摘要:

Perfused rat hearts release or accumulate approximately 10% of total Mg2+content when stimulated with norepinephrine (NE) or carbachol, respectively. Collagenase-dispersed rat ventricular myocytes increase or decrease total cell Mg2+by 1 mM within 5 minutes when stimulated with these same transmitters. Measurements of Mg2+transport using28Mg or atomic absorbance spectrophotometry indicate that the rate and the extent of both stimulated Mg2+efflux and influx are independent of the concentration of extracellular Mg2+(0 to 1.2 mM). Mg2+release induced by NE is rapidly reversed by the addition of carbachol, and Mg2+uptake induced by carbachol is reversed by NE. Decreasing extracellular Na+or Ca2+decreases or abolishes Mg2e efflux from myocytes. Cd2+or other Ca2+channel blockers also inhibit Mg2+efflux in the presence of a physiological concentration of extracellular Ca2+. Replacement of extracellular Ca2+with Sr2+or with Mn2+decreases or abolishes both stimulated efflux and influx of Mg2+. Redistribution of85Sr in myocytes and in the supernatant indicates that under those conditions Sr2+is released or accumulated by NE or carbachol in a manner similar to that of Mg2+. Hence, at least in the case of Sr2+, the inhibition of Mg2+fluxes can be explained by the transport of Sr2+rather than Mg2+through the transport(s) systems. By contrast, replacement of extracellular Ca2+with Ba2+inhibits stimulated Mg2+uptake but not Mg2+release. These results indicate that cardiac myocytes have a major pool of Mg2+that can be rapidly mobilized upon hormonal stimulation. The net uptake and release of Mg2+are quantitatively similar and appear to be independent of the extracellular Mg2+concentrations but are affected, to various degrees, by the presence of other cellular or extracellular cations.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

To determine the effects of acute myocardial infarction on the regulation of angiotensin II (Ang II) receptors and contractile performance of left and right ventricular myocytes, coronary artery ligation was surgically induced in rats, and Ang II receptor density and affinity and the mechanical properties of surviving muscle cells were examined 1 week later. Physiological determinations of cardiac pump function revealed the presence of ventricular failure, which was associated at the cellular level with a depression in the velocity of myocyte shortening and relengthening, a prolongation of time to peak shortening, and a reduction in the extent of cell shortening. These abnormalities in single-cell function were more prominent in left than in right ventricular myocytes. Cellular hypertrophy was documented by increases in cell length and width, which were also greater in the spared myocytes of the infarcted left ventricle. Reactive hypertrophy was accompanied by a 1.84- and 1.85-fold increase in the density of Ang II receptors on left and right myocytes, respectively. On the other hand, the affinity of Ang II receptors for the radiolabeled antagonist was not altered. However, Ang II-stimulated phosphoinositol turnover was enhanced by 3.7- and 2.5-fold in left and right myocytes, respectively, after infarction. Ventricular myocytes were found to possess the AT1receptor subtype exclusively. In conclusion, myocardial infarction leads to impairment in the contractile behavior of the remaining cells and to the activation of Ang II receptors and effector pathway associated with these receptors, which may be involved in the reactive growth adaptation of the viable myocytes.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

The initiation of receptor-mediated small artery contraction is dependent on inositol 1,4,5-trisphosphatestimulated release of stored calcium. The role of the other product of inositol lipid hydrolysis, 1,2-diacylglycerol, in maintaining contraction remains controversial. Therefore, we have determined the contractile response of rat subcutaneous small arteries (<300 μm i.d.), when mounted as ring preparations in a myograph, to noradrenaline, angiotensin II, KCl-induced membrane depolarization, and a cell-permeable diglyceride, dioctanoylglycerol. In parallel experiments, the conversion of this diglyceride to dioctanoylphosphatidate was studied in32P-labeled vessels. Dioctanoylglycerol produced a slow-onset sustained contraction that was dependent on extracellular calcium. This was accompanied by the generation of the lipid dioctanoylphosphatidate. Noradrenaline and KCI induced rapid-onset sustained contractions and increased the production of dioctanoylphosphatidate (75% and 91%, respectively). In addition, dioctanoylglycerol levels were reduced (41%) after noradrenaline stimulation, suggesting activation of diacylglycerol kinase. In contrast, the contractile response to angiotensin II was transient, and this agonist did not significantly affect the conversion of dioctanoylglycerol to phosphatidate. Noradrenaline markedly increased (fourfold) the formation of endogenous phosphatidate, whereas endogenous 1,2-diacylglycerol was increased (47%) with angiotensin II. These results demonstrate that phosphatidate formation is regulated by vasoconstrictor hormones during receptor-mediated contraction, independent of diglyceride mass. Modulation of the levels of lipid second messengers downstream from phospholipid hydrolysis may represent a mechanism by which agonists that act through the same signaling system produce different contractile responses.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

