摘要:

Free radical-induced oxidant stress has been implicated in ischemia and reperfusion-induced injury in the heart. A number of studies have reported that oxidant stress reduces the activity of isolated Na+,K+-ATPase enzyme. We have studied the effects of oxidant stress on the Na+-K+pump current recorded in isolated rabbit ventricular myocytes using the whole-cell voltage-clamp technique. Singlet oxygen and superoxide were generated by the photoactivation of rose bengal (50 nM). The compositions of Tyrode's and pipette solutions were designed to block channel currents and electrogenic Na+-Ca2+exchange. Cells were dialyzed with a pipette solution containing 30 mM sodium via wide-tipped (1–2-MΩ) electrodes, and outward Na+-K+pump current was recorded during a voltage-ramp protocol. The validity of using such a ramp protocol was confirmed by comparison with steady-state Na+-K+pump current measurements made at the end of 200-msec square-clamp steps. Active currents were abolished by potassium-free Tyrode's solution or ouabain (100 μM), and Na+-K+pump current was defined as the Ko.-sensitive fraction of recorded currents. The activation of Na+-K+pump current by intracellular sodium and extracellular potassium revealed a concentration of potassium necessary for half-maximal activation of 18.7 mM for Naiand 1.88 mM for Ko. Oxidant stress inhibited Na+-K+pump current at all voltages, such that after a 10-minute exposure to photoactivated rose bengal, Na+-K+pump current measured at 0 mV was reduced by approximately 50%. The voltage dependence of Na+-K+pump current was, however, not profoundly affected by oxidant stress. Passive membrane currents recorded in the absence of all major electrogenic ion channels, exchangers, or pumps were unaffected by oxidant stress. This observation suggests that, over the time course during which Na+-K+pump inhibition and calcium overload occur, oxidant stress does not cause nonspecific membrane damage and changes in the passive resistance of the lipid bilayer. The inhibition of Na+-K+pump activity by oxidant stress may contribute to ischemia/reperfusion injury and reperfusion-induced cellular calcium overload.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Previous studies have shown that the inotropic response of the heart to β-adrenergic stimulation declines with aging. This alteration has been attributed partly to an age-related impairment in the activation of the β-adrenoceptor-G protein-adenylate cyclase complex. To further understand the mechanisms underlying the age-related deficit, the present study compared β-adrenergic-mediated contractile response, cAMP accumulation, and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins in isolated perfused hearts from adult (6–8 months) and aged (28–30 months) Fischer 344 rats. In isometrically contracting, electrically paced (240 beats per minute) hearts perfused at constant flow rate (9 ml/min per gram ventricle), the baseline contractile performance differed significantly between adult and aged hearts. Thus, contraction duration was prolonged (≈15%,p< 0.001) in the aged relative to the adult heart, and this was due to increases in time to peak tension and relaxation time. Further, developed peak tension, normalized per gram ventricular wet weight, was significantly lower (≈20%, p < 0.05) in the aged compared with the adult heart. In these isolated perfused heart preparations, β-adrenergic stimulation with isoproterenol (ISO, 0.001–1 μM) evoked concentration-dependent positive inotropic and lusitropic responses, both of which were significantly lower (15–20%, p < 0.05–0.001) in the aged compared with the adult heart. These age-related differences were manifested as relatively smaller ISO-induced increases in 1) developed peak tension, 2) maximum rate of tension development (+dT/dt), and 3) maximum rate of relaxation (-dT/dt) in the aged compared with the adult heart. The ISO-induced abbreviation of time to half relaxation was also less marked in the aged heart. Under similar experimental conditions, ISO (0.1 μM)-induced increase in tissue cAMP content was also lower (≈18%,p< 0.05) in the aged heart. ISO (0.1 μM) -induced phosphorylation of the sarcoplasmic reticulum protein phospholamban and myofibrillar protein troponin I was significantly diminished (≈38% and 25% decline, respectively, for phospholamban and troponin I; p < 0.05–0.001) in the aged compared with the adult heart. No significant age-related difference was, however, evident in ISO-induced phosphorylation of C protein of myofibrils. These data suggest that age-related decrements in β-adrenergic-mediated cAMP accumulation and phosphorylation of phospholamban and troponin I contribute to the diminished contractile responses of the aged heart to β-adrenergic stimulation.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

