Dual action of ZD6169, a novel K(+) channel opener, on ATP-sensitive K(+) channels in pig urethral myocytes

ATP-sensitive K+ channels in pig urethral smooth muscle cells are heteromultimers of Kir6.1 and Kir6.2

The inwardly rectifying properties and molecular basis of ATP-sensitive K(+) channels (K(ATP) channels) have now been established for several cell types. However, these aspects of nonvascular smooth muscle K(ATP) channels still remain to be defined. In this study, we investigated the molecular basis of the pore of K(ATP) channels of pig urethral smooth muscle cells through a comparative study of the inwardly rectifying properties, conductance, and regulation by PKC of native and homo- and heteroconcatemeric recombinant Kir6.x channels coexpressed with sulfonylurea receptor subunit SUR2B in human embryonic kidney (HEK) 293 cells by the patch-clamp technique (conventional whole-cell and cell-attached modes). In conventional whole-cell clamp recordings, levcromakalim (> or = 1 microM) caused a concentration-dependent increase in current that demonstrated strong inward rectification at positive membrane potentials. In cell-attached mode, the unitary amplitude of levcromakalim-induced native and recombinant heteroconcatemeric Kir6.1-Kir6.2 K(ATP) channels also showed strong inward rectification at positive membrane potentials. Phorbol 12,13-dibutyrate, but not the inactive phorbol ester, 4alpha-phorbol 12,13-didecanoate, enhanced the activity of native and heteroconcatemeric K(ATP) channels at -50 mV. The conductance of the native channels at approximately 43 pS was consistent with that of heteroconcatemeric channels with a pore-forming subunit composition of (Kir6.1)(3)-(Kir6.2). RT-PCR analysis revealed the expression of Kir6.1 and Kir6.2 transcripts in pig urethral myocytes. Our findings provide the first evidence that the predominant K(ATP) channel expressed in pig urethral smooth muscle possesses a unique, heteromeric pore structure that differs from the homomeric Kir6.1 channels of vascular myocytes and is responsible for the differences in inward rectification, conductance, and PKC regulation exhibited by the channels in these smooth muscle cell types.

Molecular determinants for the activating/blocking actions of the 2H-1,4-benzoxazine derivatives, a class of potassium channel modulators targeting the skeletal muscle KATP channels

The 2H-1,4-benzoxazine derivatives are modulators of the skeletal muscle ATP-sensitive-K(+) channels (K(ATP)), activating it in the presence of ATP but inhibiting it in the absence of nucleotide. To investigate the molecular determinants for the activating/blocking actions of these compounds, novel molecules with different alkyl or aryl-alkyl substitutes at position 2 of the 1,4-benzoxazine ring were prepared. The effects of the lengthening of the alkyl chain and of branched substitutes, as well as of the introduction of aliphatic/aromatic rings on the activity of the molecules, were investigated on the skeletal muscle K(ATP) channels of the rat, in excised-patch experiments, in the presence or absence of internal ATP (10(-4) M). In the presence of ATP, the 2-n-hexyl analog was the most potent activator (DE(50) = 1.08 x 10(-10) M), whereas the 2-phenylethyl was not effective. The rank order of efficacy of the openers was 2-n-hexyl > or =2-cyclohexylmethyl >2-isopropyl = 2-n-butyl > or = 2-phenyl > or = 2-benzyl = 2-isobutyl analogs. In the absence of ATP, the 2-phenyl analog was the most potent inhibitor (IC(50) = 2.5 x 10(-11) M); the rank order of efficacy of the blockers was 2-phenyl > or = 2-n-hexyl > 2-n-butyl > 2-cyclohexylmethyl, whereas the 2-phenylethyl, 2-benzyl, and 2-isobutyl 1,4-benzoxazine analogs were not effective; the 2-isopropyl analog activated the K(ATP) channel even in the absence of nucleotide. Therefore, distinct molecular determinants for the activating or blocking actions for these compounds can be found. For example, the replacement of the linear with the branched alkyl substitutes at the position 2 of the 1,4-benzoxazine nucleus determines the molecular switch from blockers to openers. These compounds were 100-fold more potent and effective as openers than other KCO against the muscle K(ATP) channels.