In previous work with the method of multiple indicator dilution (MID), we have established that a spatially distributed model of transcapillary exchange proposed by Goresky, Ziegler, and Bach (GZB) accurately describes, at the in vivo whole-organ level, the handling of extracellular indicators in the canine renal cortex. To date, however, it has not been possible to assess the key hypothesis that GZB corresponds to the actual local mechanism of exchange in vivo and is not just a compact summary of the kidney's average whole-organ behavior. By adapting the MID method to high speed computed tomography (CT), we are now able to report that the GZB mechanism is an accurate description of renal cortical transcapillary exchange down to volumes of cortical tissue comprising no more than a few per cent of the total cortical mass, i.e., containing no more than a few thousand nephrons. A small bolus of iohexol (radiopaque extracellular indicator) or iodipamide ethyl ester microparticles (radiopaque plasma indicator) injected into the renal artery was followed by CT as it passed through the kidney and into the renal vein. Time-attenuation value curves of the two contrast media obtained from the renal vein and from regions of interest in the cortex were then modeled with the GZB mechanism and with a more complex formulation that includes GZB as a limiting case. When applied to the data, the models converged to GZB as the best fit for each region examined. The GZB mechanism is found to provide excellent agreement with the regional data.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Platelets are a source of vasoactive mediators that regulate vascular tone. Platelets also play a role in intravascular thrombus formation and dynamic coronary constriction that result in myocardial ischemia. However, effects of platelets on myocardial function after ischemia and reperfusion are unknown. In this study, we examined the effects of platelets on myocardial dysfunction caused by ischemia/reperfusion. Buffer-perfused isolated rat hearts, after global ischemia (15 minutes) and reperfusion (10 minutes), developed marked myocardial dysfunction, indicated by a 65±4% decrease in the force of cardiac contraction (FCC) and a 26±7% increase in coronary perfusion pressure (CPP). Ischemia/reperfusion was also associated with release of creatine kinase (CK) and ATP metabolites in the coronary effluents. Perfusion of hearts with buffer containing washed rat platelets (3–8×107cells/ml) protected hearts against dysfunction from ischemia/reperfusion, indicated by minimal changes in CPP (−1±1%) and FCC (−1±3%). Release of CK in the coronary effluent was also reduced, as was the release of ATP metabolites in the platelet-perfused hearts. Perfusion of hearts with serotonin receptor antagonist LY53,857 (10−6M), thromboxane A2receptor antagonist SQ29,548 (10−6M), adenine nucleotide scavenger apyrase (0.4 units/ml), or nitric oxide synthetase inhibitorNG-monomethyl-L-arginine (2×10−4M) attenuated (p< 0.05) the platelet-mediated cardioprotective effects. Perfusion of the hearts with L-arginine (2×10−4M) instead of platelets also showed modest protective effects on FCC (−4.3±13%), CPP (+18±7%), and CK release. Prolongation of the ischemic period to 30 minutes and reperfusion to 20 minutes also demonstrated marked cardiac dysfunction (FCC, −58±10%; CPP, +36±8%) in buffer-perfused hearts. Perfusion of hearts with platelets in this setting of prolonged ischemia/reperfusion also exhibited protective effects on FCC (−24±10%), CPP (+12±6%), and CK release. Thus, platelets protect myocardium from ischemia/reperfusion-induced injury, and these protective effects of platelets are evident regardless of the duration of ischemia/reperfusion. Furthermore, these cardioprotective effects of platelets seem to be related to the release of serotonin, thromboxane A2, and adenine nucleotides. These substances most likely elicit release of endothelium-derived relaxing factor, with its attendant tissue-protective effects, from the microvascular endothelium of hearts.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Using front-surface fluorometry, we determined the effects of cocaine on force and cytosolic Ca2+concentration ([Ca2+]i) in the rat aorta. We also examined the effects of cocaine on45Ca2+influx. Cocaine (10−7to 10−4M) alone did not alter the resting level of [Ca2+]iand force. Cocaine (<10−4M), in a concentration-dependent manner, potentiated the 10−6M serotonin (5-HT)-induced or 10−8M norepi-nephrine (NE)-induced sustained increase in [Ca2+]iand force in the presence of extracellular Ca2+, whereas it had no potentiating effects in Ca2+-free solution. Similar potentiating effects of cocaine were observed in pharmacologically denervated strips. Cocaine (10−5M) produced a leftward shift of concentration-response curves for both 5-HT- and NE-induced increases in [Ca2+]iand force with no effect on the maximal response or the relations between [Ca2+]iand force. Cocaine (10−5M) also accelerated the45Ca2+influx during activation by 10−6M 5-HT or by 10−8M NE. Cocaine (>10−3M) inhibited 5-HT-, NE-, and high-K+depolarization-induced contractions accompanied by decreases in [Ca2+]iin normal physiological salt solution and 5-HT- or NE-induced transient increase in [Ca2+]iand force in Ca2+-free physiological salt solution. Thus, low concentrations of cocaine potentiate NE- or 5-HT-induced contraction by augmenting the increase in [Ca2+]i. These potentiating effects may derive from either an increase in the affinity of the receptors to agonists or an increase in the Ca2+influx. On the other hand, high concentrations of cocaine (>10−3M) have a relaxant effect on vascular smooth muscle, as a result of a decrease in (Ca2+]i.