The mechanism by which preconditioning (brief intermittent periods of ischemia and reflow) improves recovery of function and reduces enzyme release after a subsequent 30-minute period of ischemia was investigated in perfused rat hearts. Specifically, it was hypothesized that ischemia after preconditioning would result in a decreased production of H+and therefore a smaller rise in [Na+]iand [Ca2+]ivia Na+-H+and Na+-Ca2+exchange. To test this hypothesis we measured pHi, [Na+]i, [Ca2+]i, and cell high-energy phosphates during ischemia and reflow, and we correlated this with recovery of contractile function and release of creatine kinase during reflow.31P nuclear magnetic resonance (NMR) was used to measure pHiand cell phosphates. [Na+]iwas measured by23Na NMR using the shift reagent thulium 1,4,7,10-tetraazacyclododecane-N,N′,N′′,N′′′-tetramethylenephosphonate to distinguish intracellular from extracellular sodium. [Ca2+] was measured by19F NMR using hearts loaded with 1,2-bis(2-amino-5-fluorophenoxy) ethane-N,N,N′,N′-tetraacetic acid, termed 5F-BAPTA. Basal time-averaged levels of pHi, [Na+]i, and [Ca2+]iwere 7.07±0.08, 9.4±0.8 mM, and 715±31 nM, respectively. After 30 minutes of ischemia, in preconditioned hearts, pHiwas 6.5±0.06, [Na+]iwas 20.9±4.4 mM, [Ca2+]iwas 2.1±0.4 μM, and ATP was negligible. In untreated hearts, after 30 minutes of ischemia, pH. was 6.3±0.08, [Na+]iwas 26.7±3.8 mM, [Ca2+]iwas 3.2±0.6 μM, and ATP was undetectable. During reperfusion after 30 minutes of ischemia, preconditioned hearts had significantly better recovery of contractile function than untreated hearts (71±9% versus 36±8% initial left ventricular developed pressure), and after 60 minutes of ischemia, preconditioned hearts had significantly less release of the intracellular enzyme creatine kinase (102±12 versus 164±17 IU/g dry wt). We also found that unpreconditioned hearts arrested with 16 mM MgCI2(to inhibit calcium entry via calcium channels and Na+-Ca2+exchange) before 30 minutes of ischemia recover function on reflow to the same extent as preconditioned hearts with or without magnesium arrest. Thus, preconditioning has no additional benefit in addition to magnesium arrest. In addition, in hearts that received 16 mM MgCI2just before the 30-minute period of ischemia, preconditioning had no effect on the rise in [Ca2+]iduring the 30-minute period of ischemia. These data support the hypothesis that preconditioning attenuates the increase in [Ca2+]i, [Na+]i, and [H+]iduring ischemia, most likely because of reduced stimulation of Na+-H+and Na+-Ca2+exchange. The data suggest that interventions that minimize ionic derangements during ischemia are associated with improved recovery of contractility and less enzyme release on reflow.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Recent studies have indicated that renal arteries can produce 20-hydroxyeicosatetraenoic acid (20-HETE) and suggest the potential involvement of a P450 metabolite of arachidonic acid in the myogenic activation of canine renal arteries. In the present study, the effects of 20-HETE on isolated canine renal arcuate arteries were studied. Administration of 20-HETE to the bath or the lumen at concentrations of 0.01–1 μMproduced a graded reduction in the diameter of these vessels. In contrast, 19(R)-HETEwas a vasodilator, whereas 19(S)-HETEwas relatively inactive. The vasoconstrictor response to 20-HETE was not altered by the cyclooxygenase inhibitor indomethacin, endoperoxide/thromboxane receptor antagonist SQ29548, or combined blockade of the cyclooxygenase, lipoxygenase, and P450 pathways using indomethacin, baicalein, and 7-ethoxyresorufin. The response to 20-HETE was associated with depolarization and a sustained increase in the intracellular calcium concentration in renal vascular smooth muscle cells. Patch-clamp studies indicated that 20-HETE significantly reduced mean open time, the open-state probability, and the frequency of opening of a 117-pS K+channel recorded from renal vascular smooth muscle cells in the cell-attached mode. Microsomes prepared from the renal cortex of dogs produced 20-HETE and 20-carboxyarachidonic acid when incubated with [14Clarachidonic acid. These results indicate that 20-HETE is an endogenous constrictor of canine renal arcuate arteries. The vasoconstrictor response to 20-HETE resembles the myogenic activation of these vessels after elevations in transmural pressure and suggests a potential role for this substance in the regulation of renal vascular tone.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

We studied the effects of intense sympathetic stimulation on the chronotropic responses of the heart to subsequent test stimulations of the cardiac autonomic nerves in dogs anesthetized with α-chloralose. Such intense sympathetic stimulations (which we refer to as “release stimulations”) are known to release neuropeptide Y as well as norepinephrine. The changes in cardiac cycle length evoked by vagal and sympathetic test stimulations were progressively more attenuated as we increased the frequency and duration of the antecedent sympathetic release stimulations. We found that 2.5 minutes after a maximal release stimulation (30 Hz for 5 minutes), the mean±SEM chronotropic responses to the vagal and sympathetic test stimulations were diminished to 36.5±1.6% and 54.7±1.3%, respectively, of the prestimulation responses. The mean times for the chronotropic responses to the vagal and sympathetic test stimulations to recover to their control values were 52.0±1.3 and 63.2±2.9 minutes, respectively. This enduring effect suggests the action of a neuropeptide, such as neuropeptide Y. Phentolamine potentiated the inhibitory effects of the sympathetic release stimulations. The chronotropic responses to isoproterenol infusions were not affected appreciably by antecedent sympathetic release stimulation. We conclude, therefore, that the inhibitory effects of antecedent sympathetic release stimulation on cardiac sympathetic neurotransmission are mediated prejunctionally, probably via an inhibition of the neuronal release of norepinephrine by neuropeptide Y.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

To study the mechanism of defibrillation and the reason for the increased defibrillation efficacy of biphasic waveforms, the potential gradient in a 32×30-mm region of the right ventricle in 15 dogs was progressively lowered in four steps while a strong potential gradient field was maintained throughout the rest of the ventricular myocardium. The volume of right ventricle beneath the plaque was 10±2% of the total ventricular mass. A 10-msec monophasic (eight dogs) or 5/5-msec biphasic (seven dogs) truncated exponential shock 30% above the defibrillation threshold voltage was given via electrodes on the left ventricular apex and right atrium to create the strong potential gradient field. Simultaneously, a weaker shock with the same waveform but opposite polarity was given via mesh electrodes on either side of the small right ventricular region to cancel part of the potential difference in the region and to create one of the four levels of potential gradient fields. Shock potentials and activations were recorded from 117 epicardial electrodes in the small region, and in one dog global epicardial activations and potentials were recorded from a sock containing 72 electrodes. Each gradient field was tested 10 times for successful defibrillation after 10 seconds of electrically induced fibrillation. For both monophasic and biphasic shocks, the percentage of successful defibrillation attempts decreased (p<0.05) as the potential gradient decreased in the small region. Defibrillation was successful approximately 80% of the time for a mean±SD potential gradient of 5.4±0.8 V/cm for monophasic shocks and 2.7±0.3 V/cm for biphasic shocks (p<0.05). No postshock activation fronts arose from the small region for either waveform when the gradient was more than 5 V/cm. For both waveforms, the postshock activation fronts after the shocks were markedly different from those just before the shock and exhibited either a focal origin or unidirectional conduction. Thus, 