摘要:

Depression of excitability and responsiveness provoked by the action of high K+and epinephrine on short bundles of excised canine Purkinje fibers yields reentrant excitation. An impulse entering a depressed area undergoes marked slowing of conduction; the impulse then may continue forward while also returning through the pathway by which the initiating impulse entered the depressed area. The impulse may be delayed or blocked in either the forward or the retrograde direction. The reentrant excitation can occur in the absence of premature excitation. Various methods of depression of excitability produce return extrasystoles in Purkinje fibers. The common factor is very slow conduction which depends upon the abolition of the fast upstroke and the appearance of a low-voltage, slowly propagated action potential that is readily blocked.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

Closed loops of fibers of the ventricular conducting system of canine or bovine hearts were used to study circus movement of excitation. Action potentials were recorded at three sites with intracellular microelectrodes. Discrete segments were depressed by application of K+-rich agar or the entire loop was depressed by modified Tyrode's solution containing 15−17 mM K+and 1 to 5 × 10−6M epinephrine. The loops were 12−35 mm long and the effective conduction velocity was 0.02−0.08 m/sec. Impulses entering some loops traveled in one direction only, circling around the loop and returning to produce a second response at one or more sites (single circus movement). In other loops the impulse traveled around the circuit repeatedly (sustained circus movement). Circus movement around short loops requires a low conduction velocity and must be initiated by an impulse that travels around the loop in only one direction. Single circus movement can cause extrasystoles. Sustained circus movement can cause idioventricular rhythms and ventricular tachycardia.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

The objective of the study was (1) to measure systematically the orientation, morphology, and population density of endothelial nuclei of the canine thoracic aorta and its major branches and (2) to obtain evidence in a chronic in vivo preparation that altered flow patterns do indeed change patterns of nuclear orientation. For this purpose, a segment of the descending thoracic aorta was removed, opened longitudinally, and reclosed to form a tube with a new longitudinal axis 90° from the original vessel axis. The new segment was then sutured back in place. The animals were killed at suitable postoperative periods. Endothelial nuclear patterns were studied fromen facephotomicrographs of preparations stained with Evans blue dye. Results indicated: (1) In uniform vessel segments, e.g., middle and lower descending thoracic aorta, the nuclei were oriented parallel to the axis of the blood vessel, and the ratio of major to minor axes of the nucleus was large. The flow in these regions is known to be stable. (2) Nonaxial, less-ordered nuclear orientation with smaller ratios of major to minor axes were found in entrance regions of many major arteries and in the ascending aorta. (3) In chronic studies in which the flow pattern was altered, the nuclear pattern realigned in the direction of flow within 10 days after surgery.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

The present study was undertaken to reevaluate the effects of preload on maximum velocity of shortening at zero load, Vmax, relative to the length-tension curve. Force-velocity relations were measured from afterloaded isotonic contractions and were calculated from isometric contractions of isolated cat papillary muscles. Results were interpreted in the light of three alternative mechanical models of muscle. Vmaxwas obtained by mathematical extrapolation of each force-velocity relation to zero load using a hyperbolic least-squares analysis performed on an IBM 360 computer. With the application of all three muscle models to isotonic force-velocity relations, Vmaxwas relatively constant at low preloads but was reduced substantially as muscle length approached Lmax(the length at the peak of the active length-tension curve). In force-velocity relations from isometric contractions, similar results were obtained with the two-element and Voigt models of muscle. With the Maxwell model, Vmaxremained more nearly constant near Lmax. Peak developed force (isometric contraction), maximum dP/dt, peak calculated velocity of the contractile element (VCE), and Vmaxwere compared in terms of their dependence on preload and length over the entire length-tension curve (using the Maxwell model). Peak VCEand Vmaxwere similar and were less dependent on preload than maximum dP/dt or developed force.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

Lanthanum inhibits45Ca efflux from internally injected squid axons. Evidence for La3+blockade of Ca2+fluxes across the cell membrane was also obtained for aortic smooth muscle: 1mM La3+completely inhibited contractions which otherwise resulted from adding Ca2+to calcium-free depolarizing solutions, and 2 mM La3+caused a 50% inhibition of the late45Ca efflux. Ca2+bound extracellularly could be displaced by La3+, and the binding sites preferred La3+over Ca2+. We could thus use La3+to eliminate extracellular bound Ca2+from our calcium-influx measurements as follows: after exposure of the aortic strips to experimental solutions labeled with45Ca, the extracellular Ca2+label was displaced by putting the strips in a calcium-free solution containing 2 mM La3+for 60 minutes. The loss of intracellular Ca2+was minimized during this hour by the La3+blockade of Ca2+membrane fluxes. Tissue weight,45Ca, and total Ca2+were then measured using standard techniques. Studies employing this new method showed that depolarization by high K+, Na+replacement, and high pH activate smooth muscle contraction by stimulating Ca2+influx. Low pH and La3+block Ca2+influx. Norepinephrine initiates aortic contractions by release of intracellularly sequestered Ca2+. Angiotensin and histamine appear to release Ca2+from this same fraction.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

