P-2-RECEPTOR MODULATION OF NORADRENERGIC NEUROTRANSMISSION IN RAT-KIDNEY

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 P-2-rec eptors which modulate noradrenaline release. Rat kidneys were perfused with Krebs-Henseleit solution containing the noradrenaline uptake blo ckers cocaine and corticosterone and the alpha(2)-adrenoceptor antagon ist rauwolscine. The renal nerves were electrically stimulated, in mos t experiments by 30 pulses applied at 1 Hz. The outflow of endogenous noradrenaline (or, in some experiments, of ATP and lactate dehydrogena se) as well as the perfusion pressure were measured simultaneously. 2 The P-2-receptor agonist adenosine-5'-O-(3-thiotriphosphate) (ATP gamm a S, 3-30 mu M) reduced the renal nerve stimulation (RNS)-induced outf low of noradrenaline (estimated EC50 = 8 mu M). The P-2-receptor antag onist cibacron blue 3GA (30 mu M) shifted the concentration-inhibition curve for ATP gamma S to the right (apparent pK(B) value 4.7). 3 Ciba cron blue 3GA (3-30 mu M) and its isomer reactive blue 2 (3-30 mu M) s ignificantly increased RNS-induced outflow of noradrenaline in the pre sence of the P-1-receptor antagonist 8-(p-sulphophenyl)theophylline (8 -SPT, 100 mu M) by about 70% and 90%, respectively. The P-2-receptor a ntagonist suramin (30-300 mu M) only tended to enhance RNS-induced out flow of noradrenaline. When the nerves were stimulated by short pulse trains consisting of 6 pulses applied at 100 Hz (conditions under whic h autoinhibition is inoperative), reactive blue 2 did not affect the R NS-induced outflow of noradrenaline. 4 RNS (120 pulses applied at 4 Hz ) induced the outflow of ATP but not of the cytoplasmatic enzyme lacta te dehydrogenase. 5 ATP gamma S (3-30 mu M) concentration-dependently reduced presser responses to RNS at 1 Hz. Cibacron blue 3GA, reactive blue 2 as well as suramin also reduced presser responses to RNS (maxim ally by 50 to 70%). 6 This study in rat isolated kidney, in which the release of endogenous noradrenaline was measured, demonstrates that re nal sympathetic nerves possess prejunctional P-2-receptors that mediat e inhibition of transmitter release. These prejunctional P-2-receptors are activated by endogenous ligands, most likely ATP, released upon n erve activity. Both, P-2-receptor agonists and P-2-receptor antagonist s reduced presser responses to RNS either by inhibiting transmitter re lease or by blocking postjunctional vasoconstrictor P-2-receptors.