The ATP-sensitivity of K+ channels in rat pancreatic B-cells is modulated by ADP

Reduction of ischemia-induced electrophysiologic abnormalities by glucose-insulin infusion

OBJECTIVES

This study was designed to determine the effects of glucose-insulin infusion on ischemia-induced changes in extracellular potassium ([K+]o) accumulation and the associated electrophysiologic abnormalities in the canine heart.

BACKGROUND

Although glucose-insulin-potassium infusion has been shown to limit myocardial injury in acute ischemia, its effect on ischemia-induced electrophysiologic alterations has not been investigated.

METHODS

Recordings of [K+]o and local electrograms from the normal, border and ischemic zones were obtained during serial (10-min) left anterior descending coronary artery occlusions in the control state and after infusion of glucose-insulin (eight dogs), glucose alone (six dogs) or insulin alone (eight dogs).

RESULTS

Glucose-insulin infusion caused significant reduction in the rise of [K+]o during the entire period of ischemia in both ischemic and border zones associated with significant improvement in the degree of intramyocardial conduction delay. At 10 min of ischemia, [K+]o was reduced from a mean control level of 15.9 +/- 3.7 to 10.1 +/- 4.3 mmol/liter (p < 0.005) in the ischemic zone and from 6.8 +/- 1.9 to 5.5 +/- 1.1 mmol/liter (p < 0.05) in the border zone. The electrogram duration was shortened from a mean control value of 102 +/- 13 to 78 +/- 12 ms in the ischemic zone and from 79.2 +/- 7.8 to 58.1 +/- 6.6 ms in the border zone (p < 0.005). Glucose alone caused significant reduction in [K+]o during the initial 6 min of ischemia, only in the ischemic zone. Conversely, insulin caused no changes in [K+]o accumulation during ischemia. Neither glucose nor insulin alone had any effect on ischemia-induced intramyocardial conduction delay.

CONCLUSIONS

The present study demonstrated that the combination of glucose and insulin is essential for the salutary effect of reducing [K+]o accumulation during ischemia and improving the associated intramyocardial conduction delay. It could be postulated that glucose in the presence of insulin increases the glycolytic flux, thereby providing adequate adenosine triphosphate for suppressing the cardiac adenosine triphosphate-sensitive potassium ion channels. The latter are, at least partially, responsible for the [K+]o rise in the early phase of ischemia. This study highlights the antiarrhythmic potential of interventions that modulate the metabolic consequences of ischemia.

Inhibition of the ATP-sensitive potassium channel by class I antiarrhythmic agent, cibenzoline, in rat pancreatic beta-cells

1. Cibenzoline, a class I antiarrhythmic agent, was investigated for its effect on the ATP-sensitive K+ channel of pancreatic beta-cells by the patch clamp technique. 2. In perforated patch clamp experiments, cibenzoline depolarized the membrane of single beta-cells and thereafter, caused firing of action potentials in the presence of 2.8 mM glucose. 3. Cibenzoline inhibited the activity of the ATP-sensitive K+ channel in cell-attached recordings in the presence of 2.8 mM glucose and evoked repetitive fluctuations of the baseline current, apparently reflecting the action potentials of the beta-cell. 4. In whole-cell clamp experiments, time-independent outward current was induced by depleting cytoplasmic ATP with 0.1 mM ATP and 0.1 mM ADP in the solution contained in the pipette. The outward current was inhibited by cibenzoline in a dose-dependent manner in the concentration range of 1 microM to 100 microM and half maximum inhibition occurred at 1.5 microM. 5. Cibenzoline blocked substantially the ATP-sensitive K+ channel current when applied at the inner side of the membrane in isolated inside-out membrane patches. 6. It is concluded that cibenzoline blocks the ATP-sensitive K+ channel of pancreatic beta-cells and, thereby, stimulates insulin secretion at sub-stimulatory levels of glucose.

The ATP-sensitive K+ channel

A small conductance K+ channel, that is inactivated by ATP, was recently found in the inner membrane of rat liver mitochondria (Inoue et al., 1991). This finding clearly indicates that a variety of K+ channels, showing ATP-sensitivity, are widely distributed. ATP is an important compound in view of its participation in oxidative phosphorylation and as the source of high-energy phosphate for nearly every energy-requiring reaction in the cell. Therefore, it is easy to speculate that transducing the ATP concentration within a cell into an electrical signal is vital for most living cells. The opening of the ATP-sensitive K+ channel by a decrease in the ATP level shifts the membrane potential in a negative direction and in general depresses cell function. The closing of the channel by an increase in ATP depolarizes the membrane and enhances membrane excitability. It might be speculated that a sequence of amino acids common for the binding site of ATP is preserved and combined with different types of K+ channels, so that the gating with ATP is quite similar between different K+ channels, but the conductance properties are different. The large variability in the value of K1/2ATP in the same cells or between different tissues might be due to modulation of the reaction of ATP and the binding site. These ideas will be substantiated by clarifying the molecular structure of the ATP-sensitive K+ channel in the near future. The molecular mechanisms for the selective channel blockers, sulfonylureas, and for the K+ channel openers should also be clarified.

Modification of K-ATP channels in pancreatic beta-cells by trypsin

The inside-out configuration of the patch-clamp method was used to study the effects of trypsin on the activity of ATP-sensitive potassium (K-ATP) channels from isolated mouse pancreatic beta-cells. Trypsin (20 micrograms/ml) irreversibly enhanced channel activity around twofold by reducing the interburst intervals without altering the burst kinetics. No effect on the single channel conductance or the inward rectification produced by internal Mg2+ was observed: however, the protease did reduce the inhibitory effect of Mg2+ on channel activity. Trypsin both prevented rundown of K-ATP channel activity and reactivated the channels after complete rundown. These effects of trypsin were absent in the presence of trypsin inhibitor. The protease also reduced the inhibitory effect of ATP on channel activity, increasing the dissociation constant from 7 to 49 microM. Trypsin removed the activating effect of ADP (0.1 mmol/l) on channel activity and reduced the inhibitory effect of tolbutamide (0.5 mmol/l). Carboxypeptidase A did not activate K-ATP channels in excised patches, although it was able to slightly reactivate channels after complete rundown, whereas chymotrypsin increased K-ATP channel activity but it did not produce reactivation. The effects of papain were similar to those of trypsin.

Two types of potassium channel regulated by ATP in pancreatic B cells isolated from a type-2 diabetic human

Two types of K channel regulated by ATP were observed in pancreatic beta cells from a type-2 diabetic man. One type had a conductance of 67 pS at -70 mV in symmetrical 140 mM KCl and was inhibited by intracellular ATP with a half-maximal concentration of 40 microM. ATP inhibition was antagonised by ADP. Tolbutamide inhibited the whole-cell K currents half-maximally at 25 microM. This channel has properties similar to those found for the ATP-sensitive K channel in rodent and normal human beta cells. The second channel type observed was an ATP-activated K channel. It had a conductance of 37 pS at -70 mV in symmetrical 140 mM KCl and was activated half-maximally by 9 microM intracellular ATP. This channel was unaffected by 1 mM tolbutamide. In cell-attached patches, one beta cell out of four tested responded to 20 mM glucose with depolarization. The role of the ATP-activated K channel with respect to the (patho)physiology of the beta cell is uncertain.

Negative cooperativity may explain flat concentration-response curves of ATP-sensitive potassium channels

Blockage of ATP-sensitive K+ channels by various drugs has been reported to exhibit a weak concentration dependence with Hill coefficients below unity. This phenomenon is interpreted by a negative cooperativity between K+ channels whereby drug binding to one channel lowers the drug affinities of neighbouring channels. Results are presented for a dimeric and a tetrameric channel model and compared with published experimental data.

ATP-regulated K+ channels are modulated by intracellular H+ in guinea-pig ventricular cells

1. The ATP-regulated potassium channel (K+ATP) was investigated with respect to modulation by intracellular pH (pHi) by using the inside-out membrane patch clamp technique in ventricular cells isolated from the heart of the guinea-pig. Channels which had been closed by internal ATP (0.3-3 mM) were dose-dependently activated by decreasing the pHi over the range of pH 7.6-6.0. However, the channel was conversely inhibited when the pHi was further decreased below 6.0. Inwardly rectifying K+ channels were also decreased in activity when pHi fell from 7.2 to 6.0. 2. The channel activation was also observed with constant concentration of free Ca2+ (1 nM) and Mg2+ (1 mM) in the bathing solution, suggesting that a change in divalent cation concentration is not involved in channel modulation by pHi. 3. When the dose-response relations of the channel activity for ATP concentrations at different pHi were examined, the channel activity obtained at 1 microM ATP was increased by decreasing pH from 7.2 to 6.4. The half-maximal inhibition for ATP concentration at pH 7.2 and 6.4 was 20 and 40 microM, respectively, and the Hill coefficient was 2.5 in both curves. 4. In the absence of ATP, internal H+ was able to reactivate run-down channels but it had less effect on the channel as long as the activity was maintained at a higher level. The increase in the channel activity by H+ was facilitated with a proceeding of the run-down. However, after the channel was completely inactivated by a long exposure of the membrane patch to ATP-free solution, a reduction of pH could not activate the channel. 5. The decrease of pH from 7.2 to 6.4 reduced single channel conductance from 89.0 to 77.7 pS in the absence of Mg2+, whereas it reduced the conductance only at the negative membrane potentials in the presence of 2 mM Mg2+. 6. Mean open and closed times within the burst-like openings of the channel remained unaffected during the change in pHi. 7. We conclude that the cardiac K+ATP channel is modulated by a change in the intracellular pH. The channel modulation consisted of the increase in the channel activity and a decrease in the permeability. The former effect was due to the decrease in the sensitivity of the channel to ATP and the reactivation of the channel which is during the process of run-down in activity.

Stimulation of the KATP channel by ADP and diazoxide requires nucleotide hydrolysis in mouse pancreatic beta-cells

1. The mechanisms by which ADP and the hyperglycaemic compound diazoxide stimulate the activity of the ATP-regulated K+ channel (KATP channel) were studied using inside-out patches isolated from mouse pancreatic beta-cells maintained in tissue culture. 2. The ability of diazoxide and ADP to increase KATP channel activity declined with time following patch excision and no stimulation was observed after 15-40 min. 3. Activation of KATP channels by ADP required the presence of intracellular Mg2+. The stimulatory effect of ADP was mimicked by AMP but only in the presence of ATP. Replacement of ATP with the non-hydrolysable analogue beta, gamma-methylene ATP did not interfere with the ability of ADP to stimulate KATP channel activity. By contrast, enhancement of KATP channel activity was critically dependent on hydrolysable ADP and no stimulation was observed after substitution of alpha,beta-methylene ADP for standard ADP. 4. The ability of diazoxide to enhance KATP channel activity was dependent on the presence of both internal Mg2+ and ATP. Diazoxide stimulation of KATP channel activity was not observed after substitution of beta,gamma-methylene ATP for ATP. However, in the presence of ADP, at a concentration which in itself had no stimulatory action (10 microM), diazoxide was stimulatory also in the presence of the stable ATP analogue. 5. The stimulatory action of diazoxide on KATP channel activity in the presence of ATP was markedly enhanced by intracellular ADP. This potentiating effect of ADP was not reproduced by the stable analogue alpha,beta-methylene ADP and was conditional on the presence of intracellular Mg2+. A similar enhancement of channel activity was also observed with AMP (0.1 mM). In the absence of ATP, diazoxide was still capable of stimulating channel activity provided ADP was present. This effect was not reproduced by AMP. 6. In both nucleotide-free solution and in the presence of 0.1 mM ATP, the distribution of the KATP channel open times were described by a single exponential with a time constant of approximately 20 ms. Addition of ADP or diazoxide resulted in the appearance of a second component with a time constant of > 100 ms which comprised 40-70% of the total number of events. Under the latter experimental conditions, the open probability of the channel increased more than fivefold relative to that observed in the presence of ATP alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of the ATP-sensitive K+ channel by decavanadate in guinea-pig ventricular myocytes

To evaluate the effects of decavanadate on the ATP-sensitive K+ (KATP) channel, we applied the inside-out membrane patch-clamp technique to ventricular myocytes isolated from guinea-pig hearts. Decavanadate increased the probability of the KATP channel being open in a dose-dependent manner over the range of 0.1 to 5 mM in the presence of 0.3 mM ATP. Half-maximal activation occurred at 540 microM decavanadate and a Hill coefficient of 1.3 was obtained when the Hill equation was used to fit the dose-dependent activation for the channel by decavanadate. The half-maximum inhibition for the channel by ATP (K1/2) in the presence of 2 mM Mg2+ was 19 and 74 microM in its absence. In the presence of decavanadate, both curves shifted toward the higher concentration of ATP without a change in steepness of the slope (Hill coefficient = 2). The effect of decavanadate could be expressed by a model in which its binding prevents ATP binding from closing the channel. The estimated dissociation constant of decavanadate was 1.5 microM in the presence and 22.8 microM in the absence of Mg2+. Decavanadate reactivated the rundown channel in the absence of Mg2+ and ATP. Neither the single channel slope conductance nor the mean open and closed lifetime within the bursts of channel openings were affected by decavanadate. We conclude that internal Mg2+ is not required for the modulation produced by decavanadate, but this ion influences the channel and changes the dissociation constant of both ATP and decavanadate to the channel.

