摘要:

Ventricular repolarization significantly influences contractility, refractoriness, and ion channel state. Factors affecting repolarization will thus affect these secondary phenomena. To understand the influence of age on ventricular repolarization, we studied neonatal, young, and adult dogs using electrocardiogram, action potential, and whole-cell voltage-clamp recordings from single epicardial myocytes. Hearts of neonatal and 57–58-day-old dogs require a significantly longer time for repolarization than those of adult dogs, as determined by analysis of rate-corrected QT and JT (QT minus QRS) intervals. Epicardial action potentials of neonates are significantly longer than those of adults, as determined by measurements of duration at 50% and 90% repolarization. The adult action potential is characterized by a large phase 1 notch that is absent from neonatal recordings. This notch develops between 58 and 64 days of age, and by 64–68 days of age, it is equal to that in adults. In addition, action potentials recorded from adult and young epicardial muscle are more greatly affected by rapid pacing and superfusion of 2 mM 4-aminopy-ridine than are potentials recorded from neonatal tissue. Whole-cell voltage-clamp recordings reveal a 4-aminopyridine-sensitive transient outward current in adult myocytes that is absent from neonatal myocytes. The correlation between developmental changes in the 4-aminopyridine-sensitive current, the action potential, and the QT interval suggests that the transient outward current may be an important determinant in the relation between age and repolarization.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The sarcomeric proteins and organization of cardiac myofibrils appeared intact in multiple unrelated patients with hypertrophic cardiomyopathy. In two subjects demonstrating the missense mutation at position 403 (Arg to Gln) in the β-myosin heavy chain gene, total myosin and immunoreactive β-myosin heavy chain levels were similar to those found in other patients with hypertrophic cardiomyopathy and various disease control subjects. No alteration in expression of the cardiac α-myosin heavy chain gene was observed. These results are consistent with the examined myosin heavy chain mutation, permitting proper accumulation and assembly of myosin while primarily impairing contractile function. The characteristic myocyte disarray would appear likely to be a secondary consequence of the mutations.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Vascular endothelium, which produces endothelium-derived relaxing and constricting factors, plays an important role in regulating the vascular tone. We recently demonstrated that oxidized low density lipoprotein inhibited endothelium-dependent relaxation and that lysophosphatidylcholine accumulated during the oxidative modification of low density lipoprotein was the essential substance for the inhibition of endothelium-dependent relaxation. To clarify the mechanisms of the inhibitory effect of lysophosphatidylcholine, we used a bioassay system to investigate the effect of lysophosphatidylcholine on the production and/or release of endothelium-derived relaxing factor and its effect on the cytosolic Ca2+level ([Ca2+]i) and phosphoinositide hydrolysis in cultured bovine aortic endothelial cells. [Ca2+]iwas monitored by the fura 2 method, and the accumulation of inositol phosphates in cells labeled with myo-[2−3H] inositol was measured. Bioassay experiments showed that lysophosphatidylcholine inhibited the production and/or release of endothelium-derived relaxing factor from cultured endothelial cells. Lysophosphatidylcholine (5–20 μg/ml) induced a biphasic increase in [Ca2+]i, which consisted of a rapid increase followed by a sustained increase, and the initial component was a result of mobilization from intracellular Ca2+stores without detectable synthesis of inositol 1,4,5-trisphosphates. Furthermore, lysophosphatidylcholine (5–20 μg/ml) dose-dependently inhibited both phosphoinositide hydrolysis and the increases in [Ca2+]ievoked by bradykinin. These results indicate that the impairment of endothelium-dependent relaxation induced by lysophosphatidylcholine is due to the inhibition of phosphoinositide hydrolysis and the subsequent increases in [Ca2+]iin endothelial cells. Lysophosphatidylcholine that accumulates in oxidized low density lipoprotein and atherosclerotic arteries may play an important role in the modification of endothelial function.