Abstract

Follicle-enclosed Xenopus oocytes were used to describe the ATP-sensitive K+ (KATP) channel-blocking properties of U-37883A (4-morpholinecarboximidine-N-1-adamantyl-N'-cyclohexyl), in comparison with glibenclamide. In follicular oocytes, the KATP channel opener P1060 (30 microM), a pinacidil analog, activated a large outward K+ current that was blocked by glibenclamide (IC50 = 0.33 microM) and U-37883A (IC50 = 0.26 microM). P1060 activation was inhibited by both U-37883A and glibenclamide in a noncompetitive manner. U-37883A also blocked the KATP channel activation by cAMP (300 microM) and adenosine (10 microM). Single-channel studies on isolated follicular cells showed that U-37883A (10 microM) reduced the open probability of the KATP channel by 76%, without significantly modifying the single-channel current amplitude. Receptor binding studies with [3H]U-37883 in membranes from follicle-enclosed oocytes demonstrated a single class of low affinity binding sites, with a Kd of 450 nM and a Bmax of 17 pmol/mg of protein. Studies with analogs of U-37883A showed that U-52090A inhibited KATP current and displaced [3H]U-37883 from its binding site with similar potencies. In contrast, U-42069D neither inhibited KATP current nor competed with [3H]U-37883 binding. In RINm5F cells (an insulinoma cell line), U-37883A, unlike glibenclamide, failed to inhibit KATP current. Furthermore, there was no significant specific binding of [3H]U-37883 in RINm5F cell membranes, which displayed high levels of specific binding of [3H]glibenclamide. These data demonstrate the presence of a receptor for U-37883A-type guanidines that controls the activity of the endogenous KATP channels in follicle-enclosed oocytes. The available data collectively suggest that U-37883A is a more selective blocker of the follicular KATP channel, which is very similar to that in smooth muscle, than of the pancreatic beta cell KATP channel.