α2‐Autoreceptor subclassification in rat isolated kidney by use of short trains of electrical stimulation

Rat kidneys were perfused with Krebs‐Henseleit solution and incubated with [3H]‐noradrenaline. The renal nerves were electrically stimulated at either 1 Hz for 30 s or 100 Hz for 0.06 s. The stimulation induced (S‐I) outflow of radioactivity was taken as an index of endogenous noradrenaline release. At a frequency of 1 Hz for 30 s the α‐adrenoceptor antagonists BRL 44408 (0.01, 0.1 μm) and imiloxan (0.1, 1.0 μm) enhanced S‐I outflow of radioactivity. However, at a frequency of 100 Hz for 0.06 s the α‐adrenoceptor antagonists, idazoxan (0.1, 1.0 μm), imiloxan (0.1, 1.0 μm), BRL 44408 (0.1, 1.0 μm), BRL 41992 (0.1, 1.0 μm) and prazosin (0.01 μm) failed to enhance S‐I outflow of radioactivity. Thus, the rat isolated kidney stimulated at 100 Hz for 0.06 s, avoids autoinhibition by endogenous noradrenaline and α‐adrenoceptor antagonist affinities (pKB) at the prejunctional α‐autoreceptor were estimated without disturbance by the endogenous activator. The α2‐adrenoceptor agonist, clonidine, inhibited the S‐I outflow of radioactivity with a maximum of 90% and an EC50 of 7.2 nm. All α‐adrenoceptor antagonists used caused parallel shifts of the concentration‐response curve for clonidine to the right. The rank order of potencies was: rauwolscine (α2A/B) > idazoxan (α2A/B) > phentolamine (α2A/B) > WB 4101 (α2A) > BRL 44408 (α2A) > BRL 41992 (α2B) > prazosin (α2B) = imiloxan (α2B). These data, when compared with binding, molecular and functional data of various other tissues and cell lines, indicate that prejunctional α2‐autoreceptors in rat kidney do not belong to the α2B‐ or α2C‐subtype. The α2‐autoreceptor of rat kidney seems to be of the α2A‐subtype. However, α2‐autoreceptor affinities of the present study correlate also well with binding affinities of the recently described α2D‐ligand binding site in bovine pineal gland.