To assess the immediate and long-term effects of exercise on factors regulating blood flow, we measured plasma viscosity (eta p) and plasma renin activity (PRA) in 17 trained runners and 16 sedentary healthy subjects before and 10 min after graded treadmill exercise. Resting eta p was lower in runners primarily because of significantly lower fibrinogen concentration. Compared to nonrunners with similar 24-h urine electrolyte excretion rates, runners were characterized by lower PRA at rest. In view of the overall correlation between heart rate and PRA before exercise, reduced adrenergic tone was probably a major factor contributing to the lower PRA in runners. After exercise, plasma viscosity and PRA exceeded control levels, and were similar in magnitude in runners and sedentary subjects. Changes in plasma viscosity were less than expected from the degree of hemoconcentration, primarily because enhanced fibrinolysis maintained fibrinogen level constant. To the extent that plasma viscosity affects viscous flow resistance, the results suggest that tissue perfusion and oxygen delivery rate at rest are greater in trained runners than in sedentary subjects, but these variables become similar after maximum exertion.

Adult↗

PubMedDate not supplied

Effects of lipoproteins on plasma viscosity.

Patients with hypertriglyceridemia and mixed hyperlipidemia have been found to have mean plasma viscosities significantly higher than controls (P less than 0.005). In a group of 70 hyperlipidemic patients and controls, plasma viscosity was correlated with plasma triglyceride concentration (r = 0.56, P less than 0.01) and to a lesser extent with the concentration of plasma cholesterol (r = 0.29, P less than 0.05). When isolated lipoprotein fractions were added to lipoprotein-free plasma in increasing concentration over a physiological range, a highly significant linear relationship between plasma viscosity and chylomicron concentrations (r = 0.98, P less than 0.001) was apparent. Furthermore, when chylomicrons were removed by ultracentrifugation, viscosity returned to baseline levels. Added VLDL produced a lesser effect (r = 0.70, P less than 0.001) and added LDL, over the range of cholesterol concentration studied, had no influence on viscosity. These studies indicate that chylomicrons in particular can increase plasma viscosity. Viscosity increases of the magnitude demonstrated may in turn alter blood flow and thus contribute to symptoms such as intermittent claudication. Chylomicron-induced increases in plasma viscosity and subsequent decreases in local pancreatic blood flow may be one of the factors involved in the known relationship between severe chylomicronemia and acute pancreatitis.

Blood Viscosity↗

PubMedDate not supplied

Effects of pressure on vesicle size, density, and distribution in the canine carotid arterial endothelium.

The purpose of the present experiment was to study the effects of internal hydrostatic pressure on vesicle size, density, and distribution in the canine carotid arterial endothelium by transmission electron microscopy. The pressures applied in this study were 0 (control), 40, 60, 80, 100, and 150 mm Hg. The results of transmission electron microscopy and computer analysis on the plasmalemmal vesicles of aortic endothelium showed that luminal, abluminal, and junctional vesicles all increased their diameter as the pressure was raised from 0 mm Hg, reaching a maximum at 80 mm Hg, and then decreased in size with further increases in pressure to 150 mm Hg. There was a significant difference in diameter among vesicles in different regions of the endothelium, with the diameter of luminal vesicles larger than those of abluminal and junctional vesicles. The densities of vesicles showed very little change from 0 to 80 mm Hg; but they increased markedly as the pressure was further raised from 80 to 150 mm Hg. These results indicate that pressure is an important mechanical factor governing the size and density of plasmalemmal vesicles in aortic endothelium.

Animals↗

PubMedDate not supplied

Richard Skalak 1923-1997.

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Biomedical Engineering↗

PubMedDate not supplied

Spectrin properties and the elasticity of the red blood cell membrane skeleton.

Two models of spectrin elasticity are developed and compared to experimental measurements of the red blood cell (RBC) membrane shear modulus through the use of an elastic finite element model of the RBC membrane skeleton. The two molecular models of spectrin are: (i) An entropic spring model of spectrin as a flexible chain. This is a model proposed by several previous authors. (ii) An elastic model of a helical coiled-coil which expands by increasing helical pitch. In previous papers, we have computed the relationship between the stiffness of a single spectrin molecule (K) and the shear modulus of a network (mu), and have shown that this behavior is strongly dependent upon network topology. For realistic network models of the RBC membrane skeleton, we equate mu to micropipette measurements of RBCs and predict K for spectrin that is consistent with the coiled-coli molecular model. The value of spectrin stiffness derived from the entropic molecular model would need to be at least 30 times greater to match the experimental results. Thus, the conclusion of this study is that a helical coiled-coil model for spectrin is more realistic than a purely entropic model.

Cytoskeleton↗