摘要:

We studied the developmental changes in the β-adrenergic modulation of L-type calcium current (ICa) in enzymatically isolated adult (AD) and newborn (NB, 1–4-day-old) rabbit ventricular cells using the whole-cell patch-clamp method. ICawas measured as the peak inward current at a test potential of +15 mV by applying a 180–450-msec pulse from a holding potential of −40 mV with Cs+-rich pipettes and a K+-free bath solution at room temperature. In control, ICadensity (obtained by normalizing ICato the cell capacitance) was significantly higher in AD cells (5.5±0.2 [mean±SEM] pA/pF,n=65) than in NB cells (2.6±0.1 pA/pF,n=60). Isoproterenol (ISO, 1 nM-30 μM) increased ICain a dose-dependent manner for both groups. The maximal effect (Emax) of ISO, expressed as percent increase in ICaover control levels, and the concentration for one half of the maximal effect (EC50) were 203% and 51 nM, respectively, for AD cells and 111% and 81 nM, respectively, for NB cells. The effect of ISO (1 μM) on ICawas decreased as the test potential was increased from −10 to +40 mV. However, the ratio of the percent increase in ICafor AD versus NB cells was almost constant (2.09–2.45) at each test potential. Dose-response curves of forskolin (FOR, 0.3–50 μM) gave Emaxand EC50of 268% and 0.74 μM, respectively, for AD cells and 380% and 1.15 μM, respectively, for NB cells. After stimulating ICaby 10 μM ISO, the addition of 10 μM FOR produced a further increase in ICaof only 12±2% in AD cells (n=4) but a further increase of 140±41% in NB cells (n=6). FOR (10 μM) did not produce any increase in ICafor AD and NB cells after stimulating ICaby intracellular application of 200 μM cAMP. ICadensity stimulated by 10 μM ISO (17.8±1.1 pA/pF,n=7), 10 μM FOR (21.0±1.3 pA/pF,n=8), or 200 μM cAMP (18.0±1.3 pA/pF,n=5) was equivalent in AD cells, whereas ICadensity stimulated by 10 μM ISO (5.8±0.6 pA/pF,n=9) was significantly lower than that stimulated by either 10 μM FOR (13.8±1.5 pA/pF,n=7) or 200 μM cAMP (13.4±0.7 pA/pF,n=7) in NB cells. The ICadensity for FOR or cAMP was significantly higher for AD than NB cells. Pretreatment of AD and NB cells with pertussis toxin markedly increased the ISO effect on ICafor NB cells, whereas the enhancement was relatively small for AD cells. We conclude that the effect of ISO to stimulate L-type ICaincreases after birth in rabbit ventricular cells probably as a consequence of the reduction of tonic Giinhibition of adenylate cyclase rather than the postnatal maturation of the β-receptor-adenylate cyclase system.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Changes in conditioning mean arterial pressure (cMAP) selectively alter the set point of arterial baroreceptors and baroreflexes without affecting gain. Changes in smooth muscle tone at constant cMAPs shift the pressure-discharge curves of aortic baroreceptors in a similar manner. Using an in vitro preparation of the rat aortic arch, we tested whether near maximal changes in smooth muscle tone affect rapid resetting in single regularly discharging aortic baroreceptors. Discharge, pressure, and aortic diameter were simultaneously measured. By using vasoactive drugs (phenylephrine, angiotensin II, Bay K 8644, and nitroprusside), rapid resetting to cMAP changes was tested during different smooth muscle tone conditions (control, constricted, and dilated). Baroreceptor discharge-response curves were periodically assessed with slow ramps of increasing pressure at each of three different cMAP levels (10–15 minutes each). Rapid resetting relations were constructed for pressure threshold (Pth) and diameter threshold (Dth) plotted against cMAP and conditioning diameter (cD), respectively. Vasocon-striction decreased Pth in all baroreceptors (n= 13,p<0.05). Baroreceptor resetting ability as indicated by the slopes of the resetting relations (pressure- or diameter-resetting ratios, ΔPth/ΔcMAP or ΔDth/ΔcD, respectively) was unaffected by large increases in smooth muscle tone (p>0.12). Vasoconstriction, however, offset the pressure-resetting relation, shifting the linear relation in a parallel manner to higher Pth values. In contrast, Dth values during vasoconstriction were not offset but instead fell along a single diameter-resetting relation coincident with the control relation for each baroreceptor. This last result suggests that acute alteration of vessel mechanics by vasoconstriction does not alter the basic rapid resetting process. These new results reinforce the notion that changes in the prevailing degree of baroreceptor distortion are the direct stimulus for a rapid resetting and that modulation of distortion threshold and baroreceptor performance are similar whether conditioning is produced passively by changes in cMAP or actively by altering local smooth muscle tone.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

