Characterization of adenosine receptors mediating the vasodilator effects of adenosine receptor agonists in the microvasculature of the hamster cheek pouch in vivo

Pre-exposure to adenosine, acting via A(2A) receptors on endothelial cells, alters the protein kinase A dependence of adenosine-induced dilation in skeletal muscle resistance arterioles

Adenosine (ADO) is an endogenous vasodilatory purine widely recognized to be a significant contributor to functional hyperaemia. Despite this, many aspects of the mechanisms by which ADO induces dilation in small resistance arterioles are not established, or appear contradictory. These include: identification of the primary receptor subtype; its location on endothelial (EC) or vascular smooth muscle cells; whether ADO acts on KATP channels in these resistance vessels; and the contribution of cAMP/protein kinase A (PKA) signalling to the response. In intravital microscopy studies of intact or EC-denuded skeletal muscle arterioles, we show that ADO acts via A2A receptors located on ECs to produce vasodilation via activation of KATP channels located on vascular smooth muscle cells. Importantly, we found that the signalling pathway involves cAMP as expected, but that a requirement for PKA activation is demonstrable only if the vessel is not pre-exposed to ADO. That is, PKA-dependent signalling varies with pre-exposure to ADO. Further, we show that PKA activation alone is not sufficient to dilate these arterioles; an additional EC calcium-dependent signalling mechanism is required for vasodilation to ADO. The ability of arterioles in situ to respond to occupancy of a specific receptor by utilizing different cell signalling pathways under different conditions to produce the same response allows the arteriole to respond to key homeostatic requirements using more than a single signalling mechanism. Clearly, this is likely to be physiologically advantageous, but the role for this signalling flexibility in the integrated arteriolar response that underlies functional hyperaemia will require further exploration.

Selective deletion of the A1 adenosine receptor abolishes heart-rate slowing effects of intravascular adenosine in vivo

Objective

Intravenous adenosine induces temporary bradycardia. This is due to the activation of extracellular adenosine receptors (ARs). While adenosine can signal through any of four ARs (A1AR, A2AAR, A2BAR, A3AR), previous ex vivo studies implicated the A1AR in the heart-rate slowing effects. Here, we used comparative genetic in vivo studies to address the contribution of individual ARs to the heart-rate slowing effects of intravascular adenosine.

Methods and Results

We studied gene-targeted mice for individual ARs to define their in vivo contribution to the heart-rate slowing effects of adenosine. Anesthetized mice were treated with a bolus of intravascular adenosine, followed by measurements of heart-rate and blood pressure via a carotid artery catheter. These studies demonstrated dose-dependent slowing of the heart rate with adenosine treatment in wild-type, A2AAR^−/−, A2BAR^−/−, or A3AR^−/− mice. In contrast, adenosine-dependent slowing of the heart-rate was completely abolished in A1AR^−/− mice. Moreover, pre-treatment with a specific A1AR antagonist (DPCPX) attenuated the heart-rate slowing effects of adenosine in wild-type, A2AAR^−/−, or A2BAR^−/− mice, but did not alter hemodynamic responses of A1AR^−/− mice.

Conclusions

The present studies combine pharmacological and genetic in vivo evidence for a selective role of the A1AR in slowing the heart rate during adenosine bolus injection.

Adenosine A2A receptor modulation of juvenile female rat skeletal muscle microvessel permeability

Little is known of the regulation of skeletal muscle microvascular exchange under resting or stimulating conditions. Adenosine (ADO) levels in skeletal muscle increase during physiological (exercise) and pathological (hypoxia, inflammation, and ischemia) conditions. Later stages of these pathologies are characterized by the loss of vascular barrier integrity. This study focused on determining which ADO receptor mediates the robust reduction in microvessel permeability to rat serum albumin (P(s)(RSA)) observed in juvenile female rats. In microvessels isolated from abdominal skeletal muscle, ADO suffusion induced a concentration-dependent reduction in arteriolar [log(IC(50)) = -9.8 +/- 0.2 M] and venular [log(IC(50)) = -8.4 +/- 0.2 M] P(s)(RSA). RT-PCR and immunoblot analysis demonstrated mRNA and protein expression of ADO A(1), A(2A), A(2B), and A(3) receptors in both vessel types, and immunofluorescence assay revealed expression of the four subtype receptors in the microvascular walls (endothelium and smooth muscle). P(s)(RSA) responses of arterioles and venules to ADO were blocked by 8-(p-sulphophenyl)theophylline, a nonselective A(1) and A(2) antagonist. An A(2A) agonist, CGS21680, was more potent than the A(1) agonist, cyclopentyladenosine, or the most-selective A(2B) agonist, 5'-(N-ethylcarboxamido)adenosine. The ability of CGS21680 or ADO to reduce P(s)(RSA) was abolished by the A(2A) antagonist, ZM241385. An adenylyl cyclase inhibitor, SQ22536, blocked the permeability response to ADO. In aggregate, these results demonstrate that, in juvenile females (before the production of the reproductive hormones), ADO enhances skeletal muscle arteriole and venule barrier function predominantly via A(2A) receptors using activation of adenylyl cyclase-signaling mechanisms.

