Mechanism of wortmannin-induced inhibition of secretory responses in rat adrenal medullary cells.

The effect of wortmannin (WT), an inhibitor of myosin light chain kinase (MLCK) as well as PI3-kinase, on catecholamine (CA) secretion in the perfused rat adrenal medulla was studied. After a 35-min application of 10 microM WT, secretory response to repetitive stimulation with 30 mM extracellular K+ was reduced to 30% of that obtained with intact medullae, and the response to 200 nM bradykinin (BK) was almost completely abolished. Aiming to identify the target for the WT effect, the WT derivative KT7692, which retains the same potency to inhibit MLCK as that of WT but its potency to PI3-kinase is one-hundredth that of WT, was used. Unlike WT, KT7692 at 10 microM did not affect the high-K+-evoked secretion and slightly potentiated the BK-evoked secretion. These results oppose the notion that WT inhibits the secretory responses through inhibition of MLCK. However, the alternative idea, that PI3-kinase is a target for WT, is also difficult to accept since WT concentrations required for the inhibition of the secretions are much higher than those needed to inhibit PI-3-kinase by WT.