The effect of oxygen on arteriolar red cell velocity and capillary density in the rat cremaster muscle

The purpose of this study was to examine the effect of varying oxygen tension upon microcirculatory flow. The cremaster muscle of the rat was exposed and suffused with a control Ringer's solution equilibrated with nitrogen. During this procedure the red cell velocity was measured in 10–30 μm arterioles by the dual-slit photometric method, the tissue and solution pO₂ were measured with an oxygen microelectrode, and the number of flowing capillaries was determined from analysis of video tapes. For a 5-min period the suffusion solution was changed to one containing 5, 10, or 21% oxygen. As the pO₂ of the suffusion solution immediately above the muscle rose from 24 to 85 mm Hg, the arteriolar red cell velocity fell from 2.36 ± 0.29 to 0.69 ± 0.30 mm/sec. The tissue pO₂ averaged 19 mm Hg under the 0, 5, and 10% oxygen solutions and rose to 40 mm Hg under the 21% oxygen solution. When the solution pO₂ was increased from 38 to 97 mm Hg, the capillary density fell by 48%. When the suffusion solution was equilibrated with nitrogen the tissue pO₂ was 2.9 mm Hg or higher, which is above the currently accepted values for the critical pO₂ in tissue.     

Capillary recruitment and flow velocity in skeletal muscle after contractions

The number of perfused capillaries at rest and during contractions was estimated in rat extensor digitorum muscle (EDL) by staining for erythrocytes and expressed as a capillary/muscle fiber ratio (C/F) either in muscles taken quickly out of the animal and fixed in glutaraldehyde, or in muscle frozen in situ. At the same time, the C/F ratio was estimated for all capillaries using staining for capillary endothelium. The capillary/fiber ratio for all capillaries in rat EDL was 1.035 ± 0.029 (SE), for perfused capillaries at rest 0.904 ± 0.019 and during contractions 0.945 ± 0.016. The values in muscles frozen in situ were 0.97 ± 0.01, 0.72 ± 0.03, and 0.90 ± 0.02, respectively; thus 75–87% of all capillaries were already perfused at rest. The number of perfused capillaries was also compared at rest and immediately after contractions in rat spinotrapezius muscle where velocity of capillary flow was also measured. Sixty-nine capillaries were studied in 25 different areas in the spinotrapezius muscle of seven rats. Five capillaries which could not be seen at rest appeared after contractions, and three had stationary flow at rest. Contractions of 30 sec duration at 6 Hz in an area of about 0.04 mm² resulted in the increase in flow velocity from the average resting value of 0.274 ± 0.020 mm/sec to 0.384 ± 0.031 mm/sec, but capillary flow velocities fluctuated both at rest and after contractions. The percentage increase ranged from 0 (in 5 capillaries) to 400% after contractions and was 55.9 ± 4% in all capillaries studied (P < 0.001); this is in agreement with values for blood flow in a mixed cat muscle measured in similar circumstances (contraction of a limited number of motor units at 5 Hz for 30 sec). It is concluded that the increase in blood flow during and after muscle contractions is to a great extent due to increased velocity of flow.     

Functional capillary density in skeletal muscle during vasodilation induced by isoprenaline and muscular exercise

Functional capillary density in skeletal muscle was studied in an isolated, autoperfused preparation of the abdominal muscles of the rat during different forms of vasodilation. The macromolecule hydroxyethyl starch (MW 450,000), labeled with the fluorochrome lissamine-rhodamine B 200, was intravenously injected. When a certain volume of blood had passed the muscle the tissue was fixed by snap freezing. In histological sections those capillaries which had been perfused by the dye could be visualized in the fluorescence microscope. Increase in total muscular blood flow, measured by arterial drop counting, was induced by intraarterial infusion of isoprenaline and by muscular work (control 30.0 ± 2.6, isoprenaline 48.2 ± 4.4, postcontraction hyperemia 44.2 ± 7.4 ml/min × 100g). During hyperemia the functional capillary density (stained capillaries per muscle fiber) was affected in a different way: 0.81 ± 0.02 control, 0.71 ± 0.03 isoprenaline, and 0.93 ± 0.04 postcontraction hyperemia. The data support the view that a rise in total blood flow is not necessarily associated with an increase in functional capillary density.     

