ATP suppresses activity of Ca(2+)-activated K+ channels by Ca2+ chelation

Role of vascular potassium channels in the regulation of renal hemodynamics

K+ conductance is a major determinant of membrane potential ( Vm) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by Vm through the action of voltage-operated Ca2+ channels (VOCC) in VSMC. Increased K+ conductance leads to hyperpolarization and vasodilation, while inactivation of K+ channels causes depolarization and vasoconstriction. K+ channels in EC indirectly participate in the control of vascular tone by several mechanisms, e.g., release of nitric oxide and endothelium-derived hyperpolarizing factor. In the kidney, a change in the activity of one or more classes of K+ channels will lead to a change in hemodynamic resistance and therefore of renal blood flow and glomerular filtration pressure. Through these effects, the activity of renal vascular K+ channels influences renal salt and water excretion, fluid homeostasis, and ultimately blood pressure. Four main classes of K+ channels [calcium activated (KCa), inward rectifier (Kir), voltage activated (KV), and ATP sensitive (KATP)] are found in the renal vasculature. Several in vitro experiments have suggested a role for individual classes of K+ channels in the regulation of renal vascular function. Results from in vivo experiments are sparse. We discuss the role of the different classes of renal vascular K+ channels and their possible role in the integrated function of the renal microvasculature. Since several pathological conditions, among them hypertension, are associated with alterations in K+ channel function, the role of renal vascular K+ channels in the control of salt and water excretion deserves attention.

Organization of Ca2+ release units in excitable smooth muscle of the guinea-pig urinary bladder

Ca(2+) release from internal stores (sarcoplasmic reticulum or SR) in smooth muscles is initiated either via pharmaco-mechanical coupling due to the action of an agonist and involving IP3 receptors, or via excitation-contraction coupling, mostly involving L-type calcium channels in the plasmalemma (DHPRs), and ryanodine receptors (RyRs), or Ca(2+) release channels of the SR. This work focuses attention on the structural basis for the coupling between DHPRs and RyRs in phasic smooth muscle cells of the guinea-pig urinary bladder. Immunolabeling shows that two proteins of the SR: calsequestrin and the RyR, and one protein the plasmalemma, the L-type channel or DHPR, are colocalized with each other within numerous, peripherally located sites located within the caveolar domains. Electron microscopy images from thin sections and freeze-fracture replicas identify feet in small peripherally located SR vesicles containing calsequestrin and distinctive large particles clustered within small membrane areas. Both feet and particle clusters are located within caveolar domains. Correspondence between the location of feet and particle clusters and of RyR- and DHPR-positive foci allows the conclusion that calsequestrin, RyRs, and L-type Ca(2+) channels are associated with peripheral couplings, or Ca(2+) release units, constituting the key machinery involved in excitation-contraction coupling. Structural analogies between smooth and cardiac muscle excitation-contraction coupling complexes suggest a common basic mechanism of action.

Protein kinases: tuners of the BKCa channel in smooth muscle

Large-conductance, Ca(2+)-activated K(+) (BK(Ca)) channels in smooth muscle cells are unique because they integrate changes in both intracellular Ca(2+) and membrane potential. Protein kinases such as cAMP-dependent protein kinase, cGMP-dependent protein kinase and protein kinase C can affect tissue function by 'tuning' the apparent Ca(2+)- and/or voltage-sensitivity of the BK(Ca) channel to physiological changes in both Ca(2+) concentrations and membrane potential. However, despite the central importance of kinase-mediated modulation of BK(Ca) channels in different smooth muscle tissues, many key issues, including the sites and mechanisms of actions of protein kinases, remain unresolved. In this article, the role of protein kinases in the regulation of BK(Ca) channels is discussed.

