摘要:

ATP-dependent, inorganic phosphate-supported45Ca2+uptake by digitonin-lysed adult rat ventricular cardiomyocytes was used to evaluate the effects of simulated ischemia and reperfusion on the physically intact sarcoplasmic reticulum. Mitochondrial reactions were inhibited with rotenone and oligomycin.45Ca2+accumulation in the presence of the calcium efflux inhibitors, procaine (10 mM) and ruthenium red (30 μM), was used to characterize unidirectional uptake kinetics. A decrease in pH from 7.2 to 6.6 increased the [Ca2+] K0.5from 0.5 to 2.0 μM and reduced the apparent Vmaxby 28%. In the absence of procaine and ruthenium red, at a free [Mg2+] of 0.5 mM, maximum net uptake occurred at pCa 6.2 when pH was 7.2 and at pCa 6.0 when pH was 6.6. At lower pCa, net Ca2+accumulation declined. Increasing free [Mg2+] from 0.5 to 1 mM at pH 6.6 or to 2.5 mM at pH 7.2 increased net45Ca2+accumulation in the absence of procaine and ruthenium and shifted maximum uptake to pCa 5.6 and 6.0, respectively. Increases in cytosolic free [Mg2+] thought to occur during myocardial ischemia are therefore capable of inhibiting calcium efflux from the sarcoplasmic reticulum. Reducing [ATP] from 10 to 1 mM reduced maximum net45Ca2+uptake by 30% both in the presence and absence of efflux inhibitors. Preincubation of intact myocytes under conditions designed to simulate ischemia and reperfusion decreased45Ca2+uptake ≥.50%. The data indicate that myocardial ischemia and reperfusion can alter both Ca2+accumulation and calcium release by the sarcoplasmic reticulum.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The atrial appendages contain most of the atrial natriuretic factor (ANF) in the mammalian heart, and atrial appendage mechanical function predicts ANF secretion during volume loading. To demonstrate the crucial role of the atrial appendages in ANF release, we first measured hemodynamics and changes in plasma ANF after injection of 1,000 ml i.v. normal saline in conscious dogs and again after bilateral atrial appendectomy; we next measured changes in renal function using infusions of atriopeptin 24 to achieve plasma levels corresponding to levels achieved during volume loading; and we lastly measured renal function during acute volume expansion and also after atrial appendectomy. Plasma ANF increased from 65±11 to 246±54 pg/ml after volume loading but did not increase after atrial appendectomy. Atrial appendectomy did not alter the tachycardia or hemodynamic effects of volume loading. Infusion of 10 ng/kg/min atriopeptin 24 increased plasma ANF from 50±9 to 234±54 pg/ml, increased urine output 34±10%, and increased sodium excretion 62±10% in dogs with intact atrial appendages. Renal function was compared in dogs before atrial appendectomy: 20, 40, and 60 minutes after volume loading, urine flow rate increased by 5.9±0.5, 6.9±0.4, and 4.4±0.8 ml/min, while sodium excretion increased by 717±60, 839±84, and 582±57 μeq/min. After atrial appendectomy urine flow rate increased 2.1±0.7,2.7±0.7, and 2.0±0.6 ml/min, and sodium excretion increased only by 327±110, 324±77, and 340±92 μeq/min (p<0.01) during volume loading. The total fraction of water and sodium, 63±5% and 60±4%, respectively, of the load excreted in 4 hours was significantly reduced to 30±5% and 35±3% of the load (p<0.01) after bilateral atrial appendectomy. We conclude that bilateral atrial appendectomy eliminates ANF release and blunts renal excretion of sodium and water after a large acute volume load in conscious dogs.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Conduction disturbances frequently accompany both acute and chronic Chagas' disease. To explore the possibility that changes in gap junction distribution or abundance might play a role in these disturbances, we have investigated intercellular communication between rat neonatal cardiac myocytes in cultures infected withTrypanosoma cruzi. Contractile activity of infected cells was characterized by regional asynchrony within the culture as well as by irregular contraction patterns. Junctional conductance between infected cell pairs was found to be significantly lower than in uninfected cell pairs, and the rapidity and extent of intercellular transfer of the dye lucifer yellow was markedly reduced between infected cells. Immunocytochemical studies demonstrated that the parasitic infection significantly decreased connexin43 expression at junctional membrane regions, correlating with the detected functional uncoupling. These findings of reduced gap junction abundance and function in trypanosome-infected cells may provide important insight into the pathogenesis of the cardiac arrhythmias that attend Chagas' disease.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