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Lysophosphatidylcholine (lysoPC) transferred from oxidatively modified low density lipoprotein (Ox-LDL) to the endothelial surface membrane has been shown to produce a selective unresponsiveness to cell surface receptor-regulated endothelium-dependent relaxation (EDR) in the rabbit aorta. To determine its mechanism we examined the effects of lysoPC on endothelial surface receptor-mediated transmembrane signals. Incubation for 1 minute with palmitoyl lysoPC (5–10 μM) decreased thrombin (Th, 2 units/ml)-or histamine (His, 0.1 mM)-stimulated inositol 1,4,5-trisphosphate (IP3) production in primary cultures of human umbilical vein endothelial cells (HUVECs). LysoPC also decreased Th- or His-induced intracellular calcium ([Ca2+]i, fura 2) elevation. Pretreatment with protein kinase C (PKC) inhibitors staurosporine (100 nM) or H-7 (50 μM) prevented the inhibitory actions of lysoPC, but HA-1004 had no effect. Incubation for 5 minutes with phorbol 12-myristate 13-acetate (PMA, 100 nM) produced the inhibitory actions on the Th- or His-induced intracellular signals, which closely mimic those exhibited by lysoPC. However, the inhibitory effect of lysoPC was lost in cells that were depleted of PKC by pretreatment for 24 hours with 100 nM PMA. Furthermore, incubation of the cells for 1 minute with lysoPC stimulated PKC activity in the membrane fraction. In organ chamber experiments with porcine coronary artery rings, pretreatment with staurosporine (20 nM) attenuated lysoPC-induced impairment of EDR in response to Th. These results indicate that lysoPC, which accumulates in Ox-LDL and atherosclerotic arterial walls, inhibits the early transmembrane signaling pathway in endothelial cells, and PKC activation could at least partially be involved in the negative regulation by lysoPC. These intracellular actions of lysoPC may play a role in the mechanism of the lysoPC-induced impairment of EDR in response to cell surface receptor-mediated stimulations.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Rapid washout of extracellular H+accumulated during preceding ischemia (i.e., the abrupt restoration of extracellular pH) has been implicated as an arrhythmogenic factor during reperfusion. Therefore, we hypothesized that by limiting the rate at which extracellular pH was restored during early reperfusion it should be possible to protect against reperfusion-induced arrhythmias. To test this, we used isolated rat hearts (n=12 per group) and a novel dual coronary perfusion cannula that permitted the induction of regional ischemia (10 minutes) and the selective reperfusion (8 minutes) of the ischemic zone with modified solutions. We examined the antiarrhythmic efficacy of 1) acidic (pH 6.6) reperfusion with stepwise restoration of extracellular pH to 7.4 (stepped pH) and 2) transient (2-minute) acidic (pH 7.1, 6.8, 6.6, or 6.4) reperfusion with subsequent abrupt restoration of extracellular pH to 7.4. Hearts in two contemporary control groups were reperfused with solution at pH 7.4 throughout. In all groups, 100% of hearts exhibited ventricular tachycardia (VT) on reperfusion. VT degenerated into ventricular fibrillation (VF) in 100% of hearts in the control group but in only 42% of hearts in the stepped-pH group (p<0.05). In the groups subjected to transient acidic reperfusion, there was a pH-dependent prolongation of VT cycle length (measured at 15 seconds of reperfusion), which was 47.1±3.9, 51.1±5.5, 56.0±1.9, 60.4±2.8 (p<0.05), and 68.8±5.0 (p<0.05) msec in the pH 7.4 (control), 7.1, 6.8, 6.6, and 6.4 groups, respectively. In these groups, VT degenerated into VF in 92%, 92%, 83%, 42% (p<0.05), and 33% (p<0.05) of hearts, respectively. In conclusion, limiting the rate at which extracellular pH is restored during early reperfusion does not affect the rapid induction of VT but inhibits the degeneration of VT into VF and promotes spontaneous reversion to normal sinus rhythm. This is consistent with a major arrhythmogenic role, during uncontrolled reperfusion, for the rapid washout of extracellular H+.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The effect of α1-adrenoceptor stimulation on the delayed rectifier K+current (Ik) was examined in isolated guinea pig ventricular cells by use of the patch-clamp method. IKwas evoked by a 3-second depolarizing pulse from a holding potential of −30 mV in a Na+- and K+-free solution containing 3 μM nifedipine. Phenylephrine (30 μM) in the presence of propranolol (1 μM) produced an increase in IK. In five cells, phenylephrine increased the tail current of IKby 23±5%. This effect of phenylephrine was blocked by prazosin (0.3 μM), a selective α1-blocker. Phenylephrine produced only a small effect on the voltage and time dependence of IK. Pretreatment with 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7, 10 μM) abolished the phenylephrine-induced increase in IK. In addition, pretreatment with a maximally effective concentration of 12-O-tetradecanoylphorbol 13-acetate (100 nM) abolished the phenylephrine-induced increase in IK. In conclusion, α1-adrenoceptor stimulation increases IKin guinea pig cardiomyocytes. This cvl-adrenoceptor-mediated response may be related to an activation of protein kinase C. The increase in IKmay explain a shortening of action potential duration observed after α1-adrenoceptor stimulation in guinea pig cells.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

