Regulation of flow and wall shear stress in arteriolar networks of the hamster cheek pouch

Nitrosative stress uncovers potent β2-adrenergic receptor-linked vasodilation further enhanced by blockade of clathrin endosome formation

This study investigated the effect of sodium nitroprusside (SNP) preexposure on vasodilation via the β-adrenergic receptor (BAR) system. SNP was used as a nitrosative/oxidative proinflammatory insult. Small arterioles were visualized by intravital microscopy in the hamster cheek pouch tissue (isoflurane, n = 45). Control dilation to isoproterenol (EC50: 10-7 mol/l) became biphasic as a function of concentration after 2 min of exposure to SNP (10-4 M), with increased potency at picomolar dilation uncovered and decreased efficacy at the micromolar dilation. Control dilation to curcumin was likewise altered after SNP, but only the increased potency at a low dose was uncovered, whereas micromolar dilation was eliminated. The picomolar dilations were blocked by the potent BAR-2 inverse agonist carazolol (10-9 mol/l). Dynamin inhibition with dynasore mimicked this effect, suggesting that SNP preexposure prevented BAR agonist internalization. Using HeLa cells transfected with BAR-2 tagged with monomeric red fluorescent protein, exposure to 10-8-10-6 mol/l curcumin resulted in internalization and colocalization of BAR-2 and curcumin (FRET) that was prevented by oxidative stress (10-3 mol/l CoCl2), supporting that stress prevented internalization of the BAR agonist with the micromolar agonist. This study presents novel data supporting that distinct pools of BARs are differentially available after inflammatory insult. NEW & NOTEWORTHY Preexposure to an oxidative/nitrosative proinflammatory insult provides a "protective preconditioning" against future oxidative damage. We examined immediate vasoactive and molecular consequences of a brief preexposure via β-adrenergic receptor signaling in small arterioles. Blocked receptor internalization with elevated reactive oxygen levels coincides with a significant and unexpected vasodilation to β-adrenergic agonists at picomolar doses.

Uniform-intensity, visible light source for in situ imaging

A flexible, low-cost, high-brightness light source for biological and biomedical imaging is presented. The illuminating device consists of a custom-size square plastic pouch 10 to 20 mm on a side and 1 to 3 mm thick that can be inserted fully or partially into both in situ or in vitro specimens to be imaged. The pouch contains a silicone-based gel medium embedded with silica particles that scatters light and provides a reasonably uniform, planar light source. Light is delivered to the pouch using a multimode optical fiber and a high-intensity tungsten lamp. Pouch size and geometry can be readily altered as needed for a particular application. Benefits of the device include reasonably uniform light intensity, low temperature rise (<2 degrees C), a nearly white light spectrum, and a thin (<2 mm thick) flexible form factor. The design, fabrication, and preliminary results from the device are presented using hamster cheek pouch tissue, with comparisons to standard intravital microscopy, along with suggestions for further improvement and potential uses.

Multiple receptor subtypes and multiple mechanisms of dilation are involved in vascular network dilation caused by adenosine

We previously demonstrated a vascular network response initiated by elevated tissue concentrations of adenosine that is distinct from the dilation caused when adenosine is applied directly to the arteriole. The purpose of this study was to elucidate the potential mechanism(s) for the different responses. In the cheek pouch of anesthetized hamster, arteriolar responses were measured when adenosine (10(-4)M) was applied with micropipette into the tissue 500 microm from the arteriole (n=67, baseline diameter 22+/-0.6 microm) or onto the arteriole itself. Application of adenosine to the vessel or into the tissue caused arteriolar dilation with similar concentration profiles. In stark contrast, the concentration profiles were significantly different for vessel and tissue initiated dilation when either sodium nitroprusside or methacholine was tested. Arteriolar dilation was not enhanced when adenosine was simultaneously applied with two pipettes at along a single arteriole; however, the dilation doubled when adenosine was applied simultaneously at arteriole and tissue. Control dilations caused by tissue adenosine (5+/-0.4 microm) were not altered by superfusion of the A(1) receptor antagonist DPCPX (10(-6)M; 4.6+/-0.3 microm), A(2B) receptor antagonist alloxazine (10(-6)M; 6+/-0.8 microm), or A(3) receptor antagonist MRS1220 (5 x 10(-9)M; 6+/-0.8 microm) but were abolished by the selective A(2A) receptor antagonist ZM241385 (10(-7)M; 1+/-0.2 microm), suggesting that activation of A(2A) receptors mediates these network responses. Disruption of arteriolar endothelium and direct arteriolar application of ZM241385 (10(-7)M; 5+/-0.4 microm) did not alter the dilation caused by tissue adenosine. However, local application of ZM241385 into the tissue inhibited adenosine-induced network responses (2+/-0.3 microm). Furthermore, application into the tissue of A(2A) receptor agonist CGS21680 (10(-5)M), but not A(1) (CPA; 10(-4)M), A2b (NECA, 10(-4)M) or A3 (IB-MECA; 10(-4)M) receptor agonists mimicked the adenosine network response. These data demonstrate dual, complimentary, yet distinct pathways for network dilations induced by increases in tissue adenosine.

Application of local heat induces capillary recruitment in the Pallid bat wing

Skin blood flow increases in response to local heat due to sensorineural and nitric oxide (NO)-mediated dilation. It has been previously demonstrated that arteriolar dilation is inhibited with NO synthase (NOS) blockade. Flow, nonetheless, increases with local heat. This implies that the previously unexamined nonarteriolar responses play a significant role in modulating flow. We thus hypothesized that local heating induces capillary recruitment. We heated a portion (3 cm2) of the Pallid bat wing from 25°C to 37°C for 20 min, and measured changes in terminal feed arteriole (∼25 μm) diameter and blood velocity to calculate blood flow ( n = 8). Arteriolar dilation was reduced with NOS and sensorineural blockade using a 1% (wt/vol) NG-nitro-l-arginine methyl ester (l-NAME) and 2% (wt/vol) lidocaine solution ( n = 8). We also measured changes in the number of perfused capillaries, and the time precapillary sphincters were open with ( n = 8) and without ( n = 8) NOS plus sensorineural blockade. With heat, the total number of perfused capillaries increased 92.7 ± 17.9% ( P = 0.011), and a similar increase occurred despite NOS plus sensorineural blockade 114.4 ± 30.0% ( P = 0.014). Blockade eliminated arteriolar dilation (−4.5 ± 2.1%). With heat, the percent time precapillary sphincters remained open increased 32.3 ± 6.0% ( P = 0.0006), and this increase occurred despite NOS plus sensorineural blockade (34.1 ± 5.8%, P = 0.0004). With heat, arteriolar blood flow increased (187.2 ± 28.5%, P = 0.00003), which was significantly attenuated with NOS plus sensorineural blockade (88.6 ± 37.2%, P = 0.04). Thus, capillary recruitment is a fundamental microvascular response to local heat, independent of arteriolar dilation and the well-documented sensorineural and NOS mechanisms mediating the response to local heat.

Mechanisms initiating integrin-stimulated flow recruitment in arteriolar networks

Our purpose was to investigate the local mechanisms involved in network-wide flow and diameter changes observed with localized downstream vitronectin receptor ligation; we tested specific K or Cl channels known to be involved in either dilation or elevated permeability following vitronectin receptor activation and tested integrin-linked pathway elements of tyrosine phosphorylation and protein kinase C (PKC). Arteriolar networks were observed in the cheek pouch tissue of anesthetized (pentobarbital sodium, 70 mg/kg) hamsters ( n = 86) using intravital microscopy. Terminal arteriolar branches of the networks were stimulated with micropipette LM609 (0.5–10 μg/ml, 60 s) alone or with inhibitors (separate micropipette). Hemodynamic changes (diameter, red blood cell flux, velocity) were observed at the upstream entrance to the network. LM609 alone stimulated first an increase in wall shear stress (WSS), followed by a dilation that recovered WSS to baseline or below. K channel inhibition (glybenclamide, 4-AP) had no effect on the initial peak in WSS, but decreased remote vasodilation. Cl channel inhibition (DIDS, IAA-94, niflumic acid) or inhibition of PKC (chelerythrine) prevented the initial peak in WSS and decreased remote vasodilation. Inhibition of tyrosine phosphorylation (genistein) prevented both. With the use of nitro-arginine at the observation site, the initial peak in WSS was not affected, but remote vasodilation was decreased. We conclude the remote response consists of an initial peak in WSS that relies on both PKC activity and depolarization downstream, leading to an upstream flow mediated dilation and a secondary remote dilation that relies on hyperpolarization downstream at the stimulus site; both components require tyrosine phosphorylation downstream.

Arteriolar flow recruitment with vitronectin receptor stimulation linked to remote wall shear stress

Our purpose was to investigate the hemodynamic mechanism of flow recruitment within arteriolar networks as stimulated by the vitronectin receptor. Since remote stimulation of the vitronectin receptor initiates a flow-mediated dilation, we tested whether the response could be mimicked by exogenous NO. Arteriolar diameter, red blood cell flux, and velocity were measured in terminal branch arterioles arising from a central feed (8-10 microm) arteriole (together previously defined as an arteriolar network) in the cheek pouch tissue of anesthetized hamsters (n = 29, pentobarbital 70 mg/kg). Terminal branch arterioles were stimulated (micropipette, 10 s) with 10 microg/ml LM609 (agonist, vitronectin receptor), 10(-4) M nitroprusside (SNP), or control. The same amount of flow was recruited into the network by applying LM609 or SNP to only one branch of the network; however, flow distribution differed. With LM609, all recruited flow went directly to the branch that was stimulated. With SNP, recruited flow was distributed evenly throughout the network, no matter where the stimulus was applied. Thus, flow-mediated dilation induced by vitronectin receptor stimulation initiates a robust flow recruitment response, directing flow to the stimulus flow path. In contrast, NO alone initiates flow recruitment to the entire network, along multiple parallel flow paths.