This study examines the cellular basis and specificity of the effects of adenosine on early afterdepolarizations (EADs), delayed afterdepolarizations (DADs), and triggered activity (TA) induced by various drugs with different mechanisms of action. Membrane potential and currents were measured in isolated guinea pig ventricular myocytes. Adenosine (10–100 μM) significantly (p<0.05) reduced the amplitude of DADs and suppressed TA induced by isoproterenol (10–50 nM) and forskolin (1 μM) but not those induced by dibutyryl cAMP (1 μM), ouabain (1–5 μM), and 7.2 mM [Ca2+]0. Adenosine also abolished EADs and TA induced by isoproterenol. In contrast, adenosine failed to abolish EADs and TA induced by quinidine (3 μM) or those that occurred spontaneously (i.e., in the absence of drugs). Transient inward current (ITi) was induced on repolarization after 2-second-long single depolarizing voltage steps or after 12-second-long trains of 300-msec depolarizing pulses. Concomitant with the attenuation of DADs, adenosine suppressed ITicaused by isoproterenol and forskolin but not those induced by ouabain, dibutyryl cAMP, and elevated [Ca2+]0The amplitude of ITiwas dependent on the magnitude of the activating voltage step, but the suppression of ITiby adenosine was not. The selective A1-adenosine receptor antagonist N-0861 (9-methyladenine derivative) antagonized the effects of adenosine on afterdepolarizations, ITi, and TA. In myocytes from guinea pigs treated with pertussis toxin, adenosine failed to attenuate DADs and ITior abolish TA induced by isoproterenol or forskolin. In parallel experiments, isoproterenol (10 nM) raised cellular cAMP from 5.7±0.2 to 8.1±0.1 pmol and the selective A1receptor agonist cyclopentyladenosine (5 μM) reduced it to 6.5 ± 0.2 pmol (p < 0.05). Thus, adenosine specifically attenuates afterdepolarizations and abolishes TA by suppressing ITiS that are associated with stimulation of adenylate cyclase via a pertussis toxin-sensitive A, receptor-mediated action. In conclusion, the response of TA to adenosine may identify a mechanism of afterdepolarizations related to stimulation of adenylate cyclase.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The influence of selective removal of the endocardial endothelium (by a 1-second exposure to the detergent Triton X-100, 0.5%) on myofilament Ca2+responsiveness and intracellular Ca2+transients was studied in ferret papillary muscles loaded with the Ca2+-regulated bioluminescent indicator aequorin. The removal of endocardial endothelium produced three major effects: 1) a decrease in peak developed tension and an early onset in isometric relaxation without corresponding changes in the intracellular Ca2+transient; 2) a rightward shift in the peak [Ca2+]i-peak tension relation with no change in maximum Ca2+-activated twitch tension; and 3) a decrease in steady-state tetanic force with a slight increase in the steady-state [Ca2+]i(at 4 mM [Ca2+]) and an unchanged steady-state tetanic force with a clear increase in the steady-state [Ca2+]i(at 10 mM [Ca2+]). These results suggest that intact endocardium may enhance performance of the heart by increasing the myofilament Ca2+responsiveness through endothelium-derived compounds such as endothelin. This hypothesis is supported by our observations that endothelin 1) induced a leftward shift in peak [Ca2+]i-peak tension curve and 2) could reverse the characteristic changes produced by the removal of endocardium.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The relations between pulsatile pressures and flows in the umbilicoplacental circulation have been investigated using chronically instrumented fetal sheep. Under resting conditions, mean arterial pressure fell by 30±6%, from 44±2 to 31±2 mm Hg between the aortic termination and the arteries feeding the cotyledons, and pressure waves were substantially damped during propagation between the two recording sites. This high flow resistance and wave attenuation are attributed to the very thick walls and extreme length of the umbilical arteries. Unique relations between pulsatile components of pressure and flow, characterized as vascular impedance spectra, were also observed. At rest, impedance to pulsatile flow was only slightly below resistance to steady flow, and impedance phase was positive at low frequencies. Pulse-wave reflections had more modest effects in this bed than others. Thus, oscillations in impedance spectra and percent wave transmission with increasing frequency, which are widely accepted manifestations of wave reflections, were relatively small. Positive impedance phases at low frequencies indicated that novel mechanisms influence phase relations between pressure and flow. A significant vascular compliance residing in the peripheral vascular beds could account for this finding. The vasodilator nitroprusside enhanced wave-reflection effects, whereas the vasoconstrictor angiotensin II reduced these effects. These changes were opposite to the effects of vasoactive substances in other systems, probably because these drugs act predominantly on the supply (umbilical) arteries rather than on the peripheral placental vasculature. When peripheral vascular