We investigated the mechanisms that are responsible for the basal release of endothelium-derived relaxing factor (EDRF), which is likely to be identical with nitric oxide, in the intact coronary circulation. The increase in cGMP content of platelets passing through the coronary bed of the isolated rabbit heart was used as an index of EDRF release. Platelet cGMP content after passage through the heart under control conditions (flow rate of 20 ml/min) amounted to 0.50±0.10 pmol/mg protein. Inhibition of endothelial nitric oxide synthesis by 30 μMNG-nitro-l-arginine (L-NNA) reduced this amount by more than 60%. Increasing flow rate from 20 ml/min to 40 and 60 ml/min led to flow-dependent dilation as reflected by the subsequent drop in perfusion pressure after an initial rise. The flow-dependent dilation was associated with a significant increase in the normalized platelet cGMP content. L-NNA abolished completely both the flow-dependent dilation and the increase in platelet cGMP content. Increasing shear stress by a strong vasoconstriction (1 nM endothelin-1) at constant flow was also accompanied by a 2.5-fold increase in platelet cGMP content. To investigate whether mechanical forces applied to the vascular wall by the myocardial contraction cycle were also a stimulus for EDRF release, cardiac arrest was induced by a continuous infusion of mepivacaine (final concentration, 0.02%). Under these conditions, a decrease in platelet cGMP content comparable to that after nitric oxide synthesis inhibition was observed in the arrested heart. The reappearance of mechanical activity after washout of mepivacaine was associated with the recovery of platelet guanylate cyclase activation, indicating the contribution of the mechanical activity on basal EDRF release. To study further the effects of wall deformation on EDRF release, experiments were performed in isolated saline-perfused rabbit arterial segments. Rhythmic squeezing of the segments led to a significant increase in EDRF release as assessed by assay of guanylate cyclase activity. These data indicate that at least two principles contribute to the basal agonist-independent release of EDRF in the intact coronary bed. Besides the continuously acting wall shear stress imposed by the streaming fluid, the periodic compression of intramyocardial vessels stimulates the formation of EDRF. This EDRF formation may result from the high shear stress imposed on the endothelial lining by the periodic diameter reduction and from the direct deformation of the endothelium.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

This study was designed to answer two questions. First, does the left ventricular contractile dysfunction resulting from mitral regurgitation (MR) reflect a primary defect in the cardiac muscle cell? Second, what is the basis for any change in cellular contractile function that might be observed? Left ventricular volume overload was produced in 10 dogs by catheter transection of mitral chordae tendineae. Three months later in these and in seven control dogs, left ventricular contractile function was characterized by the end-ejection stress-volume relation (EESVR). Investigators who were blinded to these results then characterized the contractile performance of cardiac muscle cells, or cardiocytes, from these same left ventricles in terms of the viscosity (graded external load) -velocity relation. Finally, the tissue and cellular components of these same left ventricles were analyzed morphometrically. Both the left ventricles from the MR group and their constituent cardiocytes showed marked contractile abnormalities. By matching ventricles with cells from the same MR dogs, ventricular EESVR was correlated with cardiocyte peak sarcomere shortening velocity (SSV). The correlation coefficient between EESVR and SSV was 0.63, but between a size-independent measure of active ventricular stiffness and SSV, it was 0.88. No change in left ventricular interstitial volume fraction was found in MR dogs, but both ventricular and cellular contractile dysfunction strongly correlated with a decreased volume fraction of cardiocyte myofibrils. Last, in an attempt to relate the degree of contractile dysfunction to the hypertrophic response, left ventricular mass in the MR dogs was correlated with both cellular and ventricular contractile indexes; no significant correlation was found. Three conclusions are warranted by these studies. First, chronic left ventricular volume overload from mitral regurgitation leads to contractile defects at both the ventricular and cellular levels, the extent of which correlates well in individual animals. Second, no quantitative interstitial change resulted from MR. Taken together, these two findings strongly suggest that the contractile defect is intrinsic to the cardiocyte. Third, while the contractile abnormality in MR remains undefined, the most basic defects appear to be a combination of myofibrillar loss with the failure of compensatory hypertrophy to occur in response to progressive decrements in cellular and ventricular function.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

