Role of A1 receptors in renal sympathetic neurotransmission in the mouse kidney

Abstract

A(1) receptors may participate in renal sympathetic neurotransmission by enhancing the postjunctional effects of norepinephrine. The purpose of this study was to test this concept using A(1) receptor knockout (A(1)AR-/-) mice. In isolated kidneys from nontransgenic mice perfused with Tyrode's solution at a constant rate, renal sympathetic nerve stimulation (RSNS) increased (P < 0.0001) renal venous perfusate levels of inosine (adenosine metabolite) from 23.9 ± 3.7 to 32.7 ± 5.1, 68.2 ± 12.4, and 94.0 ± 14.3 ng/ml at 3, 5, and 7 Hz, respectively (n = 28), suggesting frequency-dependent production of adenosine. Conversely, RSNS decreased (P < 0.0001) renal venous perfusate levels of 5'-AMP (adenosine precursor) from 1.4 ± 0.3 to 1.1 ± 0.3, 0.80 ± 0.2, and 0.6 ± 0.2 ng/ml at 3, 5, and 7 Hz, respectively (n = 28), suggesting frequency-dependent increased metabolism of 5'-AMP. In kidneys from nontransgenic mice, blockade of adenosine receptors with 1,3-dipropyl-8-p-sulfophenylxanthine attenuated (P = 0.0130) vasoconstrictor responses to RSNS at 3, 5, and 7 Hz [control (n = 29): 22 ± 4, 34 ± 6, 42 ± 6 mmHg, respectively; 1,3-dipropyl-8-p-sulfophenylxanthine-treated (n = 11): 6 ± 1, 12 ± 3, 15 ± 3 mmHg, respectively]. In A(1)AR-/- kidneys (n = 10), vasoconstrictor responses to RSNS at 3, 5, and 7 Hz were 7 ± 3, 20 ± 5, and 36 ± 9 mmHg, respectively. In kidneys from wild-type littermates (n = 9), responses were 27 ± 9, 58 ± 14, and 59 ± 11 mmHg, respectively (effect of genotype: P = 0.0363). In kidneys from nontransgenic mice, 2-chloro-N(6)-cyclopentyladenosine (CCPA; highly selective A(1) receptor agonist) increased renal vasoconstriction induced by norepinephrine (P = 0.0008; n = 28). In kidneys from A(1)AR-/- the response to norepinephrine was attenuated and the ability of CCPA to enhance responses to norepinephrine was abolished. In conclusion, adenosine formed during RSNS enhances the postjunctional effects of released norepinephrine by activating A(1) receptors.

Fig. 1.

Line graphs show effects of renal sympathetic nerve stimulation (RSNS) on changes in renal perfusion pressure (A) and renal venous perfusate levels of 5′-AMP (B), adenosine (C), and inosine (D) in 28 kidneys from C57BL/6 mice. P values in panels are from repeated-measures 1-factor ANOVA; ^aP < 0.05 compared with basal value before RSNS [Fisher's least significant difference (LSD) test]. All values represent means ± SE.

Fig. 2.

Line graph illustrates changes in renal perfusion pressure in response to exogenous norepinephrine (NE) in 28 kidneys from C57BL/6 mice both before and during treatment with 2-chloro-N⁶-cyclopentyladenosine (CCPA; highly selective A₁ receptor agonist). P values in panels are from repeated-measures 2-factor ANOVA; ^aP < 0.05 compared with corresponding value in the absence of CCPA (Fisher's LSD test). All values represent means ± SE.

Fig. 3.

Line graph summarizes changes in renal perfusion pressure in response to RSNS in 2 sets of control C57BL/6 kidneys and in a group of C57BL/6 kidneys treated with 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX; 100 μmol/l; adenosine receptor antagonist that does not penetrate cell membranes). P values in panels are from nested 2-factor ANOVA comparing the 2nd control set to the DPSPX-treated group; ^aP < 0.05 compared with corresponding value in the 2nd control set (Fisher's LSD test). All values represent means ± SE.

Fig. 4.

Line graph shows changes in renal perfusion pressure in response to RSNS in homozygous A₁ receptor knockout (A₁AR−/−), littermate heterozygous (A₁AR−/+), and littermate wild-type (A₁AR+/+) kidneys. P values in panels are from nested 2-factor ANOVA comparing the 3 different genotypes; ^aP < 0.05 compared with corresponding value in the A₁AR+/+ group (Fisher's LSD test). All values represent means ± SE.

Fig. 5.

Top: bar graph compares concentration of NE necessary to achieve a target response (change in perfusion pressure of ∼30 mmHg) in A₁AR−/− vs. A₁AR+/+ kidneys. P value is for unpaired Student's t-test. Bottom: bar graph illustrates effects of CCPA (highly selective A₁ receptor agonist) on renovascular responses (changes in perfusion pressure) to exogenous NE in A₁AR−/− vs. A₁AR+/+ kidneys. P values in panel are from nested 2-factor ANOVA comparing the 2 different genotypes; ^aP < 0.05 compared with corresponding basal value (Fisher's LSD test). All values represent means ± SE.