ALPHA(2)-AUTORECEPTOR SUBCLASSIFICATION IN RAT ISOLATED KIDNEY BY USE OF SHORT TRAINS OF ELECTRICAL-STIMULATION

1 Rat kidneys were perfused with Krebs-Henseleit solution and incubated with [H-3]-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-1) outflow of radioactivity was taken as an index of endogenous noradrenaline release. 2 At a frequency of 1 Hz for 30 s the alpha-adrenoceptor antagonists BRL 44408 (0.01, 0.1 muM) and imiloxan (0.1, 1.0 muM) enhanced S-1 outflow of radioactivity. However, at a frequency of 100 Hz for 0.06 s the alpha-adrenoceptor antagonists, idazoxan (0.1, 1.0 muM), imiloxan (0.1, 1.0 muM), BRL 44408 (0. 1, 1.0 muM), BRL 41992 (0.1, 1.0 muM) and prazosin (0.01 muM) failed to enhance S-I outflow of radioactivity. 3 Thus, the rat isolated kidney stimulated at 100 Hz for 0.06 s, avoids autoinhibition by endogenous noradrenaline and alpha-adrenoceptor antagonist affinities (pK(B)) at the prejunctional alpha-autoreceptor were estimated without disturbance by the endogenous activator. 4 The alpha2-adrenoceptor agonist, clonidine, inhibited the S-1 outflow of radioactivity with a maximum of 90% and an EC50 of 7.2 nM. 5 All alpha-adrenoceptor antagonists used caused parallel shifts of the concentration-response curve for clonidine to the right. The rank order of potencies was: rauwolscine (alpha2A/B) > idazoxan (alpha2A/B) > phentolamine (alpha2A/B) > WB 4101 (alpha2A) > BRL 44408 (alpha2A) > BRL 41992 (alpha2B) > prazosin (alpha2B) = imiloxan (alpha2B). 6 These data, when compared with binding, molecular and functional data of various other tissues and cell lines, indicate that prejunctional alpha2-autoreceptors in rat kidney do not belong to the alpha2B- or alpha2C-subtype. The alpha2-autoreceptor of rat kidney seems to be of the alpha2A-subtype. However, alpha2-autoreceptor affinities of the present study correlate also well with binding affinities of the recently described alpha2D-ligand binding site in bovine pineal gland.