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Marked neointima formation occurs after balloon injury to the intima of rat arteries. Angiotensin II has been implicated as a growth factor in this process, since angiotensin converting enzyme (ACE) inhibitors block neointima formation after injury. However, ACE is an important kininase, and its inhibitors may act in part by a kinin-mediated mechanism. Kinins are also known to stimulate synthesis of endothelium-derived relaxing factor/nitric oxide (EDRF/NO) and prostacyclin, both of which have antigrowth effects. To determine whether the effect of ACE inhibitors on neointima formation is due to blockade of angiotensin II synthesis alone and/or inhibition of kinin inactivation, we followed two approaches. First, we compared the inhibition of neointima formation induced by the AT1-type angiotensin II receptor antagonist losartan with that caused by the ACE inhibitor ramipril. We also studied whether a kinin receptor antagonist, Hoe 140, blocks the effect of two different ACE inhibitors, ramipril and enalapril, on neointima formation. In addition, we studied whether the effect of ramipril is blocked by an NO synthesis inhibitor,Nω-nitro-L-arginine-methyl ester (L-NAME). Although both ramipril and losartan significantly reduced neointima formation, ramipril had a more marked effect (p< 0.05 for ramipril versus losartan). The kinin antagonist Hoe 140 reduced the inhibitory effect of ramipril and enalapril by 73% and 62%, respectively. The remaining effect of the ACE inhibitors was now similar to that of losartan. Inhibition of neointima formation by ramipril was also blocked by the NO synthesis inhibitor L-NAME. Therefore, we suggest that the protective effect of ACE inhibitors is due to both blockade of angiotensin II formation and kinin degradation. We also suggest that NO may mediate the effect of kinins. The fact that L-NAME blocked the effect of ramipril on neointima formation suggests that NO may play a major role in the inhibitory effect of ACE inhibitors. (Circulation Research 1993;72:1202-1210)

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Transcriptional thyroid hormone responsiveness of the cardiac α-myosin heavy chain (α-MHC) gene has been demonstrated in transfections into fetal and neonatal cardiomyocytes and in transgenic mice. However, the correspondence between the regulation of MHC expression in dissociated cells with that in the intact heart is unclear. Given the cost and time involved in generating multiple transgenic lines for the characterization of gene regulatory elements, we used direct cardiac gene transfer to identify elements regulating both basal and thyroid hormone responsive cardiac α-MHC gene expression in the adult heart in vivo. Sequences upstream of the rat α-MHC gene linked to a luciferase reporter gene were injected into the hearts of adult rats subjected to various thyroid manipulations. The 161-bp sequence upstream of the transcription start site, which contains a TATA box, a CCAATT box, and a thyroid hormone response element, was transcriptionally active but not thyroid hormone responsive. The expression of a construct containing 388 bp of upstream sequence was increased by thyroid hormone administration, a response that required an intact thyroid hormone response element. However, expression of this construct failed to decrease to basal levels in a hypothyroid state. To confer complete (positive and negative) thyroid hormone regulation, 2,936 bp of upstream sequence was sufficient. These results demonstrate that, although necessary, the thyroid hormone response element is not sufficient for complete thyroid hormone regulation of this gene in vivo. In addition, DNA sequences regulating the quantitative expression of cardiac α-MHC in the euthyroid state have been demonstrated. One sequence, an MEF-2 site, which has been shown to be essential for high levels of expression of at least one other cardiac gene in neonatal cardiocytes, was mutated and found not to affect α-MHC expression in the adult heart. These data emphasize the complexity of gene regulation in an intact organ, aspects of which cannot be simulated in culture. (Circulation Research 1993;72:1211-1217)