We compared premature and nonpremature action potentials in Purkinje and ventricular fibers from the moderator band of the pig heart to determine if the duration of premature action potentials depended on factors other than preceding cycle length. In Purkinje fibers, the duration of premature action potentials was cycle-length dependent in responses originating more than 100 msec after the preceding repolarization, but the duration of earlier responses was less than the cycle-length−dependent duration. This cycle-length−independent shortening of premature responses increased with greater proximity to the preceding repolarization and increasing duration of the preceding action potential. In ventricular fibers, the duration of premature action potentials was greater than the cycle-length−dependent duration. This cycle-length−independent lengthening increased as the duration of the preceding action potential increased; it also depended on proximity, being greatest when proximity ranged between 26 and 275 msec. The difference between the durations of simultaneously recorded Purkinje and ventricular action potentials decreased as prematurity increased, but the earliest premature Purkinje action potential was consistently shorter than the simultaneously recorded ventricular action potential. Thus, premature stimulation produced different effects in Purkinje and ventricular fibers. However, in both fibers, the deviation of the duration of premature action potentials from the cycle-length-dependent duration was determined, at least in part, by the duration of the preceding action potential and proximity.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

Subendocardial ischemia without anatomic coronary artery obstruction may result from a discrepancy between metabolic needs and available blood supply. We studied this in open-chest anesthetized dogs and measured pressures in aorta and left ventricle (LV), phasic left coronary arterial blood flow (CBF) by electromagnetic flowmeter, total CBF and LV subendocardial (endo) and subepicardial (epi) flow with radioactive microspheres 8−10μ, in diameter. Since LV Subendocardial flow is mainly or entirely diastolic, it should depend on coronary driving pressure and duration of diastole (i.e., the area between aortic and left ventricular diastolic pressures). This diastolic pressure time index (DPTI) was varied by opening arteriovenous fistulas to lower aortic diastolic pressure, constricting the ascending aorta to raise LV diastolic pressure and pacing to shorten diastole. Myocardial oxygen needs were estimated from the tension time index (TTI). Normal endo-epi flow ratios per gram (1:1) fell to 0.1:1 with these procedures and paralleled a fall in diastolic flow fraction (often nearly zero) and postischemic coronary reactive hyperemic responses. These changes occurred despite normal or raised mean CBF and 300−500% increase in systolic CBF. The altered flow ratios were best predicted by relating them to the ratio of DPTI (supply) to TTI (demand).

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

In an excised cat kidney, the subcapsular veins were clipped near their connections with the renal vein and cut open distal to the clamps. The kidney was then placed in an oil-filled lucite chamber and perfused from a donor cat via tubes entering the chamber through its close-fitting lid. The blood leaking from the cut subcapsular veins was led to one flowmeter; that leaving via the cannulated renal vein (supplied now only by the arcuate vein system) was led to another flowmeter. Catecholamines and stimulation of the renal nerves reduced the arcuate blood flow more than the subcapsular blood flow, the two average reductions being 28% and 20% with epinephrine and 22% and 14% with stimulation of the renal nerves. Angiotensin reduced both flows equally. Microspheres reduced the subcapsular flow more than the arcuate flow, the two average reductions being 42% and 25% of the control flows. This effect was probably caused by trapping of the microspheres in the distal ends of the interlobular arteries beyond the inlets to the afferent arterioles, since these vessels are too narrow to allow free entrance of the microspheres. The preferential effect of epinephrine and nerve stimulation on the deep circulation is thought to be the result of the special structure of the juxtamedullary glomerular units.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

The renal arterial pressure was changed in steps of 25 mm Hg, or it was successively varied between two levels (100 and 150 mm Hg) to obtain four or five stable flow recordings at each level. Statistical calculations indicated that the blood flows in the two drainage areas of the cat kidney were autoregulated with equal effectiveness (whether judged from inflow or outflow rates). The flows at 100 mm Hg were approximately 95% of the flows at 150 mm Hg. To establish whether the accuracy of the method was sufficient to detect a possible nonautoregulated fraction of the arcuate flow similar in magnitude to that traversing the medulla, the flow data were compared with the flows calculated from a simple mathematical model consisting of three homogeneous compartments (outer cortical, inner cortical, and medullary), each with its own pressure-flow relationship. If the flows through all cortical glomeruli are similarly autoregulated, the experimental results are incompatible with a flow through juxtamedullary glomeruli which is proportional to pressure and constitutes 5−7%, or more, of the total renal blood flow.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID

摘要:

In our previous studies a twofold cardiomegaly associated with a proportional increase in cardiac DNA was found in rats subjected to a low-iron diet after weaning. To localize the newly synthesized DNA, control and anemic rats 24−34 days of age were given three intraperitoneal injections of3H-thymidine (1 μc/g body weight; 22c/mM) at 2-hour intervals. The radioautographs were prepared from sections of the left ventricle stained with PAS-hematoxylin. Between 26 and 34 days of age, significantly larger numbers of labeled nuclei were found in both muscle (31−160/1000) and nonmuscle cells, (29−139/1000) in the hearts of anemic rats as compared to controls (11−49/1000 and 6−35/1000, respectively). When 80-day-old rats were given a low-iron diet, a cardiomegaly of approximately 50% developed. A nonsignificant increase in total cardiac DNA and a small significant increase in labeled cardiac nonmuscle nuclei (16−35/1000) was observed in the anemic groups. We conclude that hyperplasia of cardiac muscle and nonmuscle cells is responsible for the increased cardiac mass if cardiomegaly is induced during early postweaning life, while cardiomegaly induced in adult life gives rise to hypertrophy of cardiac muscle cells and hyperplasia of cardiac nonmuscle cells.

ISSN:0009-7330    

出版商:OVID    

年代:1972

数据来源: OVID