Different glibenclamide-sensitivity of ATP-sensitive K+ currents using different patch-clamp recording methods

Electorophysiological and pharmacological properties of the levcromakalim-induced inward ATP-sensitive K+ currents (K(ATP) currents) in pig proximal urethra were investigated by use of two different whole-cell patch-clamp techniques, namely conventional whole-cell and nystatin-perforated patch recordings. In conventional whole-cell configuration, the levcromakalim (100 microM)-induced K(ATP) current decayed by about 30% in 8 min at a holding potential of -50 mV. In contrast, with the nystatin-perforated patch, 96% of the levcromakalim-induced K(ATP) current still remained even after 8 min application of levcromakalim. The peak amplitude of the levcromakalim-induced inward K(ATP) currents in nystatin-perforated patch was approximately half of those observed in conventional whole-cell configuration. When cytosolic extract of pig urethra was included in the pipette solution, approximately 90% of the levcromakalim (100 microM)-induced K(ATP) current remained at 8 min, even after the establishment of conventional whole-cell configuration. In conventional whole-cell configuration, glibenclamide suppressed the levcromakalim-induced K(ATP) currents in a concentration-dependent manner (Ki=175 nM). Inclusion of 1 mM uridine 5'-diphosphate (UDP) in the pipette solution shifted the glibenclamide-sensitivity (Ki=640 nM) to the right in comparison with that in the absence of UDP (i.e., control). In contrast, using nystatin-perforated patch, glibenclamide inhibited the levcromakalim-induced K(ATP) currents with two affinity sites (high-affinity site, Ki1=10 nM; low-affinity site, Ki2=9 microM). The concentration response curves regarding the inhibitory effects of K(ATP) channel pore blockers (Ba2+ and flecainide) on the levcromakalim-induced K(ATP) currents in conventional whole-cell recording nearly overlapped with those in nystatin-perforated patch recording. These results indicate that the glibenclamide-sensitivity of pig urethral K(ATP) channels in nystatin-perforated patch recording was significantly different from that in a conventional whole-cell configuration, and that the glibenclamide-sensitivity may be modified by some cytosolic factor(s).

The effects of MCC-134 on the ATP-sensitive K+ channels in pig urethra

We have investigated the effects of MCC-134 (1-[4-(1H-imidazol-1-yl)benzoyl]-N-methyl-cyclobutanecarbothioamide) on membrane currents and ATP-sensitive K(+) channel (K(ATP) channel) opener-induced currents in pig urethra by use of patch-clamp techniques (conventional whole-cell configuration and nystatin perforated patch recordings). Tension measurement was also performed to study the effects of MCC-134 on the resting tone of pig urethral strips. MCC-134 reduced the resting tone of pig urethra in a concentration-dependent manner (EC(50)=6 microM). The MCC-134 (30 microM)-induced relaxation was suppressed by glibenclamide. In voltage-clamp experiments, MCC-134 produced a concentration-dependent inward K(+) current which was suppressed by application of glibenclamide at a holding potential of -50 mV (symmetrical 140 mM K(+) conditions). Application of MCC-134 enhanced diazoxide-induced inward currents and inhibited pinacidil-induced inward currents in a concentration-dependent manner at -50 mV. These results suggest that MCC-134 induces glibenclamide-sensitive K(ATP) currents in pig urethra.

Blocking actions of glibenclamide on ATP-sensitive K+ channels in pig urethral myocytes

The inhibitory effects of glibenclamide on the levcromakalim-induced ATP-sensitive K+ (K(ATP)) channels were investigated with cell-attached configuration in pig proximal urethra. Application of 10 microM glibenclamide reversibly inhibited the activity of the 100 microM levcromakalim-induced K(ATP) channel, decreasing not only the channel open probability but also the amplitude of unitary current. The inhibitory concentration-response curve of the glibenclamide-induced sublevel conductance of K(ATP) channel was shifted to the right (IC50 = 4.7 microM), compared with the levcromakalim-induced K(ATP) channel (full conductance, IC50 = 0.5 microM). Glibenclamide is the first reported sulphonylurea to selectively block K(ATP) channel, not only by decreasing the channel activity but also by reducing the unitary amplitude in smooth muscle.

Multiple effects of mefenamic acid on K(+) currents in smooth muscle cells from pig proximal urethra

The effects of mefenamic acid on both membrane potential and K+ currents in pig urethral myocytes were investigated using patch-clamp techniques (conventional whole-cell, cell-attached, outside-out and inside-out configuration). In the current-clamp mode, mefenamic acid caused a concentration-dependent hyperpolarization, which was inhibited by preapplication of 1 microm glibenclamide. In the voltage-clamp mode, mefenamic acid induced an outward current that was blocked by glibenclamide even in the presence of iberiotoxin (IbTX, 300 nm) at -50 mV. ATP-sensitive K+ channels (KATP channels) could be activated in the same patch by mefenamic acid and levcromakalim, with the same unitary amplitude and the similar opening gating at -50 mV in cell-attached configuration. In outside-out recording, external application of mefenamic acid activated intracellular Ca2+-activated IbTX-sensitive large-conductance K+ channels (BKCa channels). Mefenamic acid (<or=30 microm) activated spontaneous transient outward currents (STOCs). In contrast, mefenamic acid (>or=100 microm) increased sustained outward currents, diminishing the activity of STOCs. Over the whole voltage range, mefenamic acid caused opposite effects on the membrane currents in the absence and presence of 5 microm glibenclamide. In the presence of 10 mm 4-aminopyridine (4-AP), mefenamic acid only increased the outward currents. These results indicate that mefenamic acid increases the channel activities of two distinct types of K+ channels (i.e. BKCa channels and KATP channels) and decreased 4-AP-sensitive K+ channels in pig urethral myocytes.

Functional involvement of sulphonylurea receptor (SUR) type 1 and 2B in the activity of pig urethral ATP-sensitive K+ channels

(1) We have investigated the possible roles of sulphonylurea receptor (SUR) type 1 and 2B in the activity of pig urethral ATP-sensitive K(+) channels (K(ATP) channels) by use of patch-clamp and reverse transcriptase-polymerase chain reaction (RT-PCR) techniques. (2) In voltage-clamp experiments, not only diazoxide, a SUR1 and weak SUR2B activator, but also pinacidil, a selective SUR2 activator, caused an inward current at a holding potential of -50 mV in symmetrical 140 mM K(+) conditions. (3) Gliclazide (</=1 micro M), a selective SUR1 blocker, inhibited the 10 micro M pinacidil-induced currents (K(i)=177 micro M) and the 500 micro M diazoxide-induced currents (high-affinity site, K(i1)=5 nM; low-affinity site, K(i2)=108 micro M) at -50 mV. (4) Application of tolbutamide (</=100 micro M) reversibly caused an inhibition of the 500 micro M diazoxide-induced current at -50 mV. (5) MCC-134, a SUR type-specific K(ATP) channel regulator (1-100 micro M), produced a concentration-dependent inward K(+) current, which was suppressed by the application of glibenclamide at -50 mV. The amplitude of the MCC-134 (100 micro M)-induced current was approximately 50% of that of the 100 micro M pinacidil-induced currents. (6) Using cell-attached configuration, MCC-134 activated a glibenclamide-sensitive K(ATP) channel which was also activated by pinacidil. (7) RT-PCR analysis demonstrated the presence of SUR1 and SUR2B transcripts in pig urethra. 8 These results indicate that both SUR1 and SUR2B subunits play a functional role in regulating the activity of pig urethral K(ATP) channels and that SUR1 contributes less than 25% to total K(ATP) currents.

[Molecular and electrophysiological investigation of ATP-sensitive K+ channels in lower urinary tract function: the aims for clinical treatment of unstable detrusor]

It is a common sequelae of bladder outlet obstruction caused by benign prostatic hyperplasia in adult males and gives rise to significant bladder dysfunction such as frequency and urgency of micturition. The unstable detrusor contractions may lead to urge incontinence. Since it has been reported that experimentally-induced bladder instability can be abolished by ATP-sensitive K+ channel (KATP channel) openers, various types of detrusor-selective KATP channels have been newly synthesized, targeting KATP channels in urinary bladder. Thus, the significant differences in molecular and pharmacological properties of KATP channels between urinary bladder and urethra hold out some hope for the development of tissue-selective KATP channel openers for urge urinary incontinence, and detrusor-selective KATP channel openers should be screened against urethral as well as vascular smooth muscle. In functional expression experiments, pharmacological and electrophysiological studies have reported that SUR1/Kir6.2 represents the pancreatic beta-cell KATP channel and that SUR2A/Kir6.2 is thought to represent the cardiac KATP channel, whereas SUR2B/Kir6.1 represents the smooth muscle-type KATP channel. In general, the smooth muscle type-KATP channel is (i) of a relatively small conductance (about 20 pS under quasi-physiological conditions, approximately 40 pS in symmetrical 140 mM K+ conditions), (ii) intracellular Ca(2+)-insensitive, (iii) inhibited by intracellular ATP, (iv) abolished by glibenclamide at a submicromolar concentration, and (v) reactivated by intracellular nucleoside diphosphates (NDPs). There has been no report concerning the properties of KATP channels in human detrusor by use of single-channel recordings. We would like to introduce our recent evidence of novel synthesized detrusor-selective KATP channel openers and properties of KATP channels in the lower urinary tract.

Dualistic actions of cromakalim and new potent 2H-1,4-benzoxazine derivatives on the native skeletal muscle K ATP channel

1 New 2H-1,4-benzoxazine derivatives were synthesized and tested for their agonist properties on the ATP-sensitive K(+) channels (K(ATP)) of native rat skeletal muscle fibres by using the patch-clamp technique. The novel modifications involved the introduction at position 2 of the benzoxazine ring of alkyl substituents such as methyl (-CH(3)), ethyl (-C(2)H(5)) or propyl (-C(3)H(7)) groups, while maintaining pharmacophore groups critical for conferring agonist properties. 2 The effects of these molecules were compared with those of cromakalim in the presence or absence of internal ATP (10(-4) M). In the presence of internal ATP, all the compounds increased the macropatch K(ATP) currents. The order of potency of the molecules as agonists was -C(3)H(7) (DE(50)=1.63 x 10(-8) M) >-C(2)H(5) (DE(50)=1.11 x 10(-7) M)>-CH(3) (DE(50)=2.81 x 10(-7) M)>cromak-slim (DE(50)= 1.42 x 10(-5) M). Bell-shaped dose-response curves were observed for these compounds and cromakalim indicating a downturn in response when a certain dose was exceeded. 3 In contrast, in the absence of internal ATP, all molecules including cromakalim inhibited the K(ATP) currents. The order of increasing potency as antagonists was cromakalim (IC(50)=1.15 x 10(-8) M)> or =-CH(3) (IC(50)=2.6 x 10(-8) M)>-C(2)H(5) (IC(50)=4.4 x 10(-8) M)>-C(3)H(7) (IC(50)=1.68 x 10(-7) M) derivatives. 4 These results suggest that the newly synthesized molecules and cromakalim act on muscle K(ATP) channel by binding on two receptor sites that have opposite actions. Alternatively, a more simple explanation is to consider the existence of a single site for potassium channel openers regulated by ATP which favours the transduction of the channel opening. The alkyl chains at position 2 of the 2H-1,4-benzoxazine nucleus is pivotal in determining the potency of benzoxazine derivatives as agonists or antagonists.

The involvement of L-type Ca(2+) channels in the relaxant effects of the ATP-sensitive K(+) channel opener ZD6169 on pig urethral smooth muscle

1. The effects of ZD6169, a novel ATP-sensitive K(+) channel (K(ATP) channel) opener, were investigated on membrane currents in isolated myocytes using patch-clamp techniques. Tension measurement was also performed to study the effects of ZD6169 on the resting tone of pig urethral smooth muscle. 2. Levcromakalim was more potent than ZD6169 in lowering the resting urethral tone. Relaxation induced by low concentrations of ZD6169 (< or =3 microM) was completely suppressed by additional application of glibenclamide (1 microM). In contrast, glibenclamide (1-10 microM) only partially inhibited the relaxation induced by higher concentrations of ZD6169 (> or = microM). 3. Bay K8644 (1 microM) reduced the maximum relaxation produced by ZD6169 (> or =10 microM). 4. In whole-cell configuration, ZD6169 suppressed the peak amplitude of voltage-dependent Ba(2+) currents in a concentration- and voltage-dependent manner, and at 100 microM, shifted the steady-state inactivation curve of the voltage-dependent Ba(2+) currents to the left at a holding potential of -90 mV. 5. In cell-attached configuration, open probability of unitary voltage-dependent Ba(2+) channels (27 pS, 90 mM Ba(2+)) was inhibited by 100 microM ZD6169 and by 10 microM nifedipine. 6. Reverse transcriptase-polymerase chain reaction (RT - PCR) analysis revealed the presence of the transcript of the alpha(1C) subunit of L-type Ca(2+) channels in pig urethra. 7. These results demonstrate that ZD6169 causes urethral relaxation through two distinct mechanisms, activation of K(ATP) channels at lower concentrations and inhibition of voltage-dependent Ca(2+) channels at higher concentrations (about 10 microM).