Renal haemodynamic responses to exogenous and endogenous adenosine in conscious dogs

Afferent arteriolar vasodilator effect of adenosine predominantly involves adenosine A2B receptor activation

Adenosine is an important paracrine agent regulating renal vascular tone via adenosine A(1) and A(2) receptors. While A(2B) receptor message and protein have been localized to preglomerular vessels, functional evidence on the role of A(2B) receptors in mediating the vasodilator action of adenosine on afferent arterioles is not available. The present study determined the role of A(2B) receptors in mediating the afferent arteriolar dilation and compared the effects of A(2B) and A(2A) receptor blockade on afferent arterioles. We used the rat in vitro blood-perfused juxtamedullary nephron technique combined with videomicroscopy. Single afferent arterioles of Sprague-Dawley rats were visualized and superfused with solutions containing adenosine or adenosine A(2) receptor agonist (CV-1808) along with adenosine A(2B) and A(2A) receptor blockers. Adenosine (10 micromol/l) caused modest constriction and subsequent superfusion with SCH-58261 (SCH), an A(2A) receptor blocker, at concentrations up 10 micromol/l elicited only slight additional decreases in afferent arteriolar diameter with maximum effect at a concentration of 1 micromol/l (-11.0 +/- 2.5%, n = 6, P < 0.05). However, superfusion of adenosine-treated vessels with MRS-1754 (MRS), an A(2B) receptor blocker, elicited greater decreases in afferent arteriolar diameter (-26.0 +/- 4.7%, n = 5, P < 0.01). SCH did not significantly augment the adenosine-mediated afferent constriction elicited by MRS; however, adding MRS after SCH caused further significant vasoconstriction. Superfusion with CV-1808 dilated afferent arterioles (17.2 +/- 2.4%, n = 6, P < 0.01). This effect was markedly attenuated by MRS (-22.6 +/- 2.0%, n = 5, P < 0.01) but only slightly reduced by SCH (-9.0 +/- 1.1%, n = 5, P < 0.05) and completely prevented by adding MRS after SCH (-24.7 +/- 1.8%, n = 5, P < 0.01). These results indicate that, while both A(2A) and A(2B) receptors are functionally expressed in juxtamedullary afferent arterioles, the powerful vasodilating action of adenosine predominantly involves A(2B) receptor activation, which counteracts A(1) receptor-mediated vasoconstriction.

Failure to upregulate the adenosine2A receptor-epoxyeicosatrienoic acid pathway contributes to the development of hypertension in Dahl salt-sensitive rats

Adenosine-activated renovascular dilatation in Sprague-Dawley (SD) rats is mediated by stimulating adenosine(2A) receptors (A(2A)R), which is linked to epoxyeicosatrienoic acid (EET) synthesis. The A(2A)R-EET pathway is upregulated by high salt (HS) intake in normotensive SD rats. Because this pathway is antipressor, we examined the role of the A(2A)R-EET pathway in Dahl salt-sensitive (SS) rats. Male Dahl salt-resistant (SR) and SS rats were fed either HS (8.0% NaCl) or normal salt (NS; 0.4% NaCl) diet for 7 days. On day 8, isolated kidneys were perfused with Krebs-Henseleit buffer containing indomethacin and N(G)-nitro-l-arginine methyl ester and preconstricted with phenylephrine. Bolus injections of the stable adenosine analog 2-chloroadenosine (2-CA; 0.1-20 microg) elicited dose-dependent dilation in both Dahl SR and SS rats. Dahl SR rats fed a HS diet demonstrated a greater renal vasodilator response to 10 microg of 2-CA, as measured by the reduction in renal perfusion pressure, than that of Dahl SR rats fed a NS diet (-104 +/- 6 vs. -77 +/- 7 mmHg, respectively; P < 0.05). In contrast, Dahl SS rats did not exhibit a difference in the vasodilator response to 2-CA whether fed NS or HS diet (96 +/- 6 vs. 104 +/- 13 mmHg in NS- and HS-fed rats, respectively). In Dahl SR but not Dahl SS rats, HS intake significantly increased purine flux, augmented the protein expression of A(2A)R and the cytochrome P-450 2C23 and 2C11 epoxygenases, and elevated the renal efflux of EETs. Thus the Dahl SR rat is able to respond to HS intake by recruiting EET formation, whereas the Dahl SS rat appears to have exhausted its ability to increase EET synthesis above the levels observed on NS intake, and this inability of Dahl SS rats to upregulate the A(2A)R-EET pathway in response to salt loading may contribute to the development of salt-sensitive hypertension.

Adenosine A2 receptor activation attenuates afferent arteriolar autoregulation during adenosine receptor saturation in rats

Adenosine is an important paracrine agent regulating renal hemodynamics via adenosine A1 and A2 receptors. To determine the interactions between adenosine A1 and A2 receptors and the possible role of adenosine as a modulator of afferent arteriolar autoregulatory responses, videomicroscopic measurements of afferent arteriolar dimensions were performed at different perfusion pressures (from 100 to 125 and 150 mm Hg) using the isolated-blood-perfused rat juxtamedullary nephron preparation. Single afferent arterioles were visualized and superfused with low or high concentrations of adenosine, either alone or with the adenosine A1 receptor antagonist 8-noradamantan-3-yl-1,3-dipropylxanthine (10 micromol/L) or the adenosine A2 receptor antagonist dimethyl-1-propargylxanthine (10 micromol/L). Adenosine (20 micromol/L) decreased afferent arteriolar diameter by -9.0+/-0.9%, and this effect was enhanced by dimethyl-1-propargylxanthine (10 micromol/L) to -16.1+/-1.2%. However, autoregulatory capability was maintained. Adenosine-induced vasoconstriction was prevented by 8-noradamantan-3-yl-1,3-dipropylxanthine (10 micromol/L) with diameter increasing by 9.6+/-1.2%. Adenosine receptor saturation with a high concentration of adenosine (120 micromol/L) or blocking A1 receptors with 8-noradamantan-3-yl-1,3-dipropylxanthine in the presence of adenosine resulted in marked vasodilation and marked impairment of autoregulatory responses to increases in perfusion pressure (-1.5+/-1.1% and -3.5+/-0.9%). However, afferent arteriolar autoregulatory responses to elevations in perfusion pressure were restored after blockade of A2 receptors alone or in combination with A1 receptor blockade. During treatment with dimethyl-1-propargylxanthine in the presence of adenosine receptor saturation (120 micromol/L), afferent arteriolar autoregulatory responses were intact (-16.5+/-1.6% and -26.4+/-2.1%). These results indicate that the interactions between adenosine A1 and A2 receptors exert important modulatory influences on afferent arteriolar tone and autoregulatory capability. Activation of A2 receptors abrogates the counteracting influences of A1 receptors leading to marked vasodilation and decreased afferent arteriolar autoregulatory efficiency.

Adenosine and Kidney Function

In this review we outline the unique effects of the autacoid adenosine in the kidney. Adenosine is present in the cytosol of renal cells and in the extracellular space of normoxic kidneys. Extracellular adenosine can derive from cellular adenosine release or extracellular breakdown of ATP, AMP, or cAMP. It is generated at enhanced rates when tubular NaCl reabsorption and thus transport work increase or when hypoxia is induced. Extracellular adenosine acts on adenosine receptor subtypes in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate (GFR) by constricting afferent arterioles, especially in superficial nephrons, and acts as a mediator of the tubuloglomerular feedback, i.e., a mechanism that coordinates GFR and tubular transport. In contrast, it leads to vasodilation in deep cortex and medulla. Moreover, adenosine tonically inhibits the renal release of renin and stimulates NaCl transport in the cortical proximal tubule but inhibits it in medullary segments including the medullary thick ascending limb. These differential effects of adenosine are subsequently analyzed in a more integrative way in the context of intrarenal metabolic regulation of kidney function, and potential pathophysiological consequences are outlined.

Evidence that the renin decrease during hypoxia is adenosine mediated in conscious dogs

This study investigated whether adenosine mediates the decrease in plasma renin activity (PRA) during acute hypoxia. Eight chronically tracheotomized, conscious beagle dogs were kept under standardized environmental conditions and received a low-sodium diet (0.5 mmol.kg body wt(-1).day(-1)). During the experiments, the dogs were breathing spontaneously via a ventilator circuit: first hour, normoxia (21% inspiratory concentration of O(2)); second and third hours, hypoxia (10% inspiratory concentration of O(2)). Each of the eight dogs was studied twice in randomized order in control and theophylline experiments. In theophylline experiments, theophylline, an A(1)-receptor antagonist, was infused intravenously during hypoxia (loading dose: 3 mg/kg within 30 min, maintenance: 0.5 mg. kg(-1). h(-1)). In theophylline experiments, PRA (5.9 +/- 0.8 ng ANG I. ml(-1). h(-1)) and ANG II plasma concentration (15.9 +/- 2.3 pg/ml) did not decrease during hypoxia, whereas plasma aldosterone concentration decreased from 277 +/- 63 to 132 +/- 23 pg/ml (P < 0.05). In control experiments, PRA decreased from 6.8 +/- 0.8 during normoxia to 3.0 +/- 0.5 ng ANG I. ml(-1). h(-1) during hypoxia, ANG II decreased from 13.3 +/- 1.9 to 7.3 +/- 1.9 pg/ml, and plasma aldosterone concentration decreased from 316 +/- 50 to 70 +/- 13 pg/ml (P < 0.05). Thus infusion of the adenosine receptor antagonist theophylline inhibited the suppression of the renin-angiotensin system during acute hypoxia. The decrease in aldosterone occurred independently and is apparently directly related to hypoxia. In conclusion, it is likely that adenosine mediates the decrease in PRA during acute hypoxia in conscious dogs.

Effect of the selective adenosine A1-receptor antagonist KW-3902 on lipopolysaccharide-induced reductions in urine volume and renal blood flow in anesthetized dogs

We investigated the effects of KW-3902 (8-noradamantan-3-yl-1,3-dipropylxanthine), a potent and selective adenosine A1-receptor antagonist, on lipopolysaccharide (LPS)-induced reduction of urine volume (UV) in anesthetized dogs, in comparison with those of furosemide. LPS was intravenously administered at a dose of 0.5 mg/kg; and the heart rate (HR), systemic blood pressure (BP), renal blood flow (RBF) and UV were measured every 15 min for 4 h. Administration of LPS continuously decreased HR, BP, RBF and UV. KW-3902, furosemide or their corresponding vehicle was given as a bolus injection 5 min after the LPS injection. Treatment with KW-3902 (1 mg/kg, i.v.) ameliorated the LPS-induced decline of UV and RBF. Furosemide (3.2 mg/kg, i.v.) tended to ameliorate the LPS-induced decline of UV but not RBF, the duration of the effect being shorter than that of KW-3902. These results suggest that KW-3902 can ameliorate the oliguria and the decrease in RBF during the early phase of LPS-induced shock. Endogenous adenosine may be involved in the endotoxin-induced oliguria via the adenosine A1-receptor.