Nonselective cation channels in exocrine gland cells

The nonselective cation channel has been described in a wide variety of nonexcitable cells. However even in such closely related tissues as the pancreatic acinar cell and the lacrimal acinar cell, which both possess a superficially similar channel, recent work has shown fundamental differences in channel regulation (Sasaki and Gallacher, 1992; Thorn and Petersen, 1992). These differences are a reflection of a diverse function of the nonselective channel in different tissues.

The principal islet of the Coho salmon (Oncorhyncus kisutch) contains the BB isoenzyme of creatine kinase

The importance of creatine kinase (E.C. 2.7.3.2) in endocrine tissues has been generally overlooked. Using a specific radiometric assay, we have demonstrated the existence of CK in the Brockmann body (principal islet) of the Coho salmon. We have purified this protein from insular tissue and concurrently purified CK from brain and muscle of the salmon. Purification characteristics, immunological cross-reactivity, and N-terminal sequence analysis have demonstrated that the predominant cytosolic CK from the Brockmann body is indistinguishable from the BB (brain) isoenzyme. Immunocytochemical studies indicated that the enzyme resides in the endocrine parenchyma. Phosphocreatine may serve as a reservoir of energy in the islet and augment its capacity to secrete hormones. The induction of CK-BB in the islet by other hormones could influence the secretion of insular hormones. Interorgan flux of the substrate creatine may be an undescribed mechanism of physiological regulation.

Intracellular ATP can regulate afferent arteriolar tone via ATP-sensitive K+ channels in the rabbit

Studies were performed to assess whether ATP-sensitive K+ (KATP) channels on rabbit preglomerular vessels can influence afferent arteriolar (AA) tone. K+ channels with a slope conductance of 258 +/- 13 (n = 7) pS and pronounced voltage dependence were demonstrated in excised patches from vascular smooth muscle cells of microdissected preglomerular segments. Channel activity was markedly reduced by 1 mM ATP and in a dose-dependent fashion by glibenclamide (10(-9) M to 10(-6) M), a specific antagonist of KATP channels. 10(-5) M diazoxide, a K+ channel opener, activated these channels in the presence of ATP, and this effect was also blocked by glibenclamide. To determine the role of these KATP channels in the control of vascular tone, diazoxide was tested on isolated perfused AA. After preconstriction from a control diameter of 13.1 +/- 1.1 to 3.5 +/- 2.1 microns with phenylephrine (PE), addition of 10(-5) M diazoxide dilated vessels to 11.2 +/- 0.7 microns, which was not different from control. Further addition of 10(-5) M glibenclamide reconstricted the vessels to 5.8 +/- 1.5 microns (n = 5; P less than 0.03). In support of its specificity for KATP channels, glibenclamide did not reverse verapamil induced dilation in a separate series of experiments. To determine whether intracellular ATP levels can effect AA tone, studies were conducted to test the effect of the glycolytic inhibitor 2-deoxy-D-glucose. After preconstriction from 13.4 +/- 3.2 to 7.7 +/- 1.3 microns with PE, bath glucose was replaced with 6 mM 2-deoxy-D-glucose. Within 10 min, the arteriole dilated to a mean value of 11.8 +/- 1.4 microns (n = 6; NS compared to control). Subsequent addition of 10(-5) M glibenclamide significantly reconstricted the vessels to a diameter of 8.6 +/- 0.5 micron (P less than 0.04). These data demonstrate that KATP channels are present on the preglomerular vasculature and that changes in intracellular ATP can directly influence afferent arteriolar tone via these channels.

Two sites for adenine-nucleotide regulation of ATP-sensitive potassium channels in mouse pancreatic beta-cells and HIT cells

ATP-inhibited potassium channels (K(ATP)) were studied in excised, inside-out patches from cultured adult mouse pancreatic beta-cells and HIT cells. In the absence of ATP, ADP opened K(ATP) channels at concentrations as low as 10 microM and as high as 500 microM, with maximal activation between 10 and 100 microM ADP in mouse beta-cell membrane patches. At concentrations greater than 500 microM, ADP inhibited K(ATP) channels while 10 mM virtually abolished channel activity. HIT cell channels had a similar biphasic response to ADP except that more than 1 mM ADP was required for inhibition. The channel opening effect of ADP required magnesium while channel inhibition did not. Using creatine/creatine phosphate solutions with creatine phosphokinase to fix ATP and ADP concentrations, we found substantially different K(ATP)-channel activity with solutions having the same ATP/ADP ratio but different absolute total nucleotide levels. To account for ATP-ADP competition, we propose a new model of channel-nucleotide interactions with two kinds of ADP binding sites regulating the channel. One site specifically binds MgADP and increases channel opening. The other, the previously described ATP site, binds either ATP or ADP and decreases channel opening. This model very closely fits the ADP concentration-response curve and, when incorporated into a model of beta-cell membrane potential, increasing ADP in the 10 and 100 microM range is predicted to compete very effectively with millimolar levels of ATP to hyperpolarize beta-cells. The results suggest that (i) K(ATP)-channel activity is not well predicted by the "ATP/ADP ratio," and (ii) ADP is a plausible regulator of K(ATP) channels even if its free cytoplasmic concentration is in the 10-100 microM range as suggested by biochemical studies.

ATP-sensitive K+ channels mediate an IPSP in dorsal horn neurones elicited by sensory stimulation

Nociceptive dorsal horn neurones, which are involved in the processing of pain-related information, are inhibited by input from vibration-sensitive, large diameter primary sensory fibres (Wall and Cronly-Dillon, 1960; Salter and Henry, 1990a,b). We have reported previously that the inhibition of spinal nociceptive neurones by vibration is mediated by adenosine acting through P1-purinergic receptors (Salter and Henry, 1987). In a number of different types of cell, adenosine is known to activate K+ currents (Gerber et al., 1989; Greene and Haas, 1985; Proctor and Dunwiddie, 1987; Segal, 1982; Trussell and Jackson, 1987) and we have recently found that the adenosine-mediated inhibition of nociceptive neurones by vibration is the result of an inhibitory postsynaptic potential (IPSP), which is, indeed, caused by a K+ conductance (De Koninck and Henry, 1988, 1992). It has been reported that adenosine-activated K+ channels in cardiac muscle cells are the ATP-sensitive K+ channels (Kirsch et al., 1990). Therefore, we questioned whether these channels might mediate the purinergic IPSP we have observed in nociceptive dorsal horn neurones. We report here that glibenclamide, a blocker of ATP-sensitive K+ channels (Ashcroft, 1988; Schmid Antomarchi et al., 1987a,b), blocks the inhibition of nociceptive neurones by vibratory stimulation when this compound is administered locally by iontophoresis or systemically by intravenous injection. In addition, direct intracellular injection of ATP was found to block the IPSP evoked by vibratory stimulation. These data indicate that the purinergic IPSP in nociceptive spinal neurones is mediated via ATP-sensitive K+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of energy metabolism by 3-nitropropionic acid activates ATP-sensitive potassium channels

3-Nitropropionic acid (1 mM), which inhibits succinate dehydrogenase activity and reduces cellular energy, produces in the pyramidal cell layer of the hippocampal region CA1 a hyperpolarization for variable lengths of time before evoking an irreversible depolarization. Hyperpolarization is caused by an increased potassium conductance that is attenuated by glibenclamide (1-10 microM), a selective antagonist of ATP-sensitive potassium channels; in contrast, diazoxide (0.5 mM), an agonist at this channel, induces a hyperpolarization in CA1 neurons of rat hippocampal slices. The transient hyperpolarization after prolonged (ca. 1 h) application of 3-NPA is followed by a depolarization that is incompletely reversed by brief application of the glutamate antagonists (D-2-amino-5-phosphonopentanoic acid (APV), 6,7-dichloroquinoxaline-2,3-dione (CNQX), 3-(+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), 7-chloro-kynurenic acid (7Cl-KYN)). Early application of glibenclamide (within the initial 5 min) blocked or reduced hyperpolarization and accelerated the depolarization. These data suggest that metabolic inhibition by 3-NPA initially activates ATP-sensitive potassium channels. Events other than activation of glutamate receptors participate in the final depolarization resulting from uncoupling of oxidative phosphorylation.

Potassium channels of the insulin-secreting B cell

Ionic and electrical events play a central role in the stimulus-secretion coupling of the pancreatic B cell. Potassium permeability is critically involved in the regulation of B cell membrane potential and insulin secretion. In the absence of glucose, membrane potential remains stable, around -65 mV. This resting potential is mainly determined by the high potassium conductance of the membrane. The ATP generated by glucose metabolism in B cells blocks the K+(ATP) channels controlling resting membrane potential. Thus, glucose metabolism leads to closure of the ATP-dependent potassium channels; the resulting decrease in K+ permeability induces depolarization and opening of voltage-activated Ca-channels. The subsequent increase in Ca2+ influx raises the cytoplasmic concentration of free Ca2+, which in turn triggers exocytosis of secretory granules. Other types of K+ channels have also been identified in the B cell, such as voltage- and Ca(2+)-dependent K+ channels, which are not a target for the action of glucose, but may play a role in the repolarization of spikes. The modulation of insulin release by some hormones and neurotransmitters involves, among other mechanisms, an interference with the plasma membrane K+ conductance. Thus, galanine, somatostatin and adrenaline, which inhibit insulin release, increase K+ conductance by a G protein-dependent mechanism; both peptides were reported to open ATP-sensitive K+ channels in insulin-secreting cell line RINm5F. It was also observed that extracellular purine nucleotides could interfere with K+ channels. Among the various drugs interfering with insulin secretion, sulfonylureas, such as tolbutamide and glibenclamide, directly inhibit ATP-dependent K+ channels in the B cell membrane and thereby initiate insulin release. In contrast, potassium channel openers such as diazoxide, antagonize the effects of glucose by increasing K+ permeability of the B cell membrane. Furthermore, other classes of drugs have recently been shown to interact with K+ (ATP) channels. Thus, K+ channels of the pancreatic B cell, particularly ATP-dependent ones, play a crucial role in the electrophysiology of insulin secretion; they are an important target for pharmacological agents designed to modulate this secretion.

Early changes in extracellular potassium in ischemic rabbit myocardium. The role of extracellular carbon dioxide accumulation and diffusion

The role of local accumulation and diffusion of CO2 to modify cellular loss and extracellular accumulation of K+ during the initial, reversible phase of myocardial ischemia was investigated in isolated, cylindrical papillary muscles of the rabbit. The muscles were blood-perfused through their vascular tree and placed in a (permanently flowing) humidified gas mixture with predetermined partial pressures of N2, O2, and CO2. Ischemia was produced by total arrest of perfusion and O2 withdrawal from the gas mixture. With surface PCO2 kept constant during ischemia, [K+]o varied markedly with muscle geometry. After 10 minutes of ischemia, K+ accumulation was approximately 2.5 mM in muscles with a radius of 0.35 mm and approximately 14 mM in muscles with a radius of 0.9 mm, indicating that a large fraction of K+ accumulation was dependent on diffusion of a volatile metabolite. Computer simulation of CO2 accumulation and diffusion within a tissue cylinder suggested a close phenomenological relation between PCO2 and [K+]o in ischemia. This was confirmed by the finding that an increase of tissue PCO2 in small cylinders before or during ischemia by externally applied CO2 produced an increase in K+ accumulation. The importance of CO2 diffusion for local inhomogeneities in K+ within the same preparation was demonstrated by showing [K+]o gradients with simultaneous or consecutive measurements between the papillary muscle cylinders and the adjacent septum and within 300 microns from the surface of the papillary muscle cylinders. These gradients predict an inhomogeneity of impulse conduction that might contribute to the genesis of ventricular arrhythmias. Besides the demonstration that accumulation and diffusion introduce inhomogeneities of [K+]o in ischemia, our results suggest that a significant component of cellular ischemic K+ loss is associated with production and extrusion of metabolic acid. On the basis of previous measurements of pHo and pHi in identical conditions, possible mechanisms of ischemic cellular K+ loss are discussed.

Perturbation of cellular energy state in complete ischemia: relationship to dissipative ion fluxes

Loss of cellular ion homeostasis during anoxia, with rapid downhill fluxes of K+, Ca2+, Na+ and Cl-, is preceded by a slow rise in extracellular K+ concentration (Ke+), probably reflecting early activation of a K+ conductance. It has been proposed that this conductance is activated by either a rise in intracellular calcium concentration (Cai2+), or by a fall in ATP concentration. In a previous study from this laboratory (Folbergrová et al. 1990) we explored whether the early activation of a K+ conductance could be triggered by a rise in Cai2+. To that end, labile metabolites and phosphorylase a, a calcium sensitive enzyme, were measured after 15, 30, 60 and 120 s of complete ischemia ("anoxia"). In the present study, we investigated whether brief anoxia is accompanied by changes in ATP/ADP ratio, or in the phosphate potential, which could cause activation of a K+ conductance. To provide information on this issue, we added a group with 45 s of anoxia to the previously reported groups, and derived changes in intracellular pH (pHi). This allowed calculations of the free concentrations of ADP (ADPf) and AMP (AMPf) from the creatine kinase and adenylate kinase equilibria, and hence the derivation of ATP/ADPf ratios. In performing these calculations we initially assumed that the free intracellular Mg2+ concentration remained unchanged at 1 mM. However we also explored how a change in Mgi2+ of the type described by Brooks and Bachelard (1989) influenced the calculation. The results showed that ADPf must have risen to 150-200% of control within 15 s, and to 330-350% of control within 45 s of anoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurements of cytoplasmic free Ca2+ concentration in human pancreatic islets and insulinoma cells

In human pancreatic islets an increase in the glucose concentration from 3 to 20 mM raised the free cytoplasmic Ca2+ concentration [( Ca2+]i), an effect being reversible upon withdrawal of the sugar. Depolarization with a high concentration of K+ or the sulphonylurea tolbutamide also raised [Ca2+]i. Addition of extracellular ATP produced a transient rapid rise in [Ca2+]i. Oscillations in [Ca2+]i were observed in the presence of 10 mM glucose. Insulinoma cells responded to glucose and tolbutamide with increases in [Ca2+]i, whereas the sulphonamide diazoxide caused a decrease in [Ca2+]i. These findings confirm previous results obtained in rodent beta-cells.

Analysis and use of the perforated patch technique for recording ionic currents in pancreatic beta-cells

We have used the nystatin perforated patch technique to study ionic currents in rat pancreatic beta-cells. The access resistance (Ra) between the pipette and the cell cytoplasm, measured by analyzing capacitive currents, decreased with a slow exponential time course (tau = 5.4 +/- 2.7 min) after seal formation. As Ra decreased, the magnitude of voltage-dependent K and Ca currents increased with a similar time course, and their activation kinetics became faster. After Ra stabilized, the macroscopic currents remained stable for up to an hour or more. When the final Ra was sufficiently low, Ca tail currents could be resolved which had properties similar to those recorded with the classical whole-cell technique. Two types of K channels could be characterized with perforated patch recordings of macroscopic K currents: (i) ATP-blockable K (KATP) channels which generate a time and voltage independent current that is blocked by glyburide and enhanced by pinacidil and (ii) voltage-dependent K (Kv) channels. Whole-cell recordings of KATP currents in the absence of ATP in the pipette showed that the maximum KATP conductance of the beta-cell was 83.8 +/- 40 nS. Perforated patch recordings show that the resting KATP conductance is 3.57 +/- 2.09 nS, which corresponds to about 4% of the channels being open in the intact beta-cell. In classical whole-cell recordings. Kv activation kinetics become faster during the first 10-15 min of recording, probably due to a dissipating Donnan potential. In perforated patch recordings where the Donnan potential is very small, Kv activation kinetics were nearly identical to the steady-state whole cell measurements.

On the mechanism of nucleotide diphosphate activation of the ATP-sensitive K+ channel in ventricular cell of guinea-pig

1. Effects of intracellular nucleotide diphosphates (NDPs) on the ATP-sensitive K+ channel (K+ATP channel) were examined in ventricular cells of guinea-pig heart, using the inside-out patch clamp technique. On formation of inside-out patches in the ATP-free internal solution, the K+ATP channel appeared and then ran down spontaneously. This run-down of the K+ATP channel activity was probably due to dephosphorylation. 2. Millimolar concentrations of various NDPs, e.g. UDP (uridine diphosphate), IDP (inosine diphosphate), CDP (cytidine diphosphate) and GDP (guanosine diphosphate), applied to the internal side of the patch membrane, induced openings of the K+ATP channel after run-down, i.e. in the dephosphorylated state. ADP opened the channel weakly at low concentrations (100 microM) but inhibited it at higher concentrations (1-10 mM). 3. NDP-induced openings of the channel were Mg2+ dependent and inhibited by ATP (100 microM) and glibenclamide (1 microM). None of nucleosides, nucleotide monophosphates nor nucleotide triphosphates induced openings of the channel. Thus, the K+ATP channel may have a Mg(2+)-dependent NDP-binding site, which induces openings of the dephosphorylated channel in ATP-free solution, in addition to the Mg(2+)-independent ATP-binding inactivation site and phosphorylation site. 4. In inside-out patches, pinacidil (a K+ATP channel opener) activated the K+ATP channel in the phosphorylated state but not in the dephosphorylated state. In the presence of NDPs (UDP, IDP, CDP, GDP), however, pinacidil (30 microM) enhanced openings of the dephosphorylated K+ATP channel prominently. 5. From the above results, we concluded that NDP-binding to the specific site has similar effects to channel phosphorylation, i.e. it keeps the K+ATP channel in an operative state in ATP-free solution and enhances the pinacidil-induced channel openings.

Skeletal muscle ATP-sensitive K+ channels recorded from sarcolemmal blebs of split fibers: ATP inhibition is reduced by magnesium and ADP

A new, nonenzymatically treated preparation of amphibian sarcolemmal blebs has been used to study the regulation of skeletal muscle ATP-sensitive K+ [K(ATP)] channels. When a frog skeletal muscle fiber is split in half in a Ca(2+)-free relaxing solution, large hemispherical membrane blebs appear spontaneously within minutes without need for Ca(2+)-induced contraction or enzymatic treatment. These blebs readily formed gigaseals with patch pipettes, and excised inside-out patches were found to contain a variety of K+ channels. Most prominent were K(ATP) channels similar to those found in the surface membrane of other muscle and nonmuscle cells. These channels were highly selective for K+, had a conductance of approximately 53 pS in 140 mM K+, and were blocked by internal ATP. The presence of these channels in most patches implies that split-fiber blebs are made up, at least in large part, of sarcolemmal membrane. In this preparation, K(ATP) channels could be rapidly and reversibly blocked by glibenclamide (0.1-10 microM) in a dose-dependent manner. These channels were sensitive to ATP in the micromolar range in the absence of Mg. This sensitivity was noticeably reduced in the presence of millimolar Mg, most likely because of the ability of Mg2+ ions to bind ATP. Our data therefore suggest that free ATP is a much more potent inhibitor of these channels than MgATP. Channel sensitivity to ATP was significantly reduced by ADP in a manner consistent with a competition between ADP, a weak inhibitor, and ATP, a strong inhibitor, for the same inhibitory binding sites. These observations suggest that the mechanisms of nucleotide regulation of skeletal muscle and pancreatic K(ATP) channels are more analogous than previously thought.

Separate processes mediate nucleotide-induced inhibition and stimulation of the ATP-regulated K(+)-channels in mouse pancreatic beta-cells

The mechanisms by which nucleotides stimulate the activity of the ATP-regulated K(+)-channel (KATP-channel) were investigated using inside-out patches from mouse pancreatic beta-cells. ATP produces a concentration-dependent inhibition of channel activity with a Ki of 18 microns. The inhibitory action of ATP was counteracted by ADP (0.1 mM) and GDP (0.2 mM) but not GTP (1 mM). Stimulation of channel activity was also observed when ADP, GDP and GTP were applied in the absence of ATP. The ability of ADP and GDP to reactivate KATP-channels blocked by ATP declined with time following patch excision and after 30-60 min these nucleotides were without effect. During the same time period the ability of ADP and GTP to stimulate the channel in the absence of ATP was lost. In fact, ADP now blocked channel activity with 50% inhibition being observed at approximately 0.1 mM. By contrast, GDP remained a stimulator in the absence of ATP even when its ability to evoke channel activity in the presence of ATP was lost. These observations show that nucleotide-induced activation of the KATP-channel does not involve competition with ATP for a common inhibitory site but involves other processes. The data are consistent with the idea that nucleotides modulate KATP-channel activity by a number of different mechanisms that may include both regulation of cytosolic constituents and direct interaction with the channel and associated control proteins.

Glucose-induced excitation in molluscan central neurons producing insulin-related peptides

The light green cells (LGCs) are a group of identified central neurons in the pond snail, Lymnaea stagnalis, that produce a number of insulin-related peptides. Freshly dissociated LGCs are activated by physiological concentrations of extracellular glucose. The response to glucose consists of a slow depolarization, which, at concentrations of 1 mM or more, rapidly induces regular spiking activity. The response persists during prolonged application of glucose but is completely reversed upon washing. The threshold concentration is 0.2 mM; the maximal effect occurs at 5 mM. In LGCs in central nervous system preparations kept in organ culture for 16-24 h, glucose causes a similar depolarization, which may lead to spiking activity. In freshly isolated preparations, which have very inexcitable LGCs, no direct response to glucose was seen. The response is specific to the LGCs; no other central neurons in Lymnaea showed consistent responses. The glucose response is evoked by D-glucose and the non-metabolized analogue 2-deoxy-D-glucose, but not by related hexoses, including L-glucose, nor pentoses or disaccharides. The response is not affected by interfering with the glucose metabolism, nor is the response mimicked by the metabolite D-glyceraldehyde or by injection of glucose. This suggests that glucose metabolites are not involved in the response. The glucose response depends on the presence of extracellular Na+ and is blocked by phlorizin, which specifically inhibits Na(+)-coupled glucose transport. This suggests that the response is due to activation of an electrogenic Na(+)-coupled glucose transporter.

Physiological changes in the running greyhound (Canis domesticus): influence of race length

1. Racing greyhounds were allowed to run 402 m, 503 m, and 704 m to determine the influence of increased race length on physiological changes during and after exercise. 2. Plasma and whole blood variables that changed significantly with increased race length were [K+], [total protein], [lactate], hematocrit, and arterial pH. 3. Those variables that changed with increased race distance showed no indication of reaching a plateau; thus, the maximum changes that these variables might undergo with longer exercise duration remains unknown.

Cromakalim and K+ channels in canine trachealis

The effects of cromakalim and glibenclamide on membrane properties and responses to acetylcholine of canine trachea were studied in the double sucrose gap to evaluate the presence and function of ATP-sensitive K+ channels. Cromakalim produced a concentration-dependent hyperpolarization of muscle membrane potential which at maximum brought the membrane potential near the potassium equilibrium potential. Current clamping by hyperpolarizing current to this equilibrium potential abolished the hyperpolarization but not the membrane resistance decrease to cromakalim. Glibenclamide had no effect on resting membrane properties but reduced or abolished effects of cromakalim. Another K+ channel antagonist, tetraethylammonium at 20 mM, also reduced the effects of cromakalim, but 4-aminopyridine (5 mM), Ba2+ (1 mM), and apamin (10(-6) M) had no antagonistic effect. The EJP produced on stimulation of cholinergic nerves sometimes increased just after cromakalim-induced hyperpolarization, but within 5-10 min as membrane resistance dramatically fell it was reduced, as was the depolarization to infused acetylcholine. Initially the reduction in EJP amplitude could be partially overcome by applying hyperpolarizing currents or by applying a second field stimulation; later the EJP was reduced further and was unaffected by these procedures. Even when depolarization to acetylcholine was markedly reduced, the contraction was not. Glibenclamide had no effects alone but antagonized all the effects of cromakalim. These results suggest that ATP-sensitive cromakalim activated K+ channels are present in canine trachea but are usually closed during resting conditions under our experimental conditions. When they are opened by cromakalim, they hyperpolarize to near EK, markedly decrease membrane resistance and reduce the depolarization response to acetylcholine, probably by short circuiting the acetylcholine-induced current.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple actions of pinacidil on adenosine triphosphate-sensitive potassium channels in guinea-pig ventricular myocytes

1. The patch-clamp method was used to study the effects of pinacidil on the adenosine 5'-triphosphate (ATP)-sensitive K+ channel current in guinea-pig ventricular myocytes. 2. In the inside-out configuration of the patch membranes, the channel activity revealed a nearly fully open state in the absence of ATP, whereas application of ATP (0.1-5 mM) markedly suppressed the channel opening. Addition of pinacidil (0.02-1.0 mM) antagonized the inhibitory action of ATP and induced channel opening without marked change in conductance. An increase in ATP concentration depressed the maximal effect of pinacidil. Consequently, the dose-response relationship of ATP inhibition was shifted to the right, but the shift approached a limiting value as pinacidil concentration was increased. The results indicate that the antagonism between pinacidil and ATP is not competitive. 3. The dose-response curve for activation of the channel by pinacidil examined at -50 mV showed a sigmoidal shape but at +50 mV it had a convex shape, revealing asymmetry in the activating effects of pinacidil at these two voltages. 4. In the absence of ATP, pinacidil produced a voltage-dependent block at positive voltages by decreasing the mean open time and increasing the mean closed time, whereas no such effects were observed at negative voltages. The concentration-block relation at a given voltage was fitted to a first-order Hill saturation function. The Kd (dissociation constant) decreased with depolarization from 2.2 mM at +20 mV to 0.15 mM at + 80 mV. 5. The kinetics of block and unblock by pinacidil were shown to be slow, and were expressed by a first-order transition model. The blocking and unblocking rate constants were voltage dependent. 6. The slow block of single-channel current showed an exponential decay in the ensemble current. The time constant of the decay was voltage dependent, reaching a maximal value at around +50 mV. 7. In the absence of ATP, the channel activity gradually decreased and eventually stopped within 12-20 min, a process known as run-down of channel activity. Calcium accelerated this run-down process. Application of pinacidil partially reactivated the channel. Such channel reactivation by pinacidil during the course of run-down depended upon the conditions of the patch and the time course of the run-down. Pretreatment of the channel with ATP markedly strengthened the reactivation effect of pinacidil. 8. These results indicate that there are multiple sites or processes for interaction of pinacidil with the ATP-sensitive K+ channel.

An ATP, calcium and voltage sensitive potassium channel in porcine coronary artery smooth muscle cells

There is increasing interest in the roles played by potassium channels of smooth muscle in protecting against ischemic and anoxic insults. Hence, potassium-selective channels were studied in freshly dispersed porcine coronary artery smooth muscle cells using the inside-out variant of the patch-clamp technique. The most abundant potassium channel had a conductance of 148 pS in a 5.4/140 mM K+ gradient, at 0 mV, and was regulated by cytoplasmic ATP (0.05-3.0 mM), cytoplasmic Ca2+ (0.1-10 microM) and voltage. ATP and AMP-PNP (0.5 mM) reduced the probability of channel opening (Po) by 87 and 92%, respectively. This inhibition was partially reversed by the addition of 0.5 mM ADP. ADP on its own (2 mM) reduced Po by 46%. It appears, therefore, that this channel shares properties with both the ATP-sensitive and the calcium-regulated potassium channels, raising the possibility that it plays a central role in the regulation of coronary blood flow.