To determine the effects of chronic nonocclusive coronary constriction on cardiac hemodynam-ics, structural integrity, and contractile protein enzyme activity, the left coronary artery was narrowed in rats, and measurements of ventricular performance, magnitude, and distribution of tissue damage and myofibrillar Mg2+and Ca2+myosin ATPase activities were evaluated 1 month later. In the presence of coronary artery stenosis averaging 58%, three levels of involvement of global cardiac performance were identified, and the rats were divided accordingly. In the first group, only left ventricular end-diastolic pressure (LVEDP) was increased; in the second group, LVEDP and left ventricular +dP/dt and/or -dP/dt were affected; and in the third group, LVEDP, left ventricular +dP/dt and -dP/dt, and right ventricular end-diastolic pressure were impaired. Thus, left ventricular moderate dysfunction, severe dysfunction, and failure occurred with coronary narrowing. On a structural basis, coronary constriction resulted in an ongoing process characterized by acute myocytolytic necrosis and foci of replacement fibrosis in different stages of healing. The number of these lesion profiles in the left ventricular myocardium increased 4.7-, 4.4-, and 8.3-fold in rats with moderate dysfunction, severe dysfunction, and failure, respectively. Biochemically, Mg2+-ATPase activity of myofibrils increased biventricularly when moderate dysfunction was present. However, this parameter decreased with the appearance of severe dysfunction, reaching control values in ventricular failure. Ca2+myosin ATPase activity was reduced in the left ventricle of rats with severe dysfunction and failure, whereas it was elevated in the right ventricle of rats with severe dysfunction. In conclusion, a fixed lesion of the left main coronary artery with a modest reduction in vessel luminal diameter generates a conditioned state of the heart characterized by a continuous loss of myocytes and replacement scarring, which, in combination with alterations in contractile protein enzyme activity, may be responsible for a number of abnormalities in cardiac dynamics ranging from moderate dysfunction to pump failure.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The effect of normothermic ischemia and ischemia/reperfusion on the function of cardiac sarcoplasmic reticulum (CSR) was investigated using a modified Langendorff perfusion of isolated rat hearts. The function of the CSR was assessed by the oxalate-supported Ca2+uptake rate of ventricular homogenates. The contribution of the ryanodine-sensitive portion of the CSR was determined by using 20 μM ruthenium red or 625 μM ryanodine to close the CSR Ca2+release channel. The Ca2+uptake rate of the CSR decreased progressively with increasing duration of ischemia, but this depression was much less when uptake was assayed in the presence of ryanodine. The depression in CSR Ca2+uptake preceded ischemic contracture. Ryanodine and ruthenium red stimulated uptake almost equally in control hearts, but ruthenium red was much less effective than ryanodine after ischemia. This difference could not be overcome by increasing the ruthenium red concentration. These results confirm the suggestion that the Ca2+release channel is inappropriately opened after ischemia. The CSR uptake rates were almost completely restored at 15 minutes of reperfusion after 5 and 10 minutes of ischemia but were only partially restored after 15 minutes of ischemia. At reperfusion, mechanical function (end-diastolic pressure and peak systolic developed pressure) was markedly depressed after only 15 minutes of ischemia. The degree of “stunning” correlated well with the depression of CSR function in individual hearts. The decreased Ca2+uptake of the CSR was not due to a buildup of ADP in the homogenates. These results suggest that the reversible ischemic damage of the CSR may be due to a reversible regulation of CSR function and that this regulation may be involved in the causal cascade of events that give rise to postischemic stunning.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Naloxone, an opioid antagonist, augments baroreflex mechanisms in animals; this occurrence suggests that endogenous opioids blunt baroreflex responses. Limited human studies suggest an inhibitory action of endogenous opioids on baroreflex-mediated vagal responses during arterial baroreceptor deactivation. To evaluate the potential effect of endogenous opioids on cardiopulmonary baroreflex mechanisms in humans, we measured arterial and central venous pressures, heart rate, and efferent muscle sympathetic nerve activity (MSNA, by peroneal microneurography) during unloading of cardiopulmonary baroreceptors with incremental lower body negative pressure (LBNP, from 0 to −15 mm Hg) and during the cold pressor test in 21 normal subjects (aged 24±1 [mean±SEM] years). In 14 subjects, we performed LBNP before and after naloxone (0.15 mg/kg i.v.) and placebo (n=11) on separate days. In six of these 14 subjects and an additional seven subjects (n= 13), studies were also performed before and after administration of a lower dose of naloxone (0.075 mg/kg i.v.) on separate days. Neither dose of naloxone significantly altered control arterial or central venous pressures or heart rate. Control MSNA was reduced after the higher but not after the lower dose of naloxone. Comparable reductions in central venous pressure were produced by LBNP in all groups before and after naloxone or placebo, whereas LBNP did not alter arterial pressure. Cardiopulmonary baroreflex sympathetic sensitivity, which was derived as the slope of the linear regression relation between percent change in total MSNA (units) per absolute change in central venous pressure (mm Hg) during incremental LBNP, was significantly augmented after both the high dose (from 18.6±4.7%o/mm Hg to 39.3±8.1%o/mm Hg,p=0.001) and low dose of naloxone, whereas placebo had no effect. MSNA responses to the cold pressor test were not altered by either dose of naloxone. Thus, naloxone selectively potentiates cardiopulmonary baroreflex regulation of sympathetic neural activity in normal humans. These findings suggest that endogenous opioids exert a tonic inhibitory effect on sympathetic responses to orthostatic stress in normal humans.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