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

The present study was designed to investigate the effect of O2on intracellular Ca concentration ([Ca]i) in the ductus arteriosus and the mechanisms for O2-induced ductal contraction. The force of isometric contraction of the ring of the ductus arteriosus isolated from fetal rabbits at 30 days of gestation (term, 31 days) was measured. The ductus arteriosus was loaded with fura 2, a calcium-sensitive dye, and [Ca]iwas determined from the ratio of fluorescence intensity at 340 and 380 nm excitation wavelengths. The ductus arteriosus was initially superfused with hypoxic control solutions and contraction was induced by application of oxygenated solutions. The O2-induced contraction of the ductus arteriosus was associated with increases in [Ca]i; and was eliminated in the absence of extracellular calcium. An increase in [K]ofrom 5 to 50 mM, which causes membrane depolarization, induced ductal contraction. The calcium channel blockers verapamil, diltiazem, and nickel caused a similar inhibition of O2-induced contraction as well as KCl-induced contraction. The role of intracellular calcium stores in O2-induced ductal contraction was examined using ryanodine, an inhibitor of calcium uptake and release from the sarcoplasmic reticulum. The inhibition of O2-induced contraction byryanodine was minimal. Infusion of glibenclamide, an inhibitor for opening the ATP-sensitive potassium channel, caused contraction of the ductus arteriosus in the hypoxic solution. Cromakalim, an opener of ATP-sensitive potassium channels, completely relaxed the contraction induced by O2. These data suggest that O2increases [Ca]iand causes contraction in the ductus arteriosus. Application of 0, may change from anaerobic to aerobic metabolism and depolarize membrane potential by closing the ATP-sensitive potassium channel, which in turn increases calcium influx via the voltage-dependent calcium channel. Mechanisms other than the ATP-sensitive potassium channel may also be involved in the O2-induced contraction and remain to be studied. (Circulation Research 1993;72:1218-1228)

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID

摘要:

Oxygen-derived free radicals (O-Rs) are thought to induce alterations in cardiac electrical activity; however, the underlying membrane ionic currents affected by O-Rs and the mechanisms by which O-Rs induce their effects on ion channels in the heart are not well defined. In this study, we investigated the time-dependent changes in resting membrane potential and action potential configuration and changes in steady-state membrane currents in guinea pig ventricular myocytes after intracellular application of an O-R-generating system. O-Rs were generated from the combination of dihydroxyfumaric acid (3 mM) and FeCl3:ADP (0.05:0.5 mM) added to the pipette solution that was used to record membrane potential and currents via the whole-cell variant of the patch-clamp technique. Intracellular exposure of myocytes to the O-R-generating solution induced three stages of changes: 1) an early depolarization (5–10 mV) and an increase in action potential duration accompanied by a decrease in resting inward rectifying K+current conductance, 2) delayed afterdepolarizations and triggered activity caused by the activation of transient inward current mediated by Na+-Ca2+exchange, with failure to repolarize and sustained depolarization between −35 and −20 mV, reflecting the stimulation of nonselective cation current, and 3) a late stage of marked decline in action potential duration, hyperpolarization, and loss of excitability accompanied by activation of the outward current through ATP-sensitive K+channels. These alterations in electrical activity and membrane currents could be prevented by pretreatment withN-(2-mercaptopropionyl)glycine (500 μM), a scavenger of hydroxyl free radicals. The alterations associated with stages 1 and 2 but not stage 3 were completely abolished on intracellular Ca21 chelation (5 mM EGTA in the pipette solution) or disruption of sarcoplasmic reticulum Ca2+handling with ryanodine (10 μM). This study shows that intracellular O-R stress causes specific alterations in membrane ionic currents, leading to changes in resting membrane potential and action potential configuration. Moreover, the data indicate that an elevation in intracellular Ca2+due to abnormal Ca2+handling by the sarcoplasmic reticulum is a cause of some of the alterations in membrane currents during O-R stress. (Circulation Research 1993;72:1229-1244)

ISSN:0009-7330    

出版商:OVID    

年代:1993

数据来源: OVID