resistance was selectively elevated by infusing 50-μm microspheres, reflection effects were enhanced: the pressure pulse in the umbilical artery was transmitted without attenuation, or was amplified, and impedance spectra more closely resembled patterns typical of other vascular beds. Specifically, impedance modulus fell sharply with increasing frequency, and impedance phase was negative at low frequency. In addition, we observed coordinated oscillations in impedance modulus and phase that are characteristic of beds that exhibit wave-reflection effects. These findings indicate that the specialized anatomy and control mechanisms observed in the umbilical circulation result in unique hemodynamic function, in which wave-propagation effects exert influences not readily predictable from studies on other systems.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Although sodium channels have been reported to be inactive after 5–10 minutes of ventricular fibrillation (VF), their state during early VF is unknown. In 12 open-chest dogs, a floating glass microelectrode was used to record intracellular action potentials from the right ventricle during pacing and during electrically induced VF. Before any drug was administered, an initial episode of VF was continuously recorded for at least 20 seconds followed by defibrillation. Recordings were made during VF episodes after superfusion for 15 minutes around the microelectrode site by low (2.8±10−5M) and high (10−4M) concentrations of tetrodotoxin (TTX) in five dogs, or by low (4 μM) and high (100 μM) concentrations of verapamil in another four dogs. In three dogs, VF was induced without drugs three times to determine if the effects observed in the previous dogs were caused by the drugs or by successive episodes of VF. Ten consecutive action potentials were analyzed at the onset and after 5, 10, 15, and 20 seconds of VF. Action potential amplitude and duration during paced rhythm or VF were not changed by the local perfusion of either TITX or verapamil. In the ITX group, the maximum upstroke rate of depolarization of an action potential (Vmax) during paced rhythm was 104±14 V/sec for control cycles before any drug was given, 86±15 V/sec for the low TIX concentration, and 55±14 V/sec for the high TTX concentration (p<O.OS versus other two). Vmaxdecreased from 55±32 V/sec at the beginning of VF to 37±27 V/sec after 20 seconds of VF for predrug VF, from 39±20 V/sec to 18±11 V/sec for low-dose TTX VF, and from 18±13 V/sec to 12±7 V/sec for high-dose TTX VF (p<0.05 among the three groups). In the dogs receiving verapamil, VF was still inducible with Vmaxnot significantly different from predrug VF at the onset and after 5 or 20 seconds of VF but with Vmaxsmaller (p<0.05) for verapamil than for predrug VF after 10 or 15 seconds of VF. In three dogs, Vmaxwas not significantly different during three successive episodes of VF when no drug was given between the episodes. Thus, the ability of TTX to decrease Vmaxindicates that some sodium channel activity remains in early VF and the inability of verapamil or TTX to prevent VF suggests that both fast and slow channel activity maintains VF during the first 20 seconds.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The importance of perfusion of the coronary vasculature in the regulation of ATPase activity of myosin in rat myocardial cells has been studied. Quantitative histochemistry was used to determine the activity of the enzyme among cells in tissues that had been either perfused through the coronary system or superfused over the surface of the tissue. Enzymatic activity was measured in cryostatic sections from three different preparations: 1) hearts frozen immediately after removal from the animal; 2) isolated hearts frozen after they had been perfused through the coronary circulation; and 3) isolated papillary muscles or trabeculae that had been superfused after dissection and then frozen. ATPase activity was measured in the isolated tissues at different times after dissection. Both calcium- and actin-activated myosin ATPase activities were uniform among cells in both the ventricles of the hearts frozen immediately after dissection and those that had been perfused through the coronary system. In the superfused tissues, although calcium-activated myosin ATPase activity was uniform, actin-activated ATPase activity was not uniform for about 90 minutes after the dissection, the period required for stabilization of the contraction. The pattern of nonuniformity was complex. In all bundles the lowest enzymatic activity was found in the most superficial cells. In very thin bundles, the cells in the center had the highest activity. In the medium and thicker bundles, there were three concentric zones of actin-activated ATPase activity, the superficial zone with the lowest activity, an intermediate zone with high activity, and a central zone with lower activity. Within each zone, the activity was often greatest in