P₂-receptor modulation of noradrenergic neurotransmission in rat kidney

Abstract

1. ATP has previously been shown to act as a sympathetic cotransmitter in the rat kidney. The present study analyses the question of whether postganglionic sympathetic nerve endings in the kidney possess P₂-receptors which modulate noradrenaline release. Rat kidneys were perfused with Krebs-Henseleit solution containing the noradrenaline uptake blockers cocaine and corticosterone and the α₂-adrenoceptor antagonist rauwolscine. The renal nerves were electrically stimulated, in most experiments by 30 pulses applied at 1 Hz. The outflow of endogenous noradrenaline (or, in some experiments, of ATP and lactate dehydrogenase) as well as the perfusion pressure were measured simultaneously.

2. The P₂-receptor agonist adenosine-5′-O-(3-thiotriphosphate) (ATPγS, 3–30 μM) reduced the renal nerve stimulation (RNS)-induced outflow of noradrenaline (estimated EC₅₀=8 μM). The P₂-receptor antagonist cibacron blue 3GA (30 μM) shifted the concentration-inhibition curve for ATPγS to the right (apparent pK_B value 4.7).

3. Cibacron blue 3GA (3–30 μM) and its isomer reactive blue 2 (3–30 μM) significantly increased RNS-induced outflow of noradrenaline in the presence of the P₁-receptor antagonist 8-(p-sulphophenyl)theophylline (8-SPT, 100 μM) by about 70% and 90%, respectively. The P₂-receptor antagonist suramin (30–300 μM) only tended to enhance RNS-induced outflow of noradrenaline. When the nerves were stimulated by short pulse trains consisting of 6 pulses applied at 100 Hz (conditions under which autoinhibition is inoperative), reactive blue 2 did not affect the RNS-induced outflow of noradrenaline.

4. RNS (120 pulses applied at 4 Hz) induced the outflow of ATP but not of the cytoplasmatic enzyme lactate dehydrogenase.

5. ATPγS (3–30 μM) concentration-dependently reduced pressor responses to RNS at 1 Hz. Cibacron blue 3GA, reactive blue 2 as well as suramin also reduced pressor responses to RNS (maximally by 50 to 70%).

6. This study in rat isolated kidney, in which the release of endogenous noradrenaline was measured, demonstrates that renal sympathetic nerves possess prejunctional P₂-receptors that mediate inhibition of transmitter release. These prejunctional P₂-receptors are activated by endogenous ligands, most likely ATP, released upon nerve activity. Both, P₂-receptor agonists and P₂-receptor antagonists reduced pressor responses to RNS either by inhibiting transmitter release or by blocking postjunctional vasoconstrictor P₂-receptors.