ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

An ineffective stimulus applied to cardiac tissue within the relative refractory period can alter the response to an immediately subsequent stimulus. We observed three response patterns that can coexist at different sites of stimulation in the same heart. In the first pattern, a stimulus of two to ten times diastolic threshold, applied too early to elicit a propagated response, becomes effective when a stimulus of equal strength is delivered 10 msec earlier. In the second pattern, a stimulus applied just late enough to evoke a response fails to do so when a stimulus of equal strength precedes it by as much as 30 msec. Finally, in the third pattern, two stimuli, separated by 10 msec, both of which are late enough to be effective when they are given alone, fail to yield a propagated response when they are applied together. These results have a bearing on the use of trains of stimuli to assess the ventricular fibrillation threshold. Possible interpretations are based on the temporal dispersion of recovery from the refractory state.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

Microelectrode and isometric recording techniques were used to evaluate the effects of acetylcholine (ACh) on depressed isolated preparations of dog and cat atrial muscle. Atrial muscles were maintained at 36–37°C with warmed Tyrode's solution and were stimulated at frequencies of 30 or 60solmin. Depolarization to resting potentials of approximately −50 mv was noted (1) after excessive stretch was applied, (2) in muscles obtained from cats in overt right heart failure, and (3) during exposure of the muscles to excessive concentrations of acetylstrophanthidin or lidocaine. Depolarized muscles demonstrated action potentials of smaller amplitude and rate of rise. Exposure to ACh (2.7 × 10−6m) had a minimal effect on resting potential in normal dog and cat atrial muscle and was accompanied by significant negative inotropic actions. The same concentration of ACh markedly increased resting potential and action potential amplitude and induced positive inotropic effects in depolarized muscles; these effects also occurred during beta-adrenergic blockade. We suggest that the positive inotropic effect of ACh in depressed muscles may result from (1) a more synchronous contraction of cells within each muscle, (2) recruitment of previously quiescent cells in contraction, (3) possibly increased calcium inflow in individual cells during depolarizations of greater magnitude, and (4) an increase in the number of interacting sites between actin and myosin after resting potential is improved.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

The influence of the sympathetic nervous system on the cerebral circulatory response to graded reductions in mean arterial blood pressure was studied in anesthetized baboons. Cerebral blood flow was measured by the133Xe clearance method, and arterial blood pressure was decreased by controlled hemorrhage. In normal baboons, the constancy of cerebral blood flow was maintained until mean arterial blood pressure was approximately 65lpercent of the base-line value; thereafter, cerebral blood flow decreased when arterial blood pressure was reduced. Superior cervical sympathectomy of 2–3 weeks duration did not affect the normal response. In contrast, both acute surgical sympathectomy (cervical trunk division) and α-receptor blockade (1.5 mgsolkg of phenoxybenzamine) enhanced the maintenance of cerebral blood flow in the face of hemorrhagic hypotension in that cerebral blood flow did not decrease until mean arterial blood pressure was approximately 35percent of the base-line value. The results indicate that the sympathetic nervous system is not involved in the maintenance of cerebral blood flow in the face of a fall in arterial blood pressure. Indeed, the implication is that the sympathicoadrenal discharge accompanying hemorrhagic hypotension is detrimental to, rather than responsible for, cerebral autoregulation.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

In this study, we characterized the sequence of several intrarenal events and evaluated their relative importance in the pathogenesis of unilateral oliguric acute renal failure induced experimentally in rats by complete occlusion of a renal artery for 1 hour. Kidneys were studied prior to occlusion and 1–3 hours and 22–26 hours after release of the temporary occlusion. Renal blood flow measured by an electromagnetic flow transducer was reduced to 40-50percent of control during both postocclusion periods. Flow of tubular fluid was markedly reduced, and the damaged kidneys were oliguric. Proximal and distal convolutions were filled with fluid and dilated 1–3 hours after occlusion; their pressures were greatly heterogeneous and were elevated, on the average, to 31 and 16 mm Hg, respectively. Glomerular capillary pressure at this time was normal or slightly increased. Histological sections showed extensive tubular obstruction. We conclude that initially the oliguria is primarily due to intraluminal obstruction in the absence of predominant increases in preglomerular vascular resistance. Observations at 22–26 hours after occlusion indicated acute tubular necrosis. Moreover, the combined involvement of preglomerular vasoconstriction, persisting tubular obstruction, and passive backflow of tubular fluid appeared to be important in the maintenance of the oliguria. Glomerular capillary, proximal intratubular, and peritubular capillary hydrostatic pressures were reduced below control values. After acute volume expansion, the reduced pressures and renal blood flow were reversed, yet the experimental kidneys remained oliguric. Thus, it is clear that tubular obstruction is a significant factor responsible for both the genesis and the maintenance of oliguria in this experimental model of ischemia-induced acute renal failure.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

The role of the sympathetic nervous system in the development of deoxycorticosterone-sodium chloride (DOCA-saline) hypertension was investigated by measuring plasma levels of norepinephrine, total catecholamines, and dopamine-β-hydroxylase activity at intervals after the initiation of the DOCA-saline regimen. Plasma norepinephrine was significantly higher in DOCA-saline-treated rats at 4 and 7 weeks and in rats treated with saline alone at 4 weeks compared with that in untreated controls. Total plasma catechol-amine levels (epinephrine and norepinephrine) and dopamine-β-hydroxylase activity were similar in hypertensive rats, untreated controls, and rats that received either DOCA or saline alone. The increases in plasma norepinephrine levels may have resulted from centrally mediated increases in peripheral sympathetic neuronal activity, since the destruction of central catecholaminergic neurons with intracerebroventricularly administered 6-hydroxydopamine (6-OHDA) prevented both the DOCA-saline-induced rise in blood pressure and the increases in plasma norepinephrine. Rats treated with 6-OHDA consistently drank less water or saline than did vehicle-treated controls. The actions of centrally administered 6-OHDA on blood pressure and plasma norepinephrine levels were not secondary to a reduction in salt intake, however, since intact rats given a similar reduced saline intake became hypertensive and demonstrated elevated plasma norepinephrine concentrations. Chronic salt loading may cause a centrally mediated increase in peripheral sympathetic neuronal activity with raised plasma concentrations of norepinephrine. The increased adrenergic activity in the presence of mineralocorticoid-induced sodium retention leads to the development of hypertension.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

This study examined the calcium transport properties of two subcellular fractions of bovine aorta obtained by differential centrifugation. The vesicular fraction had a high-affinity (Km= 1.05 × 10−6m) calcium transport mechanism which could be potentiated by using the calcium-precipitating anion, oxalate. The mitochondria-enriched fraction's calcium transport system had a lower affinity for calcium than did that of the vesicular fraction. The calcium capacity of the vesicular fraction was determined by comparing the steady-state calcium uptake in the presence of oxalate in the whole homogenate with the same uptake by the vesicular fraction alone. A calcium capacity of about 100 mgrmoles Ca2+solkg aorta was obtained. It is concluded that the vesicular fraction has all of the properties of a major site of subcellular calcium regulation in vascular smooth muscle.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

An isolated perfused canine lung preparation in which determinants of vascular caliber could be individually controlled was developed. The relation of pulmonary arterial (Pa), venous (Pv), and alveolar (PA) pressures was such that Pa > PA> Pv throughout the whole lung. The addition of isoprenaline to the perfusate abolished vascular reactivity. Once stability was reached, vascular cross-sectional area remained acceptably constant for 2.25 hours as judged by normalized conductance. The influence of perfusate hematocrit, blood gas tensions, and pH on pressure-flow relations was then studied in 15 isolated canine lungs. The hematocrit-vascular conductance relation was derived at constant perfusion pressure. Conductance varied linearly with hematocrit over a range of 16.5 to 89.5percent. Mean pulmonary arterial blood gas tensions were: Po2= 121 mm Hg, Pco2= 28 mm Hg, and pH = 7.46. Acute respiratory acidosis (Po2= 30 mm Hg, Pco2= 81 mm Hg, pH = 7.17) and lactic acidosis and hypoxemia (Po2= 32 mm Hg, Pco2= 21 mm Hg, pH = 6.96) did not significantly alter this relation. Transformation of the conductance-hematocrit data indicated that hematocrit was the most important determinant of relative apparent viscosity of the blood. Both acute respiratory and lactic acidosis failed to significantly increase relative viscosity within the range of hematocrit usually found in secondary polycythemia.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

A method for measuring inwardly directed transmembrane tracer flow during the cardiac cycle was developed and applied to a study of42K influx in frog atrial trabeculae. A fine frog atrial fiber was suspended in a stream of nonisotopic perfusate into which a small tracer-containing bolus could be injected, subjecting the fiber to a brief, controlled exposure to the tracer at any desired point in the cardiac cycle. In an experiment, the tissue was exposed to a fixed number of radioactive pulses at a selected point in the cardiac cycle; a brief flush with nonradioactive perfusate removed ambient and extracellular label and an extended wash removed the remaining intracellular tracer for radioassay. The same procedure was repeated at different points in the cycle, and the resulting tracer uptake at each point measured the relative influx of the particular ion. In this way, a characteristic and reproducible “K influx profile was demonstrated which exhibited a marked drop below diastolic values during the first 500 msec or so of the action potential followed by a rise and an overshoot above resting values. The time of return to the resting level was somewhat uncertain but was tentatively placed in the vicinity of rapid repolarization. We suggest that the rise and the overshoot reflect the activity of the membrane Na+, K+-adenosinetriphosphatase.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID

摘要:

Active or passive immunization has been used repeatedly as a tool in studies on the role of the renin-angiotensin system in the control of blood pressure and kidney function. The results are not consistent among each other, and they are also at variance with other studies using different approaches. To evaluate the possible causes of these discrepancies, the biological characteristics of angiotensin antibodies were studied in rats. Following the intravenous injection of angiotensin II antibodies (purified by affinity chromatography), the plasma concentration of the antibodies declined in a two-exponential curve with half times of 11 hours and 7 days, which probably reflect distribution in the extracellular space and elimination, respectively. Plasma angiotensin II levels rose from preinjection levels of 100 pgsolml to 12,000 pg/ml within 10 minutes and then declined concomitantly with the decline in antibody concentration. We calculated that only a small fraction of the circulating antibody was occupied by angiotensin II. Plasma renin concentrations were initially elevated both in controls and in antibody-injected rats, but they returned to the control level after 30 minutes and remained at that level throughout the rest of the experiment (10 days). This fact indicates that feedback mechanisms which control renin secretion, such as free plasma angiotensin II concentrations, are in the normal range. We therefore concluded that the renin-angiotensin system in antibody-injected rats was regulated at a normal level. Similar conditions seemed to exist in rabbits actively immunized against angiotensin II; these animals exhibited high concentrations of total immunoreactive angiotensin II (up to 200,000 pg/ml) and a small increase in plasma aldosterone and corticosterone concentrations.

ISSN:0009-7330    

出版商:OVID    

年代:1975

数据来源: OVID