Inosine-induced vasoconstriction is mediated by histamine and thromboxane derived from mast cells

Mast cell degranulation has been shown to release products that cause arteriolar constriction. We previously reported that two nucleosides, adenosine and inosine, cause vasoconstriction of isolated hamster cheek pouch arterioles by stimulating degranulation of periarteriolar mast cells. The objectives of the present study were to characterize the nucleoside-dependent vasoconstriction in vivo and to determine the mediator or mediators responsible. We examined the vasomotor effect of inosine on arterioles in the cheek pouches of anesthetized hamsters (70 mg/kg pentobarbital sodium) in the control situation and in the presence of receptor antagonists for histamine (H1), thromboxane A2 (Tx), and leukotrienes (LT). Most experiments were carried out using inosine applied once locally via micropipette to arterioles and observing the subsequent response. Over a range of inosine concentrations from 10(-5) to 10(-3) M in the pipette, we observed a dose-dependent increase in the incidence and magnitude of constriction. In addition, mast cell staining with ruthenium red was observed after stimulation with inosine, an indication of mast cell degranulation. Neither the H1, Tx, nor LT antagonist alone had a significant effect on the vasomotor response to inosine. However, combined H1 and Tx blockade significantly reduced the incidence and magnitude of inosine-induced constriction. These data establish that inosine-induced constriction occurs in vivo and support the role of mast cells in this response. Furthermore they suggest that multiple mediators, primarily histamine and thromboxane, are responsible for the observed constriction.

Electromechanical coupling and the conducted vasomotor response

Conducted vasomotor responses are viewed as one mechanism that functionally integrates the microvasculature. It is hypothesized that the conducted vasomotor response is the result of an electrical current and its passive electrotonic spread along the length of a microvessel. We tested this hypothesis in isolated, unpressurized arterioles from the hamster cheek pouch using conventional intracellular membrane potential recording techniques. The mean resting membrane potential (RMP) was -67 mV. KCl and phenylephrine (PE) pulse-stimulation applied through micropipettes could both induce transient depolarizations and vasoconstrictions at the site of stimulation (local) and at conducted (560 microns) sites. It was noted, however, that the conducted vasomotor response could not be induced until the conducted electrical response exceeded a threshold of -45 mV for a minimum amount of time. The relationship between the amplitude of constriction and the amplitude-time area of depolarization above -45 mV was the same for local and conducted KCl and for conducted PE but was significantly different from that for local PE. Nifedipine greatly reduced the local and conducted mechanical but not electrical responses. Our results indicate that the conducted vasomotor responses are the result of the generation and subsequent conduction of electrical signals along the vessel but that the corresponding mechanical response occurs only when the electrical response exceeds a threshold level.

Use of Ruthenium Red staining to detect mast cell degranulation in vivo

OBJECTIVE

To establish a method of detecting mast cell degranulation in tissues during in vivo microscopy.

METHODS

Hamster tissues were prepared for intravital microscopy. Ruthenium red (RR) was superfused over the cheek pouch at concentrations of 0.0001-0.01% to determine the optimal concentration. Mast cells were stimulated with compound 48/80, as well as with vasoactive agents not known to be stimulatory to mast cells, following which, mast cell staining was observed. Mesenteries were stained with Toluidine Blue (TB) or RR and mast cell degranulation was assessed during treatment with compound 48/80, or control.

RESULTS

During superfusion with varying concentrations of RR, a dose dependence for background staining of unstimulated cells was observed. A RR concentration of 0.001% was optimal for in vivo detection of mast cell degranulation. Mast cells exposed to 0.001% RR were stained following stimulation with compound 48/80 but not after treatment with KCl or acetylcholine. The latter agents are not known to stimulate mast cells. Thus, arteriolar vasomotor responses, per se, did not appear to play a role in mast cell RR uptake. Comparable results were obtained with RR versus TB in control or 48/80-treated mesenteries.

CONCLUSIONS

This RR technique facilitates rapid detection of mast cell degranulation in vivo and provides an opportunity to assess both mast cell and microvascular function simultaneously.

Cellular pathways of the conducted electrical response in arterioles of hamster cheek pouch in vitro

We have previously shown that conducted vasomotor responses follow patterns that are consistent with a passive spread of electrical current along the length of the arterioles [(Xia and Duling, Am. J. Physiol. 269 (Heart Circ. Physiol. 38): H2022-H2030, 1995]. In this study, we define the cells through which the current flows. Isolated arterioles of hamster cheek pouch were used. The mean resting membrane potential (RMP) for randomly sampled arteriolar cells was -67 mV. When cell types were identified by dye injection, the RMPs were -68 and -67 mV for smooth muscle (SM) and endothelium (EC), respectively. Pulses of KCl induced transient, monophasic depolarizations at the site of stimulation (local), which were conducted decrementally along the length of the arteriole over several millimeters. During electrical conduction, three patterns of responses could be observed, but identical patterns of the conducted electrical responses were always observed in SM and EC. Phenylephrine stimulation also caused transient local and conducted depolarizations in both SM and EC. As with KCl stimuli, shapes of conducted electrical responses were identical in records made in both cell types. The results suggest that SM and EC are electrically coupled both homocellularly and heterocellularly.

Method to create small photo-bleached volumes to monitor blood plasma flow in capillaries

A method has been developed to examine the movement of plasma in capillaries using intravital microscopy. Spatial transients in fluorescence properties are instantaneously induced by laser photo-bleach pulses after which the convective recovery can be monitored. The plasma is tagged with fluorescent dyes coupled to bovine serum albumin, which is injected well before the measurements and circulates with the blood stream. A laser beam from an argon laser source, set to emit light with a wavelength of 488 nm, is focused on the illumination field diaphragm and creates a spot in the object plane of the microscope. At low laser power, the laser spot is aimed at a blood plasma gap between red blood cells in a capillary segment, using a steerable mirror. Light sensors, coupled to photo-multipliers in the secondary image plane of the microscope, record the light intensity of the moving plasma/dye while the preparation is continuously illuminated with a xenon epi-illuminating set-up. The laser photo-bleach spot is then used to bleach the dye complex within a 5.4 microns segment of the capillary for less than 20 ms. The movement of the bleached plasma bolus is tracked by the photo-sensors, placed sequentially along the capillary. Both dye and red blood cell passage can be detected in the photo-multiplier signals, and the relative velocities of the two blood components can be measured. Measurements reveal that the ratio of transit times between blood plasma and red blood cells is 1.23 (SD = 0.22, N = 18), which is in good agreement with measurements by other techniques.

Dye tracers define differential endothelial and smooth muscle coupling patterns within the arteriolar wall

Dye tracers were chosen, based on net charge, chemical structure, and reactive groups, to test for the existence of and to provide novel insight into channel selectivities of junctional pathways connecting smooth muscle and endothelial cells of the arteriolar wall. Dyes were injected into individual smooth muscle or endothelial cells of hamster cheek pouch arterioles using microiontophoresis. Coupling, independent of tracer net charge, was seen both within and between cell layers. Endothelial cells were well coupled by all of the tested dyes. Smooth muscle junctions appeared less effective in dye transfer than endothelial junctions. Lucifer yellow was confirmed to be a poor tracer of smooth muscle gap junctions, and remarkably this dye and other related sulfate-containing molecules interfered with dye movement through smooth muscle but not endothelial junctions. Myoendothelial junctions showed a striking polarity of dye movement, with dye transfer from endothelial to smooth muscle cells but little or no transfer in the reverse direction. Because the dyes have size and charge characteristics similar to those of known cellular second messengers, these findings have important implications for cell-cell signaling in the vessel wall.

Connexin 43 and connexin 40 gap junctional proteins are present in arteriolar smooth muscle and endothelium in vivo

The distributions of connexin 43 (Cx43) and connexin 40 (Cx40) in smooth muscle and endothelium of resistance vessels were examined using indirect immunofluorescence techniques coupled with confocal microscopy. Cx43 and Cx40 were found in smooth muscle and endothelium. Similar staining patterns were found in microvessel samples from brain and cremaster of the rat and from arterioles of the hamster cheek pouch. Double-labeling studies showed a high degree of colocalization of Cx40 with Cx43, suggesting the presence of multiple connexins within a single junctional plaque. Quantitative comparisons were made of the fluorescent patterns in the endothelium and smooth muscle of rat brain arterioles. Cx43 and Cx40 plaque diameters were 0.9 +/- 0.1 and 0.8 +/- 0.1 (SE) microns, respectively, in the endothelial layer and 0.5 +/- 0.1 and 0.5 +/- 0.1 microns, respectively, in the smooth muscle. There was no difference between mean plaque diameters of Cx43 and Cx40 in endothelium or smooth muscle. However, plaques were significantly larger in endothelium than in smooth muscle (P < 0.05). These findings demonstrate the potential for cell-cell communication in both cell types of the wall of arterioles from three different tissues. The data also suggest a greater level of coupling within the endothelium.

Vulnerability of conducted vasomotor response to ischemia

Many vasoactive substances induce two responses, a direct effect at the site of application and a conducted response that spreads along the vessel length. In the microcirculation, we find that these two components of the vasomotor response display quite different sensitivities to occlusion and/or ischemia. Conducted vasomotor responses were induced in arterioles of the hamster cheek pouch by micropipette application of two test agents: phenylephrine (PE), which causes a receptor-mediated vasomotor response, and KCl, which causes an alteration in the membrane potential by a simple change in the K+ gradient. Ischemia was produced either by total occlusion of the vascular supply, which resulted in a complete cessation of flow in all vessels, or by venous occlusion, which was achieved by gradually inflating a pressurized cuff positioned across the pedicle of the pouch until venous return from the pouch was arrested while the feed arterioles remained patent. Both types of occlusion produced ischemia, the former with low intravascular pressure, the latter with high intravascular pressure. During both types of occlusion, arterioles were initially maximally dilated and unresponsive to both agonists, but over a subsequent 3- to 5-min period, resting arteriolar tone and local responses to both agonists returned. With total occlusion, the conducted response to KCl returned in parallel with the local response, whereas the conducted response to PE was diminished or absent. With venous occlusion, the local responses recovered as with total occlusion, but the conducted responses to both PE and KCl recovered as well.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of capillary tube hematocrit during arteriolar microperfusion

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.

Nucleoside-induced arteriolar constriction: a mast cell-dependent response

Adenosine (Ado) is a potent vasodilator that has occasionally been shown to cause vasoconstriction. Constrictor responses are generally attributed to A1-receptor stimulation or interactions with the renin-angiotensin system. We describe a previously unreported vasoconstrictor action of Ado and inosine (Ino) in hamster cheek pouch arterioles and examine the mechanism by which these nucleosides induce constriction. Arterioles were dissected from male Golden hamster cheek pouches, transferred to a 37 degrees C tissue chamber, and cannulated at both ends. Changes of luminal diameter in response to Ado were measured to generate cumulative concentration-response curves. The concentration-response curves were biphasic: 10(-6) M Ado elicited an intense, transient constriction, and higher concentrations induced dilator responses. Pretreatment with 8(p-sulfophenyl)theophylline, an Ado receptor antagonist, inhibited the dilator responses but did not alter the constriction. Inhibition of Ado uptake with S-(4-nitrobenzyl)-6-thio-inosine eliminated the constrictor response without altering dilator responses. Similar effects were found after pretreatment with an Ado deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride. Finally, Ino, a metabolite of Ado, induced constrictions of similar magnitude to those seen with Ado, but at higher concentrations. The constrictor response was focal in nature, suggesting discrete sites of action of Ado. Methylene blue staining after Ado application revealed degranulated mast cells closely associated with the vessel wall, indicating a possible role for mast cell degranulation in the constrictor response. Supporting this idea were the observations that inhibition of degranulation by 10 microM cromolyn blocked the constrictor response, and compound 48/80 (a mast cell secretagogue) caused constriction similar to that elicited by Ado.(ABSTRACT TRUNCATED AT 250 WORDS)

A technique for the estimation of plasma flow in single capillaries using photobleached dyes

In order to estimate plasma flow in single capillaries, an "indicator bolus" was optically inserted into individual microvessels of the hamster cremaster muscle. This was accomplished using short-duration (200 msec), argon laser pulses to photobleach a 5-microns segment of fluorochrome circulating with the plasma. The subsequent motion of the bleached plasma bolus was then tracked using photomultipliers positioned at three sites along the capillary. The transient passage of the dye appeared as a steep fall in light intensity as the downstream edge of the bleached area entered the sensor field, followed by a steep rise in light intensity as trailing unbleached plasma flowed under the sensor. The behavior of light intensity as the photobleached bolus flowed past a sensor was analyzed using a theoretical model developed to predict the behavior of this type of plasma flow indicator in single capillaries. The characteristic time, tau, which equals the capillary segment volume divided by the plasma flow, was taken as an estimate of plasma flow. The model predicts that, for this system, tau of the capillary corresponds closely to the time at which 50% of the full sensor response to the bolus is attained, that is, the t50. The ratio between the t50 and the characteristic time is found to be a function of the relative sensor width and the flow velocity profile. A procedure is also described to assess the flow velocity profile from in vivo measurements. Using this technique, the ratio of the velocity of the red cell compared to that of plasma is found to be about 1.3.

A light-emitting diode light standard for photo- and videomicroscopy

A light calibration system consisting of a compact light-emitting diode (LED) source with feedback control of intensity is described. The source is positioned in the focal plane of the microscope objective and produces flat-field illumination of up to 31 microW. The source can be easily used to determine the performance of microscope optics and camera response. It can also be used as a standard light source for calibration of experimental systems. Selectable light intensities are produced by controlling the LED input power via a feedback circuit consisting of a photodiode that detects output light intensity. Spectral coverage extends between 550 and 670 nm using green, yellow and red LEDs mounted side by side, which are selected individually. The LED chips are encapsulated in plastic diffusers which homogenize the light, and a flat field of illumination is obtained through a thin 1-mm-diameter aperture positioned directly over each chip. Provision is made for insertion of Ronchi rulings over the aperture to enable measurements of contrast modulation in a uniform field. The light may be pulse-modulated to assess camera response times and the device can be synchronized with video frames. Narrow bandpass interference filters can be placed between the objective lens and the LED source to produce monochromatic light without affecting the spacing of controlled light intensities since emission spectra do not shift appreciably over the range of LED powers chosen in this design. Results of tests using controlled light intensity and uniform illumination are presented.

Modification of vascular reactivity by alteration of intimal permeability: effect of TNF-alpha

We investigated the effects of alterations in intimal permeability on microvascular reactivity to small hydrophilic agents in isolated, cannulated, perfused arterioles (65 +/- 6 microns ID) from hamster cheek pouches. Arterioles are 300-fold less responsive to the hydrophilic alpha 1-agonist, phenylephrine, applied to the lumen than when applied to the adventitia. Luminal treatment with tumor necrosis factor-alpha (TNF-alpha, 0.625 micrograms/ml, 1-2 h) potentiated reactivity to luminally applied phenylephrine, but the treatment did not change reactivity to adventitially applied phenylephrine. Similar results were obtained with a brief treatment with the detergent, 3-[(3-cholamidopropyl)dimethylammonio]-1- propanesulfonate (CHAPS; 0.3%, < 30 s). To confirm that a change in permeability had occurred, we measured the movement across the arteriolar wall of a low-molecular-weight hydrophilic fluorescent molecule, fluorescein, before and after luminal treatment with TNF-alpha or CHAPS. Either TNF-alpha or CHAPS significantly increased the rate of movement of fluorescein across the arteriolar wall. These data suggest that one element in the pathophysiology of TNF-alpha is an increase in arteriolar permeability to small, water-soluble agents, which may modify reactivity to circulating vasoactive substances.

Comparison of conduit vessel and resistance vessel reactivity: influence of intimal permeability

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.

A micropipette which allows in situ perfusion of arterioles and capillaries

In order to investigate capillary physiology, a glass micropipette system was developed that allowed in situ perfusion of microvessels as well as rapid changes of perfusion solutions. Theta tube (WPI, Inc.; 1.5-mm o.d. glass stock capillary tubing which is divided into two hemicylindrical sides by a central glass septum) was pulled to a smaller diameter of approx 300-600 microns and inserted into the shank of a sharpened cannulating micropipette tip constructed from large-bore glass stock (1.6 mm i.d.). The resulting dead volume between the end of the Theta supply tube and the tip of the outer cannulating tip was approximately 90 nl. The perfusate was driven in a circuit from a pressurized feed reservoir down one side of the Theta supply tube pipette and back through the second side into a reservoir maintained at a lower pressure. The pressure gradient between the two reservoirs established a high-volume flow rate and subsequently a short perfusate transit time from the feed to the collection reservoir. The average pressure in the two reservoirs determined the pressure which drove the perfusate from the cannulating tip. At normal pressures and flows, the time required to change perfusion fluid composition at the pipette tip was less than 1 min, and discharge hematocrit of a red blood cell suspension was indistinguishable from the hematocrit measured in the feed reservoir.(ABSTRACT TRUNCATED AT 250 WORDS)

Access of blood-borne vasoconstrictors to the arteriolar smooth muscle

In vitro experiments have shown that luminally applied water-soluble vasoactive materials have limited access to arteriolar smooth muscle cells, and as a result, the responses to such agents applied luminally are less than the responses to those applied adventitially. To determine the extent to which this 'compartmentation' influences arteriolar responsiveness to blood-borne water-soluble vasoconstrictors in vivo, we applied phenylephrine, vasopressin and angiotension II to arterioles in the hamster cheek pouch both by luminal perfusion, and by topical application to the arteriolar smooth muscle via micropipettes. The arterioles were about 2 orders of magnitude more sensitive to these water-soluble vasoconstrictors when they were applied topically than when they were applied luminally. In contrast, the arterioles were almost equally sensitive to the lipid-soluble alpha 1-adrenoceptor agonist SKF 89748-A applied by either route. The venular wall appears to be much less effective as a barrier than the arteriolar endothelium. Phenylephrine and vasopressin both elicited large arteriolar constrictions when perfused through venules in close proximity to the arteriole, and these constrictions were larger than those observed when the drug was applied to the arteriole's own lumen. Our observations confirm that the arteriolar endothelium can inhibit the direct access of water-soluble blood-borne agents to the arteriolar smooth muscle in vivo, and they suggest that the capillaries and venules could be the primary routes of access for water-soluble agents from the blood to the arteriolar smooth muscle.

Arteriolar endothelial cell barrier separates two populations of muscarinic receptors

The endothelium of arterioles can function as a barrier to diffusion of hydrophilic molecules when studied in vitro. Thus a substance applied to one side of the arteriole is relatively ineffective in reaching receptors on the opposite side of the vessel wall unless it is lipid soluble. To study the receptor populations on the two sides of the arteriolar endothelium, we used micropipettes to apply methacholine (MCh; 1.0 microM), either luminally or adventitially, for 5 s to the arterioles of the cheek pouch of pentobarbital-anesthetized hamsters. MCh equally dilated the arterioles regardless of the side of application. That different populations of receptors are located on either side of the arteriole was shown by the fact that adventitially applied hydrophilic methscopolamine was ineffective in blocking the effects of the luminally applied MCh but completely blocked the effects of abluminally applied MCh. In contrast, the luminal population of receptors was easily blocked by adventially applied scopolamine, which is lipophilic. Separate and independent populations of receptors in the vessel wall suggests the potential for differential control between humoral and adventitial sources of vasoactive metabolites.

Microcirculatory dysfunction following perfusion with hyperkalemic, hypothermic, cardioplegic solutions and blood reperfusion. Effects of adenosine

BACKGROUND

Cardioplegic solutions have been used to enhance myocardial preservation during cardiac surgery. The benefits derived from preventing myocardial ischemia with cardioplegic solutions may, however, be countered by tissue damage that occurs when the myocardium is reperfused with oxygenated blood. Furthermore, cardioplegia-induced endothelial dysfunction may contribute to depressed myocardial function postoperatively. The endothelium of coronary arteries and vein grafts is damaged by crystalloid cardioplegic solutions. There is less known about the effects of cardioplegic solutions on the microvasculature.

METHODS AND RESULTS

The hypothesis that microvascular damage occurs following perfusion with hyperkalemic, crystalloid, cardioplegic solutions and blood reperfusion, leading to decreased blood flow and increased neutrophil accumulation, was tested in a model system. Intravital microscopic observations were performed during a 20-minute perfusion of the hamster cremaster muscle with cardioplegic solutions (10 degrees C) via the femoral artery with the iliac occluded and during a subsequent 2-hour blood reperfusion period (iliac open). Arteriolar vasoconstriction (27% decrease in diameter, p less than 0.05) and a 25% decrease in the density of perfused capillaries (p less than 0.05) occurred during reperfusion in hamsters receiving crystalloid cardioplegic solution (16 meq K+) compared to control hamsters (no cardioplegic solution given). Neutrophils accumulated on venular endothelium in treated animals (250% increase, p less than 0.05) and extravascularly (myeloperoxidase levels 2.0 +/- 0.4 U/g versus 1.3 +/- 0.3 U/g in control, p less than 0.05). The addition of adenosine (10(-4) M) and albumin (2 g%) to the cardioplegic perfusate, accompanied by the administration of adenosine (10(-4) M) during reperfusion, produced arteriolar vasodilation (34% diameter increase, p less than 0.05) and inhibited extravascular neutrophil accumulation (myeloperoxidase level of 1.5 +/- 0.2 U/g, p greater than 0.05 versus control). Capillary perfusion, however, was still significantly diminished (28% decrease, p less than 0.05.)

CONCLUSIONS

We conclude that injury manifest by decreased microvascular blood flow and increased neutrophil accumulation in tissues occurs after perfusion with hypothermic, hyperkalemic, crystalloid cardioplegic solutions and blood reperfusion. Adenosine seems to partially attenuate this injury by dilating arterioles and decreasing extravascular neutrophil accumulation.

Heterogeneity in conducted arteriolar vasomotor response is agonist dependent

Microiontophoresis of acetylcholine onto cheek pouch arterioles of the pentobarbital-anesthetized hamster results in both a local response at the pipette tip and a conducted dilator response. The conducted response is not dependent on blood flow, and its magnitude decays with distance from the site of stimulation. In an attempt to define the mechanism responsible for activation of arteriolar conduction, vasoactive agonists directed toward different vascular wall cell types, receptor types, and second messengers were applied to arterioles by pressure-pulse microejection. As expected, microapplication caused a consistent arteriolar response at the site of application with each of the agonists tested (local response). However, a high degree of variability was observed among agonists in their ability to produce conducted responses. Acetylcholine, muscarine, and phenylephrine, invariably induced both local and conducted responses. In contrast, bradykinin, substance P, papaverine, isoproterenol, and adenosine, though consistently inducing local responses, displayed a highly variable ability to induce the conducted responses. When conduction was observed, the arteriolar response was similar regardless of the agonist used to induce the response. Microejection of sodium nitroprusside or arginine vasopressin produced local arteriolar responses with no evidence of a conducted response regardless of the dose. These studies reveal previously undetected heterogeneity among microvessel responses and may reflect variations in the coupling mechanisms linking the local vasomotor response to the conducted response.

Ca2+ sensitivity of isolated arterioles from the hamster cheek pouch

When isolated from the hamster cheek pouch, cannulated, and perfused, 60- to 90-microns arterioles spontaneously contracted to 67 +/- 4% of maximum diameter. Vessel sensitivity to variations in extracellular Ca2+ was then evaluated. Tone, regardless of its source, was highly dependent on the concentration of Ca2+ in the bathing solution. The magnitude of responses to changing Ca2+ depended upon which vessel surface (luminal or abluminal) the change was made. For K(+)-induced tone the Ca2+ concentration-response curve was right shifted 60-fold for luminal vs. abluminal changes. These results suggest that restricted diffusion of Ca2+ from lumen to smooth muscle dramatically reduces smooth muscle Ca2+ concentration and that under standard in vitro conditions the smooth muscle layer is effectively isolated from luminal contents. Both the cytosolic and stored Ca2+ in these microvessels were dependent on the Ca2+ concentration in the bathing solution. Abrupt removal of Ca2+ from bath produced a rapid maximal dilation with a mean time to half-maximal response (t1/2 max) of 14 +/- 4 s. Ca2+ replacement induced a return to the previous level of tone with a mean t1/2 max of 8 +/- 3 s. The magnitude of transient responses to caffeine (10 mM) was inversely related to the time of exposure to zero Ca2+ with a rapid decay in magnitude (t1/2 max = 2.7 +/- 0.8 min). These data suggest that the smooth muscle cells of arterioles have a particularly rapid transmembrane Ca2+ flux that is tightly controlled by an intracellular regulatory mechanism, which may explain the generally increased dependence of smaller vessels on extracellular Ca2+.

Arteriolar reactivity in vivo is influenced by an intramural diffusion barrier

The endothelium of the hamster cheek pouch arteriole in vitro is able to greatly reduce the potency of luminally applied water-soluble drugs by acting as a barrier to diffusion from the lumen to the smooth muscle [Lew, Rivers, and Duling. Am. J. Physiol. 257 (Heart Circ. Physiol. 26): H10-H16, 1989]. Lipid-soluble drugs appear unaffected by the diffusion barrier, presumably because their ability to cross cell membranes allows them to freely cross the endothelium. We compared the effects of two alpha 1-adrenoceptor agonists, phenylephrine (water soluble) and SKF 89748A (lipid soluble), on systemic blood pressure and the arterioles of the hamster cheek pouch in vivo. Both agonists were able to activate the arterioles when applied topically to the outside of the arterioles (extraluminal application). The agonists were also injected as a brief bolus into the aortic arch at doses chosen to elicit similar peak pressor responses. At all levels of pressor response, the arteriolar responses to phenylephrine were smaller than those to SKF 89748A. In the cremasteric vasculature SKF 89748A was similarly found to be more effective in activating the arterioles after intravascular administration than was phenylephrine. We conclude that an intramural diffusion barrier exists in the arteriolar wall in vivo and that it can influence vascular reactivity.

Heparinase treatment suggests a role for the endothelial cell glycocalyx in regulation of capillary hematocrit

Physiological stimuli induce rapid and unexplained increases in the number of red blood cells within capillaries of skeletal muscle. We hypothesized that such alterations in intracapillary red cell numbers might be due to an undefined interaction between one or more components of blood and the luminal surface of the capillary. This proposition was tested by in situ microperfusion of capillaries with enzymes directed against macromolecules likely to be expressed on the surface of endothelial cells. The instantaneous fractional volume of red blood cells within a capillary (tube hematocrit) was used as an index of a capillary's response to enzyme microperfusion. Five to 8 min of perfusion with enzyme vehicle (0.25% albumin-Ringer solution) produced no significant alteration in capillary tube hematocrit. Perfusion with solutions containing heparinase raised the tube hematocrit at least twofold (P less than 0.05) without a significant change in red cell velocity. Heat-denatured heparinase and other enzymes such as neuraminidase, hyaluronidase, papain, pronase E, and clostripain had no detectable effect on the tube hematocrit (P greater than 0.05). After enzyme treatment, application of adenosine (10(-4) M) or oxygen caused brisk vasomotor responses in arterioles feeding perfused capillary units, but the usual changes in the tube hematocrit were not observed. Thus heparinase treatment results in a sustained elevation in the capillary tube hematocrit and a dissociation of the typical relationship between vasomotor changes and red cell distribution in capillaries. These findings suggest that physiological stimuli which alter the number of red blood cells within capillaries may operate by modifying interactions between plasma and one or more components on the luminal surface of capillaries.

Microcirculatory responses to exogenous endothelial cell-derived relaxing factor

Endothelium-derived relaxing factor (EDRF) plays an important role in the vasodilatory responses of large blood vessels. However, such a role has yet to be conclusively shown for the microvasculature. In this study we tested the sensitivity of arterioles in the cheek pouch of pentobarbital-anesthetized hamsters to the EDRF-dependent agonists bradykinin and A23187, as well as to exogenous EDRF from cultured bovine aortic endothelial cells. The pouch superfusion fluid was arranged to first pass through a column containing endothelial cells and then on to the tissue. Bradykinin (10-30 nM) or A23187 (0.3 microM) was introduced either upstream or downstream to the endothelial cells, and the resultant responses were measured with video microscopy. Bradykinin and A23187 both caused a dose-dependent release of a microvessel dilator from cultured endothelial cells. We take this dilator to be EDRF based on the characteristics of the responses to the stimuli. Indomethacin (7.7 microM) was present in the superfusate to eliminate the production of cyclooxygenase products from the endothelial cells, and the magnitude of the response was diminished if the superfusate was first passed through a 3-min delay coil before arrival at the pouch. The arterioles dilated to the direct application of bradykinin in a dose-dependent fashion. They did not respond however to the direct application of A23187. These studies demonstrate that arteriolar smooth muscle is able to respond to exogenous EDRF and support the premise that EDRF may play an active role in the regulation of blood flow in the microcirculation.