Effects of endogenous and synthetic prostanoids, the thromboxane A2 receptor agonist U-46619 and arachidonic acid on [3H]-noradrenaline release and vascular tone in rat isolated kidney.

1. Rat kidneys were perfused with Krebs-Henseleit solution and the perfusion pressure was monitored. After incubation with [3H]-noradrenaline the renal nerves were stimulated. The stimulation-induced (S-I) outflow of radioactivity was taken as an index of noradrenaline release. The effect of prostaglandins on perfusion pressure, pressor responses to renal nerve stimulation (RNS) and S-I outflow of radioactivity was assessed. 2. Prostaglandin E2 (PGE2, 0.06 and 0.6 microM), PGF2 alpha (0.6 microM), PGI2 (0.6 and 3 microM) and iloprost (0.6 microM) increased perfusion pressure and enhanced pressor responses to RNS. These facilitatory effects of the prostaglandins were not a result of an enhanced transmitter release. In contrast, PGE2 dose-dependently inhibited, whereas the other prostaglandins failed to modulate S-I outflow of radioactivity. PGE2 (0.6 microM) also enhanced pressor responses to exogenous noradrenaline. 3. Arachidonic acid (1 microM) increased perfusion pressure and enhanced pressor responses to RNS. These effects were abolished in the presence of indomethacin (10 microM) suggesting that local production of prostaglandins from exogenous arachidonic acid was responsible for this facilitation. However, arachidonic acid (1 microM) did not modulate S-I outflow of radioactivity. Arachidonic acid (10 microM), despite causing a marked increase in perfusion pressure, failed to alter pressor responses to RNS and only slightly inhibited S-I outflow of radioactivity. 4. The thromboxane A2 (TxA2) receptor agonist U-46619 (0.1 microM) increased vascular tone and enhanced pressor responses to RNS. These effects were blocked by the newly developed selective TxA2 receptor antagonist, daltroban (BM 13505; 3 microM), suggesting that these facilitatory effects of U-46619 were due to activation of TxA2 receptors. However, U-46619 failed to alter the S-I outflow of radioactivity from rat isolated kidney. 5. The alpha 1-adrenoceptor agonist methoxamine (1 microM) also increased perfusion pressure and enhanced pressor responses to RNS without affecting the S-I outflow of radioactivity in the presence of the prostaglandin synthesis inhibitor indomethacin (10 microM). 6. The results suggest that PGE2 modulates noradrenaline release through an inhibitory prejunctional receptor mechanism. There is no evidence for prejunctional PGF2 alpha, PGI2 or TxA2 receptors in the rat isolated kidney. All prostaglandins increased vascular tone in the rat isolated kidney and this alone may provide a condition for enhanced pressor responses to RNS since methoxamine also enhanced pressor responses to RNS without affecting S-I outflow of radioactivity. It is probable that postjunctionally active PGF2 and PGI2 is formed locally from exogenous arachidonic acid, but not enough prejunctionally active PGE2 is synthesized to modulate renal transmitter release.