Flow modulates the transport of K+ through the walls of single perfused mesenteric venules in anaesthetised rats

Mechanosensing at the vascular interface

Mammals are endowed with a complex set of mechanisms that sense mechanical forces imparted by blood flow to endothelial cells (ECs), smooth muscle cells, and circulating blood cells to elicit biochemical responses through a process referred to as mechanotransduction. These biochemical responses are critical for a host of other responses, including regulation of blood pressure, control of vascular permeability for maintaining adequate perfusion of tissues, and control of leukocyte recruitment during immunosurveillance and inflammation. This review focuses on the role of the endothelial surface proteoglycan/glycoprotein layer-the glycocalyx (GCX)-that lines all blood vessel walls and is an agent in mechanotransduction and the modulation of blood cell interactions with the EC surface. We first discuss the biochemical composition and ultrastructure of the GCX, highlighting recent developments that reveal gaps in our understanding of the relationship between composition and spatial organization. We then consider the roles of the GCX in mechanotransduction and in vascular permeability control and review the prominent interaction of plasma-borne sphingosine-1 phosphate (S1P), which has been shown to regulate both the composition of the GCX and the endothelial junctions. Finally, we consider the association of GCX degradation with inflammation and vascular disease and end with a final section on future research directions.

Microvascular permeability to water is independent of shear stress, but dependent on flow direction

Endothelial cells in a cultured monolayer change from a "cobblestone" configuration when grown under static conditions to a more elongated shape, aligned with the direction of flow, after exposure to sustained uniform shear stress. Sustained blood flow acts to protect regions of large arteries from injury. We tested the hypothesis that the stable permeability state of individually perfused microvessels is also characteristic of flow conditioning. In individually perfused rat mesenteric venular microvessels, microvascular permeability, measured as hydraulic conductivity (Lp), was stable [mean 1.0 × 10(-7) cm/(s × cmH2O)] and independent of shear stress (3-14 dyn/cm(2)) for up to 3 h. Vessels perfused opposite to the direction of normal blood flow exhibited a delayed Lp increase [ΔLp was 7.6 × 10(-7) cm/(s × cmH2O)], but the increase was independent of wall shear stress. Addition of chondroitin sulfate and hyaluronic acid to perfusates increased the shear stress range, but did not modify the asymmetry in response to flow direction. Increased Lp in reverse-perfused vessels was associated with numerous discontinuities of VE-cadherin and occludin, while both proteins were continuous around the periphery of forward-perfused vessels. The results are not consistent with a general mechanism for graded shear-dependent permeability increase, but they are consistent with the idea that a stable Lp under normal flow contributes to prevention of edema formation and also enables physiological regulation of shear-dependent small solute permeabilities (e.g., glucose). The responses during reverse flow are consistent with reports that disturbed flows result in a less stable endothelial barrier in venular microvessels.

Erythrocyte-derived sphingosine-1-phosphate stabilizes basal hydraulic conductivity and solute permeability in rat microvessels

Exogenous sphingosine-1-phosphate (S1P), a lipid mediator in blood, attenuates acute microvascular permeability increases via receptor S1P1 to stabilize the endothelium. To evaluate the contribution of erythrocytes as an endogenous source of S1P to the regulation of basal permeability, we measured permeability coefficients in intact individually perfused venular microvessels of rat mesentery. This strategy also enabled the contributions of other endogenous S1P sources to be evaluated. Apparent permeability coefficients (P(S)) to albumin and α-lactalbumin and the hydraulic conductivity of mesenteric microvessels were measured in the presence or absence of rat erythrocytes or rat erythrocyte-conditioned perfusate. Rat erythrocytes added to the perfusate were the principal source of S1P in these microvessels. Basal P(S) to albumin was stable and typical of blood-perfused microvessels (mean 0.5 × 10(-6) cm/s) when erythrocytes or erythrocyte-conditioned perfusates were present. When they were absent, P(S) to albumin or α-lactalbumin increased up to 40-fold (over 10 min). When exogenous S1P was added to perfusates, permeability returned to levels comparable with those seen in the presence of erythrocytes. Addition of SEW 2871, an agonist specific for S1P1, in the absence of red blood cells reduced P(S)(BSA) (40-fold reduction) toward basal. The specific S1P1 receptor antagonist (W-146) reversed the stabilizing action of erythrocytes and increased permeability (27-fold increase) in a manner similar to that seen in the absence of erythrocytes. Erythrocytes are a primary source of S1P that maintains normal venular microvessel permeability. Absence of erythrocytes or conditioned perfusate in in vivo and in vitro models of endothelial barriers elevates basal permeability.

Endothelial glycocalyx: permeability barrier and mechanosensor

Endothelial cells are covered with a polysaccharide rich layer more than 400 nm thick, mechanical properties of which limit access of circulating plasma components to endothelial cell membranes. The barrier properties of this endothelial surface layer are deduced from the rate of tracer penetration into the layer and the mechanics of red and white cell movement through capillary microvessels. This review compares the mechanosensor and permeability properties of an inner layer (100-150 nm, close to the endothelial membrane) characterized as a quasi-periodic structure which accounts for key aspects of transvascular exchange and vascular permeability with those of the whole endothelial surface layers. We conclude that many of the barrier properties of the whole surface layer are not representative of the primary fiber matrix forming the molecular filter determining transvascular exchange. The differences between the properties of the whole layer and the inner glycocalyx structures likely reflect dynamic aspects of the endothelial surface layer including tracer binding to specific components, synthesis and degradation of key components, activation of signaling pathways in the endothelial cells when components of the surface layer are lost or degraded, and the spatial distribution of adhesion proteins in microdomains of the endothelial cell membrane.

Modeling of biopterin-dependent pathways of eNOS for nitric oxide and superoxide production

Endothelial dysfunction is associated with increase in oxidative stress and low NO bioavailability. The endothelial NO synthase (eNOS) uncoupling is considered an important factor in endothelial cell oxidative stress. Under increased oxidative stress, the eNOS cofactor tetrahydrobiopterin (BH(4)) is oxidized to dihydrobiopterin, which competes with BH(4) for binding to eNOS, resulting in eNOS uncoupling and reduction in NO production. The importance of the ratio of BH(4) to oxidized biopterins versus absolute levels of total biopterin in determining the extent of eNOS uncoupling remains to be determined. We have developed a computational model to simulate the kinetics of the biochemical pathways of eNOS for both NO and O(2)(•-) production to understand the roles of BH(4) availability and total biopterin (TBP) concentration in eNOS uncoupling. The downstream reactions of NO, O(2)(•-), ONOO(-), O(2), CO(2), and BH(4) were also modeled. The model predicted that a lower [BH(4)]/[TBP] ratio decreased NO production but increased O(2)(•-) production from eNOS. The NO and O(2)(•-) production rates were independent above 1.5μM [TBP]. The results indicate that eNOS uncoupling is a result of a decrease in [BH(4)]/[TBP] ratio, and a supplementation of BH(4) might be effective only when the [BH(4)]/[TBP] ratio increases. The results from this study will help us understand the mechanism of endothelial dysfunction.

Acute laminar shear stress reversibly increases human glomerular endothelial cell permeability via activation of endothelial nitric oxide synthase

Laminar shear stress is a key determinant of systemic vascular behavior, including through activation of endothelial nitric oxide synthase (eNOS), but little is known of its role in the glomerulus. We confirmed eNOS expression by glomerular endothelial cells (GEnC) in tissue sections and examined effects of acute exposure (up to 24 h) to physiologically relevant levels of laminar shear stress (10-20 dyn/cm(2)) in conditionally immortalized human GEnC. Laminar shear stress caused an orientation of GEnC and stress fibers parallel to the direction of flow and induced Akt and eNOS phosphorylation along with NO production. Inhibition of the phophatidylinositol (PI)3-kinase/Akt pathway attenuated laminar shear stress-induced eNOS phosphorylation and NO production. Laminar shear stress of 10 dyn/cm(2) had a dramatic effect on GEnC permeability, reversibly decreasing the electrical resistance across GEnC monolayers. Finally, the laminar shear stress-induced reduction in electrical resistance was attenuated by the NOS inhibitors l-N(G)-monomethyl arginine (l-NMMA) and l-N(G)-nitroarginine methyl ester (l-NAME) and also by inhibition of the PI3-kinase/Akt pathway. Hence we have shown for GEnC in vitro that acute permeability responses to laminar shear stress are dependent on NO, produced via activation of the PI3-kinase/Akt pathway and increased eNOS phosphorylation. These results suggest the importance of laminar shear stress and NO in regulating the contribution of GEnC to the permeability properties of the glomerular capillary wall.

Vascular permeability modulation at the cell, microvessel, or whole organ level: towards closing gaps in our knowledge

Multiple processes modulate net blood-to-tissue exchange in a microvascular unit in normal and pathophysiological conditions. These include mechanisms that control the number and type of microvessels perfused, the balance of adhesion and contractile forces that determine the conductance of the spaces between endothelial cells to water and solutes, the pressure and chemical potential gradients determining the driving forces through these conductive pathways, and the organization of barriers to macromolecules in the endothelial glycocalyx. Powerful methods are available to investigate these mechanisms at the levels of cultured endothelial monolayers, isolated microvessels, and the microvascular units within intact organs. Here we focus on current problems that limit the integration of our knowledge of mechanisms investigated in detail at the cellular level into a more complete understanding of modulation of blood-to-tissue exchange in whole organs when the endothelial barrier is exposed to acute and more long-term inflammatory conditions. First, we review updated methods, applicable in mouse models of vascular permeability regulation, to investigate both acute and long-term changes in permeability. Methods to distinguish tracer accumulation due to change in perfusion from real increases in extravascular accumulation are emphasized. The second part of the review compares normal and increased permeability in individually perfused venular microvessels and endothelial cell monolayers. The heterogeneity of endothelial cell phenotypes in the baseline state and after exposure to injury and inflammatory conditions is emphasized. Lastly, we review new approaches to investigation of the glycocalyx barrier properties in cultured endothelial monolayers and in whole-body investigations.

Shear stress and the endothelial transport barrier

The shear stress of flowing blood on the surfaces of endothelial cells that provide the barrier to transport of solutes and water between blood and the underlying tissue modulates the permeability to solutes and the hydraulic conductivity. This review begins with a discussion of transport pathways across the endothelium and then considers the experimental evidence from both in vivo and in vitro studies that shows an influence of shear stress on endothelial transport properties after both acute (minutes to hours) and chronic (hours to days) changes in shear stress. Next, the effects of shear stress on individual transport pathways (tight junctions, adherens junctions, vesicles and leaky junctions) are described, and this information is integrated with the transport experiments to suggest mechanisms controlling both acute and chronic responses of transport properties to shear stress. The review ends with a summary of future research challenges.

In vivo visualization of nitric oxide and interactions among platelets, leukocytes, and endothelium following hemorrhagic shock and reperfusion

OBJECTIVES

We examined changes in nitric oxide (NO) distribution in the mesenteric microcirculation after hemorrhagic shock and reperfusion (H/R), and correlated NO production to leukocyte and platelet behavior.

MATERIALS AND METHODS

The behavior of leukocytes and platelets in mesenteric venules was observed by intravital microscopy at 0.5 and 24 h after H/R in male Wistar rats. Transvascular leakage of fluorescein isothiocyanate-labeled albumin was assessed by epi-illumination. The NO-sensitive dye, 4,5-diaminofluorescein diacetate, was used for imaging NO release.

RESULTS

H/R significantly increased vascular albumin leakage and adhesion of leukocytes and platelets (P < 0.05). In H/R 0.5 h rats, NO production in the venular endothelium declined. However, NO production was elevated in H/R 24 h rats in mast cells (P < 0.05). Leukocyte adherence, platelet adherence, and venular permeability were attenuated by iNOS inhibition.

CONCLUSION

Mesenteric endothelial cell dysfunction after H/R 0.5 h is associated with reduced NO, whereas after H/R 24 h is related to increase NO in mast cells.

Microvascular solute and water transport

OBJECTIVE

This review evaluates (1) the regulation of water and solute transport across the endothelial barrier in terms of pore theory and the glycocalyx-junction-break model of capillary permeability; and (2) the mechanisms regulating permeability based on experiments using cultured endothelial cells and intact microvessels.

CONCLUSIONS

The current form of the glycocalyx-junction-break model of capillary permeability describes the selectivity of the capillary wall (pore size) in terms of the space between the fibers of a quasi-periodic matrix on the endothelial cell surface, and the area for exchange (pore number) in terms of the length and frequency of breaks in the tight junction strands. An independent test of this model in a range of mammalian microvascular beds is new experimental evidence that the colloid osmotic pressure of plasma proteins is developed across the glycocalyx, not across the whole microvessel wall. We are beginning to understand that endothelial cells may change their phenotype in response to physical and chemical stresses. Such changes in phenotype may explain changes in the regulation of endothelial barrier function in intact microvessels that have previously been exposed to injury and differences in the regulation of contractile mechanisms between endothelial cells in vivo and in vitro.

Mass transport in arteries and the localization of atherosclerosis

Atherosclerosis is a disease of the large arteries that involves a characteristic accumulation of high-molecular-weight lipoprotein in the arterial wall. This review focuses on the mass transport processes that mediate the focal accumulation of lipid in arteries and places particular emphasis on the role of fluid mechanical forces in modulating mass transport phenomena. In the final analysis, four mass transport mechanisms emerge that may be important in the localization of atherosclerosis: blood phase controlled hypoxia, leaky endothelial junctions, transient intercellular junction remodeling, and convective clearance of the subendothelial intima and media. Further study of these mechanisms may contribute to the development of therapeutic strategies for atherosclerotic diseases.

Blood flow limitation in vivo of small solute transfer during peritoneal dialysis in rats

The aim of this study was to determine whether or to what extent transperitoneal flux of small solutes is reduced at low blood flows during peritoneal dialysis (PD) in rats. Peritoneal blood flow reductions were achieved by bleeding anesthetized (300 g) rats by 25% of their blood volume. After bleeding, a 2 h PD dwell was started using standard PD fluid. The permeability-surface area product (PS) for (51)Cr-EDTA and glucose were assessed, as well as the transperitoneal clearance (Cl) of albumin. Control animals were not bled. After bleeding, peritoneal blood flow declined from 145 +/- 17 perfusion units (PU) to 59 +/- 12 PU (P = 0.001). Concomitant with this reduction, PS for (51)Cr-EDTA fell from 0.284 +/- 0.01 ml/min to 0.216 +/- 0.01 ml/min (P = 0.006) and PS for glucose from 0.338 +/- 0.02 ml/min to 0.294 +/- 0.01 ml/min (P = 0.046). Mean arterial BP (MAP) dropped from 133 +/- 4 mmHg to 61 +/- 5 mmHg (P = 0.008). Cl of albumin fell largely in proportion to the estimated capillary hydrostatic pressure drop, i.e., from 6.1 +/- 0.7 microl/min to 2.3 +/- 0.3 microl/min (P = 0.001). The results demonstrate that the transperitoneal clearances of small solutes are blood flow limited during PD, when peritoneal perfusion is markedly reduced. The level of flow limitation was, however, much lower than expected and observed in other tissues. Albumin transport, which is not blood flow limited, was reduced largely in proportion to the calculated capillary hydrostatic pressure decrease.

Nonendothelial source of nitric oxide in arterioles but not in venules: alternative source revealed in vivo by diaminofluorescein microfluorography

This study aimed to examine topographic distribution of microvascular NO generation in vivo. To this end, nitrosonium ion (NO+)-sensitive diaminofluorescein diacetate was superfused continuously on the rat mesentery and the fluorescence was visualized in the microvessels through laser confocal microfluorography. Two major sites exhibited a time-dependent elevation of the fluorescence: microvascular endothelia and mast cells. As judged by the fluorescence sensitivity to local application of different inhibitors of NO synthase (NOS), NO availability in arteriolar endothelium and mast cells appeared to be maintained mainly by NOS1, whereas that in venular endothelium greatly depends on NOS3. In venules, the magnitude of inhibitory responses elicited by the inhibitors was positively correlated with the density of leukocyte adhesion. NOS inhibitors significantly reduced, but did not eliminate, the NO+-associated fluorescence in arterioles, capillaries, and venules, suggesting alternative sources of NO in circulation for these microvessels. Immunohistochemistry for NOS isozymes revealed that NOS1 occurred not only in nerve fibers innervated to arterioles but also abundantly in mast cells. Laser flow cytometry of peritoneal cells in vitro revealed abundant expression of NOS1 in mast cells. Interestingly, NOS3 occurred in endothelia of capillaries and venules but not in those of distal arterioles with comparable diameters. These results suggest that the arterioles receive NO from nonendothelial origins involving NOS1 present in nerve terminals and mast cells, whereas venules depend on the endothelial NOS as a major source. Furthermore, nonenzymatic sources of NO from circulating reservoirs constitute a notable fraction throughout different classes of microvessels. The full text of this article is available at http://www.circresaha.org.

Effects of perfusion rate on permeability of frog and rat mesenteric microvessels to sodium fluorescein

The permeability, P(S), to sodium fluorescein (Stokes-Einstein radius = 0.45 nm) has been measured in single mesenteric capillaries of pithed frogs and anaesthetised rats as perfusion velocity, U, was varied over a range from 400 up to 2000-10,000 microm s(-1). P(S) increased linearly with U. In 20 frog capillaries, mean (+/- S.E.M.) P(S) (in microm s(-1)) = 9.35 (+/- 1.55)U x 10(-5) + 0.244 (+/- 0.0291). Similarly, in nine rat venules, mean P(S) = 1.62 (+/- 0.385)U x 10(-4) + 0.375 (+/- 0.025). The flow-dependent component of permeability could be reversibly abolished in frog capillaries by superfusing with 100 microM noradrenaline and by superfusing rat venules with the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine (20 microM). It was shown that changes in microvascular pressure accompanying changes in U during free perfusion could account for only 15 % of the changes in P(S), i.e. 85 % of the changes in P(S) were changes in the permeability coefficient itself. A comparison between the changes in P(S) with U and the previously described changes in microvascular permeability to K(+) with U, suggest that if the flow-dependent component of permeability is modelled as a population of pores of constant size, these have radii of 0.8 nm. Such a pathway would limit flow-dependent permeability to small hydrophilic molecules and have minimal effect on net fluid exchange.

Measurement of hydraulic conductivity of single perfused Rana mesenteric microvessels between periods of controlled shear stress

A new method for the determination of hydraulic conductivity in individually perfused microvessels in vivo is described. A vessel is cannulated at both ends with glass micropipettes and the fluid filtration rate across the vessel wall measured from the velocities of red cells when the pressure in the micropipettes is balanced. Hydraulic conductivity measured using this double-cannulation method (2.6 (+/- 0.9) x 10(-7) cm s(-1) cmH(2)O(-1)) was not significantly different from that measured using a previously described technique in the same vessel (2.4 (+/- 0.9) x 10(-7) cm s(-1) cmH(2)O(-1) using the Landis-Michel method). Shear stress on the vessel wall was controlled by changing the difference between the inflow and outflow pressures during periods of perfusion. The volume flow through the vessel, calculated from red cell velocity either in the vessel or in the pipette, was linearly proportional to this pressure difference. Higher flow rates could only be calculated from red cell velocities in the micropipette. There was no relationship between the imposed shear stress and intervening measurements of hydraulic conductivity (r = 0.029). This novel technique has advantages over the Landis-Michel method, which include the control of outflow resistance, the measurement of shear stress under conditions of controlled pressure, the elimination of compression damage to the vessel (since vessel occlusion is not necessary) and assessment of hydraulic conductivity over the same length of vessel throughout the experiment. The measurement of solute concentrations by indwelling micropipette electrodes and the collection of perfusate for analysis are other possibilities.

Similar permeability responses to nitric oxide synthase inhibitors of venules from three animal species

The influence of nitric oxide (NO) on microvascular permeability remains unclear. NO synthase (NOS) inhibitors have been reported to increase as well as to decrease permeability in different experimental models and animal species. We tested the hypothesis that NOS inhibitors influence venular permeability differently in amphibians and mammals. Permeability coefficients to albumin (P(alb)(s)) were measured on in situ mesenteric venules of the frog and rat and excised pig coronary venules before and after exposure to NOS inhibitors. Despite individual variability in magnitude of responses, NOS inhibitors resulted in a reduction in P(alb)(s) in each species. Superfusion with 10(-5) M N(G)-monomethyl-l-arginine (l-NMMA) reduced P(alb)(s) of frog mesenteric venules by 42% (from a median of 11.4 x 10(-7) cm s(-1), n = 12, P < 0.01) and by 67% in porcine coronary venules (from 12.5 x 10(-7) cm s(-1), n = 5, P < 0.05). The response was attenuated in rat mesenteric venules; 10(-4) M N(G)-nitro-l-arginine methyl ester (l-NAME) reduced P(alb)(s) by 23% (from 7.6 x 10(-7) cm s(-1), n = 9, P = 0.01). The inactive d-enantiomers of the NOS inhibitors were without effect on P(alb)(s) in each model. In pig venules, perfusion with blood modified the permeability responses to l-NMMA, suggesting that effects of NO on permeability are modified by one or more elements of blood. These data support a role of nitric oxide release on venular permeability to albumin that is conserved among the three animal species.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Flow modulates the transport of K+ through the walls of single perfused mesenteric venules in anaesthetised rats

1. We have investigated the effects of varying flow velocity (U) upon permeability to potassium ions (PK) of single perfused mesenteric venules in anaesthetised rats. PK was estimated using a development of the single bolus microperfusion technique at chosen flow velocities in the range of 300 to 6000 microm s-1. 2. In an initial study on 12 vessels, there was a strong positive correlation between PK and U. This was described by the relation: PK = 0.0053U + 8.86, where PK and U are both expressed in micrometres per second (microm s-1). 3. The addition of the nitric oxide (NO) synthase inhibitors (20 micromol l-1) N G-monomethyl-L-arginine (L-NMMA) and N G-nitro L-arginine (L-NNA) to the superfusate abolished the positive correlation between PK and U. The addition of D-NNA (20 micromol l-1) did not change the relation between PK and U where the median value for the slope of the relation was 57.7 (+/- 58.7 interquartile (IQR)) x 10-4 (n = 4). The addition of L-arginine (200 micromol l-1) restored the relation between PK and U where the slope of the relation was increased from 3.9 (+/- 16.3 IQR) x 10-4 to 69.2 (+/- 13.5 IQR) x 10-4 (n = 7). 4. The addition of the guanylate cyclase inhibitor LY83583 (10 micromol l-1) abolished the positive correlation between PK and U (n = 6). 5. Our data suggest that the flow modulates the potassium permeability through the walls of single perfused rat mesenteric venules via a NO-cGMP-dependent process.

Inhibition of effects of flow on potassium permeability in single perfused frog mesenteric capillaries

1. We have investigated the effects of various potential inhibitors on flow-dependent K+ permeability (PK) of single perfused mesenteric microvessels in pithed frogs. 2. Neither superfusion with a nitric oxide synthase inhibitor, NG-monomethyl-L-arginine (10 or 100 micromol l-1), nor the addition of indomethacin (30 micromol l-1) to both perfusate and superfusate reduced the positive correlation between PK and flow velocity (U). 3. In the presence of agents known to raise intracellular levels of adenosine 3',5'-cyclic monophosphate (noradrenaline, 8-bromo-cAMP and a combination of forskolin and rolipram) the slope of the relation between PK and U was no longer significant, so that PK was no longer flow dependent. 4. These results confirm that the flow dependence of PK is a biological process and not an artefact of measurement and suggest a role for intracellular cAMP rather than nitric oxide or prostacyclin in the flow-dependent modulation of PK in frog mesenteric microvessels.