete abolition of CFVs was achieved in 81% (13/16) of dogs with 30-minute CFVs and in 83% (five of six) of dogs with 2-hour CFVs. In other dogs, a potent inhibitor of ADP-induced platelet aggregation, clopidogrel, was administered as a 10 mg/kg i.v. bolus and a 2.5 mg/kg/hr infusion 30 minutes and 3 hours after the establishment of CFVs. This treatment resulted in complete abolition of CFVs in 14 dogs (100%) with either 30-minute or 3-hour CFVs. Epinephrine was infused into some dogs after CFVs had ceased as a result of either apyrase or clopidogrel administration and into some dogs in whom SQ29548, a thromboxane A2receptor antagonist, had been given when apyrase failed to abolish CFVs. Epinephrine restored CFVs in all dogs treated with apyrase alone, 67% (four of six) of dogs treated with the combination of apyrase and SQ29548, and 290 (two of seven) of dogs treated with clopidogrel. The plasma epinephrine levels required for CFV restoration were 20 times higher than baseline values in dogs receiving apyrase alone, 100 times higher when a combination of apyrase and SQ29548 had been given, and more than 5,000 times higher in dogs receiving clopidogrel. In vitro studies showed that apyrase only inhibited ADP-induced platelet aggregation, whereas clopidogrel not only inhibited ADP-induced platelet aggregation, but also reduced platelet aggregation induced by the thromboxane mimetic U46619 and serotonin. These data suggest that 1) ADP is an important mediator of platelet aggregation and CFVs in vivo and 2) combined inhibition of thromboxane A2and ADP's effects provides marked protection against CFVs in experimentally stenosed and endothelium-injured canine coronary arteries. These data and our previous observations are consistent with the possibility that specific antagonists of thromboxane A2, serotonin, and ADP, alone and together, may provide substantial protection against platelet aggregation leading to CFVs at sites of endothelial injury and coronary artery stenosis.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Arterial smooth muscles behave like a syncytium, since they are electrically coupled. It is generally assumed that electrical coupling and dye coupling are mediated by gap junctions. No gap junctions could be detected by transmission electron microscopy in media of coronary arteries. We looked for the presence of gap junction protein in vascular smooth muscle by immunohistochemistry with light microscopy. Immunohistologically detectable connexin is expressed by smooth muscle cells of the media of pig coronary arteries, where staining occurs as a discrete punctation. We investigated the dye coupling in strips of pig coronary artery. The fluorescent dye lucifer yellow was microiontophoretically injected into a smooth muscle cell through an intracellular microelectrode. The dye was visualized on the entire strip, then on semithin sections with a fluorescence microscope, and at the ultrastructural level by using an anti-lucifer yellow antibody revealed by the protein A-gold technique. In all the tissues examined, the cells were dye-coupled. We conclude that in arterial media the smooth muscle cells are dye-coupled, despite the absence of detectable gap junctions by transmission electron microscopy, and suggest that dye coupling could occur via isolated gap junction channels.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Ventricular and atrial myocytes cultured from chick embryos 14 days in ovo were used as model systems to study cardiac adenosine receptors. In membranes of ventricular cultures, blocking of the A1-adenosine receptor pathway by the A1-selective antagonist 8-cyclopentyl-1,3-dipropyl-xanthine (DPCPX) or by pertussis toxin treatment of the myocyte resulted in a significant adenosine agonist-mediated stimulation of the adenylate cyclase activity. The maximal increases in adenylate cyclase activity caused by the equipotent or the A2-adenosine receptor-selective agonists (from 52.1±3% to 63±10% [mean±SEM]) were significantly greater than those caused by the A1-selective agonists (from 11±5% to 34.6±7%) (p<0.01, byttest,n=4–8). However, in membranes of atrial myocytes, when Al-subtype had been blocked, the various adenosine agonists had no effect on the adenylate cyclase activity. Whether the stimulatory adenylate cyclase-coupled adenosine receptor is also capable of stimulating contractility in the intact ventricular myocyte was next investigated. In