Oxygen sensitivity of vascular smooth muscle. I. In vitro studies

The mechanical responses of isolated arterial smooth muscle strips to alterations in bathing solution oxygen tension (PO₂) were investigated to determine the extent to which diffusion of oxygen limits in vitro contractile performance of vascular smooth muscle. Isometric tension was measured for 17 isolated hog carotid artery strips suspended in a physiological salt solution aerated with gas mixtures of varying PO₂. Strip thickness varied between 0.017 and 0.053 cm. Strips were stimulated with 10^?6M norepinephrine (bath concentration), and PO₂ was alternately varied between control and lower levels while the steady-state mechanical tensions were recorded at each PO₂ level. For each strip studied, there was a PO₂ below which the mechanical tension declined from the control level. This PO₂ varied with strip thickness, ranging from 15 mm Hg for a strip 0.017 cm thick to 240 mm Hg for one 0.053 cm thick. The data were consistent with the assumption that diffusion limited the oxygen supply to the smooth muscle cells. Extrapolation of these data to the thickness of arteriolar walls (10 μm) indicates that the smooth muscle cells would not be affected by the changes in oxygen tension at levels above 2 ± 6 mm. Hg. Since the PO₂'s found at this level of the microcirculation are probably greater than 20 mm Hg, it follows that oxygen per se cannot exert a direct influence on blood flow regulation by altering vascular tone unless the arteriolar smooth muscle possesses a special sensitivity to oxygen or the flow is intermittent.     

Skeletal muscle microcirculatory structure and hemodynamics in diabetes

Within skeletal muscle, insulin-dependent (Type 1) diabetes produces straighter, narrower capillaries. To test the hypothesis that these microvascular alterations would be associated with impaired capillary hemodynamics, intravital microscopy techniques were used to study the in vivo spinotrapezius muscle microcirculation of age-matched control (C) and streptozotocin (STZ) induced diabetic (D) rats. D rats exhibited a marked reduction in body weight (C, 266±5 g; D, 150±6 g; P<0.001). At resting sarcomere lengths (i.e. ≈2.7 μm), the additional capillary length arising from tortuosity and branching was less in D muscle (C, 10.5±0.8%; D, 5.3±1.0%, P<0.01). Capillary diameter was reduced in D muscle (C, 5.4±0.1 μm; D, 4.6±0.1μm; P<0.001), and was positively correlated (r=0.71) with the decreased proportion of capillaries sustaining flow (C, 85±5%; D, 53±3%; P<0.001). Within those ‘flowing’ capillaries, red blood cell (RBC) velocity and flux were reduced 29 and 43%, respectively in D muscle (both P<0.05). This reduced calculated O₂ delivery by 57% per unit tissue width and 41% per unit muscle mass. Capillary ‘tube’ hematocrit was unchanged from control values (C, 0.22±0.02; D, 0.22±0.02). We conclude that, in the diabetic state, microvascular remodeling is associated with a reduced proportion of ‘flowing’ capillaries and a reduction in RBC velocity and flux in these vessels such that skeletal muscle O₂ delivery is markedly reduced.     

Pressure effects on the flow behavior of sickle (HbSS) red cells in isolated (ex-vivo) microvascular system

The effect of varying arterial perfusion pressure (P_a) on flow behavior of human normal (HbAA) and sickle (HbSS) erythrocytes was evaluated in isolated rat mesoappendix vasculature. Red cell velocity (V_rbc) and wall shear rate in single arterioles (i.d. 20.4 ± 4.5 μm, $\text{X}$ and SD) were determined and total peripheral vascular resistance (PRU) calculated. The vasculature initially perfused with Ringer's solution was then perfused with red cells suspended (HCT 2%) in the same medium. At P_a of 100 mm Hg, oxy HbSS cells resulted in higher (50%) PRU and lower V_rbc (7.1 ± 2.2 mm/sec) and wall shear rates (1800 ± 490 sec^?1) than those recorded with HbAA cells which show a more rapid microvascular passage, i.e., V_rbc (14.4 ± 2.8 mm/sec) and wall shear rates (3810 ± 360 sec^?1). At the same P_a, partial deoxygenation (PO₂ 40 mm Hg) of HbSS cells caused marked (300%) increase in PRU, and decrease in V_rbc (3.2 ± 0.9 mm/sec), and wall shear rates (820 ± 440 sec^?1). During stepwise decrement of P_a (100-30 mm Hg), PRU for oxy HbSS cells remains elevated but the overall trend is similar to that for HbAA cells and Ringer's perfusion. At P_a of 30 mm Hg, oxy HbSS cells caused some microvascular obstruction. In contrast, with decrement in P_a below 80 mm Hg partially deoxy HbSS cells resulted in progressive increase in PRU and drastic decrease in V_rbc, coupled with progressive capillary obstruction and stasis. An increased propensity of these cells to cause irreversible vasoocclusion is demonstrated when low-pressure conditions prevail.     

Blood flow velocity in capillaries of brain and muscles and its physiological significance

Intravital microfilming by means of a dark-field contact epiobjective was used for measuring capillary blood flow velocity in the brain and skeletal muscles of the rat. The linear flow rate in capillaries was determined by measuring the rate of motion of plasma-filled “gaps” in the continuous erythrocyte flow. The mean linear red cell velocity for 100 cerebral capillaries 2–5 μm in diameter was found to be 0.79 ± 0.03 mm/sec. In the temporalis muscle the velocity was equal to 1.14 ± 0.04 mm/sec in 123 capillaries and 2.43 ± 0.08 mm/sec in 34 arterioles and precapillaries not more than 5 μm in luminal diameter. The experimentally obtained average values of blood flow velocities in cerebral capillaries indicate that these velocities vary mainly from 0.5 to 1.5 mm/sec. This agrees with previously performed calculations based on the mathematical model which suggests that this range of velocities is optimal to adequately supply neurons with oxygen.     

Effect of intraaortic balloon counterpulsation on regional myocardial blood flow and oxygen consumption in the presence of coronary artery stenosis: Observations in an awake animal model

The mechanism by which intraaortic balloon pumping ameliorates myocardial ischemia in patients with unstable angina pectoris is uncertain. Accordingly, the following study was performed to determine the effect of intraaortic balloon pumping on regional myocardial blood flow and myocardial oxygen consumption (MVO₂) distal to severe coronary artery stenosis. Nine closed chest conscious pigs were instrumented with a 7.5 mm long plastic stenosis which reduced vessel diameter by 82%. Measurements of hemodynamics, regional myocardial blood flow (microsphere technique) and MVO₂ were made (1) before intraaortic balloon pumping, (2) at the end of 15 to 20 minutes of intraaortic balloon pumping, and (3) 20 minutes after its discontinuation.Control endocardial blood flow (ml · min^{? 1} · g^{? 1}) distal to the stenosis (1.04 ± 0.20, mean ± 1 standard deviation [SD]) was less than endocardial flow in myocardium perfused by the unobstructed circumflex coronary artery (1.67 ± 0 0.77, p < 0.01). Likewise, control distal zone epicardial flow (1.16 ± 0.36) was reduced in comparison with control circumflex zone epicardial flow (1.48 ± 0.60, p < 0.01In response to intraaortic balloon pumping rate-pressure product declined versus control (10,300 ± 2,090 [SD] mm Hg · min^{? 1} to 9,110 ± 2,010, p < 0.005), whereas aortic mean diastolic pressure (mm Hg) increased versus control (109.0 ± 9.9 to 121.0 ± 13.8, p < 0.01). Distal coronary mean diastolic pressure did not change in response to intraaortic balloon pumping (61.9 ± 13.0 to 68.7 ± 16.5, p = NS). Likewise, endocardial blood flow (ml · min^{? 1} · ^{? 1}) distal to the stenosis did not change during intraaortic balloon pumping (1.00 ± 0.24) versus control (1.04 ± 0.20). In contrast, during intraaortic balloon pumping epicardial blood flow distal to the stenosis declined versus control (1.16 ± 0.36 to 1.01 ± 0.27, p < 0.05). Regional MVO₂ (ml · min^{? 1} · 100 g^{? 1}) distal to the stenosis also decreased versus control in response to intraaortic balloon pumping (12.90 ± 3.55 to 10.30 ± 2.52, p < 0.05). Furthermore, regional MVO₂ correlated well (r = 0.74, p < 0.002) with rate-pressure product.Thus, intraaortic balloon pumping reduces myocardial oxygen demand but does not improve blood flow distal to a severe coronary stenosis; (2) blood flow distal to a severe stenosis may fail to increase with intraaortic balloon pumping because (A) distal coronary mean diastolic pressure may not increase, and (B) blood vessels distal to the stenosis tend to autoregulate in response to a decline in myocardial oxygen demand; and (3) intraaortic balloon pumping ameliorates myocardial ischemia in patients with unstable angina pectoris primarily by reducing oxygen demand rather than by increasing oxygen supply.     