1) for both monophasic and biphasic waveforms, defibrillation efficacy depends on the potential gradient fields; 2) a low potential gradient in approximately 10% of the ventricular mass can cause defibrillation to fail, suggesting that the critical mass for defibrillation is over 90% of the ventricular mass in dogs; 3) this low gradient field halts the activation fronts during fibrillation and then leads to new activation fronts to reinduce fibrillation; 4) postshock activation fronts are prevented by a potential gradient of more than 5 V/cm for both waveforms; and 5) the strength of the minimum potential gradient field required for defibrillation is less for the biphasic waveform.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Exposure of isolated arteries to oxidatively modified low density lipoprotein (LDL) has been reported to suppress endothelium-dependent relaxation (EDR). To determine whether lipid degradation products in oxidized LDL contribute to impaired relaxation, we have tested the responsiveness of isolated rabbit aortas to endothelium-dependent relaxants (acetylcholine, ATP, and calcium ionophore A23187) and nitroglycerin before and after 2-hour incubations with selected lipids and LDL preparations. Concentrations (10 μM) of lecithin, phosphatidylserine, lysophosphatidylserine, sphingomyelin, phosphatidic acid, palmitate, arachidonate, and auto-oxidized arachidonate had no effect on EDR. Concentrations (10 μM) of lysolecithin, lyso-platelet activating factor, and sphingosine significantly suppressed endothelium-dependent relaxation. Native LDL (100 μg/ml incubation buffer) containing only small amounts of lysophosphatidylcholine exerted no effect on EDR. In contrast, LDL preparations oxidatively modified by exposure to cultured endothelial cells or copper inhibited EDR. When modified LDL was depleted of its lysolecithin by treatment with a selective phospholipase B (lysolecithinase), the inhibitory effects were attenuated. In contrast, native LDL accumulating lysolecithin under the influence of a phospholipase A2(lecithinase) exerted inhibitory effects mimicking those of oxidized LDL. Lipids and lipoproteins had no effect on the responsiveness to nitroglycerin, an endothelium-independent vasodilator. We conclude that lysolecithin in oxidatively modified LDL contributes importantly to its vasomotor effects.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Previous studies have shown regional differences in atrial distensibility. We studied 12 open-chest dogs to test the hypothesis that left atrial compliance is decreased after removal of the left atrial appendage and to determine the effect of altered atrial compliance on atrial reservoir and conduit function. Sonomi-crometer crystal pairs were used to measure the long- and short-axis diameters of the left atrium over a wide range of intracardiac pressures and volumes obtained by intravenous hetastarch infusion both before and after suture ligation of the left atrial appendage (appendectomy). Pulmonary venous flow was measured with an ultrasonic flowmeter, and transmitral flow velocities were measured with transesoph-ageal Doppler echocardiography. After appendectomy, the diastolic pressure-volume relation was shifted upward and to the left in six of seven dogs. The mean dynamic stiffness constant of the left atrial diastolic pressure-volume relation was significantly greater after appendectomy than before (0.20±0.11 [mean±SD] versus 0.14±0.08 ml−l, p < 0.01); the meanyintercept was slightly, but significantly, less after appendectomy (0.6±0.3 versus 1.3±0.6 mm Hg, p < 0.05). The left atrial reservoir volume (maximum minus minimum left atrial volume) was significantly less after appendectomy at matched left atrial pressures. The systolic to diastolic flow integral ratio of pulmonary venous flow (JFTI/KFTI), an index of the relative reservoir to conduit functions of the left atrium, increased significantly with volume infusion only before appendectomy; at matched left atrial pressure, JFTI/KFTIwas significantly less afterwards. Similarly, the early diastolic transmitral velocity increased with volume infusion but was less at each level of left atrial pressure after appendectomy. We conclude that left atrial compliance decreases after appendectomy and is characterized by decreased relative reservoir to conduit function of the left atrium and decreased early left ventricular filling rate.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

