Effects of isoprenaline on functional capillary density in the subendocardial and subepicardial layer of the rat myocardium

Perfusion pressure and transmural flow distribution in the left ventricles of isolated rat hearts

1. The influence of perfusion pressure on flow distribution in the left ventricular wall was investigated with a newly developed microsphere method in Langendorff preparations of the rat heart. 2. Microspheres with a diameter of 10 microns stained with the fluorescent dye yellow-green were infused into the perfusion medium. The hearts were then relaxed with a calcium-chelator, fixed with formaldehyde and cut transversally with a freezing microtome. Photographs were taken of the 52 microns slices with a fluorescence photomacroscope. The ventricular wall was subdivided into four zones, evaluated planimetrically and the beads were counted for calculation of microsphere density. 3. Perfusion of groups of hearts, paced at 5 Hz, with Tyrode's solution at pressures of 30, 50, 80 or 120 cmH2O for 30 min revealed an increase of left ventricular pressure amplitude, coronary flow, myocardial oxygen consumption and flow redistribution towards the endocardium of the left ventricular wall with increasing perfusion pressure. 4. The quotient of the sphere densities in the inner endocardium over that in the outer epicardium, however, was already maximal at a pressure of 80 cmH2O and amounted to 1.59 +/- 0.15. This quotient was only 0.52 +/- 0.11 at a perfusion pressure of 30 cmH2O, thus indicating massive change in flow distribution upon hypoperfusion. 5. These results indicate that transmural left ventricular flow redistribution in response to hypoperfusion in the isolated rat heart is similar to that in hearts of much larger species.

Protective effects of long-term bradycardial pacing against catecholamine-induced myocardial damage in rabbit hearts

A high intravenous dose of norepinephrine (4 micrograms/kg/min for 60 minutes) to New Zealand Red rabbits produced patchy subendocardial damage (estimated stereologically in frozen sections) of about 5% of the heart volume 2 days after application. The damaged areas showed loss of staining for alkaline phosphatase, an enzyme present in normal capillary endothelium. Heart performance (cardiac output index, cardiac work [i.e., cardiac output X mean blood pressure], and dP/dtmax) were significantly lower than in control hearts. Capillary density distribution estimated in nondamaged areas of the left ventricular free wall was inhomogeneous favoring subepicardial regions, while homogeneous transmural distribution was found in control hearts. Bradycardial pacing (reduction of heart rate to 50% of normal) performed for 3-4 weeks prior to norepinephrine administration showed a protective effect against catecholamine damage manifested in a smaller extent of necrosis, in the maintenance of homogeneous transmural capillary distribution in nondamaged areas, and, most importantly, in the maintenance of normal cardiac pump performance at rest and during maximal work in response to acutely administered norepinephrine.

An examination of the measurement of flow heterogeneity in striated muscle

This review leads us to a number of conclusions and suggestions for further study. First, we find wide differences in the meaning of flow heterogeneity, arising as a result of the different methods used. These differences will have to be reconciled to form a comprehensive view of the role of heterogeneity in determining vascular function. Second, in the future, the meaning of heterogeneity must be clearly defined and related to a particular microvascular component, and it is imperative that the differences in scale of heterogeneity be appreciated when comparing data from various laboratories. These heterogeneities have different implications for function, and failure to distinguish among them leads to confusion. Third, the degree to which perfusion heterogeneity is regulated in the microcirculation remains in doubt. Reports of variations in flow heterogeneity in response to physiological stimuli are for the most part based on highly questionable indirect methods. Fourth, the heterogeneity that can be demonstrated at the capillary level within striated muscle does not appear to be large relative to the capacity for the microcirculation to exchange most diffusible solutes. Thus, the inferences regarding heterogeneity, as evidenced by diffusible indicators, are likely to be the result of different preparations, damage to the preparations, or perhaps large-scale heterogeneities in the tissue. An alternate possibility would be that the heterogeneity occurs at the microvascular level but reflects some other aspect of microcirculatory function, such as length or hematocrit heterogeneities, but not flow heterogeneities. Fifth, flow heterogeneity within microvessels implies important consequences for capillary exchange and tissue oxygenation. Heterogeneities of velocity of a magnitude comparable to those observed by direct visualization of microcirculation can clearly produce reductions in oxygen supply to small tissue regions of a degree that may limit oxygen delivery, and thereby, tissue function. Sixth, flow heterogeneity may also influence capillary hematocrit and/or red cell spacing by producing cell separation at bifurcations and a resultant reduction in mean capillary tube hematocrit. There is as yet no agreement on why and how these hematocrits influence tissue oxygenation and function. Although several hypotheses are advanced to explain the distribution of blood flow and red cells within microcirculation, each lacks a critical experimental test at present.(ABSTRACT TRUNCATED AT 400 WORDS)

A modified regionally ischemic porcine heart preparation with eligible residual blood flows

The left anterior descending coronary artery was occluded in each of 28 thoracotomized pigs around an intracoronary catheter for periods between 30 and 240 min followed by 90 min of reperfusion. The catheter was connected via an external pump with another arterial catheter. The pump rate was set to deliver 1.5 ml (group I), 3 ml (group II), or 6 ml blood/min (group III) respectively during ischemia. The distribution of the residual blood flow during ischemia was determined in group II with non-radioactive microspheres. We delineated the risk region by a fluorescent dye and the infarcted tissue with a tetrazolium stain. The higher residual blood flow in groups II and III reduced the incidence of ventricular fibrillation during ischemia from 70% (group I) to 28%, suggesting that the amount of residual blood flow is one important determinant for this rhythm disturbance. The subendocardial-subepicardial blood flow ratio in the risk region of the anterior wall was 41%. Infarcts started to develop after 30 min of ischemia (groups I and II). In all groups necrosis progressed most rapidly within the first 90 min of ischemia indicating that besides the beneficial effect of a high residual blood flow only early reperfusion is able to salvage a substantial amount of jeopardized myocardium. Compared to conventional regionally ischemic canine and porcine heart preparations the described model offers the following advantages: Accurate delineation of the risk region, eligible residual blood flow, reduction of ventricular fibrillation with higher residual blood flows, and the possibility to selectively test the metabolic influence of drugs on ischemic injury while avoiding systemic effects.

Sequential perfusion of skeletal muscle capillaries

Rats were injected intraarterially with a fluorescent dye that binds to capillary endothelium, thereby labeling any capillary through which it has passed. After 10, 15, or 30 sec of circulation of the dye blood flow was interrupted, the gastrocnemius was frozen, and the density and distribution of labeled capillaries were measured in transverse sections of the central portion of the medial head. These tissue sections were then counterstained by the myosin ATPase method for capillaries to mark all capillaries. After 10 sec, 45% of all capillaries were labeled and after 15 sec, 59% of all capillaries were labeled. Thirty seconds after injection, all capillaries were labeled with the fluorescent dye. In all three time intervals, the distributions of labeled capillaries were ordered, suggesting that there is a tissue-level control mechanism for regulating capillary perfusion to maintain relatively short maximal oxygen diffusion distances.

Flow and distribution of India ink in microvessels of the frog

Velocities of an India ink front and of RBCs moving ahead of it were studied by intravital microscopy in capillaries of frog mesentery and skeletal muscle. Measurements were made during microperfusion of single vessels or groups of connected vessels in mesentery, and following intravenous ink injection in both tissues. The presence of ink did not appear to interfere with microvascular flow or vasomotion during the period of observation. On the average, the ink spearhead moved only slightly faster than the RBCs. There was substantial variation in relative velocities of RBCs in the same vessel and in the relative velocities of ink front and RBCs. The time course of ink filling showed substantial heterogeneity of flow in mesentery, more nearly uniform flow in skeletal muscle. Comparison of the measured velocity ratios of ink to RBCs with published observations on relative velocities of RBC to blood suggest that the advancing, apparently parabolic front of ink moves at less than twice the mean blood velocity. This is due in small part to diffusive dispersion of the ink particles in the laminar flow gradient, but more largely to stochastic dispersion of the front by interaction with RBCs and by displacement at branches.

Density of plasma-perfused capillaries in the rat heart during carbocromene-induced vasodilation

Urethane-anesthetized thoracotomized rats were used to ascertain whether the density of plasma-perfused capillaries increases in the heart during pharmacologically induced vasodilation. Carbocromene, in a dose of 3.0 mg/(kg x min) i.v., infused for 5 min, raised coronary blood flow from 6.2 +/- 0.6 to 15.6 +/- 0.1 ml/(min x g); heart rate and blood pressure were only slightly changed. Capillary density was determined by timed infusions of a plasma label (FITC or RB 200 coupled with gamma-globulin), infused for different periods of time in the same animal. No significant difference could be observed in the number of capillaries marked for 10 min in the carbocromene-treated rats as compared to the controls (carbocromene-treated rats: 3,630 +/- 90 cap/mm2 subepicardium, 3,360 +/- 70 cap/mm2 subendocardium; controls: 3,750 +/- 140 cap/mm2 subepicardium, 3,210 +/- 90 cap/mm2 subendocardium). In those rats treated with the vasodilator the filling of the microcirculatory system was nearly complete within a labelling period of 1 sec (3,500 +/- 170 cap/mm2 subepicardium, 3,070 +/- 110 cap/mm2 subendocardium), whereas significantly lower values were found when the dye was infused for 1 sec in the controls (2,560 +/- 460 cap/mm2 subepicardium, 1,960 +/- 400 cap/mm2 subendocardium). The results indicate that the filling of the cardiac microcirculatory system is accelerated by a pharmacologically induced vasodilation, the maximal density of plasma-perfused capillaries is not raised, however.