It has been shown that endothelium-derived relaxing factor (EDRF) may inhibit platelet aggregation in vitro through activation of platelet-soluble guanylate cyclase. To assess whether EDRF may also affect platelet function in vivo, intravascular platelet aggregation was initiated by placing an external constrictor around endothelially injured rabbit carotid arteries. Carotid blood flow velocity was measured continuously by a Doppler flow probe placed proximal to the constrictor. After placement of the constrictor, cyclic flow reductions (CFRs), due to recurrent platelet aggregation, developed at the site of the stenosis. After CFRs were observed for 30 minutes, a solution of authentic nitric oxide (NO,n=10) was infused into the carotid artery via a small catheter placed proximally to the stenosis. Before infusion of NO, CFR frequency averaged 18.3±2.9 cycles per hour, and CFR severity (lowest carotid blood flow as percentage of baseline values) was 6±1%. NO completely inhibited CFRs in all animals, as shown by the normal and constant pattern of carotid blood flow (CFR frequency, 0 cycles per hour,p<0.001; carotid blood flow, 92±5%,p=NS versus baseline). These effects were transient; CFRs were restored spontaneously within 10 minutes after cessation of NO infusion. After CFRs returned, S-nitroso-cysteine (S-NO-cys), a proposed form of EDRF, was infused into the carotid artery. S-NO-cys also abolished CFRs in all animals but at a significantly lower dose than NO (0.3±0.1 versus 12±4 nmol/min). The role of endogenously released EDRF in modulating in vivo platelet function was then tested in additional experiments. In 10 animals, endogenous release of EDRF was stimulated by infusing acetylcholine into the aortic root during CFRs. Infusion of acetylcholine was also associated with a complete inhibition of CFRs, similar to that observed during exogenous infusion of NO orS-NO-cys. These antithrombotic effects of acetylcholine were completely lost when EDRF synthesis was prevented by administration of the l-arginine analogueNG-monomethyl l-arginine (L-NMMA). Furthermore, in six additional rabbits the basal release of EDRF was blocked by L-NMMA after CFRs had been previously abolished with aspirin or the combination of aspirin and ketanserin, a serotonin S2receptor antagonist. L-NMMA caused restoration of CFRs in all animals, indicating that even the basal release of EDRF is important in modulating platelet reactivity in vivo. Taken together, the data of the present study demonstrate that endogenous EDRF might importantly contribute to the modulation of platelet function in vivo.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Synthesis and activity of the enzymatic equivalent of the sodium pump, Na,K-ATPase, are regulated by thyroid hormone in responsive tissues. The purpose of this study was to determine whether triiodothyronine (T3) regulates the level of the messenger RNA (mRNA) coding for Na,K-ATPase α-and β-subunits in the heart. The expression of Na,K-ATPase mRNAs in in vitro myocardial cells was directly assayed by Northern and slot blot hybridization using Na,K-ATPase α- and β-isoform-specific cDNA probes. Exposure of cultured neonatal rat cardiocytes to 10-8M T3resulted in 1) threefold to fourfold increase in α1- and β1-mRNA accumulation, with a maximum elevation at 48 hours, 2) sevenfold increase in α2-mRNA accumulation with a peak elevation at 72 hours, and 3) transient threefold increase in α3-mRNA within the first 24 hours followed by a deinduction thereafter. The increase in αl-mRNA accumulation by T3occurred over the physiological T3concentration range with an EC50of 5×10−10M. This was associated with a twofold increase in αl-subunit protein accumulation and an increase in Na,K-ATPase transport activity. The half-life of α1-mRNA analyzed by actinomycin D chase was less than 3 hours and was not affected by T3. Transfection experiments with the luciferase reporter gene revealed that thyroid hormone response sequences are located within the 5'-flanking regions of each α-isoform gene. The above results suggest that thyroid hormone regulates all three Na,K-ATPase α-isoforms in cardiocytes and may play an important role in the developmental switching of the cardiac α2- and α3-isoforms. These effects are mediated, at least in part, by transcriptional regulatory factors interacting with the respective α-isoform gene promoters.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

β-Adrenergic receptor downregulation is the end result of cellular adaptation to prolonged agonist exposure. The factors mediating receptor downregulation include receptor phosphorylation, receptor movement from the plasma membrane to intracellular sites, and alterations in nascent receptor synthesis. We have previously demonstrated a downregulation of the left ventricular β-receptor during chronic hypoxia in vivo. To determine the mechanism of this downregulation, we produced chronic hypoxia in seven newborn lambs by creating right ventricular outflow obstruction and an atrial septal defect. Oxygen saturation was reduced to 65–74% for 2 weeks. Six lambs served as normoxic controls. Sarcolemmal membrane and cytosolic fractions were prepared from left ventricular free wall samples. β-Receptor density in each fraction was determined with the radioligand [125I]iodocyanopindolol. Steady-state levels of β-receptor mRNA were determined by Northern blot analysis using a β1-adrenergic receptor cDNA probe. During chronic hypoxia, left ventricular membrane β-adrenergic receptor density decreased by 55% (153±28 fmol/mg for hypoxic lambs versus 342±79 fmol/mg for control lambs,p<0.05). There was no corresponding increase in β-receptor density in the cytosolic fraction (23±3 fmol/mg for hypoxic lambs versus 33±9 fmol/mg for control lambs,p=NS), nor was there a significant change in the ratio of β1-receptor/β2-receptor subtypes as assessed by radioligand binding (β1subtype, 84.1±10.1% for hypoxic lambs versus 93.2±8.8% for control lambs;p=NS). During chronic hypoxia, steady-state levels of β1-receptor mRNA were reduced twofold (2.0±0.4 absorbency units per millimeter for hypoxic lambs versus 3.7±0.4 absorbency units per millimeter for control lambs,p<0.05). These data demonstrate that chronic hypoxia downregulates left ventricular β-adrenergic receptor density in vivo by reducing steady-state levels of β-receptor mRNA.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Abnormal coronary blood flow (CBF) in long-standing left ventricular (LV) pressure-overload hypertrophy has been associated with ischemia and LV dysfunction. Thus, goals of therapy in pressure overload are not only the relief of the overload itself but also regression in hypertrophy and subsequent improvement in CBF. However, little is known about CBF in humans or in large mammals after the relief of pressure overload, when the hypertrophy has regressed. This study was performed to test the hypothesis that, even 6 months after the relief of pressure overload in the dog, CBF would still be abnormal. Three groups of dogs were studied: 1) normal control dogs (NL group), 2) dogs with LV pressure-overload hypertrophy (LVH group), and 3) dogs that had developed LV pressure-overload hypertrophy but in whom the pressure overload was relieved 6 months before the final study (LVH Reg group). CBF was studied in conscious dogs by use of the radiolabeled microsphere technique at rest, during rapid atrial pacing, and during maximum coronary vasodilation produced by adenosine infusion. The ratio of LV weight (g) to body weight (kg) (LVBW) was 4.2±03 in the NL group, 7.1±0.6 in the LVH group, and 7.7±0.5 in the LVH Reg group before pressure-overload relief (p=NS, LVH versus LVH Reg). Six months after removal of the pressure overload, the LVBW in the LVH Reg group had fallen to 5.5±0.3 (p<0.05), but this LVBW was still greater than that in the NL group (p<0.05). During rapid atrial pacing, endocardial and epicardial CBF rose significantly in NL dogs. However, during rapid atrial pacing, endocardial CBF fell from 1.18±0.22 to 0.7±0.20 ml/min per gram in the LVH group (p<0.05) and did not rise in the LVH Reg group. During adenosine infusion, endocardial blood flow increased in NL dogs from 1.63±0.13 to 4.0±0.3 ml/min per gram and increased to a similar level in the LVH Reg group. Although CBF increased during adenosine infusion in the LVH group, the increase was less than that in the NL or LVH Reg group (p<0.05). Minimum coronary vascular resistance was similar in NL dogs (14±2 units) and LVH Reg dogs (18±3 units,p=NS) but was significantly elevated (32±10 units) in LVH dogs (p<0.05). We conclude that after significant but incomplete regression of pressure-overload hypertrophy, maximum CBF and minimum coronary vascular resistance return to normal. However, during rapid atrial pacing, significant abnormalities in CBF still exist.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID