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

Cilostazol strengthens the endothelial barrier of postcapillary venules from the rat mesentery in situ

Objective

Although cilostazol, a phosphodiesterase 3 inhibitor, has been suggested to strengthen the endothelial barrier using cultured endothelial monolayers, its effect has not been tested in vivo. We, therefore, investigated effects of cilostazol on barrier properties of postcapillary venules of the rat in situ.

Methods

Cilostazol was administered to the rats through oral gavage at 4 hours before the measurements. The hydraulic permeability ( Lp) and the effective osmotic pressure (σΔπ), molecular sieving properties of microvascular walls, were estimated in single mesenteric postcapillary venules by a micro-occlusion technique, first during control perfusion and then in the presence of histamine.

Results

When the vessels were inflamed with histamine, cilostazol attenuated a transient increase in Lp and prevented σΔπ from falling. Furthermore, it reduced baseline Lp under a control state.

Conclusion

Cilostazol appears to tighten the endothelial barrier in situ, at least in part by inhibiting the cAMP-degrading enzyme in the endothelium.

Quantification of angiogenesis by functional computed tomography in a Matrigel model in rats

RATIONALE AND OBJECTIVES

The aim was to evaluate functional computed tomography (fCT) in the quantification of angiogenesis by comparing the tissue perfusion parameters measured by CT perfusion (CTP) software with histologic vascular parameters in a Matrigel model in rats. It was hypothesized that tissue perfusion parameters and histologic vascular parameters are related.

MATERIALS AND METHODS

In vivo angiogenesis assays were performed using Matrigel supplemented with escalating doses (0 ng [control group], 250 ng, and 1,000 ng) of recombinant rat vascular endothelial growth factor (VEGF164) subcutaneously injected into the backs of Sprague Dawley rats. On day 7, rats with Matrigel plug underwent fCT following a bolus injection of iodinated contrast medium. Using CTP software, fCT parameters were generated (blood flow [BF], blood volume [BV], mean transit time, and permeability-surface area product) and functional maps on the basis of a distributed parameter tracer kinetic model, the adiabatic approximation to the tissue homogeneity model. The animals were then sacrificed. Matrigel plug was sectioned into slices corresponding to the CT scan plane and stained with CD31 immunohistochemical stain. Histologic vascular parameters, including microvascular density (MVD), vessel number (VN), vascular area, and vascular perimeter, were measured. CTP and histologic parameters were correlated.

RESULTS

The Matrigel plugs with the 1,000-ng VEGF group exhibited a higher MVD than the 250-ng VEGF and control groups (P < .05). VN differed significantly between the control versus the 250-ng VEGF groups and 250-ng versus 1,000-ng VEGF groups (P < .05), with the highest VN in the 250-ng VEGF group. BF, mean transit time, and permeability-surface area product each differed significantly to VEGF levels. Changes in BF and BV did not correspond with increases in MVD or VN; however, in the 250-ng VEGF group, there was a strong positive correlation (r = 0.9) between BV and VN, vascular area, and vascular perimeter, which was not seen in the control or 1,000-ng VEGF group. All fCT parameters significantly correlated with each other (P < .05), with strong correlations between BF and mean transit time (r = -0.7) and between BF and permeability-surface area product (r = 0.7) and a weak correlation between BF and BV (r = 0.3).

CONCLUSION

These results validate the VEGF-induced endothelial cell in a rat Matrigel model. In addition, histologic vascular parameter MVD does not correlate with fCT parameters measured by CTP software.

Effect of steady versus oscillating flow on porcine coronary arterioles: involvement of NO and superoxide anion

Coronary blood vessels are compressed by the contracting myocardium. This leads to oscillations in flow in especially the subendocardium. We examined the effects of steady and oscillating flow on isolated, cannulated subendocardial and subepicardial porcine arterioles. Steady flow-induced dilation in both vessel types, up to 12.9+/-0.8% of the passive diameter in subendocardials and 9.6+/-1.4% in subepicardials at 40 dyne/cm2. Dilation was completely abolished after treatment with 10 micromol/L L-NNA. Sinusoidal modulation of steady flow at 1.5 Hz and 50% to 200% amplitude did not affect dilation. Oscillating flow without a net forward component with peak-peak shear values up to 100 dyne/cm2 caused no dilation at all in these vessels. However, in the presence of 100 U/mL superoxide dismutase (SOD), oscillating flow induced dilation up to 19.5+/-2.3% in subendocardial vessels and 11.5+/-4.3% in subepicardials. LNNA (10 micromol/L) blocked this dilation by approximately 50%. SOD did not affect the magnitude of steady flow-induced dilation, but the response time after onset of steady flow shortened from 23.4+/-1.5 to 14.3+/-2.1 seconds. Diphenyleneiodinium, an inhibitor of NAD(P)H oxidase, uncovered dilation to oscillating flow in subendocardial vessels up to 9.5+/-1.6%. Flow causes production of both NO and O2-. During steady flow, the bioavailability of NO is sufficient to cause vasodilation. During oscillating flow, NO is quenched by the O2-, suppressing vasodilation. Considering the pulsatile nature of subendocardial flow and the vulnerability of this layer, pharmacological manipulation of the balance between NO and O2- may improve subendocardial perfusion.

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.

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.