Carl R. Honig

Direct measurement of intercapillary distance in beating rat heart in situ under various conditions of O2 supply

Abstract Intercapillary distance was measured from stop-motion color photographs of rat heart beating in situ . When PaO 2 is 100 mm Hg intercapillary distance is 20 μ and about half the available capillaries are open. If hypoxia is induced (a) by lowering pO 2 in myocardial tissue at normal PaO 2 or, (b) by lowering PaO 2 , intercapillary distance decreases significantly. Conversely, if PaO 2 is raised above the normal value, intercapillary distance increases significantly. The mechanism by which oxygen influences the function of precapillary sphincters, and hence the number of open capillaries, is discussed.

The mechanism of cardiovascular action of nitroglycerine: An example of integrated response during the unsteady state☆

Abstract The effects of nitroglycerine on phasic pressure, flow and myocardial oxygen tension were measured in anesthetized spontaneously breathing dogs. As aortic pressure fell, cardiac output increased from 10 to 55 per cent for half to three and a half minutes, then fell 10 to 25 per cent below control levels as the diastolic pressure recovered. Prolonged increases in flow depended upon adequate thoracic blood volume, sympathetic tonus and release of pressor amines. Under conditions most closely resembling those in man, measurements in dogs indicated a rise in left ventricular external work of as much as 25 per cent. Despite this additional load, myocardial oxygen tension increased. In contrast, when venous return was compromised both external work and myocardial oxygen tension fell. Indirect evidence is presented which suggests that qualitatively similar changes take place in patients with coronary artery disease. We conclude that nitroglycerine relieves ischemia by enhancing oxygen delivery rather than by altering load. Changes in chemical and/or mechanical efficiency may also play a role. The determinants of nitroglycerine responses are analyzed and the therapeutic implications briefly discussed.

O2 transport and its interaction with metabolism ; a systems view of aerobic capacity

This commentary demonstrates that VO2max depends, in part, on diffusive O2 transport; exercise hyperemia is necessary but not sufficient. Experiments and new mathematical models place the principal site of resistance to O2 diffusion between the surface of a red cell and the sarcolemma. The large drop in PO2 over this short distance is caused by high flux density and absence of heme protein O2 carrier in this region. PO2 gradients within red myocytes are shallow at high VO2 because myoglobin acts as O2 carrier and PO2 buffer. At high VO2 cell PO2 is less than 5 torr, the myoglobin P50. Low cell PO2 relative to blood PO2 is essential to a) maintain the driving force on diffusion as capillary PO2 falls, and b) to increase myoglobin-facilitated diffusion and the overall O2 conductance. O2 per se does not limit mitochondrial ATP production under normal circumstances because the low O2 drive on electron transport is compensated by greater phosphorylation and redox drives. These metabolic adaptations support transcapillary diffusion by defending VO2 at the low cell PO2 required to extract O2 from blood. Thus aerobic capacity is a distributed property, dependent on the interaction of transport and metabolism as a system.

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.

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

Abstract 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.

Control of smooth muscle actomyosin by phosphate and 5′AMP: Possible role in metabolic autoregulation

Abstract Inorganic phosphate (Pi) inhibits ATPase activity and syneresis of myofibrils and myosin B from vertebrate smooth muscle. This effect is not due to binding of Ca ++ by Pi. Smooth muscle actomyosin is also inhibited by 5′AMP, but the effect is small except at high AMP concentrations. When Pi and 5′AMP are simultaneously present in vitro , the inhibition of ATPase activity is greater than the sum of the separate effects of the two metabolites, and increases as substrate concentration decreases. Concentrations of Pi, 5′AMP and ATP which simulate those in normally oxygenated smooth muscle tissue produce little inhibition in vitro , but concentrations typical of ischemic muscle can almost completely eliminate contraction-coupled ATP hydrolysis. Since smooth muscles contain creatine kinase and adenylate kinase, but possess little 5′AMP deaminase activity, Pi and AMP could serve as oxygen-linked feed-back controls of ATP utilization. The foregoing ideas are applied to a theoretical analysis of the mechanism by which pre-capillary sphincter smooth muscle adjusts capillary density in accordance with local tissue metabolism.

Comparison of Intracellular PO2 and Conditions for Blood-Tissue O2 Transport in Heart and Working Red Skeletal Muscle

1. Neither anoxic nor hypoxic cells were found in epicardium of anaesthetized dogs, cats, rabbits and rats despite heterogeneity of flow (Wieringa et al., 1982) and haematocrit (Honig et al., in press) in the coronary capillary network. 2. Median PO2 in unstressed dog heart and cat heart are 4.8 and 5.2 torr, respectively. These values are close to the P50 of the oxymyoglobin dissociation curve, and well above PcritO2. 3. A dense, interconnected capillary network and high capillary haematocrit appear essential to achieve high O2 extraction at flows characteristic of maximally working myocardium. 4. Mb promotes O2 transport in myocardium by: a) maximizing the driving force for transcapillary diffusion, b) minimizing spatial variability in PmbO2, c) facilitating O2 diffusion in myocytes and, d) permitting close capillary packing without a diffusion shunt for O2. 5. The O2 conductance of the red cell-capillary system is a major determinant of O2 mass transfer in red muscle.

The preparation and characterization of a cardiac relaxing substance.

Abstract Techniques for preparing cardiac myofibrils, granules and soluble relaxing substance are described, and the properties of these materials outlined. Since a cardiac relaxing factor system can be made to function in vitro , it seems likely that tension development is modified by inhibition of ATP hydrolysis in heart as well as in skeletal muscle.

Precapillary O2 Loss and Arteriovenous O2 Diffusion Shunt are Below Limit of Detection in Myocardium

1. Mean intracellular PO2 is much lower than mean venous PO2 in subepicardium. 2. The drop in Hb saturation between aorta and terminal arterioles is within the 5% error of our method. 3. Arteriolar O2 has no effect on saturation in paired countercurrent venules in myocardium. 4. Saturation in coronary venules is independent of venule diameter and indistinguishable from saturation in macroscopic epicardial veins. 5. Since diffusive O2 shunting is negligible and PO2 is approximately linearly related to saturations over the observed range, mean coronary venous PO2 should closely approximate mean-end capillary PO2. 6. O2 mass transport from blood to tissue requires a steep PO2 gradient between the capillary and the surface of a tissue cell.

Myoglobin Saturation and Calculated PO2 in Single Cells of Resting Gracilis Muscles

Use of myoglobin (Mb) as an indicator of intracellular PO2 has been dormant for half a century, chiefly because of difficulty in differentiating Mb from hemoglobin (Hb) when both are illuminated. We recently devised a microspectrophotometer with which light can be collected exclusively from either Hb or Mb. Spatial resolution is 2–5 μ. Since freezing arrests chemical reaction, saturation measurements on a large cell population can be interpreted as though all the measurements had been made simultaneously. In this way the purely spatial uniformity of O2 delivery can be evaluated. Measurements can be made at several loci within one cell, at various loci in a cell cluster, or in cells selected at random from grossly different regions of the muscle. The method offers the further advantage that the contribution of local capillary recruitment to O2 delivery can be evaluated.