Hog barn dust extract increases macromolecular efflux from the hamster cheek pouch

The purpose of this study was to determine whether short-term exposure to an aqueous extract of hog barn dust increases macromolecular efflux from the intact hamster cheek pouch and, if so, to begin to determine the mechanism(s) underlying this response. By using intravital microscopy, we found that suffusion of hog barn dust extract onto the intact hamster cheek pouch for 60 min elicited a significant, concentration-dependent leaky site formation and increase in clearance of FITC-labeled dextran (molecular mass, 70 kDa). This response was significantly attenuated by suffusion of catalase (60 U/ml), but not by heat-inactivated catalase, and by pretreatment with dexamethasone (10 mg/kg iv) (P < 0.05). Catalase had no significant effects on adenosine-induced increase in macromolecular efflux from the cheek pouch. Suffusion of hog barn dust extract had no significant effects on arteriolar diameter in the cheek pouch. Taken together, these data indicate that hog barn dust extract increases macromolecular efflux from the in situ hamster cheek pouch, in part, through local elaboration of reactive oxygen species that are inactivated by catalase. This response is specific and attenuated by corticosteroids. We suggest that plasma exudation plays an important role in the genesis of upper airway dysfunction evoked by short-term exposure to hog barn dust.

A comparison of adenosine A2A agonism and methylprednisolone in attenuating neuronal damage and improving functional outcome after experimental traumatic spinal cord injury in rabbits

Object

Steroid agents remain the lone pharmacological treatment in widespread use for acute spinal cord injury (SCI), although their utility remains in dispute in the neurotrauma literature. Adenosine A2A receptor activation with ATL-146e, a selective A2A agonist, has shown potential benefit in treating SCI; however, it has not been compared with the gold standard, methylprednisolone. The authors of this study evaluated ATL-146e and methylprednisolone for their ability to preserve neuronal viability and motor function in experimental SCI.

Methods

New Zealand White rabbits sustained SCI or sham injury via the Allen weight-drop technique. Ten minutes postinjury, animals received ATL-146e (ATL group, 0.06 μg/kg/min intravenously for 3 hours), methylprednisolone (steroid group, 30 mg/kg intravenously), or saline (trauma control group). Hindlimb motor function was recorded every 12 hours using the Tarlov motor grading scale (0, paralysis–5, normal hop). At 48 hours, fixed spinal cord tissue was evaluated for neuronal viability.

Hindlimb motor function in animals treated with ATL-146e was equivalent to that of sham-injured animals and was significantly better than that of trauma control animals at all time points and that of steroid-treated animals at 12 hours (p = 0.05). Motor function in steroid-treated animals was worse than in those given ATL-146e and better than that of trauma control animals at later time points, but was not statistically significant (both p > 0.05). Neuronal viability (measured in neurons/hpf) was significantly higher in both treatment groups compared with the trauma control group (12.1 ± 1.4 neurons/hpf for the ATL and 13.3 ± 1.4 neurons/hpf for the steroid group compared with 7.5 ± 1.5 neurons/hpf for the trauma control group; both p < 0.04). Neuronal viability did not differ among ATL-146e–treated, steroid-treated, and sham-injured groups.

Conclusions

The use of ATL-146e is at least as effective as methylprednisolone in preserving function and is equivalent to methylprednisolone in preserving the structure of spinal cord tissue after blunt SCI. Adenosine A2A receptor activation may be an effective treatment for acute SCI while avoiding the adverse effects of steroid agents.