During the previous funding period we discovered that endogenous adenosine by activating A1 receptors is a major determinant of the renovascular response to renal sympathetic nerve stimulation (RSNS). This conclusion is based on our observations that selective A1-receptor antagonism in rats and A1-receptor deletion in mice suppresses renovascular responses to RSNS. Moreover, we discovered that there are three reasons A1 receptors importantly contribute to RSNS-induced renal vasoconstriction: 1) RSNS triggers adenosine formation; 2) Preglomerular microvessels express high levels of vasoconstrictor A1 receptors; and 3) In the renal vasculature, the Gi signaling pathway (which A1 receptors engage) converges with the Gq signaling pathway (which ?1-adrenoceptors engage) to trigger ?coincident signaling? at phospholipase C leading to augmentation by adenosine of the vasoconstrictor response to released norepinephrine (NE). Because ATP is released from noradrenergic varicosities, as well as from vascular smooth muscle and endothelial cells, the main precursor of adenosine in the sympathetic neuroeffector junction is likely ATP. CD39 catalyzes the metabolism of ATP to ADP and ADP to AMP, and CD73 metabolizes AMP to adenosine; thus these twin ecto-enzymes acting in tandem are considered the most important mechanism for producing extracellular adenosine from ATP. Surprisingly, however, our experiments show that neither pharmacological inhibition of CD39 nor genetic deletion of CD73 attenuates renovascular responses to RSNS. Instead, our preliminary findings suggest that inhibition of kidney non-specific alkaline phosphatase (KAP) markedly attenuates both renovascular responses to RSNS and adenosine release by RSNS. We hypothesize that renovascular KAP, rather than CD39/CD73, is necessary for the formation of the pool of adenosine that participates in RSNS-induced renal vasoconstriction. If our hypothesis is correct then KAP inhibition, knockdown (KAP-/+) or knockout (KAP-/-) should attenuate RSNS- and NE-induced renovascular responses and purine release. Moreover, our hypothesis predicts that A1-receptor stimulation should rescue (reverse) the suppression of renovascular responses to NE induced by KAP inhibition or knockdown/knockout. These predictions will be tested in Aims #1 and #2, respectively. We have strong preliminary evidence that coincident signaling between A1 receptors and NE not only enhances renal vasoconstriction but also induces the renovascular release of soluble KAP ? which could further enhance adenosine formation and potentiate renovascular responses to NE.
Aim #3 of this proposal will determine whether and how A1-receptor stimulation augments RSNS- or NE-induced release of soluble KAP. Using the world?s first A1-receptor knockout Dahl salt sensitive rat, our final aim (Aim #4) will ?put our hypothesis to work? by determining whether A1-receptor knockout and long-term KAP inhibition attenuate in vivo RSNS-induced renal vasoconstriction and chronically lower salt-induced hypertension and target organ damage.
Although current drugs can control most hypertension, there is a subgroup of hypertensive patients with resistant hypertension. In such patients, reducing renal sympathetic nerve activity using catheter-based renal denervation or baroreceptor activation therapy is promising; but technical problems remain. Current sympatholytic drugs cause system-wide inactivation of sympathetic neurotransmission leading to multiple adverse effects. An alternative approach is to develop drugs that attenuate ONLY renovascular sympathetic neurotransmission. It may be possible by blocking renovascular A1 receptors or inhibiting kidney alkaline phosphatase to attenuate sympathetic nerve stimulation-induced renal vasoconstriction while leaving systemic sympathetic neurotransmission intact, thus yielding safe, convenient, and effective drugs for resistant hypertension.
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