To investigate the effect of lactate, pyruvate, and glucose on the endogenous levels of lipids in the normoxic, ischemic, and reperfused myocardium, isolated working rat hearts were exposed to various grades of ischemic insult (15, 30, or 45 minutes). Glucose was present as the basal substrate in the perfusion medium, and lactate (5 mM) or pyruvate (5 mM) was added as the cosubstrate. Lipid metabolism was evaluated by fatty acid accumulation, triacylglycerol turnover, and phospholipid homeostasis. Exogenous lactate significantly increased fatty acid content above preischemic levels after 45 minutes of ischemia. In glucose-perfused hearts, fatty acid levels were even slightly higher than in lactate-perfused hearts, whereas pyruvate-perfused hearts demonstrated less accumulation of fatty acids. By reperfusion, fatty acid levels in glucose-perfused hearts returned to control values. In lactate- and pyruvate-perfused hearts, fatty acid accumulation was further enhanced by reperfusion. When the fatty acid content exceeded 400 nmol/g dry wt during reperfusion, hemodynamic function was impaired, whereas fatty acid levels below 400 nmol/g dry wt did not correlate with hemodynamic recovery. The total triacylglycerol content did not change during ischemia and reperfusion. However, accumulation of glycerol was remarkable during the first 15 minutes of ischemia in all hearts, and release of glycerol by reperfusion was considerable in lactate-perfused hearts after 30 minutes of ischemia and in all groups of hearts after 45 minutes of ischemia. Release of glycerol in association with maintained levels of triacylglycerols suggests turnover of the triacylglycerol pool. The rate of triacylglycerol cycling correlated poorly with hemodynamic recovery. Accumulation of arachidonic acid revealed disturbances in phospholipid turnover. Arachidonic acid accumulation during reperfusion demonstrated a strong relation with impairment of cardiac function. Hence, derangements in phospholipid homeostasis during reperfusion might be involved in myocardial damage, which is influenced by the substrates available.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

A major determinant of oxygen transport to the myocardium is the time spent by the erythrocytes (red blood cells [RBCs]) traversing the microcirculation. Although it has been shown that the myocardium has regional differences in blood volume, blood flow, metabolism, and sensitivity to ischemic injury, the regional distribution of RBC transit times through the myocardial capillaries has not been previously measured. The present study was designed to measure the regional myocardial capillary RBC transit time by a new technique to determine whether there are regional differences in the capillary RBC transit time in the normal resting heart. Anesthetized open-chest male New Zealand White rabbits (3.0–3.7 kg,n=8) were studied. Regional myocardial blood volume was determined using chromium-51-labeled RBCs, and regional blood flow was measured using a reference flow technique and a left atrial injection of 15-gm-radiolabeled (gadolinium-153, 10–20 μCi) microspheres. Capillary blood volume was determined by multiplying the regional blood volume by the histologically determined fraction of the total blood volume that was in the capillaries. Capillary RBC transit time was calculated as the quotient of capillary blood volume and blood flow. The myocardial capillary blood volume was the same in the endocardium and the epicardium (4.67±0.67 ml/100 g for endocardium versus 4.52±0.70 ml/100 g for epicardium,p=NS), whereas myocardial blood flow tended to be greater in the endocardium (6.09±0.73 ml/sec per 100 g for endocardium versus 5.47±0.75 ml/sec per 100 g for epicardium), although this was not statistically significant. Myocardial capillary RBC transit times ranged from 0.22 to 2.58 seconds with a mean of 0.89±0.13 and 0.83±0.13 seconds and a median of 0.72 and 0.70 seconds in the epicardium and endocardium, respectively. We conclude that there are no regional differences in the myocardial capillary RBC transit time in the normal heart at rest.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID