摘要:

The possible role of plasma low-density protein (LDL) receptors in the uptake and degradation of LDL in the whole arterial wall was investigated by comparison of the in vitro uptake ofI25l-native LDL (nLDL) and131l-methylated LDL (mLDL) by the media of deendothelialized rabbit thoracic aorta excised at in vivo length and pressurized to 70 mm Hg, taking the advantage that mLDL is not recognized by the LDL receptor. The distribution of the relative concentrations of nLDL (C± and mLDL (Cm) across the wall was obtained using a serial frozen sectioning technique. The aorta was incubated under three different conditions for varying periods of incubation in order to analyze separately the processes of binding, binding-internalization, and degradation. At 39°C, in which binding-internalization and degradation occurred, Cnwas significantly higher than Cmat each position across the media. The mean medial Cn/Cmratio was 1.36±0.15 (n=5) after 1 hour of incubation, and decreased to 1.23±0.22 (n=7) after 2 hours of incubation and to 1.13±0.11 (n=5) after 4 hours of incubation. At 4°C, in which internalization and degradation were blocked, the Cn/Cmratio reflected the surface nLDL binding alone; the CnCmratio was 1.47±0.20 (n=5) after 4 hours of incubation, higher than the value obtained at 39°C. To investigate whether degradation of nLDL occurred after receptor binding, the interstitial LDL was washed out by an LDL-free solution after 2-hour incubation at 39°C. After 30 minutes of washout, the Cn/Cmratio decreased to 1.06±0.20 (n=5) in the inner media and was unchanged in the outer media. After 1 hour of washout, the ratio declined to 0.57±0.18 (n=7) in the inner part of the media and increased progressively to 1 at the media-adventitia boundary. The CnCmratio, at 0.67±0.12 (n=5), was practically constant throughout the media after 2 hours of washout. The nLDL degradation rate across the media was obtained from the comparison of nLDL and mLDL before and after the washout. A steep decreasing gradient hi nLDL degradation rate was observed from the luminal to the external surface. The mean medial nLDL degradation rate value was 9.6±4.5 μl/cm3wet tissue/hr. We concluded that functional LDL receptors participate in the uptake and degradation of LDL in the whole aorta.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

The changes in myocardial high energy phosphates and pH during regional ischemia, and their potential role in mediating functional abnormalities, is unclear. To determine the degree of regional blood flow reduction required to induce changes in high energy phosphates and pH, and to correlate these metabolic changes with alterations hi blood flow,3IP nuclear magnetic resonance spectroscopy was employed in an in vivo porcine model of graded coronary stenosis. Simultaneous measurements of regional blood flow and phosphate compounds were made during various steady-state degrees of regional ischemia in which subendocardial blood flow was reduced by as much as 80percent;. ATP did not fall over the total range of graded ischemia, while phosphocreatine (PCr), inorganic phosphate (P,), and pH all changed progressively after blood flow was reduced below 50percent; of normal. The ratio of PCr/P, (a measure of the energy reserve of the myocardium) was strongly correlated to subendocardial blood flow (r=0.94) and declined by 25percent; when blood flow was reduced by only 21percent; below normal. These findings indicate that PCr/P, is a sensitive marker of ischemia and support the hypothesis that the in vivo energy status of the myocardium is closely coupled to myocardial blood flow.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

Early afterdepolarizations (EADs) are a type of triggered activity found in heart muscle. We used voltage-clamped sheep cardiac Purkinje fibers to examine the mechanism underlying EADs induced near action potential plateau voltages with the Ca2+current agonist Bay K 8644 and the effect of several interventions known to suppress or enhance these EADs. Bay K 8644 produced an inward shift of the steady-state current-voltage relation near plateau voltages. Tetrodotoxin, lidocaine, verapamil, nitrendipine, and raising [K]0abolish EADs and shift the steady-state current-voltage relations outwardly. Using a two-pulse voltage-clamp protocol, an inward current transient was present at voltages where EADs were induced. The voltage-dependence of availability of the inward current transient and of EAD induction were similar. The time- dependence of recovery from inactivation of the inward current transient and of EAD amplitude were nearly identical. Without recovery of the inward current transient, EADs could not be elicited. The inward current transient was enhanced with Bay K 8644 and blocked by nitrendipine, but was not abolished by tetrodotoxin or replacement of [Na], with an impermeant cation. These results support a hypothesis that the induction of EADs near action potential plateau voltages requires 1) a conditioning phase controlled by the sum of membrane currents present near the action potential plateau and characterized by lengthening and flattening of the plateau within a voltage range where, 2) recovery from inactivation and reactivation of L-type Ca2+channels to carry the depolarizing charge can occur. Our results suggest an essential role for the L-type Ca1+"window" current and provide a framework for understanding the role of several membrane currents in the Induction and block of EADs.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

Fetal head compression during normal labor can increase intracranial pressure (ICP). We studied the cerebral and peripheral blood flow responses to ICP elevation in utero in chronically catheterized fetal sheep using the radiolabeled microsphere technique. ICP was elevated, stepwise, in increments of 6±1 mm Hg by infusion of artificial cerebrospinal fluid into a lateral ventricle. When ICP was raised to within 28 mm Hg of baseline mean arterial blood pressure (i.e., ICP above 22 mm Hg), arterial pressure began to increase. Above this ICP level, up to 41 mm Hg, mean cerebral perfusion pressure was maintained by equivalent increases in arterial pressure. Cerebral blood flow and O2uptake at the highest ICP levels were not different from baseline values. Changes in peripheral organ blood flow were graded according to the level of ICP. At the highest level (ICP=41 nun Hg), renal, gastrointestinal, and skin blood flow decreased by 68percent;, 69percent;, and 65percent;, respectively. Myocardial and adrenal blood flow doubled, whereas heart rate and cardiac output were unchanged. Placenta! blood flow increased in proportion to arterial pressure. Arterial plasma epinephrine, norepinephrine and arginine vasopressin increased by nearly two orders of magnitude. Therefore, as ICP approaches baseline mean arterial pressure, fetal lambs are capable of sustaining cerebral perfusion by initiating profound visceral vasoconstriction without curtailing placental blood flow. Since cerebral O2uptake was maintained, there is no evidence that stimulation of the peripheral response requires pronounced cerebral ischemia. This highly developed Gushing response may be important for ensuring cerebral viability when the fetal head is compressed during parturition.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

In the Syrian cardiomyopathic hamster heart, abnormal cellular calcium regulation, resulting in cellular calcium overload, is believed to play a role in the pathogenesis of cardiac hypertrophy and failure. Alternatively, the primary abnormality may be coronary vasospasm, resulting in reperfusion-induced necrosis. According to the latter hypothesis, only those cells that suffer an ischemic insult would contain elevated calcium levels. To determine whether a generalized elevation in myocytic calcium exists in myopathic hamster hearts, we measured cellular and subcellular calcium concentrations by electron probe microanalysis in cryosections of 50 -day and 96-day myopathic and control hearts, rapidly frozen in vivo. Total calcium content of ventricular homogenates from each group was also measured by atomic absorption spectropho-tometry. No significant differences in subcellular calcium were found by electron probe microanalysis among 50-day and 96-day myopathics and then- age-matched controls. In 50-day myopathic and control hearts, mitochondrial calcium was 0.7±0.2 and 0.9±0.2, respectively, and A-band calcium was 3.0±0.4 and 2.6±0.4 nunol calcium/kg dry wt(±SEM). Results from 96-day animals were similar. Localized regions of elevated calcium were found only at sites of necrotic foci: in Na+-loaded cells (mitochondria: 4.7±1.3 (SEM) mmol/kg dry wt), in dying cells (mitochondria: 72±22 (SEM) mmol/kg dry wt) or as extracellular deposits (7-10 mol/kg dry wt). Total calcium content of hearts from myopathic hamsters, as determined by atomic absorption spectrophotometry, was also 13 times (50-day) and 50 times (96-day) higher than controls. These results demonstrate that there is a marked heterogeneity in cellular calcium content in myopathic hamster hearts, but the data do not support the hypothesis of a generalized cellular calcium overload.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

Previous studies have shown that the elevation of renal interstitial hydrostatic pressure by the direct expansion of renal interstitial volume increases urinary sodium excretion. The objective of the present study was to investigate whether proximal tubules respond to the elevated renal interstitial hydrostatic pressure and whether the inhibition of prostaglandin synthesis would alter the effect of elevated renal interstitial hydrostatic pressure on proximal sodium reabsorption. Expansion of renal interstitial volume by injecting 100 μl of 2.5percent; albumin solution through a chronically implanted matrix increased renal interstitial hydrostatic pressure similarly in control rats (n=8) and in indomethacin (n=8) or meclofenamate-treated (n=7) rats. In the absence of prostaglandin synthesis inhibition, renal interstitial volume expansion significantly increased the fractional delivery of sodium at the superficial late proximal tubules from 56.5±6.1 to 67.0±6.5percent; (p<0.01) with an accompanying increase in fractional excretion of sodium from 2.1±0.5 to 3.0±0.4percent; (p<0.01). In the presence of indomethacin or meclofenamate, renal interstitial volume expansion failed to augment the fractional delivery of sodium and the fractional excretion of sodium. In summary, these studies demonstrate that the synthesis of prostaglandins plays a role in the regulation of sodium reabsorption by the proximal tubules in response to elevated renal interstitial hydrostatic pressure.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