We administered ascitic fluid containing atrial natriuretic factor (ANF) monoclonal antibody to rats after 3 weeks of exposure to hypoxia while the rats were still hypoxic. In additional chronically hypoxic rats, we infused synthetic rat ANF. In conscious chronically instrumented rats, after a bolus dose of 5 μg i.v. ANF, pulmonary arterial pressure fell significantly from 26.5±2 to 21±2 mm Hg (p<0.01), reaching its nadir at 5 minutes without change of systemic arterial pressure, cardiac output, or heart rate. Pulmonary arterial pressure increased gradually from 26±4 to 34±4 mm Hg within 30 minutes (p<0.05) after acute administration of ANF monoclonal antibody and decreased transiently to return to baseline within 15 minutes after infusion of control ascitic fluid containing monoclonal antibody against an apolipopro-tein. Cardiac output and heart rate remained unchanged after both ANF monoclonal antibody and control ascitic fluid. In normoxic rats, acute administration of ANF monoclonal antibody did not cause significant changes in pulmonary arterial pressure, cardiac output, or heart rate. Rats receiving weekly intravenous injections of ANF monoclonal antibody that were started before initiation of exposure to hypoxia experienced significantly aggravated pulmonary hypertension and right ventricular hypertrophy compared with rats receiving repeated infusions of control ascitic fluid. However, there was no significant difference in small pulmonary arterial wall thickness or percentage of muscularized arteries at the alveolar duct level. These results suggest that endogenous ANF attenuates hypoxic pulmonary hypertension by decreasing pulmonary vascular tone.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Gene transfer can be achieved in the adult rat heart in vivo by direct injection of plasmid DNA. In this report we define the spatial and temporal limits of reporter gene expression after a single intracardiac injection. pRSVCAT (100 μg), in which the Rous sarcoma virus long terminal repeat is fused to the chloramphenicol acetyltransferase reporter gene, and pαMHCluc (100 μg), in which the α-cardiac myosin heavy chain promoter is fused to the firefly luciferase gene, were injected into hearts, and reporter gene activities were assayed at various times. Both chloramphenicol acetyltransferase and luciferase were detectable in 100% of the rats from 1 to 7 days, in 60% of the rats from 17 to 23 days, and in 30% of the rats from 38 to 60 days after injection. Reporter gene activity was largely limited to a 1–2-mm region of the ventricle surrounding the injection site. Closed circular DNA was far more effective than linear DNA in transfecting cells in vivo. The relative strengths of three different promoters, Rous sarcoma virus long terminal repeat, α-myosin heavy chain, and α1-antitrypsin, all fused to the luciferase reporter gene were determined. The constitutive viral promoter was ∼20-fold more active than the cardiac-specific cellular promoter, and the liver-specific cellular promoter was not active at all in the cardiac environment. Thus, direct injection of genes into the heart offers a simple and powerful tool with which to assess the behavior of genes in vivo. However, the potential of the technique to effect a phenotypic change in the heart is currently limited by the temporal and geographic extent of transfection.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Voltage-dependent sodium channels from a variety of tissues are known to be phosphorylated by the cAMP-dependent protein kinase, protein kinase A. However, the functional significance of sodium channel phosphorylation is not clearly understood. Using whole-cell voltage-clamp techniques, we show that sodium currents (INas) in rabbit cardiac myocytes are enhanced by isoproterenol (ISO). This enhancement of INaby ISO 1) is holding potential dependent, 2) can be mimicked by forskolin and dibutyrl cAMP, and 3) is accompanied by an increase in the rate of Na+channel inactivation. In single-channel, inside-out patch experiments, the catalytic subunit of protein kinase A also enhances INaand increases the rate of inactivation, suggesting that cardiac Na+channel phosphorylation may be physiologically important. Addition of the protein kinase A inhibitor to the pipette solution in whole-cell experiments blocks the stimulatory effect of forskolin without blocking the effect of ISO, suggesting that ISO also enhances INathrough a cAMP-independent pathway. To determine if ISO may stimulate INathrough a direct G protein pathway, single channels were recorded in the presence of the G-activating GTP analogue, GTPγS, and the stimulatory G protein subunit, GSα. Both of these agents enhanced INawithout affecting the rate of Na+channel inactivation. These results suggest that ISO enhances rabbit cardiac INathrough a dual (direct and indirect) G protein regulatory pathway.

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID