Microvascular hematocrit and its possible relation to oxygen supply were systematically examined. We studied the red cell volume fraction (hematocrit) in arterial blood and in capillaries under a ...variety of circumstances. Control capillary hematocrit averaged 10.4 +/- 2.0% (SE) and arteriolar (14.2 micrometer ID) hematocrit averaged 13.9 +/- 1.2% in cremaster muscles of pentobarbital-anesthetized hamsters. Carotid artery hematocrit was 53.2 +/- 0.6%. The low microvessel hematocrit could not be entirely explained by a high red cell flux through arteriovenous channels other than capillaries (shunting). Hematocrit was not only low at rest, but varied with physiological stimuli. A 1-Hz muscle contraction increased capillary hematocrit to 18.5 +/- 2.4%, and maximal vasodilation induced a rise to 39.3 +/- 9.5%. The quantitative relations between capillary red cell flux, arterial hematocrit, and total blood flow could be explained by a two-element model of microvascular blood flow that incorporated a relatively slow-moving plasma layer (1.2 micrometer). Such a model would generate a low microvessel hematocrit and might reduce the diffusion capacity of individual capillaries, but would not reduce time-averaged red cell flux or alter steady-state vascular oxygen supply.
Intracapillary hematocrit is known to be substantially lower than arterial hematocrit. We hypothesized that capillary hematocrit might be influenced by interactions between plasma macromolecules and ...the endothelial cell surface. Microvessel perfusion pipettes were inserted in second- or third-order vessels, and capillaries were perfused with three different artificial bloods composed of 50% red cells plus the following suspension media: fetal calf serum (group I), serum albumin plus serum globulins (fractions II and III; group II), and bovine serum albumin plus dextran (group III). The mean hematocrits of the pipette-perfused capillaries averaged close to 50% of the systemic value with all perfusion fluids and were not different from the hematocrits of the capillaries perfused by the animal. These data suggest that bifurcations proximal to the pipette location did not contribute to the reduction in mean tube hematocrit normally seen in the animal. Furthermore, interactions between the plasma macromolecules and the endothelial cell surface do not appear to contribute to the low intracapillary hematocrit. Analysis of the data indicate that the capillary Fåhraeus effect, the network Fåhraeus effect in terminal vessels of the arterial tree, and intracapillary events all contribute to the reduction in intracapillary hematocrit.
Gap junctions are common features in the vasculature, long thought to provide a pathway for cell-cell signaling. Emerging understanding of the gap-junctional proteins (connexins) and new tools for ...their investigation now offer the opportunity to explore the vital role that the gap junctions may play in cardiovascular homeostasis and pathophysiology.
In rat cremasteric microcirculation, mechanical occlusion of one branch of an arteriolar bifurcation causes an increase in flow and vasodilation of the unoccluded daughter branch. This dilation has ...been attributed to the operation of a shear stress-dependent mechanism in the microcirculation. Instead of or in addition to this, we hypothesized that the dilation observed during occlusion is the result of a conducted signal originating distal to the occlusion. To test this hypothesis, we blocked the ascending spread of conducted vasomotor responses by damaging the smooth muscle and endothelial cells in a 200-microm segment of second- or third-order arterioles. We found that a conduction blockade eliminated or diminished the occlusion-associated increase in flow through the unoccluded branch and abolished or strongly attenuated the vasodilatory response in both vessels at the branch. We also noted that vasodilations induced by ACh (10(-4) M, 0.6 s) spread to, but not beyond, the area of damage. Taken together, these data provide strong evidence that conducted vasomotor responses have an important role in coordinating blood flow in response to an arteriolar occlusion.
Arterioles (40-80 micron diameter) were isolated from the hamster cheek pouch, cannulated at both ends, and perfused with 3-(N-morpholino)propanesulfonic acid (MOPS)-buffered physiological salt ...solution (PSS). The vessels were observed with an inverted microscope and video system, and arteriolar diameter was measured. Arterioles were found to be 100 times more responsive to the alpha 1-adrenoceptor agonist phenylephrine when applied to the adventitial surface than when applied to the luminal surface. In contrast, SKF 89748-A, also an alpha 1-adrenoceptor selective agonist, but with a much greater lipid solubility than phenylephrine, was equipotent from either surface of the arteriole. We hypothesized that the difference between the two drugs was due to the ability of SKF 89748-A to permeate a diffusion barrier in the arteriolar wall because of its lipid-solubility. To test this hypothesis, a spectrum of antagonists with different sites of action and lipid solubilities was tested. The alpha-adrenoceptor antagonists phentolamine and benextramine and the muscarinic receptor antagonists atropine, scopolamine, and methscopolamine were all found to be more potent at blocking the action of appropriate agonists when applied to the same surface of the arteriole as the agonist than when applied to the opposite surface. Octanol-water partition coefficients were measured for each of the compounds, and these were found to be highly correlated with the ratio of luminal potency to adventitial potency for each of the drugs tested. These data support the hypothesis that the endothelial cell layer in these arterioles forms a barrier to the diffusion of small, water-soluble molecules from the lumen to the smooth muscle cell layer. Such a barrier may have a significant effect on arteriolar reactivity.
The microcirculatory anatomy of the hamster tibialis anterior muscle is based on modules (units) consisting of groups of 12-20 capillaries which run parallel to muscle fibers. The units are supplied ...by a common terminal arteriole and drained by a common terminal venule; a single terminal arteriole commonly supplies two microvascular units or a "unit pair." Regulation of the tibialis muscle microcirculation was investigated in pentobarbital-anesthetized hamsters using epifluorescence microscopy. We examined the patterns of capillary control in response to physiological and pharmacological stimuli including elevation of superfusate oxygen content, direct muscle stimulation, and topical application of phenylephrine. Changes in capillary perfusion were rarely manifested as responses of individual capillaries. The predominant response consisted of a coordinated change in virtually all the capillaries of a unit pair. For example, gradual elevation of superfusate PO2 resulted in simultaneous arrest or "derecruitment" of capillary flow in all capillaries of a unit pair in 37 of 43 such elements studied. In the 6 unit pairs showing atypical behavior, no more than four individual capillaries showed atypical behavior. Capillaries in 28 of 29 unit pairs were also recruited during muscle stimulation as members of a unit pair. In 18 of 21 unit pairs, exposure to topical phenylephrine resulted in simultaneous arrest of capillary flow in all capillaries of a unit pair. These data suggest that in this striated muscle, regulation of capillary perfusion is accomplished by control of capillary unit pairs. Accordingly, the patterns of interdigitation of units will ultimately determine the precision of control of tissue diffusion distance as well as oxygenation.
Microvessels undergo complex shape changes during constriction that could have profound implications for control of resistance. We exploited in vitro cannulation techniques in combination with ...electron microscopy to assess the effects of physiological degrees of vasoconstriction on the size and form of the lumen of isolated rat mesenteric arterioles. Photomicrographs of vasoconstricted vessels revealed that the luminal surface is folded and thrown into longitudinal ridges several hundred microns long. These ridges begin to form and encroach on the lumen as the vessel is constricted. Ridge height may increase to 5-10 microns, and as many as 50 ridges were observed around the circumference of a 70-microns vessel. Ridges are comprised of endothelial cells, basal elastic lamina, and portions of the smooth muscle cytoplasm including thick filaments. The ridges are major determinants of the relationships among stress on smooth muscle contractile elements, intraluminal pressure, and luminal diameter. The ridges may also limit the precision of measurement of microvessel diameter in situ since it is not known whether the apex or the base of the ridge is measured under typical conditions of in vivo microscopy. Our findings emphasize the need for additional detailed study of wall morphology to fully understand the regulation of microvessel flow resistance by smooth muscle function.
Arterioles of hamster cheek pouches are less reactive to luminal application of small hydrophilic agents than to adventitial application. To explore possible longitudinal variations in response ...sidedness, we compared reactivity of isolated vessels from carotid arteries to first-order arterioles. Concentration-response curves for luminally or adventitially applied phenylephrine (PE) were constructed. Arterioles were 274-fold less responsive when PE was in luminal vs. adventitial responsiveness decreased as vessel diameters increased, from 24-fold in inferior saccular arteries to 18-fold in external maxillary arteries and, finally, to 3-fold in common carotid arteries. Differences in response to luminal or adventitial application of PE could be eliminated in arterioles by perfusion with 3-(3-cholamidopropyl)-dimethylammonio-1-propanesulfonate (CHAPS), which disrupts membrane integrity. Treatment with CHAPS also increased the transmural movement of sodium fluorescein across arteriolar vessel walls. We conclude that a diffusion barrier exists in arterial walls, that there is a longitudinal variation in this barrier as expressed by the differences in movement of small hydrophilic molecules from lumen to smooth muscle cell layers, and that the site of the barrier is likely to be at the endothelial cell membrane.
Muscle blood flow increases during work. Any associated change in blood velocity that occurs during functional hyperemia can have profound effects on wall shear rate and arteriolar hemoglobin ...saturations. We measured arteriolar red cell velocity and cross-sectional area during muscle contraction to determine the physiological significance of any of these changes in calculations of wall shear rate and the in situ spectrophotometric measurement of hemoglobin oxygen saturation. Calculated cremaster muscle blood flow increased 64-236% during twitch and tetanic stimulation, respectively, which was due entirely to an increase in cross-sectional area, with muscle work producing little change in either the red cell velocity or the calculated wall shear rate. Small changes ranging from a 3% increase to a 4% decrease in hemoglobin saturation were evident in second- and third-order arterioles, which apparently reflects offsetting effects of the increase in metabolic rate and the increase in arteriolar blood volume. A simple model explaining the microcirculatory adjustments made during muscle work requires dilation of both feed arteries and arterioles if red cell velocity is to remain constant during hyperemia.