A rise in cytosolic free Ca2+is the immediate trigger for contraction in mammalian vascular smooth muscle. We used the fluorescent calcium indicator fura-2 and digital imaging microscopy to study the spatial distribution of intracellular Ca2+in arterial myocytes and the changes elicited by activation with norepinephrine (NE). Viable arterial myocytes were obtained from bovine tail arteries by enzymatic digestion. In modified Krebs' solution containing 1.8 mM Ca2+, these myocytes were relaxed and spindle-shaped. The cells contracted rapidly when exposed to NE or high-K+solution ejected from a micropipette; they relaxed slowly when the activator was washed away. NE evoked a rise hi Ca2+concentration ([Ca2+]) in the cells within 100 msec, at a time when the cells had not yet begun to contract. Maximal [Ca2+] levels were attained within 600 msec, at which time the cells were substantially contracted. Digital analysis of images of cellular fura-2 fluorescence revealed that the intracellular [Ca2+] was relatively uniformly distributed prior to activation, with an average resting level of 111±14 nM (n=6). During NE-evoked contractions, intracellular [Ca2+] increased, and the distribution of [Ca2+] became much more heterogeneous. On recovery from activation, the cells relaxed, usually attaining >90percent; of their original resting length. In contrast to the relatively uniform Ca2+distribution observed prior to NE activation, discrete regions of elevated [Ca2+] were observed throughout the recovered cells. The large spatial variation of [Ca2+] after cell activation implies that Ca was sequestered at localized sites in the cell during relaxation.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

The cerebrovascular response to cholinergic stimulation and the effect of inhibition of the lipoxygenase and cyclooxygenase pathways on the response to acetylcholine (ACh) was investigated in newborn piglets. Responsiveness of pial arterioles to ACh, methacholine, and nicotine was studied using a closed cranial window. Pial arteriolar diameter was measured using intravital microscopy. Pial arteriolar responses to ACh, 10-8-10-4M, applied to the cortical surface, were variable and dose-dependent. At low concentration, 10-7M, 45percent; of the arterioles increased diameter by 9±1percent;, 19percent; responded by decreasing diameter by 9±1percent;, and 35percent; did not respond. The response to high concentration, 10-4M, was a profound decrease in diameter, 28±3percent;, in 78percent; of the arterioles studied. These effects were abolished by atropine (0.5 mg/kg i.v.). Muscarinic agonist, methacholine, 10-5-10-3M, also resulted in a decrease in cerebral vascular diameter, while nicotine, 10-6-10-4M, had no effect. In six animals, administration of cyclooxygenase inhibitor, indomethacin (5 mg/kg i.v.), blocked the response to 10 " M ACh but did not affect the response to 10-7M ACh. In five animals, administration of lipoxygenase inhibitor, nordihydroguaiaretic acid (2 mg/kg i.v.) augmented the vasoconstrictor response at both ACh concentrations. The data suggest that, in the newborn piglet, vasoactive prostanoids released following cholinergic activation of a muscarinic-type receptor mediate vasoconstriction in cerebral arterioles.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID

摘要:

LY186126 was found to be a potent inhibitor of type IV cyclic AMP phosphodiesterase located in the sarcoplasmic reticulum of canine cardiac muscle. This compound, a close structural analogue of indolidan (LY195115), was prepared in high specific activity, tritiated form to study the positive inotropic receptor(s) for cardiotonic phosphodiesterase inhibitors such as indolidan and milri- none. A high-affinity binding site for [3H]LY186126 was observed (Kd= 4 nM) in purified preparations of canine cardiac sarcoplasmic reticulum vesicles. Binding was proportional to vesicle protein, was inactivated by subjecting membranes to proteolysis or boiling, and was dependent on added Mg2+. Scatchard analysis suggested the presence of a single class of binding sites in the membrane preparation. Indolidan, milrinone, and LY186126 (all at 1 /nM) produced essentially complete displacement of bound [3H]LY186126, while nifedipine, propranolol, and prazosin had little or no effect at this concentration. This represents the first reported use of a radioactive analogue to label the inotropic receptor for cardiotonic phosphodiesterase inhibitors. The results suggest that [3H]LY186126 is a useful radioligand for examining the subcellular site(s) responsible for positive inotropic effects of these drugs.

ISSN:0009-7330    

出版商:OVID    

年代:1989

数据来源: OVID