Pharmacologic study of muscarinic receptor subtypes and arteriolar dilations: a comparison of conducted and local responses

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.

Remote arteriolar dilations caused by methacholine: a role for CGRP sensory nerves?

Remote vasodilation caused by arteriolar microapplication of acetylcholine cannot be completely attributed to passive cell-cell communication of a hyperpolarizing signal. The present study was undertaken to ascertain whether a neural component may be involved in the remote response. In the cheek pouch of anesthetized hamsters, methacholine (100 μM) was applied to the arteriole by micropipette for 5 s, and the arteriolar responses were measured at the site of application and at remote locations: 500 and 1,000 μm upstream from the application site. Superfusion with the local anesthetic bupivacaine attenuated a local dilatory response and abolished the conducted dilation response to methacholine. Localized micropipette application of bupivacaine 300 μm from the methacholine application site also attenuated the remote dilation but did not inhibit the local dilation. Blockade of neuromuscular transmission with botulinum neurotoxin A (1 U, 3 days), micropipette application of calcitonin gene-related peptide (CGRP) receptor inhibitor CGRP-(8–37) (10 μM) 300 μm upstream from the methacholine application site, and denervation of the CGRP sensory nerve by 2 days of capsaicin treatment reduced the conducted dilation response to methacholine but did not affect the local dilatory response. Together, these data support involvement of a TTX-insensitive nerve, specifically the CGRP containing nerve, in vascular communication. Understanding the effect of regulation of a novel neural network system on the vascular network may lead to a new insight into regulation of blood flow and intraorgan blood distribution.

Increased flow precedes remote arteriolar dilations for some microapplied agonists

This study asks which occurs first in time for remote responses: a dilation or a remote change in flow. Arteriolar diameter (approximately 20 microm) and fluorescently labeled red blood cell (RBC) velocity were measured in the cremaster muscle of anesthetized (pentobarbital sodium, 70 mg/kg) hamsters (n = 51). Arterioles were locally stimulated for 60 s with micropipette-applied 10 microg/ml LM-609 (alpha(v)beta(3)-integrin agonist), 10(-3) M adenosine, or 10(-3) M 3-morpholinosydnonimine (SIN-1, nitric oxide donor) as remote response agonists or with 10(-3) M papaverine, which dilates only locally. Observations were made at a remote site 1,200 microm upstream. With LM-609 or adenosine, the RBC velocity increased first (within 5 s), and the remote dilation followed 5-7 s later. N-nitro-L-arginine (100 microM) blocked the LM-609 (100%) and adenosine (60%) remote dilations. SIN-1 induced a concurrent remote dilation and decrease in RBC velocity (approximately 10 s), suggesting the primary signal was to dilate. Papaverine had no remote effects. This study suggests that, although remote responses to some agonists are induced by primary signals to dilate, additionally, network changes in flow can stimulate extensive remote changes in diameter.

Network vascular communication initiated by increases in tissue adenosine

Vascular communication of vasomotor signals appears to coordinate the distribution of tissue blood flow. This study was performed to determine whether elevated tissue concentrations of adenosine or nitric oxide could induce vascular communicating signals. To test this, remote arteriolar responses were tested when drugs were applied either directly to an arteriole ( approximately 20 microm diameter), or into the tissue in a region (with no vessels over 10 microm in diameter) that was 500 microm away from the arteriole and that bore no defined relationship to the flow path of the remote arteriole. In anesthetized hamster cheek pouch (n = 25), or cremaster muscle (n = 10), remote arteriolar responses were measured in response to nitric oxide (NO) donors (10(-5) to 10(-3) M), adenosine (10(-5) to 10(-3) M), or papaverine (10(-5) to 10(-2) M) applied for 40-120 s. Papaverine caused no remote response when applied directly while adenosine and NO donors caused similar, late-onset (10-20 s), dose-dependent, remote responses in both preparations. Remarkably however, only adenosine initiated a consistent remote arteriolar dilation when applied to the tissue site. Thus, increases in tissue adenosine may be critical for vascular communication of metabolic demands without regard to the specific blood flow path.