ventricular but not in atrial cells, the various adenosine agonists caused an increase in the contractile amplitude in the presence of DPCPX or in myocytes preexposed to pertussis toxin. The increase in contraction amplitude caused by each agonist was expressed as percent of maximum (maximum is the increase in contractility, caused by 2.4 mM calcium). In the pertussis toxin-treated myocyte, the maximal increases caused by the equipotent or A2-agonists (NECA, MECA, CV-1808, and CGS21680, from 49.6±3% to 52.5±6%,n=8–12) were significantly greater than those elicited by the Al-agonists (2-CADO,S-PIA,R-PIA, and DCCA, from 12±4% to 37±3%,n=8) (p<0.05, byttest). These data demonstrated that a stimulatory adenosine receptor, likely the A2-adenosine receptor, was present on the ventricular but not the atrial myocytes and was linked directly to a stimulation of the cardiac contractility. The functional effects mediated by the A1-subtype became manifested in the presence of isoproterenol, as evidenced by an inhibition of the isoproterenol-stimulated increases in adenylate cyclase activity and in cardiac contractility by adenosine agonists. Thus, both subtypes of adenosine receptors, each mediating opposing responses, were present on the ventricular myocytes, whereas only the A1-subtype was found in the atria. The presence of a stimulatory functional A2-adenosine receptor may help explain the absence of a direct negative inotropic response to adenosine in the ventricle.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

We investigated the effects of μ δ, andkopioid receptor stimulation on the contractile properties and cytosolic Ca2+(Ca1) of adult rat left ventricular myocytes. Cells were field-stimulated at 1 Hz in 1.5 mM bathing Ca2+at 23°C. The μ-agonist [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (10−5M) had no effect on the twitch. The δ-agonists methionine enkephalin and leucine enkephalin (10−10to 10−6M) and theK-agonist (trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclo-hexyl]-benzeneacetamide)methanesulfonate hydrate (U-50,488H; 10−7to 2×10−5M) had a concentration-dependent negative inotropic action. The sustained decrease in twitch amplitude due to U-50,488H was preceded by a transient increase in contraction. The effects of δ- andK-receptor stimulation were antagonized by naloxone and (-)-N-(3-furyl-methyl)-α-normetazocine methanesulfonate, respectively. In myocytes loaded with the Ca2+probe indo-1, the effects of leucine enkephalin (10−8M) and U-50,488H (10−5M) on the twitch were associated with similar directional changes in the Caitransient. Myofilament responsiveness to Ca2+was assessed by the relation between twitch amplitude and systolic indo-1 transient. Leucine enkephalin (10−8M) had no effect, whereas U-50,488H (10−5M) increased myofilament responsiveness to Ca2+. We subsequently tested the hypothesis that δ andkopioid receptor stimulation may cause sarcoplasmic reticulum Ca2+depletion. The sarcoplasmic reticulum Ca2+content in myocytes and in a caffeine-sensitive intracellular Ca2+store in neurons was probed in the absence of electrical stimulation via the rapid addition of a high concentration of caffeine from a patch pipette above the cell. U-50,488H and leucine enkephalin slowly increased Caior caused Caioscillations and eventually abolished the caffeine-triggered Caitransient. These effects occurred in both myocytes and neuroblastoma-2a cells. In cardiac myocyte suspensions U-50,488H and leucine enkephalin both caused a rapid and sustained increase in inositol 1,4,5-trisphosphate. Thus, δ andKbut not μ opioids have a negative inotropic action due to a decreased Caitransient. The decreased twitch amplitude due toK-receptor stimulation is preceded by a transient increase in contractility, and it occurs despite an enhanced myofilament responsiveness to Ca2+. The effects of δ andKopioids appear coupled to phosphatidylinositol turnover and, at least in part, may be due to sarcoplasmic reticulum Ca2+depletion. Ca2+release and depletion of an intracellular store site occur in both myocytes and neurons and may represent a general mechanism for the effects of opioids.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The effects of adenine nucleotides and adenosine on DNA synthesis and cell growth have been studied in bovine aortic endothelial cells (BAECs). ATP produced a small but significant (+44%) increase of the fraction of BAECs whose nuclei are labeled by [3H]thymidine. This mitogenic effect was mimicked by ADP, the phosphorothioate analogues ATPγS and ADPβS, and the nonhydrolyzable analogue adenosine 5'-(β,γ-imido)triphosphate (APPNP), whereas adenosine 5'-(α,β-methylene)triphosphate (APCPP), a selective agonist of P2x-purinoceptors, had no effect at 10 μM and a small one at 100 μM; this profile is consistent with the involvement of P2y-receptors. Adenosine induced a mitogenic response of a magnitude similar to that of ATP. This effect was not reproduced byR-phenylisopropyl adenosine, by 5'-N-ethylcarboxamide adenosine, or by 2',5'-dideoxyadenosine, selective ligands of the A1- and A2-receptors and the P site, respectively, nor was it inhibited by 8-phenyltheophylline, an antagonist of both A1- and A2-receptors. The mechanism of this adenosine action thus remains unclear. ATP and ATPγS did not enhance the proliferation of BAECs cultured in the presence of fetal calf serum concentrations ranging from 0.5% to 10%. They inhibited the growth-promoting effect of basic fibroblast growth factor; among the various nucleotides tested, APCPP was the least effective to reproduce the action of ATP, suggesting the possible involvement of P2y-receptors. In conclusion, the action of ATP on the proliferation of BAECs is complex: an increase in the fraction of cells synthesizing DNA, no effect on the cell proliferation in the presence of serum, and inhibition of the growth-promoting effect of basic fibroblast growth factor.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Whole-cell voltage-clamp experiments and single-channel current recordings in cell-attached patch mode were performed on enzymatically dissociated single ventricular myocytes harvested from feline endocardial and epicardial surfaces. The studies were designed to compare the characteristics of inward rectifier K+current (IK1) and delayed rectifier K+current (IK) between endocardial and epicardial cells and to test the hypothesis that the differential characteristics of IK1and/or IKare responsible for the differences in action potential configuration between the two cell types. IK1in endocardial cells displayed a distinct N-shaped current-voltage (I-V) relation, with a prominent outward current at potentials between −80 and −30 mV. In epicardial cells, an outward current region was much smaller, and the I-V relation demonstrated a blunted N-shaped I-V relation. In single-channel current recordings in cell-attached patch mode, neither unitary current amplitude of IK1nor probability of channel opening was different between endocardial and epicardial cells, suggesting that the difference in the number of functional channels might be responsible for the differential IK1I-V relations. The characteristics of IKalso differed between endocardial and epicardial cells. The time course of growth of tail current of IK(IK,tall) (activation of IK) was significantly enhanced and that of IK,talldeactivation was delayed in epicardial cells compared with endocardial cells. The time constant of the slow component of IKactivation at +20 mV was 3,950±787 msec in endocardial cells and 2,746±689 msec in epicardial cells (p<0.05); the corresponding values for IKdeactivation at −50 mV were 1,041±387 msec and 1,959±551 msec, respectively (p<0.01). The voltage dependence of steady-state activation of IK,tallwas similar between endocardial and epicardial cells, suggesting that the probability of channel opening at any potential was not different in the two cell types. The amplitude and density of fully activated IK(IK,full) were significantly greater in epicardial cells than in endocardial cells. At repolarization to −20 mV, IK,fullamplitude was 452±113 pA in endocardial cells and 578±135 pA in epicardial cells (p<0.05), and the corresponding values for IK,fulldensity were 2.86±0.73 and 4.21±0.83 μA/cm2, respectively (p<0.05). A nonstationary fluctuation analysis revealed that the amplitude of IKunitary current was similar between endocardial and epicardial cells (0.23±0.07 versus 0.22±0.03 pA,p=NS). Thus, the difference in whole-cell current amplitude of IKmight be due to the difference in the number of functional channels per myocyte. These data lead us to suggest that the differential characteristics of IK1and IKcontribute to the difference of action potential configuration between endocardial and epicardial cells.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID