U-37883A potently inhibits dopamine-modulated K+ channels on rat striatal neurons

Pharmacological Profile of U-37883A, a Channel Blocker of Smooth Muscle-Type ATP-Sensitive K Channels

U-37883A (PNU-37883A, guanidine; 4-morpholinecarboximidine-N-1-adamantyl-N'-cyclohexyl hydrochloride) was originally developed as a potential diuretic with specific binding in kidney and vascular smooth muscle rather than in brain or pancreatic beta cells. U-37883A inhibits ATP-sensitive K(+) channels (K(ATP) channels) in vascular smooth muscle at submicromolar concentrations whilst even at high concentrations (> or =10 microM) it has no inhibitory effect at pancreatic, cardiac or skeletal K(ATP) channels. Thus, it is generally thought that U-37883A is a selective inhibitor of vascular smooth muscle K(ATP) channels. Approximately one decade ago, K(ATP) channels were cloned and found to consist of at least two subunits: an inwardly-rectifying K(+) channel six family (K(ir)6.x; K(ir)6.1 and K(ir)6.2) which forms the ion conducting pore and a modulatory sulphonylurea receptor (SUR.x; SUR1, SUR2A, and SUR2B) that accounts for several pharmacological properties. It is generally believed that different combinations of K(ir)6.x and SUR.x determine the molecular properties of K(ATP) channels. Thus, K(ir)6.2/SUR1 channel represents the pancreatic beta-cell K(ATP) channel, K(ir)6.2/SUR2A channel is thought to represent the cardiac K(ATP) channel, whereas K(ir)6.1/SUR2B channel is likely to represent the vascular smooth muscle K(ATP) channel. Recent molecular studies have shown that U-37883A selectively suppresses the activity of recombinant K(ATP) channels which contain K(ir)6.1 subunits in the channel pore unit. It was thus thought that U-37883A was a selective pharmacological tool which could be used to investigate the activity of vascular smooth muscle K(ATP) channels. However, due to its multiple pharmacological actions on several ion channels and poor tissue selectivity, U-37883A should not be viewed as a selective blocker of smooth muscle K(ATP) channels.

Multiple actions of U-37883A, an ATP-sensitive K+ channel blocker, on membrane currents in pig urethra

The effects of U-37883A, a vascular ATP-sensitive K(+) channel (K(ATP) channel) blocker, on membrane currents were investigated in pig urethral myocytes by use of patch-clamp techniques (conventional whole-cell recordings, nystatin-perforated patches and cell-attached configuration). Tension measurement was also performed to study the effects of U-37883A on the levcromakalim-induced urethral relaxation and the urethral resting tone in the absence and presence of Bay K 8644. Although cumulative application of U-37883A produced a concentration-dependent inhibitory effect on the levcromakalim-induced urethral relaxation, U-37883A did not abolish the relaxation. In nystatin-perforated patch recording, K(ATP) currents activated by levcromakalim were inhibited by U-37883A in a concentration-dependent manner (K(i), 4.7 microM). Approximately 10% of the K(ATP) currents still remained even in the presence of 300 microM U-37883A. In cell-attached mode, extracellular application of U-37883A (100 microM) irreversibly inhibited the activity of the levcromakalim-induced K(ATP) channels. In whole-cell configuration, U-37883A suppressed the peak amplitude of voltage-dependent Ba(2+) currents in a concentration- and voltage-dependent manner, and at 30 microM, shifted the steady-state inactivation curve of the Ba(2+) currents to the left at -90 mV. These results demonstrate that U-37883A reduces not only the activities of K(ATP) channels but also voltage-dependent Ca(2+) channels. Therefore, it is not appropriate to define U-37883A as solely a vascular K(ATP) channel blocker.

Effects of U-37883A on intracellular Ca2+-activated large-conductance K+ channels in pig proximal urethral myocytes

Kinetic studies of U-37883A (4-morpholinecarboximidine-N-1-adamantyl-N'-cyclohexyl-hydrochloride), a vascular ATP-sensitive K+ channel (KATP channel) blocker, were performed on pig urethral myocytes to investigate inhibitory effects on large-conductance intracellular Ca2+ -sensitive K+ channels (i.e., BKCa channels; 225 pS K+ channels) by use of single-channel recordings (outside-out and inside-out configuration). BKCa channels in pig urethral smooth muscles showed extracellular iberiotoxin (300 nM) sensitivity and voltage dependency. The alpha subunit of BKCa channel proteins was detected in the membrane fraction by use of Western blot technique. Application of U-37883A (> or =10 microM) reduced the activity of BKCa channels in a concentration-dependent manner, not only by decreasing mean openlife time but also by prolonging the mean closed time. These results shows that U-37883A affects channels other than the vascular KATP channel, and demonstrates how it inhibits the activities of BKCa channels in urethral smooth muscles.

Hydroxyl-substituted sulfonylureas as potent inhibitors of specific [3H]glyburide binding to rat brain synaptosomes

We are seeking to discover potent CNS-active sulfonylureas with structural features that allow for the formation of several types of prodrugs. We report herein the syntheses of compounds comprising an initial series of hydroxyl-substituted analogues of the potent ATP-sensitive potassium channel blockers glyburide (glibenclamide) and gliquidone. Somewhat unexpectedly, several of the compounds were found to be comparably potent to glyburide as inhibitors of specific [(3)H]glyburide binding in rat brain preparations.

D2 receptors inhibit the secretory process downstream from calcium influx in dopaminergic neurons: implication of K+ channels

Dopaminergic (DAergic) neurons possess D2-like somatodendritic and terminal autoreceptors that modulate cellular excitability and dopamine (DA) release. The cellular and molecular processes underlying the rapid presynaptic inhibition of DA release by D2 receptors remain unclear. Using a culture system in which isolated DAergic neurons establish self-innervating synapses ("autapses") that release both DA and glutamate, we studied the mechanism by which presynaptic D2 receptors inhibit glutamate-mediated excitatory postsynaptic currents (EPSCs). Action-potential evoked EPSCs were reversibly inhibited by quinpirole, a selective D2 receptor agonist. This inhibition was slightly reduced by the inward rectifier K(+) channel blocker barium, largely prevented by the voltage-dependent K(+) channel blocker 4-aminopyridine, and completely blocked by their combined application. The lack of a residual inhibition of EPSCs under these conditions argues against the implication of a direct inhibition of presynaptic Ca(2+) channels. To evaluate the possibility of a direct inhibition of the secretory process, spontaneous miniature EPSCs were evoked by the Ca(2+) ionophore ionomycin. Ionomycin-evoked release was insensitive to cadmium and dramatically reduced by quinpirole, providing evidence for a direct inhibition of quantal release at a step downstream to Ca(2+) influx through voltage-dependent Ca(2+) channels. Surprisingly, this effect of quinpirole on ionomycin-evoked release was blocked by 4-aminopyridine. These results suggest that D2 receptor activation decreases neurotransmitter release from DAergic neurons through a presynaptic mechanism in which K(+) channels directly inhibit the secretory process.