Intercapillary distances and capillary reserve in right and left ventricles: Significance for control of tissue pO2

Distances between capillaries perfused with erythrocytes were measured by stopmotion microcinematography in rat hearts beating in situ. In 18 animals mean intercapillary distance (ICD) was 14.44 μm in the left ventricle and 15.47 μm in the right. These values correspond to about 2815 and 2530 perfused capillaries/mm². respectively. The difference in ICD is statistically significant. Calculations indicate that the shorter diffusion path in the working left ventricle has a significant effect on O₂ transport. On the other hand, minimum ICD is longer and maximum capillary density is smaller in the left ventricle. Thus a larger fraction of the available capillaries is utilized in the normal, working left ventricle. The estimated capillary reserve is 750/mm² in the left ventricle and 1500/mm² in the right. The shorter ICD in the beating left ventricle appears to reflect control of coronary precapillary sphincters by local tissue pO₂.     

Normal transcapillary pressures in human skeletal muscle and subcutaneous tissues

Starling pressures (capillary blood pressure, interstitial fluid pressure, blood oncotic pressure, and interstitial fluid oncotic pressure) were measured in skeletal muscle and subcutaneous tissues of 19 normal humans. Due to inaccessibility of muscle and subcutis capillaries to micropuncture, continuous recording of capillary blood pressure was confined to the fingernail-fold microcirculation. Interstitial fluid pressure was determined acutely by using wick catheters introduced under local anesthesia into tissues of the leg. Oncotic pressure of venous blood was measured in 0.1-ml samples of serum by a colloid osmometer. Interstitial fluid was collected by empty wick catheters. An equation for calculation of interstitial fluid oncotic pressure was formulated which incorporates experimentally measured values of total protein and albumin concentrations. Calculated values for interstitial fluid oncotic pressure correlated well with directly measured values. In fingernail-fold tissue of one human subject mean capillary blood pressure (± SD) was 31 ± 11 mm Hg. In leg subcutaneous tissue of the same subject, interstitial fluid pressure was ?1.3 ± 0.5 mm Hg, blood oncotic pressure was 28 ± 0.6 mm Hg, and interstitial fluid oncotic pressure was 9.1 ± 1.0 mm Hg. In skeletal muscle of the same subject's leg, interstitial fluid pressure was 0.5 ± 1.5 mm Hg, and interstitial fluid oncotic pressure was 8.3 ± 1.6 mm Hg. Based on data for interstitial fluid pressure, blood oncotic pressure, and interstitial fluid oncotic pressure, and assuming the capillary membrane reflection coefficient is 0.9 for both tissues, filtration equilibrium across the capillary wall would occur at a mean capillary blood pressure of 16 mm Hg in subcutis and 18 mm Hg in skeletal muscle.     

Mechanism of action of hyaluronidase in decreasing myocardial ischemia post coronary occlusion in the isolated perfused rabbit heart

The influence of hyaluronidase (H) on subacute experimental myocardial ischemia was studied in isolated perfused rabbit hearts. Changes in ischemic area were assessed by epicardial nicotinamide adenine dinucleotide (NADH) fluorescence photography, an intrinsic high-resolution display of myocardial ischemia. Computerized determination of ischemic area was made from standardized photographs. Hyaluronidase was begun 20 minutes after coronary artery occlusion at 4 units/ml perfusate. NADH fluorophotographs were taken at 10-minute intervals up to 60 minutes of ischemia. Coronary sinus oxygen tension (P_csO₂), myocardial oxygen consumption (MV?O₂), and coronary flow were determined. After 70 minutes, the hearts were perfused with rhodamine solution to identify areas of myocardial perfusion. In 13 H-treated hearts 54.3% ± 3.7% (mean ± SEM) of the nonperfused area (rhodamine stained) was ischemic (NADH fluorescent). In 14 untreated hearts 79.8% ± 3.2% of the nonperfused area was ischemic (p < 0.0001) and the ischemic areas were uniform. The distance between perfused and ischemic tissue was 952 ± 78 μm in the H hearts and 504 ± 35 μm in the untreated hearts (p < 0.0001). In the H treated hearts P_csO₂ increased to 155% of the post-ligation control while it decreased to 79% in the untreated hearts (p < 0.0001). MV?O₂ decreased in the H-treated hearts to 62%; the untreated hearts had no further change. In the H-treated hearts, coronary flow increased to 146% of the post-ligation control while it fell to 91% in the untreated group (p < 0.0001). We conclude that H increases coronary flow while decreasing MV?O₂ during subacute ischemia. In H-treated hearts, significant amounts of myocardium remain normoxic within the nonperfused areas, and may potentially be salvaged after prolonged myocardial ischemia.     

The microvascular bed and capillary surface area in rat extensor hallucis proprius muscle (EHP)

The extensor hallucis proprius muscle (EHP) in rats can be easily dissected with minimal trauma for intravital microscopy. The muscle is very thin and can be transilluminated with good resolution.EHP is a fast contracting muscle (time to peak, 20 msec). The vascular anatomy in most of the studied muscles was of a regular type with a central artery and vein and transverse arterioles and venules. The terminal arterioles had a small number of branching straight capillaries. The capillaries were 535 ± 25 μm (SE) long with an internal diameter of 4.0 ± 0.08 μm at the arteriolar end and 5.5 ± 0.09 μm close to the collecting venule. Intercapillary branches were rare. The average capillary/muscle fibre ratio was 0.95 ± 0.03.By combining in vivo studies with histochemical staining methods it is possible to calculate the total capillary surface area (A):

$\text{A = A}{}_{\mathrm{c}}\text{×}\text{L}\text{l}{}_{\mathrm{c}}\text{×}\text{p+d}\text{2}\text{,}$

where A_c is the average surface area of individual capillaries, L total muscle length, l_c average length of individual capillaries, p number of capillaries/cross-section area at the proximal end, and d number of capillaries/cross section area at the distal end of the muscle.The total capillary surface area of the EHP muscle thus calculated was 1.3–1.9 m²/100 g tissue.     

Heterogeneity of capillary diameters in skeletal muscle of the frog

Capillary diameters in sartorius muscle of frogs were measured in vivo by means of a new computer video method, based on the passage of red blood cells (RBCs) through the capillary (C. Ellis, R. Sanfranyos, and A. Groom (1983), Microvasc. Res.26, 139–150). The distribution of capillary diameters from 21 frogs was represented by a histogram with a mean ± SD of 16.7 ± 4.4 μm (N = 83). The measured dimensions (mean ± SD) of frog RBCs, which have a flattened ellipsoidal shape, were: major axis = 24.1 ± 2.6 μm (N = 149); minor axis = 16.5 ± 1.5 μm (N = 158); thickness at center = 5.4 ± 0.8 μm (N = 32). Frog RBCs travel through capillaries with their major axes predominantly parallel to the direction of flow; therefore, RBCs pass through capillaries without deformation provided that the diameter of the capillary is larger than the minor axis of the cell. By standardizing the measured values of capillary diameter in terms of mean minor dimension of the RBCs (ratio of means for frog being 1.0, approx), we were able to compare the diameter distribution in an amphibian with that in a mammal (rat). If RBC size alone mattered, both standardized distributions should superimpose; however, that for frog was shifted to the right of that for rat, indicating that frog RBCs are less deformable than RBCs of rat. This highlights the necessity, in the microcirculation, for matching capillary diameter to both size and deformability of the red cell.     