A novel type of ATP block on a Ca(2+)-activated K(+) channel from bullfrog erythrocytes

Using the patch-clamp technique, we have identified an intermediate conductance Ca(2+)-activated K(+) channel from bullfrog (Rana catesbeiana) erythrocytes and have investigated the regulation of channel activity by cytosolic ATP. The channel was highly selective for K(+) over Na(+), gave a linear I-V relationship with symmetrical 117.5 mM K(+) solutions and had a single-channel conductance of 60 pS. Channel activity was dependent on Ca(2+) concentration (K(1/2) = 600 nM) but voltage-independent. These basic characteristics are similar to those of human and frog erythrocyte Ca(2+)-activated K(+) (Gardos) channels previously reported. However, cytoplasmic application of ATP reduced channel activity with block exhibiting a novel bell-shaped concentration dependence. The channel was inhibited most by approximately 10 microM ATP (P(0) reduced to 5% of control) but less blocked by lower and higher concentrations of ATP. Moreover, the novel type of ATP block did not require Mg(2+), was independent of PKA or PKC, and was mimicked by a nonhydrolyzable ATP analog, AMP-PNP. This suggests that ATP exerts its effect by direct binding to sites on the channel or associated regulatory proteins, but not by phosphorylation of either of these components.

ATP inhibition of a mouse brain large-conductance K+ (mslo) channel variant by a mechanism independent of protein phosphorylation

1. We investigated the effect of ATP in the regulation of two closely related cloned mouse brain large conductance calcium- and voltage-activated potassium (BK) channel alpha-subunit variants, expressed in human embryonic kidney (HEK 293) cells, using the excised inside-out configuration of the patch-clamp technique. 2. The mB2 BK channel alpha-subunit variant expressed alone was potently inhibited by application of ATP to the intracellular surface of the patch with an IC50 of 30 microM. The effect of ATP was largely independent of protein phosphorylation events as the effect of ATP was mimicked by the non-hydrolysable analogue 5'-adenylylimidodiphosphate (AMP-PNP) and the inhibitory effect of ATPgammaS was reversible. 3. In contrast, under identical conditions, direct nucleotide inhibition was not observed in the closely related mouse brain BK channel alpha-subunit variant mbr5. Furthermore, direct nucleotide regulation was not observed when mB2 was functionally coupled to regulatory beta-subunits. 4. These data suggest that the mB2 alpha-subunit splice variant could provide a dynamic link between cellular metabolism and cell excitability.

ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells

ATP-sensitive K+ (KATP) channels are therapeutic targets for several diseases, including angina, hypertension, and diabetes. This is because stimulation of KATP channels is thought to produce vasorelaxation and myocardial protection against ischemia, whereas inhibition facilitates insulin secretion. It is well known that native KATP channels are inhibited by ATP and sulfonylurea (SU) compounds and stimulated by nucleotide diphosphates and K+ channel-opening drugs (KCOs). Although these characteristics can be shared with KATP channels in different tissues, differences in properties among pancreatic, cardiac, and vascular smooth muscle (VSM) cells do exist in terms of the actions produced by such regulators. Recent molecular biology and electrophysiological studies have provided useful information toward the better understanding of KATP channels. For example, native KATP channels appear to be a complex of a regulatory protein containing the SU-binding site [sulfonylurea receptor (SUR)] and an inward-rectifying K+ channel (Kir) serving as a pore-forming subunit. Three isoforms of SUR (SUR1, SUR2A, and SUR2B) have been cloned and found to have two nucleotide-binding folds (NBFs). It seems that these NBFs play an essential role in conferring the MgADP and KCO sensitivity to the channel, whereas the Kir channel subunit itself possesses the ATP-sensing mechanism as an intrinsic property. The molecular structure of KATP channels is thought to be a heteromultimeric (tetrameric) assembly of these complexes: Kir6.2 with SUR1 (SUR1/Kir6.2, pancreatic type), Kir6.2 with SUR2A (SUR2A/ Kir6.2, cardiac type), and Kir6.1 with SUR2B (SUR2B/Kir6.1, VSM type) [i.e., (SUR/Kir6.x)4]. It remains to be determined what are the molecular connections between the SUR and Kir subunits that enable this unique complex to work as a functional KATP channel.

Suprachiasmatic nucleus neurons are glucose sensitive

The suprachiasmatic nucleus (SCN) in the hypothalamus serves as the pacemaker for mammalian circadian rhythms. In a hamster brain slice preparation, the authors were able to record spontaneous activity from SCN cells for up to 4 days in vitro and verify a self-sustained rhythm in firing. The phase of this rhythm was altered by the concentration of glucose in the bathing medium, with time of peak firing advanced for a 20 mM glucose condition and slightly delayed for a 5 mM glucose condition, relative to 10 mM. The advancing effect of 20 mM glucose and the delaying effect of 5 mM glucose were not maintained during a 2nd day in vitro after changing the bathing medium back to 10 mM glucose, thus indicating the effect was not a permanent phase shift of the underlying oscillation. In experiments recording from cell-attached membrane patches on acutely dissociated hamster SCN neurons, exchanging the bathing medium from high (20 mM) to zero glucose increased potassium (K+)-selective channel activity. With inside-out membrane patches, the authors revealed the presence of a glybenclamide-sensitive K+ channel (190 pS) and a larger conductance (260 pS) Ca(2+)-dependent K+ channel that were both reversibly inhibited by ATP at the cytoplasmic surface. Furthermore, 1 mM tetraethylammonium chloride was demonstrated to advance peak firing time in the brain slice in a similar manner to a high concentration of glucose (20 mM). The authors interpret the result to imply that SCNs are sensitive to glucose, most probably via ATP modulation of K+ channel activity in these neurons. Tonic modulation of K+ channel activity appears to alter output of the pacemaker but does not reset the phase.

Run-down of the GABAA response under experimental ischaemia in acutely dissociated CA1 pyramidal neurones of the rat

1. The effect of experimental ischaemia on the response to gamma-aminobutyric acid (GABA) was assessed in acutely dissociated CA1 pyramidal neurones of rats, using the patch-clamp technique. 2. Rapid application of 3 x 10(-5) M GABA induced a bicuculline-sensitive inward Cl- current (IGABA) at a holding potential (Vh) of -44 mV. The peak amplitude of IGABA showed a time-dependent decrease (run-down) when it was recorded with the conventional whole-cell mode without internal ATP. The run-down was not observed when the intracellular ATP concentration ([ATP]i) was maintained by the nystatin-perforated recording with an intracellular Na+ concentration ([Na+]i) of 0 mM. 3. When [Na+]i was increased to more than 30 mM, the IGABA run-down was observed even with the nystatin-perforated recording. 4. The IGABA run-down observed at 60 mM [Na+]i with the nystatin method was further enhanced under experimental ischaemia without changes in the reversal potential of IGABA. The enhanced run-down was suppressed by application of the Na+,K(+)-ATPase inhibitors, ouabain and SPAI-1. 5. IGABA run-down during ischaemia was also accompanied by an outward holding current and a concomitant increase in intracellular free Ca2+ concentration ([Ca2+]i) in 48.5% of the neurones. The outward current was a Ca(2+)-activated K+ current, which was blocked by 3 x 10(-7) M charybdotoxin. 6. In the inside-out mode of the single-channel analysis, GABA activated three subconductance states with conductances of 33.4, 22.7 and 15.2 pS. Reduction of ATP concentration from 2 to 0 mM on the intracellular side suppressed the channel activities, while an increase in Ca2+ concentration from 0.7 x 10(-9) to 1.1 x 10(-6) M had no effect. 7. These results suggest that ischaemia induces the run-down of the postsynaptic GABA response at the GABAA receptor level, and that this run-down is triggered by a decrease in [ATP]i.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

ATP increases Ca(2+)-activated K+ channel activity in isolated rat arterial smooth muscle cells

Large conductance Ca(2+)-activated K+ (Kca) channels are known to be activated by phosphorylation through cAMP- and cGMP-dependent kinase activation. In pulmonary arterial smooth muscle KCa channels are directly activated by ATP (but not by non-hydrolysable analogues) independently of the presence of cyclic nucleotides or the catalytic subunits of protein kinases. This study was designed to determine whether direct activation of KCa channels by ATP is apparent in other types of arterial smooth muscle. KCa channels of similar conductance to those of rat pulmonary artery (approximately 250 pS) were found in membrane patches excised from isolated smooth muscle cells from rat aorta, mesenteric and basilar arteries. In myocytes isolated from each of these arteries, intracellular application of ATP (in the absence of exogenous cyclic nucleotides or catalytic subunits) reversibly increased the open state probability of KCa channels: a response markedly reduced by a specific inhibitor of protein kinase A. Nucleotide sequence analysis of KCa channels revealed no homology with the majority of protein kinases. It is concluded that phosphorylation of KCa channels through the activity of a membrane tethered kinase related to protein kinase A (but lacking its regulatory subunits) may play an important role in controlling K+ flux in a range of arterial smooth muscle cell types.

Evidence that BKCa channel activation contributes to K+ channel opener induced relaxation of the porcine coronary artery

The rank order of potency of a series of benzopyran and cyanoguanidine K+ channel openers (KCOs) for causing relaxation of the PGF2 alpha-precontracted porcine coronary artery was determined. Glyburide, an inhibitor of KATP channels, showed an apparent competitive inhibition of the vasorelaxant activity of the KCOs. The pA2 values of glyburide when cromakalim and CGP 14877 (P1060) were used as vasorelaxants were 7.66 and 7.83, respectively. Charybdotoxin (40 nM), an inhibitor of BKCa channels, also caused a significant inhibition of the cromakalim mediated relaxation of the porcine coronary artery. In order to clarify the site of action of these KCOs, we identified a K+ channel current in single porcine coronary arterial cells and measured channel activity in the presence of these compounds. The prominent K+ ion current in these cells had characteristics typical of the conventional large Ca(2+)-activated K+ channel (BKCa) present in other smooth muscle cells. Using symmetrical K+ concentrations, the channel had a conductance of 214 pS and was found to be sensitive to [Ca2+]i and membrane potential. The KCOs were found to reversibly increase the open probability (P(o)) of the channel without changing channel conductance. The potency of the KCOs to increase K+ channel opening was similar to the potency of these compounds to cause coronary artery relaxation. These results indicate that the porcine coronary artery contains the BKCa channel and that this channel, along with other types of K+ channels (KATP), mediate the vasorelaxant effects of K+ channel openers.

Hypoxia inhibits myogenic reactivity of renal afferent arterioles by activating ATP-sensitive K+ channels

Recent findings implicate K+ channels as important modulators of myogenic tone and possible mediators of the vasodilatory effects of hypoxia. In the present report, we examined the effects of hypoxia on myogenic vasoconstriction of renal afferent arterioles. Using the in vitro perfused hydronephrotic rat kidney model, we observed precisely graded decreases in arteriolar diameter when renal perfusion pressure was increased. Normal myogenic reactivity was observed over PO2 levels of 150 to 80 mm Hg. Reducing PO2 to 60, 40, and 30 mm Hg resulted in a significant progressive inhibition of myogenic reactivity. At approximately 20 mm Hg, myogenic vasoconstriction was essentially abolished, whereas the vasoconstriction induced by 30 mmol/L KCl was unaffected. The addition of 1.0 mumol/L glibenclamide completely restored myogenic vasoconstriction during hypoxia. In contrast, 1.0 mmol/L tetraethylammonium did not alter the effects of hypoxia. To investigate the relation between hypoxia-induced vasodilation and smooth muscle oxidative phosphorylation, we monitored changes in arteriolar levels of reduced NADH during exposure to hypoxia. Arterioles preconstricted by elevated pressure were optically isolated for simultaneous monitoring of vessel diameter and NADH fluorescence (360-nm excitation, 450-nm emission). Reducing perfusate PO2 from 150 to 20 mm Hg resulted in progressive loss of myogenic tone with no change in arteriolar NADH. These findings indicate that lowering PO2 within a physiological range attenuates myogenic reactivity of the renal afferent arteriole by causing the activation of ATP-sensitive K+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Large conductance calcium-activated potassium channels in cultured retinal pericytes under normal and high-glucose conditions

Pericytes are considered to contribute to the regulation of retinal microcirculation which is impaired in diabetic retinopathy. Single, large-conductance, Ca(2+)-dependent K+ channels (BK) were studied in cultured bovine retinal capillary pericytes using the patch-clamp method. In excised patches with symmetrical 135-mmol/l K+ solutions a single channel conductance of 238 +/- 9.9 pS was measured. With a K+ gradient of 4/135 mmol/l (extracellular/intracellular) the slope conductance averaged 148 +/- 2.9 pS at 0 mV. The mean permeability was 4.2 X 10(-13) cm3/s. The channel was highly selective for K+ with a permeability ratio for K+ over Na+ of 1/0.02. The mean open time and the open probability (Po) of the BK channel increased with depolarization and with increasing internal [Ca2+] showing a maximal sensitivity to Ca2+ between 10(-4) and 10(-5) mol/l Ca2+. Ba2+ (5 mmol/l), quinine (5 mmol/l), and verapamil (Michaelis constant 1.5 X 10(-5) mol/l) blocked from the intracellular side. Tetraethylammonium induced a dose-dependent block from the outside only with a half-maximal blocking concentration of 2.5 X 10(-4) mol/l. Charybdotoxin (10(-8) mol/l) blocked completely from the extracellular side. The channel activity was not changed by either internal adenosine triphosphate (ATP, 10(-4) mol/l) or the putative opener of ATP-sensitive K+ channels Hoe 234 (10(-6) mol/l). In cell-attached patches channel Po was less than 3%. After a 3-day incubation in culture medium containing an elevated glucose concentration (22.5 mmol/l) the channel activity in attached patches was markedly increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of Ca(2+)-activated K+ channels in excised patches of human T lymphocytes

Ca(2+)-activated K+ [K(Ca)] channels were studied in excised patches of resting and activated human peripheral blood T lymphocytes. The K(Ca) channel had a single-channel conductance of 50 +/- 6 pS in symmetrical high-K+ solutions in the potential range of -100 to -10 mV and was inwardly rectifying at more depolarized potentials. The channel was sensitive to block by charybdotoxin (10 nM) and insensitive to apamin (3 nM). Half-maximum activation occurred at an internal free Ca2+ concentration of 360 +/- 110 nM. The concentration-effect curve had a slope factor of 0.83 +/- 0.12, suggesting a 1:1 interaction of Ca2+ ions with the channel. Ca2+ affects the open time probability of the K(Ca) channels, mainly by modulating the frequency of channel opening. The open probability did not show voltage dependence. The kinetics of the channel could be described assuming one open state and two closed states. The time constant of the exponential describing the open time distribution amounted to 2.8 +/- 1.2 ms, whereas the closed time distribution could be described with two exponentials with time constants of 0.2 +/- 0.05 ms and 8.0 +/- 2.1 ms, respectively. Resting T lymphocytes expressed a low number of channels but the density of channels increased dramatically during chronic phytohaemagglutinin stimulation.

Single channel and whole-cell K-currents evoked by levcromakalim in smooth muscle cells from the rabbit portal vein

1. Single channel and whole-cell current recordings were made from single smooth muscle cells isolated from the rabbit portal vein. 2. Application of 10 microM levcromakalim ((-)-Ckm) to single cells held with pipettes containing 1 mM GDP induced a K-current (IK(Ckm)) which occurred in addition to the current caused by GDP alone (IK(GDP)) and averaged 135 pA at -37 mV. We have investigated whether the same K channels underlie the GDP- and Ckm-induced K-currents. 3. If 1 mM GDP was in the pipette but Mg ions were omitted the effect of GDP was absent and IK(Ckm) averaged only 10 pA, suggesting that the action of (-)-Ckm was Mg-dependent. 4. Intracellular ATP was not observed to have much effect on IK(-Ckm). Loading of cells with 10 mM ATP from the recording pipette had no significant effect and flash photolysis of caged-ATP loaded into cells from the pipette, estimated to release about 1 mM free ATP, also had no effect on IK(-Ckm). 5. Bath-applied glibenclamide inhibited IK(-Ckm) with an IC50 of 200 nM, a value 8 times higher than that found for inhibition of IK(GDP). The delayed rectifier K-current (IK(DR)) was also inhibited by glibenclamide but at higher concentrations (IC50 100 microM). Bath-applied tetraethylammonium ions (TEA) inhibited IK(-Ckm) and IK(GDP) to the same extent (IC50 about 7 mM). 6. In inside-out patch recordings (- )-Ckm (10 microM) applied to the intracellular surface of the membrane potentiated the opening of K channels already stimulated by I mM GDP and all of the channel activity was abolished by 10 microM glibenclamide. The unitary conductance of the channels was 24lpS in a 60 mM: 130 mM K-gradient.7. We suggest that (-)-Ckm may hyperpolarize and relax smooth muscle cells by opening KNDP, a class of small conductance K channels that are related to the ATP-sensitive K channels seen in other tissues.

ATP-sensitive potassium channels in smooth muscle cells from guinea pig urinary bladder

We explored the possibility that ATP-sensitive potassium (KATP) channels exist in urinary bladder smooth muscle, since synthetic openers (e.g., lemakalim) of KATP channels in other tissues relax bladder smooth muscle. Unitary currents through single potassium channels and whole cell potassium currents were measured in smooth muscle cells isolated from the detrusor muscle of the guinea pig bladder. Lemakalim (10 microM) increased whole cell K+ currents by 50 pA at -80 mV with 60 mM external K+ when the cells were dialyzed with 0.1 mM ATP and 140 mM K+. Glibenclamide (10 microM), a sulfonylurea blocker of KATP channels in other tissues, inhibited the entire lemakalim-stimulated current as well as 19 pA of the steady-state K+ current. Glibenclamide-sensitive K+ currents were not dependent on voltage. Increasing intracellular ATP from 0.1 to 3.0 mM reduced the glibenclamide-sensitive K+ current in both the presence and absence of lemakalim by about fourfold. External barium (100 microM) which blocks KATP channels in skeletal muscle reduced KATP channel currents in bladder smooth muscle by 50% at -80 mV. Lemakalim (10 microM) increased the open-state probability of single K+ channels in outside-out patches (with 0.1 mM internal ATP) by sixfold. The single-channel conductance was approximately 7 pS at 0 mV with a physiological K+ gradient. This single-channel conductance was in accord with estimates of conductance made from noise analysis of the lemakalim-induced whole cell current. Glibenclamide inhibited these channels. The number of channels per cell was estimated to be approximately 425. We conclude that urinary bladder smooth muscle has KATP channels and that these channels can be opened by the K+ channel opening drug, lemakalim, and blocked by external glibenclamide and barium. We propose that modulation of these channels may regulate bladder contractility.

Potassium channels in airway smooth muscle: a tale of two channels

Potassium channels are an important determinant of smooth muscle excitability and force generation. Two potassium channels have been fully described in airway smooth muscle: large conductance, calcium-activated potassium channels and voltage-dependent delayed rectifier channels. This article will review the biophysics and pharmacology of these channels and discuss what is currently known with respect to their regulation and physiological significance.

Potassium channel modulation in rat portal vein by ATP depletion: a comparison with the effects of levcromakalim (BRL 38227)

1. The effects of levcromakalim and of adenosine 5'-triphosphate (ATP) depletion on membrane potential and ionic currents were studied in freshly-dispersed smooth muscle cells of rat portal vein by use of combined voltage- and current-clamp techniques. 2. Levcromakalim (1 microM) induced a glibenclamide-sensitive, non-inactivating K-current (IKCO) and simultaneously inhibited the slow, transient outward, delayed rectifier K-current (ITO). Levcromakalim also hyperpolarized the portal vein cells by approximately 20 mV. 3. Reduction of intracellular ATP by removal of glucose and carboxylic acids from the recording pipette and of glucose from the bath fluid, induced a slowly-developing, non-inactivating and glibenclamide-sensitive K-current (Imet) within 60-300 s after breaking the membrane patch. Imet reached peak amplitude after 300-900 s, remained at a plateau for 200-800 s and then slowly ran down. At the peak of Imet, the cells were hyperpolarized by approximately 20 mV and their input conductance was increased by 42%. 4. At the time of maximum development of Imet, the delayed rectifier current, ITO, was reduced by 48%. 5. In the absence of glucose and carboxylic acids, addition of 1 microM free ATP to the recording pipette almost doubled the magnitude of Imet. At a holding potential of -10 mV, Imet was increased from 124 +/- 11 pA to 228 +/- 54 pA whereas the time-course of development and run-down of Imet was unaffected. 6. During the development and after the run-down of Imet, levcromakalim (1-10 microM) failed to induce IKCO. 7. Stationary fluctuation analysis of the current noise associated with Imet revealed a unitary conductance of between 10-20 pS in a physiological potassium gradient. A second contaminating current with an underlying unitary conductance of approximately 150 pS remained after Imet had run down. 8. It is concluded that IKCO induced by levcromakalim and Imet are carried by the same population of relatively small conductance, glibenclamide-sensitive K-channels. The open state of these is increased by procedures designed to lower intracellular ATP concentrations. 9. The simultaneous inhibition of the delayed rectifier current (ITO) by both levcromakalim and during the development of Imet is highly significant. It suggests that levcromakalim could modify the interaction of ATP with sites linked to more than one type of K-channel. This results in the opening of those channels which underlie IKCO (and which are normally inhibited by ATP binding) together with the modulation of phosphorylation-dependent channels such as those which underlie ITO.

Lack of effect of potassium channel openers on ATP-modulated potassium channels recorded from rat ventromedial hypothalamic neurones

1. Single neuronal cells were freshly isolated from the ventromedial hypothalamic nuclei (VMHN) of the rat brain. Currents through ATP-modulated and large conductance (160 and 250 pS) calcium-activated potassium channels were recorded by the cell-attached and excised inside-out patch techniques. 2. BRL38227 (lemakalim; 30-90 microM) applied to the superfusing medium produced no change in firing rate of isolated glucose-receptive VMHN neurones in cell-attached recordings. 3. BRL38227, at concentrations of between 30-100 microM applied to the intracellular (cytoplasmic) aspect of inside-out patches, had no effect on the activity of ATP-sensitive K+ channels in the absence of ATP or in the presence of a sub-maximal inhibitory concentration (3 mM) of ATP. Cromakalim, pinacidil, minoxidil sulphate and diazoxide also produced no effect under these conditions. 4. The potassium channel openers (KCO's) were tested on ATP-activated potassium channels recorded from a further subpopulation of VMHN neurones. Application of BRL38227 (up to and including 100 microM) to this channel in inside-out patches either in the absence of ATP or when activated by 5 mM ATP had no effect on channel activity. Identical results were obtained with cromakalim and pinacidil. 5. BRL38227 had no effect on either of the large conductance (250 pS and 160 pS) calcium-activated potassium channels in VMHN neurones. 6. Intracellular recordings were made from glucose-receptive VMHN neurones in rat brain slices. Cromakalim (50 microM) or diazoxide (60 microM) did not alter the firing rate or passive membrane properties of these neurones demonstrated to be sensitive to tolbutamide (0.1 mM). 7. These results show that the KCO's tested in this study have no effect either on VMHN neurones contained in brain slices or on the activity of any of the ATP-modulated potassium channels under isolated patch conditions associated with these neurones.