myocardial cells immediately next to blood vessels even though the blood vessels had not been perfused. The transverse distribution of ATPase activity of myosin could be explained by a mechanism in which cells in blood vessels (presumably endothelium) release a substance that upregulates myosin ATPase activity, with the rate of release being related to the local oxygen tension. A downregulating substance may also be produced. The period of stabilization of the contraction coincides with the time during which the pattern of actomyosin ATPase activity is nonuniform. These data suggest that the contractile proteins are regulated by a substance produced by blood vessels in proportion to the local PO2, and possibly in relation to shear force on the vascular endothelium.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Myocardial reoxygenation injury may be attenuated by oxygen free radical scavengers, arguing for a role of oxygen radicals in this process. To determine whether free radical scavengers affect reoxygenation injury in isolated cardiac myocytes, resting rat ventricular myocytes were exposed to hypoxic (PO2<0.02 mm Hg) glucose-free buffer alone (n=50) or with the addition of the oxygen radical scavengers 1,3-dimethyl-2-thiourea (DMTU, 25 mM, n=46), human recombinant superoxide dismutase (SOD, 1,000 units/ml, n=40), or the combination of these agents (n=41). All cells responded by undergoing contracture to a rigor form. Hypoxia was then continued for a second period (T2), the duration of which correlates inversely with survival. After reoxygenation, cells either retained their rectangular shape (survival) or hypercontracted to a rounded form (death). For the group of cells with a T2period >30 minutes, no cell exposed to buffer alone (n=20) or to SOD (n=16) survived, in contrast to 15 of 24 (63%) cells exposed to DMTU. The addition of SOD to DMTU offered no advantage to DMTU alone. The protective effect of DMTU was not observed when it was added at reoxygenation, suggesting that this agent has an important effect during the hypoxic period when intracellular Ca2+is known to rise, most likely because of the reversal of Na+-Ca2+exchange. Therefore, the effects of DMTU on Ca2+regulation (indexed by indo-1 fluorescence) during hypoxia were studied. DMTU significantly blunted the [Ca2+] rise during the hypoxic period. When normoxic, electrically stimulated cells were exposed to this agent, they displayed a progressive rise in diastolic [Ca2+], an increase in the amplitude of the Ca2+transient, and a parallel increase in contractility. These findings could be explained by inhibition of Na+-Ca2+exchange. To test the hypothesis that DMITU inhibits Na+-Ca2+exchange, myocyte Ca2+loading via the exchanger was induced by exposing cells to normoxic buffer with Na+fully replaced by choline. Cells exposed in this fashion displayed an intracellular [Ca2+] rise that was nearly abolished by DMTU, consistent with pharmacological inhibition of the exchanger. We conclude that Na+-Ca2+exchange inhibition is responsible for an important part of the effect of DMTU on prevention of hypoxic injury of isolated cardiac myocytes. Although free radical scavenging may play a more important role in the intact heart than in isolated myocytes, the establishment of the role of DMTU as an inhibitor of the Na+-Ca2+exchanger suggests that previous reports of improved postischemic myocardial function with DMTU attributed to free radical scavenging should be interpreted cautiously.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

A system determining the rate of oxygen release from erythrocytes flowing in single microvessels was constructed with an inverted microscope by connecting 1) a scanning/grating spectrophotometer equipped with a photon-counting detector through a thin light guide, to obtain the visible absorption spectrum of a spot (5 μM in diameter) focused on a microvessel, 2) two photomultipliers (connected to a microcomputer via an analog-to-digital converter) through two light guides, to determine the flow velocity of erythrocytes by calculating the cross correlation between the light-intensity changes of two spots (3 μmin diameter, 5 μM apart from each other) focused on the microvessel, and 3) an image processor through a video camera, to estimate the diameter of microvessel from the digitized video images. The rate of oxygen release from single microvessels 7–25 μM in diameter in rat mesentery was measured under the superfusion of deoxygenated solution: 1) The maximal rate was obtained in capillaries, and the rate in arterial microvessels was larger than that in venous microvessels, when similar diameters were compared. 2) The rate was maximum at pH 7.0–7.2, and it decreased in more acidic and alkaline pH values. 3) The rate decreased with a decrease in temperature. The reliability of the measurement using the present apparatus was tested in detail.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID