Hemodilution causes size-dependent constriction of pial arterioles in the cat.

Cerebral blood flow (CBF) rises as hematocrit (Hct) falls. We previously attributed this rise in CBF to two independent factors of equal importance, decreased arterial O2 content and decreased blood viscosity. We hypothesized that decreased arterial O2 content would dilate cerebral arterioles and that the magnitude of the vasodilation would depend on the magnitude of the passive fall in vascular resistance attributable to decreased viscosity. The present study was designed to test the hypothesis that anemia is accompanied by cerebral vasodilation. Using a closed cranial window, we measured the diameters of 42 pial arterioles (35-305 microns) in 7 cats as serial isovolemic hemodilution lowered Hct by 44% from 31 +/- 4 to 17 +/- 3%. Hemodilution increased CBF (microsphere technique) but did not change mean arterial blood pressure or arterial blood gases. Anticipated vasodilation did not occur; instead, pial arterioles constricted as Hct fell. Maximum vasoconstriction was observed when Hct reached 65-70% of the initial value. Vasoconstriction lessened as Hct was lowered further, but arteriolar diameters at the lowest Hcts remained less than base-line levels. Constriction was greater in small (less than 100 microns) than in large (greater than or equal to 100 microns) arterioles. The initial constriction of pial arterioles may represent myogenic vasoconstriction in response to flow-induced vasodilation of more proximal portions of the cerebrovascular bed and/or to washout of an endogenous vasodilator. Arteriolar relaxation with more profound hemodilution may reflect superimposed metabolic vasodilation.

[1]  H. Kontos,et al.  Independent blockade of cerebral vasodilation from acetylcholine and nitric oxide. , 1988, The American journal of physiology.

[2]  L. Kuo,et al.  Effect of hemodilution on oxygen transport in arteriolar networks of hamster striated muscle. , 1988, The American journal of physiology.

[3]  R. Koehler,et al.  Effect of hematocrit on cerebral blood flow with induced polycythemia. , 1987, Journal of applied physiology.

[4]  G. H. Nelson,et al.  Laser-induced endothelial damage inhibits endothelium-dependent relaxation in the cerebral microcirculation of the mouse. , 1987, Circulation research.

[5]  R. Koehler,et al.  Effect of hematocrit on cerebral blood flow. , 1986, The American journal of physiology.

[6]  H. H. Lipowsky,et al.  Microvascular hemodynamics during systemic hemodilution and hemoconcentration. , 1986, The American journal of physiology.

[7]  V. Miller,et al.  Modulation of vascular smooth muscle contraction by the endothelium. , 1986, Annual review of physiology.

[8]  Kortaro Tanaka,et al.  Blood Flow Velocity in the Pial Arteries of Cats, with Particular Reference to the Vessel Diameter , 1984, Journal of cerebral blood flow and metabolism : official journal of the International Society of Cerebral Blood Flow and Metabolism.

[9]  H. Bohlen,et al.  Arterial and microvascular contributions to cerebral cortical autoregulation in rats. , 1984, The American journal of physiology.

[10]  D. Becker,et al.  Mannitol causes compensatory cerebral vasoconstriction and vasodilation in response to blood viscosity changes. , 1983, Journal of neurosurgery.

[11]  R. Traystman,et al.  Cerebral venous outflow and arterial microsphere flow with elevated venous pressure. , 1983, The American journal of physiology.

[12]  R. Traystman,et al.  Effects of changes in arterial O2 content on cerebral blood flow in the lamb. , 1981, The American journal of physiology.

[13]  S Chien,et al.  In vivo measurements of "apparent viscosity" and microvessel hematocrit in the mesentery of the cat. , 1980, Microvascular research.

[14]  S. Chien,et al.  Effects of hematocrit variations on regional hemodynamics and oxygen transport in the dog. , 1980, The American journal of physiology.

[15]  P. Gaehtgens,et al.  Flow of blood through narrow capillaries: rheological mechanisms determining capillary hematocrit and apparent viscosity. , 1980, Biorheology.

[16]  M. Marcus,et al.  Role of large arteries in regulation of cerebral blood flow in dogs. , 1978, The Journal of clinical investigation.

[17]  J. Patterson,et al.  Responses of cerebral arteries and arterioles to acute hypotension and hypertension. , 1978, The American journal of physiology.

[18]  J I Hoffman,et al.  Blood flow measurements with radionuclide-labeled particles. , 1977, Progress in cardiovascular diseases.

[19]  J. Patterson,et al.  Analysis of Vasoactivity of Local pH, Pco2 and Bicarbonate on Pial Vessels , 1977, Stroke.

[20]  J. Patterson,et al.  Determinants of Response of Pial Arteries to Norepinephrine and Sympathetic Nerve Stimulation , 1975, Stroke.

[21]  J. Patterson,et al.  Detailed Description of a Cranial Window Technique for Acute and Chronic Experiments , 1975, Stroke.

[22]  B. Siesjö,et al.  The influence of acute normovolemic anemia on cerebral blood flow and oxygen consumption of anesthetized rats. , 1975, Acta physiologica Scandinavica.

[23]  R W Gore,et al.  Pressures in Cat Mesenteric Arterioles and Capillaries during Changes in Systemic Arterial Blood Pressure , 1974, Circulation research.

[24]  D. Stromberg,et al.  Pressures in the Pial Arterial Microcirculation of the Cat during Changes in Systemic Arterial Blood Pressure , 1972, Circulation research.

[25]  W. Rosenblum Effects of Reduced Hematocrit on Erythrocyte Velocity and Fluorescein Transit Time in the Cerebral Microcirculation of the Mouse , 1971, Circulation research.

[26]  William G. Cochran,et al.  Experimental Designs, 2nd Edition , 1950 .