BOVINE ADRENAL GLOMERULOSA CELLS EXPRESS SUCH A LOW-LEVEL OF FUNCTIONAL B-2 RECEPTORS THAT BRADYKININ DOES NOT SIGNIFICANTLY INCREASE THEIRALDOSTERONE PRODUCTION

It was recently demonstrated that bradykinin (BK) stimulates aldostero ne secretion in bovine adrenal glomerulosa (BAG) cells, The aim of the present study was to characterize the mechanism of action of BK on th ese cells, Binding experiments with the radioligand I-125-[Tyr(8)]BK r evealed the presence of a relatively small amount (B-max = 180 +/- 55 fmol/mg of protein) of high affinity (K-d = 0.65 +/- 0.17 nM) binding sites. BK induced a time- and concentration-dependent increase of [H-3 ]inositol trisphosphate ([H-3]IP3) in myo-[H-3]inositol-labeled BAG ce lls. A maximal response was obtained with 10 nM BK and the EC50 value was 1.0 +/- 0.5 nM. I-125-[Tyr(8)]BK binding and BK-induced IP3 produc tion were inhibited by the selective B-2 receptor-antagonist Icatibant (1 mu M) and unaffected by the selective B-1 receptor antagonist [Des Arg(9),Leu(8)]BK (1 mu M). In fura-2 loaded BAG cells, BK (100 nM) ind uced a typical biphasic Ca2+ response composed of a rapid and transien t increase of intracellular Ca2+ concentration [Ca2+](i) which slowly declined to a level that remained above basal level for about 5 min, I n the presence of EGTA (2 mM), the rapid and transient calcium increas e was unaffected whereas the plateau phase was abolished. Angiotensin II (Ang II, 100 nM) also elicited a typical biphasic response in BAG c ells. However the rapid and transient elevation of [Ca2+](i) was follo wed by a sustained plateau phase which remained above the basal level for more than 10 min. Although BAG cells ex-press functional B-2 recep tors, no secretion aldosterone was observed after stimulation with 100 nM BK for 120 min. Under the same conditions Ang II increased by abou t 10-fold the basal level of aldosterone. The lack of effect of BK is probably attributable to its very transient effect on IP3 production. Pretreatment of BAG cells with 100 nM BK for 20 min reduced by 70 +/- 10% their total binding capacity. These results suggest a rapid and ve ry efficient desensitization process, We conclude that BAG cells expre ss functional B-2 receptors. The weak production of second messengers and the rapid desensitization process could explain why BK fails to in crease aldosterone production in these cells. Since functional B-2 rec eptors are expressed in BAG cells it is likely that under some specifi c physiological or pathological conditions these receptors may play a significant role in aldosterone secretion, However these conditions re main to be determined.