Control of muscle blood flow during exercise: local factors and integrative mechanisms.

Understanding the control mechanisms of blood flow within the vasculature of skeletal muscle is clearly fascinating from a theoretical point of view due to the extremely tight coupling of tissue oxygen demands and blood flow. It also has practical implications as impairment of muscle blood flow and its prevention/reversal by exercise training has a major impact on widespread diseases such as hypertension and diabetes. Here we analyse the role of mediators generated by skeletal muscle activity on smooth muscle relaxation in resistance vessels in vitro and in vivo. We summarize their cellular mechanisms of action and their relative roles in exercise hyperaemia with regard to early and late responses. We also discuss the consequences of interactions among mediators with regard to identifying their functional significance. We focus on (potential) mechanisms integrating the action of the mediators and their effects among the cells of the intact arteriolar wall. This integration occurs both locally, partly due to myoendothelial communication, and axially along the vascular tree, thus enabling the local responses to be manifest along an entire functional vessel path. Though the concept of signal integration is intriguing, its specific role on the control of exercise hyperaemia and the consequences of its modulation under physiological and pathophysiological conditions still await additional analysis.

Arteriolar dilations induced by contraction of hamster cremaster muscle are dependent on changes in endothelial cell calcium.

UNLABELLED: Muscle contraction initiates microvascular arteriolar dilation in regions directly overlapping the active fibres but the cells (vascular smooth muscle cells, endothelial cells) responsible for producing the dilation and the underlying signalling mechanisms are unknown. AIMS: We tested the hypothesis that changes in endothelial cell calcium (Ca2+) are involved in this dilation. METHODS: Four to five muscle fibres lying approximately perpendicular to arterioles (maximum diameter approximately 40 microm) were stimulated (4 Hz, 4-20 V, 0.4 ms duration) and observations were made at the site of muscle fibre/arteriole overlap. RESULTS: Chelation of endothelial cell Ca2+ (with BAPTA) abolished dilations to 120 s of muscle contraction (5.6 +/- 1.5 microm in controls vs. 0.51 +/- 1.2 microm with BAPTA, n = 6), indicating that changes in endothelial cell Ca2+ are required for the response. To determine the time frame of the Ca2+ signal, we monitored whole endothelial cell Ca2+ (with Fura-PE3) prior to and following either 120 (n = 13), 30 (n = 9) or 10 (n = 9) s of muscle contraction. In all instances, no changes in Ca2+ were observed despite typical dilator responses. CONCLUSIONS: These data indicate that (i) the initiation of muscle contraction-induced arteriolar dilations depends on a change in endothelial cell Ca2+, which must be a transient event that takes place early/during stimulation, and (ii) maintenance of the dilation after contraction occurs via mechanisms that are independent of changes in global Ca2+ within the cell.

Local and remote arteriolar dilations initiated by skeletal muscle contraction.

To investigate the relationship between skeletal muscle metabolism and arteriolar dilations in the region local to contracting muscle fibers as well as dilations at remote arteriolar regions upstream, we used a microelectrode on cremaster muscle of anesthetized hamsters to stimulate four to five muscle fibers lying approximately perpendicular to and overlapping a transverse arteriole. Before, during, and after muscle contraction, we measured the diameter of the arteriole at the site of muscle fiber overlap (local) and at a remote site approximately 1,000 microm upstream. Two minutes of 2-, 4-, or 8-Hz stimulation (5-10 V, 0.4-ms duration) produced a significant dilation locally (8.2 +/- 2.0-, 22.5 +/- 2.4-, and 30.9 +/- 2.1-microm increase, respectively) and at the remote site (4.2 +/- 0.8, 11.0 +/- 1.1, and 18.9 +/- 2.7 microm, respectively). Muscle contraction at 4 Hz initiated a remote dilation that was unaffected by 15-min micropipette application of either 2 microM tetrodotoxin, 0.07% halothane, or 40 microM 18-beta-glycyrrhetinic acid between the local and upstream site. Therefore, at the arteriolar level, muscle contraction initiates a robust remote dilation that does not appear to be transmitted via perivascular nerves or gap junctions.

nNOS and eNOS modulate cGMP formation and vascular response in contracting fast-twitch skeletal muscle.

Nitric oxide (NO) from Ca(2+)-dependent neuronal nitric oxide synthase (nNOS) in skeletal muscle fibers may modulate vascular tone by a cGMP-dependent pathway similar to NO derived from NOS in endothelial cells (eNOS). In isolated fast-twitch extensor digitorum longus (EDL) muscles from control mice, cGMP formation increased approximately 166% with electrical stimulation (30 Hz, 15 s). cGMP levels were not altered in slow-twitch soleus muscles. The NOS inhibitor N(omega)-nitro-l-arginine abolished the contraction-induced increase in cGMP content in EDL muscles, and the NO donor sodium nitroprusside (SNP) increased cGMP content approximately 167% in noncontracting EDL muscles. SNP treatment but not electrical stimulation increased cGMP formation in muscles from nNOS(-/-) mice. cGMP formation in control and stimulated EDL muscles from eNOS(-/-) mice was less than that obtained with similarly treated muscles from control mice. Arteriolar relaxation in contracting fast-twitch mouse cremaster muscle was attenuated in muscles from mice lacking either nNOS or eNOS. These findings suggest that increases in cGMP and NO-dependent vascular relaxation in contracting fast-twitch skeletal muscle may require both nNOS and eNOS.

Role for capillaries in coupling blood flow with metabolism.

1. Skeletal muscle blood flow is coupled with metabolism; for this coupling to be effective in matching blood flow to capillary exchange, control of capillary blood flow and recruitment must reside at the capillary level. 2. Capillaries are, indeed, capable of sensing and responding to vasoactive stimuli. We report studies that indicate that contraction of skeletal muscle fibres underneath capillaries is capable of increasing blood flow in those capillaries. 3. This presumed metabolically related signal initiates remote dilations in arterioles upstream of the stimulated capillaries. Our findings indicate that the vasodilatory signal is transmitted along the blood vessel wall. 4. Although we present evidence supporting a role for gap junctionally mediated communication of this vasodilatory signal, it appears unlikely to be primarily electrotonic spread of membrane potential changes. 5. Our studies further indicate that the transmitted signal is not dependent on changes in endothelial cell calcium.

Adaptation and survival of surface-deprived red blood cells in mice.

The consequences of lost membrane area for long-term erythrocyte survival in the circulation were investigated. Mouse red blood cells were treated with lysophosphatidylcholine to reduce membrane area, labeled fluorescently, reinfused into recipient mice, and then sampled periodically for 35 days. The circulating fraction of the modified cells decreased on an approximately exponential time course, with time constants ranging from 2 to 14 days. The ratio of volume to surface area of the surviving cells, measured using micropipettes, decreased rapidly over the first 5 days after infusion to within 5% of normal. This occurred by both preferential removal of the most spherical cells and modification of others, possibly due to membrane stress developed during transient trapping of cells in the microvasculature. After 5 days, the cell area decreased with time in the circulation, but the ratio of volume to surface area remained essentially constant. These results demonstrate that the ratio of cell volume to surface area is a major determinant of the ability of erythrocytes to circulate properly.

Remote arteriolar dilations in response to muscle contraction under capillaries.

In hamster cremaster muscle, it has been shown previously that contraction of skeletal muscle fibers underlying small groups of capillaries (modules) induces dilations that are proportional to metabolic rate in the two arteriolar generations upstream of the stimulated capillaries (Berg BR, Cohen KD, and Sarelius IH. Am J Physiol Heart Circ Physiol 272: H2693-H2700, 1997). These remote dilations were hypothesized to be transmitted via gap junctions and not perivascular nerves. In the present study, halothane (0.07%) blocked dilation in the module inflow arteriole, and dilation in the second arteriolar generation upstream, the branch arteriole, was blocked by both 600 mosM sucrose and halothane but not tetrodotoxin (2 microM). Dilations in both arterioles were not blocked by the gap junction uncoupler 18-beta-glycyrrhetinic acid (40 microM), and 80 mM KCl did not block dilation of the module inflow arteriole. These data implicate a gap junctional-mediated pathway insensitive to 18-beta-glycyrrhetinic acid in dilating the two arterioles upstream of the capillary module during "remote" muscle contraction. Dilation in the branch arteriole, but not the module inflow arteriole, was attenuated by 100 microM N(omega)-nitro-L-arginine. Thus selective contraction of muscle fibers underneath capillaries results in dilations in the upstream arterioles that have characteristics consistent with a signal that is transmitted along the vessel wall through gap junctions, i.e., a conducted vasodilation. The observed insensitivities to 18-beta-glycyrrhetinic acid, to KCl, and to N(omega)-nitro-L-arginine suggest, however, that there are multiple signaling pathways by which remote dilations can be initiated in these microvessels.

Coupling of muscle metabolism and muscle blood flow in capillary units during contraction.

Muscle blood flow is tightly coupled to the level of skeletal muscle activity: Indices of skeletal muscle metabolic rate, for example oxygen consumption or muscle work, are directly related to the magnitude of the change in muscle blood flow. Despite the large amount that is known about individual aspects of local metabolic vasodilation, the mechanisms underlying integrated aspects of the response remain largely unknown. Arteriolar dilation serves both to increase blood flow through the muscle and also to recruit capillaries and control capillary blood flow distribution. Conceptually, these two apparently separate functions of larger vs. more terminal arterioles (where larger vessels subserve conductance changes while the smaller more distal vessels have a primary role in capillary blood flow control) can be met, at least in part, by differential sensitivity of large vs. small arterioles to metabolites and agonists relevant to the metabolic response. However, longitudinal differences in sensitivity through the arteriolar network will not by themselves account for observed heterogeneities in capillary perfusion or for the close matching between blood flow and metabolism that occurs even in mixed muscles. In mixed skeletal muscles, fibres of widely different metabolic profile are dispersed throughout the muscle and even fibres of a single motor unit are not perfused by common arterioles but are matched with arterioles arising from widely disparate regions within the microvascular network. In this review we present findings that support the notion that capillaries are an integral part of the mechanism underlying this close matching between blood flow and metabolism. We review studies that indicate that capillaries are capable of responding to stimuli in their immediate environment and, importantly, are able to communicate with arterioles located remotely upstream in the arteriolar tree. Not only can skeletal muscle capillary endothelial cells induce remote arteriolar vasodilatory and vasoconstrictor responses to pharmacological stimuli such as acetylcholine or noradrenaline, but they can also initiate these remote arteriolar responses in response to skeletal muscle contraction. Capillary endothelial cells respond to skeletal muscle contraction by transmitting a dilatory signal to at least three branch orders of arterioles proximal to the capillary; these upstream dilations present a mechanism whereby capillaries can initiate their own recruitment, and whereby increased blood flow can be directed only to those exchange vessels associated with the contracting muscle fibres and where, presumably, the initiating signal is sensed. This signal involves KATP channels, although their location (on endothelial, vascular smooth muscle or skeletal muscle cells) is not yet known and has a nitric oxide-dependent component. The studies reviewed here thus indicate that capillaries have the capacity to play an active role in co-ordination of muscle blood flow responses to changed muscle metabolism. Much more remains to be learned, however, about the mechanisms underlying the signals generated by the contracting muscle and the mechanisms of transmission of the signals upstream.

Flow-induced cytoskeletal changes in endothelial cells growing on curved surfaces.

OBJECTIVE: Our purpose was to investigate the effect of the shape of the growth surface (curved versus flat) on flow-induced F-actin organization in endothelial cells. METHODS: Human umbilical vein endothelial cells were grown to confluence on curved or flat surfaces. Microchannels (curved surface, 10- to 30-microm radius) or parallel plate flow chambers were perfused (30 minutes to 6 hours) at physiological flow rates (wall shear stress 1 to 10 (dyn/cm2). RESULTS: On curved surfaces, the number of central F-actin stress fibers (for cells of equal area) decreased from 4.8+/-0.3 (mean +/- SE, n = 36) (static) to 0.9+/-0.5 per cell in perfused microchannels. Perfusion with 100 microM histamine prevented this response to flow (5.5+/-0.8 per cell, n = 12). Stress fibers were initially aligned with tile long axis of the microchannel at an angle of 9+/-0.7 degrees (static). With flow, alignment of the few remaining central F-actin stress fibers with respect to the long axis of the microchannel decreased to 19+/-4 degrees this was prevented by perfusion with histamine (5.6+/-1 degrees). The number of stress fibers per cell, for cells grown on flat surfaces (8.1+/-0.3, static, n = 36) was significantly greater than cells on curved surfaces, and did not change with flow (8.1+/-0.5 per cell, n = 6). On flat surfaces, the stress fiber orientation with respect to the longitudinal axis of the channel) was 42+/-1.4 degrees (static) and did not change with flow (38+/-4.2 degrees). CONCLUSIONS: Endothelial cells on curved growth surfaces respond to flow rapidly, with marked changes in F-actin central stress fiber formation. This implicates a tight relationship between cell shape and the environmental substrate, and suggests that the shape of the endothelial cell significantly impacts its ability to respond to its environment.

Skeletal muscle contractions stimulate cGMP formation and attenuate vascular smooth muscle myosin phosphorylation via nitric oxide.

Nitric oxide generated by neuronal nitric oxide synthase in contracting skeletal muscle fibers may regulate vascular relaxation via a cGMP-mediated pathway. Neuronal nitric oxide synthase content is greatly reduced in skeletal muscles from mdx mice. cGMP formation increased in contracting extensor digitorum longus muscles in vitro from C57 control, but not mdx mice. The increase in cGMP content was abolished with NG-nitro-L-arginine. Sodium nitroprusside treatment increased cGMP levels in muscles from both C57 and mdx mice. Skeletal muscle contractions also inhibited phenylephrine-induced phosphorylation of smooth muscle myosin regulatory light chain. Arteriolar dilation was attenuated in contracting muscles from mdx but not C57 mice. NO generated in contracting skeletal muscle may contribute to vasodilation in response to exercise.

Shear stress gradient over endothelial cells in a curved microchannel system.

Our purpose was to test a scale model of the microcirculation by measuring the shear forces to which endothelial cells were exposed, and comparing this to computer simulations. In vitro experiments were performed to measure the 2-dimensional projected velocity profile along endothelial cell lined microchannels (D-shaped, 10-30 microns radius, n = 15), or in microchannels without endothelial cells (n = 18). Microchannels were perfused with fluorescently labeled microspheres (0.5 micron dia., < 1%) suspended in cell culture media. The velocity of individual microspheres was obtained off-line (videorecording), using an interactive software program; velocity was determined as the distance traveled in one video field (1/60 s). Mass balance was verified in the microchannels by comparing the microsphere velocities to the perfusion pump rate. In confluent endothelial cell lined microchannels, a velocity profile was obtained as microspheres passed an endothelial cell nucleus (identified by fluorescent dye), and again, for a paired region 100 microns away without nuclei (cytoplasm region). The velocity profile was significantly shifted and sharpened by the endothelial cell nucleus, as anticipated. Over the nucleus, data are consistent with a normal sized nucleus extending into the lumen, further confirming that this scale model can be used to determine the wall shear stress to which endothelial cells are exposed. Using the experimental bulk phase fluid parameters as boundary conditions, we used computational fluid dynamics (CFD) to predict the expected wall shear stress gradient along an endothelial cell lined D-shaped tube. The wall shear stress gradient over the nucleus was 2-fold greater in the radial versus axial directions, and was sensitive to lateral versus midline positioned nuclei.

Direct coupling between blood flow and metabolism at the capillary level in striated muscle.

In hamster cremaster muscle, capillary networks consist of anatomically invariant subunits termed modules [Berg, B. R., and I. H. Sarelius, Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H1215-H1222, 1995]. To explore local coupling between blood flow and metabolism, we used micropipettes to stimulate five to six muscle fibers running underneath specified capillary modules. Capillary erythrocyte flow increased significantly at all stimulation frequencies because of increased erythrocyte content at 2 Hz and increased erythrocyte velocity at 4 and 8 Hz. Erythrocyte flow did not increase when the fibers underlying the module were mechanically tugged but did not actively contract at these frequencies. Increased capillary flow was accommodated by dilation of three upstream arteriolar generations: the module inflow arteriole dilated significantly at all frequencies, and further upstream, dilations were significant at higher frequencies. Other module inflow arterioles in the same capillary network as the stimulated module did not dilate. Dilations in the module inflow arteriole were abolished by 600 mosM sucrose but were unaffected by 10(-6) M tetrodotoxin. These data suggest that local coupling between capillary flow and muscle contraction includes a conducted vasodilation that is responsible for the remote upstream dilations.

Erythrocyte flux in capillary networks during maturation: implications for oxygen delivery.

Erythrocyte (RBC) flow variables were measured with videomicroscopy in hamster cremaster muscle capillary networks. Capillary networks consist of subgroups, termed modules, with architectural characteristics that are invariant with maturation [B. R. Berg and I. H. Sarelius. Am. J. Physiol, 268 (Heart Circ. Physiol. 37): H1215-H1222, 1995]. RBC flux in modules decreased from 82.0 +/- 4.3 (SE) cells/s at 51 days of age to 59.5 +/- 7.5 and 27.5 +/- 2.8 cells/s at 65 and 79 days of age, respectively. Mean cell velocity at 51 days (385 +/- 10 microns/s) was higher than at 65 or 79 days (285 +/- 15 and 241 +/- 12 microns/s, respectively). Cell content (number of cells per unit length) decreased later, between 65 and 79 days (from 0.21 +/- 0.01 and 0.23 +/- 0.02 cells/micron at 51 and 65 days, respectively, to 0.12 +/- 0.01 cells/micron at 79 days). These temporal differences in the decrease in cell velocity and cell content suggest different regulatory mechanisms. The capacity of capillary networks to deliver oxygen was modeled by using the calculated mean PO2 at the capillary wall to indicate the capacity to delivery oxygen. During maturation, the mean capillary wall PO2 remained unchanged (15.5 +/- 1.2 and 11.4 +/- 2.7 Torr in maximal dilation and 24.5 +/- 1.4 and 22.8 +/- 2.4 Torr at rest at 51 and 79 days, respectively). Thus, despite changes in RBC flow variables with maturation, the capacity for networks to deliver oxygen remains constant.

Effects of lost surface area on red blood cells and red blood cell survival in mice.

The effects of removing area from mouse red blood cells on the fate of the cells after reinfusion were investigated. When cells were made nearly spherical (by reducing cell area by approximately 35%) and then reinfused into the animal, most were cleared from the circulation within 1-2 h, although approximately 20% of the cells survived for 4 h or longer. When only 20% of the area was removed (leaving a 15% excess), more than 90% of the cells continued to circulate for 4 h. After reinfusion, the mean surface area of the surviving cells remained constant (73-75 microns2), but the mean volume decreased, from 56.6 +/- 2.1 to 19.1 +/- 1.5 microns3 (+/- SD of 5 replicates) over 4 h. These changes did not occur in cells suspended in plasma but not reinfused into the animal. Thus a loss of surface area results in a decrease in cell volume, as if to maintain a requisite degree of deformability. The results support the hypothesis that the increase in cell density associated with increasing cell age may be a consequence of surface area loss.

Endothelial cell dilatory pathways link flow and wall shear stress in an intact arteriolar network.

Our purpose was to determine whether the endothelial cell-dependent dilatory pathways contribute to the regulation of flow distribution in an intact arteriolar network. Cell flow, wall shear stress (T omega), diameter, and bifurcation angle were determined for four sequential branches of a transverse arteriole in the superfused cremaster muscle of pentobaribtal sodium (Nembutal, 70 mg/kg)-anesthetized hamsters (n = 51). Control cell flow was significantly greater into upstream than into downstream branches [1,561 +/- 315 vs. 971 +/- 200 (SE) cells/s, n = 12]. Tissue exposure to 50 microM N omega-nitro-L-arginine + 50 microM indomethacin (L-NNA + Indo) produced arteriolar constriction of 14 +/- 4% and decreased flow into the transverse arteriole. More of the available cell flow was diverted to downstream branches, yet flow distribution remained unequal. Control T omega was higher upstream than downstream (31.3 +/- 6.8 vs. 9.8 +/- 1.5 dyn/cm2). L-NNA + Indo decreased T omega upstream and increased T omega downstream to become equal in all branches, in contrast to flow. To determine whether constriction in general induced the same changes, 5% O2 (8 +/- 4% constriction) or 10(-9) M norepinephrine (NE; 4 +/- 3% constriction) was added to the tissue (n = 7). With O2, flow was redistributed to become equal into each branch. With NE, flow decreased progressively more into the first three branches. The changes in flow distribution were thus predictable and dependent on the agonist. With O2 or NE, the spatial changes in flow were mirrored by spatial changes in T omega. Changes in diameter and in cell flux were not related for L-NNA + Indo (r = 0.45), O2 (r = 0.07), or NE (r = 0.36). For all agonists, when the bifurcation angle increased, cell flow to the branch decreased significantly, whereas if the angle decreased, flow was relatively preserved; thus active changes in bifurcation angle may influence red cell distribution at arteriolar bifurcations. Thus, when the endothelial cell dilatory pathways were blocked, the changes in flow and in T omega were uncoupled; yet when they were intact, flow and T omega changed together.

Combined use of fluorescence microscopy and micromechanical measurement to assess cell and membrane properties.

The combined use of fluorescence microscopy and micromanipulation provides a powerful approach for understanding the mechanochemistry of cell membranes. Fluorescent labeling of erythrocytes has been used to identify particular populations of cells to assess the effects of abnormal deformability on cell survival. It was found that cells deprived of surface area are either eliminated rapidly from the circulation or undergo a reduction in volume to improve cellular deformability. Fluorescence microscopy can also be used to assess the distribution of specific membrane components during mechanical deformation and fragmentation of cell membranes and so lead to more fundamental understanding of the physical association between the membrane bilayer and the underlying membrane cytoskeleton.

A system for culture of endothelial cells in 20-50-microns branching tubes.

OBJECTIVE: To construct an in vitro endothelial cell culture system which would mimic the geometry and hemodynamic conditions of the arteriolar microcirculation. METHODS: Using a photolithography technique, semicircular channels (20-50 microns in diameter) were etched in mirror-image patterns on pairs of borosilicate microscope slide glass. One-half of each plate pair was predrilled with perfusion port holes at funnel-shaped fluid entrance regions. The perfusion system was constructed of micropipette glass and Teflon tubing, and imbedded in Sylgard. Two types of endothelial cells were grown to confluence within the half-channels: rabbit lung microvascular endothelial cells (a gift of Dr. M.E. Gerritsen, Miles Inc.) and human umbilical vein endothelial cells. After the cells were confluent, the two mirror images were aligned and clamped together to form a complete branching system of tubes lined with endothelial cells. RESULTS: This cell culture system can be perfused at physiological flow rates corresponding to wall shear stress values in the range 0.03-48 dyn/cm2. The fluid velocity profiles can be measured in this system by tracking the velocity and flow paths of 0.5-micron fluorescently labeled microspheres. Endothelial cells which grow within the channel exhibit F-actin alignment along the long axis of the channel by 3 days after seeding. Scanning electron micrographs indicate that 4 hr after seeding, endothelial cells commonly form cellular projections extending across the half-channel; by 5 days after seeding, the projections appear to have flattened out along the bottom of the channel. CONCLUSIONS: An in vitro endothelial cell culture system was constructed which mimics the geometry and hemodynamics conditions of resistance arterioles. This system can be used to examine endothelial cell responses to flow and flow gradients under defined and controllable conditions which mimic the arteriolar microcirculation.

Energy optimization and bifurcation angles in the microcirculation.

Our purpose was to examine the relationship between bifurcation angle and energy optimization in the arteriolar microcirculation. We measured bifurcation angles and diameters for sequential branches along a third-order feed arteriole (25 microns) in the superfused cremaster muscle of anesthetized (pentobarbital, 70 mg/kg) Golden hamsters (N = 51). Predicted bifurcation angles were calculated using the diameter data in a model designed to minimize total energy or using four different models each designed to minimize a specific energy cost (vessel wall surface area, vascular volume, wall shear stress, power losses), these models each assuming constant viscosity and that branching occurs with perfect space filling (i.e. junction exponent, x, = 3). The range of the predicted bifurcation angles for any model was small (+/- 10 degrees), and they were not different for the sequential junctions along the feed arteriole, where the observed angles significantly decreased in angle along the feed (first junction, 115 +/- 4.4 degrees; second, 88 +/- 5.2 degrees; third, 76 +/- 4.8 degrees; and last, 57 +/- 3.4 degrees). We next corrected for a nonconstant viscosity by using our in vivo tube hematocrit data and a published relationship among diameter, tube hematocrit, and apparent viscosity. Again assuming that x = 3, the total energy minimization model now predicted that the bifurcation angle was always obtuse and not different for the sequential branches along the feed arteriole (first, 125 +/- 3.3 degrees; second, 124 +/- 3.4 degrees; third, 120 +/- 6.6 degrees; and last, 132 +/- 2.7 degrees); the predicted angles were not correlated with the observed angles (r = 0.25). Using the geometric resistance (diameters) and the angles measured in vivo, and assuming constant viscosity, we next calculated the value of chi for each of the bifurcation junctions for each of the four models described above. The average value of x was not equal to 3 for any of the four models. The value of x decreased along the feed arteriole (first to last branch) from 2.7 +/- 0.26 to 1.6 +/- 0.22 (surface) and from 4.2 +/- 0.36 to 2.9 +/- 0.23 (volume), and x increased along the feed from 3.0 +/- 0.35 to 15.5 +/- 2.6 (shear stress) and from 40 +/- 31 to 82 +/- 49 (power loss). These calculations suggest that both changing viscosity and a changing value for the junction exponent are likely important when examining the energy optimization within the arteriolar microcirculation.

L-arginine-induced conducted signals alter upstream arteriolar responsivity to L-arginine.

Our purpose was to determine whether L-arginine was involved in vascular communication between downstream and upstream locations within a defined microvascular region. Arteriolar diameter was measured for the branches along a transverse arteriole in the superfused cremaster of anesthetized (pentobarbital sodium, 70 mg/kg i.p.) hamsters (N = 53). The upstream branch arterioles dilated significantly to locally applied L-arginine (100 mumol/L pipette concentration) only if the downstream branches (approximately 1400 microns away) were preexposed. With exposure order downstream to upstream, diameter change was last branch, -3.8 +/- 1.5% (of baseline); third, +58.1 +/- 27%; first, +92 +/- 26% (n = 5); with exposure order upstream to downstream: first branch, -0.4 +/- 3%; third, +5 +/- 11%; last, -5.6 +/- 7.5% (n = 4). Thus, downstream preexposure to L-arginine altered the responsivity upstream to locally applied L-arginine. Downstream-applied L-arginine also induced a conducted vasodilation (+17.8 +/- 2.8%; n = 14) 1327 +/- 166 microns upstream. This response was completely blocked by simultaneous sucrose (600 mOsm), halothane (0.0345%), or N omega-nitro-L-arginine (L-NNA, 100 mumol/L) exposure to the feed vessel (second micropipette) midway between the downstream site of L-arginine exposure and the upstream observation site. An acetylcholine-induced conducted vasodilation (+18.1 +/- 2.6%, n = 8) was also completely blocked by sucrose, halothane, or L-NNA.(ABSTRACT TRUNCATED AT 250 WORDS)