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.