Geometric Resistance and Microvascular Network Architecture of Human Colorectal Carcinoma

Objective : To measure the geometric resistance to blood flow in human colorectal carcinoma. Although tumor blood flow is of central importance in both the detection and the treatment of cancer, the determinants of blood flow through the neoplastic circulation are poorly understood. Methods : Human colorectal carcinomas (tissue weight = 272 g ± 43 g (SD), n = 6) were perfused ex vivo with a buffered physiological salt solution of known viscosity at flow rates ranging from 2.5 to 40 ml/min and perfusion pressures from 8 to 100 mm Hg. The geometric resistance was determined from the slope of the pressure-flow curve. For examination of the principal determinant of geometric resistance, the vascular architecture, one of the tumors was perfused with Batson's No. 17 polymer and macerated in KOH to produce a positive vascular cast that was used for measurement of vascular branching patterns and dimensions. Results : The pressure-flow relationship was linear at perfusion pressures above 40 mm Hg, and the geometric resistance, z₀, was constant at approximately 6.5 ± 10⁹g/cm³. Below 40 mm Hg, z₀ increased rapidly. The architecture of the arteriolar and capillary networks of human colorectal carcinoma is similar to those of experimental rodent tumors. Capillaries in planar and nonplanar mesh-works had mean segment diameters of 11 ± 2 and 9.6 ± 2 μm, lengths of 46 ± 24 and 107 ± 40 μm, and intercapillary distances of 46 ± 13 and 74 ± 24 μm, respectively. Conclusions : The geometric flow resistance in neoplastic tissue is 1–2 orders of magnitude higher than that observed in normal tissues. A decrease in functional vascular cross-sectional area may explain the additional increase in resistance at small perfusion pressures. The observed flow resistance may be due to the specialized arteriolar and capillary network architecture, pressures exerted by proliferating cancer cells, and/or coupling between vascular and extravascular flow. These observations demonstrate that tumor vascularity alone may not be indicative of flow resistance or tumor susceptibility to blood-borne therapeutic agents.     

Structural skin capillary rarefaction in essential hypertension

A reduction in the density of capillaries (rarefaction) is known to occur in many tissues in patients with essential hypertension. This rarefaction may play a role in increasing peripheral resistance. However, the mechanism underlying this capillary rarefaction is not understood. The aim of this study was to assess the extent of structural versus functional capillary rarefaction in the skin of dorsum of fingers in essential hypertension. The capillary microcirculation was examined with video microscopy before and after maximizing the number of perfused capillaries by venous congestion. The study group comprised 17 patients with essential hypertension (mean supine blood pressure, 155/96 mm Hg) and 17 closely matched normotensive controls (mean blood pressure, 127/77 mm Hg). We used intravital video microscopy with an epi-illuminated microscope to examine the skin of the dorsum of left middle phalanx before and after venous congestion at 60 mm Hg for 2 minutes. A significantly lower mean capillary density occurred at baseline in hypertensive subjects versus normotensive subjects. With venous occlusion, capillary density increased significantly in both groups; however, maximal capillary density remained significantly lower in the hypertensive subjects than in the normotensive subjects. The study strongly suggests that much of the reduction in capillary density in the hypertensive subjects is caused by structural (anatomic) absence of capillaries rather than functional nonperfusion.     

Capillary diameter and geometry in cardiac and skeletal muscle studied by means of corrosion casts