The effect of ATP-sensitive K+ channels on the electrical burst activity and insulin secretion in pancreatic beta-cells

In recent years, the electrical burst activity of the insulin releasing pancreatic beta-cells has attracted many experimentalists and theoreticians, largely because of its functional importance, but also because of the nonlinear nature of the burst activity. The ATP-sensitive K+ channels are believed to play an important role in electrical activity and insulin release. In this paper, we show by computer simulation how ATP and antidiabetic drugs can lengthen the plateau fraction of bursting and how these chemicals can increase the intracellular Ca2+ level in the pancreatic beta-cell.

Basolateral K conductance: role in regulation of NaCl absorption and secretion

In this review we explore the possible role of basolateral K conductance (gK) in the regulation of salt absorption and secretion. This inquiry is prompted by a growing body of evidence which, taken together, suggests that basolateral gK is very labile and that alterations in basolateral gK may be a key feature in both stimulatory and inhibitory regulatory mechanisms. We first consider the role of basolateral gK in relation to models for salt absorption and secretion, particularly in relation to the maintenance of cellular charge balance and the obligatory coupling between the apical and basolateral membranes that is produced by transcellular current flow. Next, we review some of the experimental evidence that suggests that changes in basolateral gK are associated with transport regulation. The cellular mechanisms that are known to impact on K channel regulation are considered in a general way, and finally, we consider the use of integrated models for understanding possible coordinate regulation of apical and basolateral cell membranes.

The beta-cell glibenclamide receptor is an ADP-binding protein

The effects of ADP on [3H]glibenclamide binding to membranes and whole cells, the activity of the ATP-sensitive K+ channel (K-ATP channel), intracellular Ca2+ concentration and insulin secretion were studied in a hamster pancreatic beta-cell line, HIT T15. ADP dose-dependently inhibited [3H]glibenclamide binding to membranes and to whole cells in a competitive manner. ADP-agarose also inhibited the binding to whole cells. The activity of the K-ATP channel was assayed by measuring 86Rb efflux from whole cells. ADP inhibited the 86Rb efflux elicited either by diazoxide or by ATP depletion. In the presence, but not in the absence, of extracellular Ca2+, ADP evoked a rapid and sustained increase in intracellular Ca2+ concentration as estimated with the fluorescent dye quin 2. Insulin release from HIT cells was also increased by 0.5-2 mM-ADP in the presence of 0.5 mM-glucose. These effects of ADP on glibenclamide binding, K-ATP channel activity and insulin release were specific for ADP, and were not reproduced by any other nucleotide so far tested. The present findings strongly suggest that ADP and sulphonylureas have common binding sites on the extracellular side of beta-cell plasma membranes, where they inhibit the activity of the K-ATP channel, resulting in an increase in intracellular Ca2+ concentration and insulin release.

Effect of compartmentalized Ca2+ ions on electrical bursting activity of pancreatic beta-cells

Patch-clamp single-channel and whole cell recordings have revealed new insights into the ionic channel properties in the pancreatic beta-cells. I have modeled the electrical events during the burst activity based on the observations that 1) the whole cell Ca2+ current has two functionally distinct components (fast and slow), 2) a fast component is inhibited by intracellular Ca2+, 3) a slow component is inactivated by depolarization, and 4) a significant fraction of the outward current is carried by the Ca2(+)-sensitive, voltage-gated K+ channels [K(Ca, V) channels]. The model contains a feature that the Ca2+ concentration in the submembrane compartment ([Ca2+]s) is higher than that in the cellular phase. At the plateau phase, [Ca2+]s is high enough to activate the K(Ca, V) channels. In addition to the K(Ca, V) channels, the model contains a voltage-activated Ca2+ channel that is quickly blocked by Ca2+ and slowly inhibited by voltage. Because the Ca2+ channel has an intracellular Ca2(+)-dependent inactivation gate, the increase in [Ca2+]s can inactivate the Ca2+ channels. According to this model, the spikes during the plateau phase are caused by a rapid movement of Ca2+ into and out of the compartment. Because of a rapid change in [Ca2+]s, the two competing currents, ICa and IK(Ca, V), fluctuate rapidly; the fluctuation leads to an emergence of spikes. The slow underlying wave is due to a voltage-dependent inactivation gate of the Ca2+ channels, which slowly closes as a result of depolarization. This model differs radically from my previous models, which featured a slowly varying intracellular Ca2+ concentration that was responsible for the underlying slow wave. Although the previous models give plateau fractions (the ratio between the plateau duration and cyclic time) to be far less than unity, the present model is the first of its kind that allows plateau fractions to be in the near-unity range.

Modulation of ATP-sensitive K+ channels in skeletal muscle by intracellular protons

Since their discovery in cardiac muscle, ATP-sensitive K+(KATP) channels have been identified in pancreatic beta-cells, skeletal muscle, smooth muscle and central neurons. The activity of KATP channels is inhibited by the presence of cytosolic ATP. Their wide distribution indicates that they could have important physiological roles that may vary between tissues. In muscle cells the role of K+ channels is to control membrane excitability and the duration of the action potential. In anoxic cardiac ventricular muscle KATP channels are believed to be responsible for shortening the action potential, and it has been proposed that a fall in ATP concentration during metabolic exhaustion increases the activity of KATP channels in skeletal muscle, which may reduce excitability. But the intracellular concentration of ATP in muscle is buffered by creatine phosphate to 5-10 mM, and changes little, even during sustained activity. This concentration is much higher than the intracellular ATP concentration required to half block the KATP-channel current in either cardiac muscle (0.1 mM) or skeletal muscle (0.14 mM), indicating that the open-state probability of KATP channels is normally very low in intact muscle. So it is likely that some additional means of regulating the activity of KATP channels exists, such as the binding of nucleotides other than ATP. Here I present evidence that a decrease in intracellular pH (pHi) markedly reduces the inhibitory effect of ATP on these channels in excised patches from frog skeletal muscle. Because sustained muscular activity can decrease pHi by almost 1 unit in the range at which KATP channels are most sensitive to pHi, it is likely that the activity of these channels in skeletal muscle is regulated by intracellular protons under physiological conditions.