Studies of microvascular geometry made from microscope observations of tissues in vivo or after perfusion with a silastic elastomer or india ink are restricted to a two-dimensional field of view. Microvascular corrosion casts, however, if of sufficient rigidity and structural integrity, can yield three-dimensional information when examined under the scanning electron microscope. We have used modified Batson's No. 17 anatomical casting compound (having a shrinkage <1% on setting) to prepare casts of the microvasculature of the heart and skeletal muscles in anesthetized rats. In casts from the L. ventricle the capillary network appeared to parallel the arrangement of the muscle fibers, but showed many capillary loops and anastomoses. In skeletal muscles (gastrocnemius and gracilis) held at full extension, in situ, the casts showed long straight capillaries with fewer branchings than in the heart. In shortened skeletal muscle the capillaries exhibited an undulatory configuration. Capillary diameters (mean ± SD) were 5.14 ± 1.42 μm (N = 202), 5.04 ± 1.45 μm (N = 294) and 4.84 ± 1.97 μm (N = 335) in L. ventricle, gastrocnemius, and gracilis muscles (both shortened), respectively. The mean values for capillary diameter in these three tissues did not differ significantly. Combining our data with those of L. Henquell, P. L. LaCelle, and C. R. Honig on erythrocyte deformability in the rat (Microvasc. Res.12, 259–274 (1976)) suggests that even when the capillary bed is fully distended the smallest capillaries, amounting to 1–2% of the total number, must be channels for plasma flow alone. In cross-sectional views of the casts from contracted skeletal muscle the capillaries appeared to form a tightly meshed network of convoluted vessels around the fibers, such that in some regions a large fraction of the surface of each fiber was in contact with blood. The Krogh cylinder geometry appears not to be appropriate for modeling O₂ transport in maximally shortened skeletal muscle; a more appropriate model may be that of a cylindrical muscle fiber supplied, at any point down its length, by a uniform peripheral O₂ supply.     

Myocardial oxygen consumption during experimental hypertrophy and congestive heart failure

The purpose of this investigation was to study the effects of experimental myocardial hypertrophy-congestive failure state on myocardial oxygen consumption (MV?O₂). Hypertrophy and heart failure were induced in nine adult cats by surgical banding of the main pulmonary artery to a lumen approximately 10% of normal (2.8 mm circumference clip). Twenty-five to 82 days later nine CHF and 12 control animals were studied, and right ventricular papillary muscles were mounted in a polarographic oxygen electrode muscle bath for simultaneous determination of myocardial mechanics and MV?O₂. Pulmonary arterial banding resulted in average peak systolic right ventricular pressure of 73 ± 7 mmHg and right ventricular/body weight ratio was increased from 0.54 ± 0.08 control to 1.22 ± 0.08 g/kg CHF (P < 0.001). Right ventricular end-diastolic pressure was increased from 1.9 ± 0.2 control to 9.0 ± 1.1 mmHg CHF (P < 0.001) while liver weight/body weight ratio was increased from 27.8 ± 1.1 control to 31.7 ± 1.5 g/kg CHF (P < 0.01). The average force velocity curve was depressed downward and to the left with maximal measured velocity (preload) changing from 1.35 ± 0.07 control to 0.62 ± 0.06 muscle lengths/s (P < 0.001). The extent of shortening and thus the external work performed was significantly depressed in CHF muscles. However, MV?O₂ of afterload contractions of CHF muscles was entirely normal. The length-active tension relationship was significantly depressed with peak developed tension at L_max in CHF muscles of 3.55 ± 0.53 g/mm² (control 6.19 ± 0.55, P < 0.01). Although the rate of tension development of isometric contractions was depressed from 34.1 ± 3.3 control to 14.7 ± 0.19 g/mm²/s CHF (P < 0.001), the MV?O₂ per gram of tension development was paradoxically increased from 0.56 to 1.17 μl/mg/contraction × 10^?3. Resting MV?O² was increased from 2.07 ± 0.19 control to 4.27 ± 0.53 μl/mg/h. The effect of acetylstrophanthidin 2 × 10^?7 g/ml was to increase both the contractile state and MV?O₂ of CHF muscles. These findings demonstrate an inefficiency of conversion of oxygen to tension development in experimental hypertrophy and congestive heart failure.     

Perfused capillary surface area in postural and locomotor skeletal muscle

A measure was made of the perfused capillary surface area per gram of tissue (S_f) in postural, anterior latissimus dorsi (ALD), and locomotor, posterior latissimus dorsi (PLD), skeletal muscles in chickens. The animals were anesthetized with L.A. Thesia and the ALD and PLD muscles were prepared for observation using a modification of the method developed by R. E. Klabunde and P. C. Johnson (1977, Amer. J. Physiol.232, H411–417). S_f was determined from the relationship S_f = πdLθ_f, where d = capillary diameter, L = capillary length, and θ_f = number of perfused capillaries per gram. Capillary lengths and diameters were measured directly through the microscope. Estimates of θ_f were measured in two ways: by a flow method (θ^Q_f), and by an histochemical method (θ^H_f). It was observed that the average capillary diameter in PLD was the same as in ALD muscles, but the average capillary length in PLD was two times longer than in ALD muscles. Both the flow and histochemical methods yielded values of θ_f that gave similar values of S_f for PLD muscles (S^Q_f = 24.7 ± 20.0 cm²/g, S^H_f = 13.7 ± 14.4 cm²/g), but different values for ALD muscles (S^Q_f = 9.8 ± 6.8 cm²/g, S^H_f = 42.8 ± 19.1 cm²/g). The difference between S^Q_f and S^H_f for ALD muscles could be explained by the more irregular and longer perfusion path observed in postural versus locomotor muscle. The flow method appears to underestimate S_f in capillary beds with irregular perfusion paths. The results indicate that only a small fraction of the total capillary bed was perfused in resting skeletal muscle and that S^H_f for ALD was approximately three times larger than S^H_f for PLD.     

Laser Doppler imaging and capillary microscopy in ischemic ulcers

The local distribution of laser Doppler flux (mainly thermoregulatory perfusion) and capillary density (nutritive circulation) within 25 ischemic leg ulcers and their adjacent skin were investigated. For this purpose the technique of laser Doppler imaging and capillary microscopy were applied. In each ulcer a non granulation tissue area (NGTA), a granulation tissue area (GTA) and in adjacent skin a skin area (SA) were defined. In these areas the average laser Doppler area flux (arbitrary units, AU) and the number of capillaries/mm² were determined for each patient. The mean±S.D. of laser Doppler area fluxes were: NGTA 1.30±1.93, GTA 2.13±1.53 and SA 1.21±0.77 AU, respectively. The differences between GTA and NGTA or SA was statistically significant (p<0.001, each) The mean±S.D. of capillary densities were as follows: NGTA: 0.56±2.06, GTA 6.76±8.39 and SA 16.80±7.38 capillaries/mm², respectively. The following differences were statistically significant: NGTA versus GTA (p<0.01) and SA versus NGTA or GTA (p<0.001, each). In conclusion following characteristics of the three areas can be described: In NGTA low laser Doppler area flux is combined with very low capillary density (ulcer area without healing). In GTA the highest laser Doppler area flux of all three areas and an intermediate capillary density (wound healing) is found. In SA an intermediate laser Doppler area flux is associated with the highest capillary density of all three areas with the healing process nearly completed and no granulation tissue.     

Capillary diameter in rat heart in situ: Relation to erythrocyte deformability,O2 transport,and transmural O2 gradients

The diameter of subepicardial capillaries was measured in stop-motion photo-micrographs of normoxic rat hearts. Mean diameter over the whole cardiac cycle was 4.41 μm (0.09, SEM). Calculations indicate that mean diameter during systole is about 4 μm and during diastole is about 5 μm. The deformability of rat erythrocytes was evaluated by aspirating the cells into micropipets of various diameters. All cells traversed a 2.8-μm pipet at a mean ΔP of 0.17 mm Hg and a 2.5-μm pipet at a ΔP of 2.9 mm Hg. Below 2.5 μm, the pressure required to aspirate 100% of the cells increased linearly as the channel diameter decreased and reached 104 mm Hg at 1.9 μm. Comparison of deformability data with frequency distributions of coronary capillary diameter indicates that all cells traverse all capillaries during diastole and traverse most superficial capillaries during systole. In the subendocardium, however, systolic tissue pressure is very high relative to erythrocyte deformability. Consequently, perfused capillaries should be compressed to the minimum thickness of an erythrocyte (about 1.8 μm). Calculated pericapillary O₂ gradients demonstrate that such narrow capillaries cannot sustain aerobic metabolism throughout the tissue. This is particularly true since capillary compression impedes erythrocyte entry, and thereby increases functional intercapillary distance. We conclude that: (1) Compression and narrowing of capillaries during systole can account for the transmural gradient in tissue pO₂. (2) During diastole, capillary dimensions are perfectly matched to the dimensions and deformability of erythrocytes.