Potassium channel openers and vascular smooth muscle relaxation

Potassium channel openers comprise a diverse group of chemical agents which open plasma-lemmal K-channels. They show selectivity for smooth muscle, although K-channels in cardiac and skeletal muscle, neurones and the pancreatic beta-cell are also affected at relatively high concentrations. In addition, at least one endogenous K-channel opener of vascular origin--endothelium-derived hyperpolarizing factor--exists and in man plays a role in modulating blood vessel tone. The type of K-channel involved in the actions of both exogenous and endogenous K-channel openers is still uncertain, although a prime candidate in smooth muscle seems similar to the [ATPi]-modulated K-channel in the pancreatic beta-cell. This review focuses attention on the action of these agents in vascular smooth muscle and on the possible clinical exploitation of their powerful vasorelaxant properties.

The effects of cromakalim on ATP-sensitive potassium channels in insulin-secreting cells

1. The single-channel current recording technique has been used to investigate the effects of cromakalim, diazoxide and ATP, separately and combined, on the opening of ATP-sensitive potassium channels in the insulin-secreting cell-line RINm5F. The actions of these drugs have been studied using the permeabilized open-cell variation of the patch-clamp technique. 2. In the absence of internal ATP, cromakalim (80-200 microM) was unable to open ATP-sensitive K+ channels but when ATP was present both cromakalim and diazoxide caused channel openings. 3. Interactions between ATP and cromakalim seemed competitive. Concentrations of cromakalim in the range 80-200 microM readily activated channels inhibited by 0.1 mM ATP, but had no effects when the concentration of ATP was increased to 0.5-2 mM. Only when the concentration of cromakalim was increased to 400-800 microM could opening of 0.5-2 mM ATP-inhibited channels be regularly observed. In the continued presence of cromakalim (400-800 microM), an increase in the internal concentration of ATP from either 0.25 to 0.5 mM or 1 to 2 mM, inhibited cromakalim-activated K+ channels. 4. Activation of ATP-inhibited K+ channels was abolished by replacing ATP with ATP gamma S and cromakalin had no effects on ATP gamma S-inhibited channels. This suggests that cromakalim may open KATP channels in insulin-secreting cells by a mechanism which involves protein phosphorylation.

Bioenergetic response of pancreatic islets to stimulation by fuel molecules

The relationship of fuel-stimulated insulin secretion and the beta-cell bioenergetic state was investigated in isolated rat islets. In islets perifused with 5 mmol/L glucose to maintain a high basal energy state, stimulation by 9 to 28 mmol/L glucose increased the [ATP]/[ADP] and [GTP]/[GDP]. The rise in the former occurred prior to, or coincident with, the onset of insulin secretion and was dependent on glucose concentration. The increase in the latter appeared to lag behind the alteration in the [ATP]/[ADP] and achieved statistical significance after 30 minutes of incubation. Addition of 20 mmol/L alpha-ketoisocaproic acid, a powerful secretagogue, also caused a rise in the [ATP]/[ADP]. By contrast, 20 mmol/L lactate, which affected insulin secretion only minimally, failed to alter nucleotide concentrations. These data support the hypothesis that an increase in the islet energy state is a metabolic signal linking fuel metabolism with initiation of insulin secretion.

Nucleotide modulation of the activity of rat heart ATP-sensitive K+ channels in isolated membrane patches

1. We have measured the ATP dependence of KATP channel activity, and the effect of various metabolites on this relationship, in inside-out membrane patches isolated from rat ventricular myocytes. 2. The inhibition of KATP channel activity by ATP could be described as a sigmoid function of [ATP] with a Hill coefficient (HATP) of 2 and a half-maximal inhibition at an ATP concentration (Ki, ATP) of 25 microM, in the presence of 0 mM, or 0.5 mM, total [Mg2+]. The non-hydrolysable ATP analogue, AMP-PNP, also inhibited the channel with Ki, AMP-PNP = 60 microM and HAMP-PNP = 2. 3. Acidosis caused a small, but significant, increase in Ki, ATP from 25 microM at pH 7.25 to 50 microM at pH 6.25, but phosphate and lactate were without effect (at 20 mM) on channel activity. 4. In the absence of ATP or Mg2+, ADP3- inhibited channel activity with Ki, ADP = 275 microM, and HADP = 1.2. Other purine and pyrimidine triphosphates, diphosphates and monophosphates also inhibited the channel with apparent order of inhibitory effectiveness ATP greater than AMP-PNP greater than ADP greater than CTP greater than GDP = AMP = ITP. 5. In the absence of Mg2+, but in the presence of 40 microM-ATP, channel inhibition by GTP, ITP, CTP, GDP, ADP or AMP was additive with inhibition by ATP. 6. In the presence of 0.5 mM-Mg2+ and 40 microM-ATP, inhibition by GTP, GMP and AMP was still additive with inhibition by ATP. The diphosphates ADP and GDP, however, paradoxically increased channel activity in the presence of ATP. This increase in channel activity appeared to result from a competitive increase in Ki, ATP, MgADP did not appear to cause any inhibition of channel activity. 7. We conclude that, in cardiac tissue, KATP channels are regulated by [ATP], and that this regulation is sensitive to other intracellular nucleotides, Mg2+, and pH, but not to phosphate or lactate. A simple, interactive two binding-site model is consistent with the nucleotide-dependent regulation that we observe.

The dependence on intracellular ATP concentration of ATP-sensitive K-channels and of Na,K-ATPase in intact HIT-T15 beta-cells

We have studied the effects of changes of intracellular ATP concentration ([ATP]i) on the activity of ATP-sensitive K-channels (IK(ATP] and of Na,K-ATPase in intact cells of the insulin-secreting cell-line HIT-T15. Pre-exposure of HIT beta-cells to oligomycin caused a dose-dependent reduction in [ATP]i. Marked activation of IK(ATP) activity was found when ATP was lowered below 3 mM. Na,K-ATPase was progressively inhibited as ATP was lowered to 1.5 mM. These data demonstrate that changes in intracellular ATP in the millimolar range markedly influence the activity of two beta-cell membrane proteins having affinities for ATP in the micromolar range. This suggests that submembrane [ATP] may be considerably below the measured bulk cytosolic concentration. The findings also support the proposed role of intracellular ATP in mediating effects of changes in glucose concentration on the activity of beta-cell IK(ATP) and insulin secretion.

ATP-sensitive K-channels in HIT T15 beta-cells studied by patch-clamp methods, 86Rb efflux and glibenclamide binding

ATP-sensitive K-channels in the cloned beta-cell line HIT T15 were studied by patch-clamp methods; by measurement of 86Rb efflux; and by [3H]glibenclamide binding to isolated membrane preparations. In inside-out patches a 50 pS K-channel was found which was blocked by ATP or tolbutamide applied to the intracellular membrane surface. A minimum estimate of about 500 channels per beta-cell was obtained by combining whole-cell and single-channel data. The rate of efflux of 86Rb from 86RbCl-loaded HIT cells was markedly increased by intracellular ATP-depletion; 86Rb-efflux was progressively inhibited by increasing concentrations of glibenclamide or tolbutamide. In non-ATP-depleted cells, diazoxide elicited a concentration-dependent stimulation of 86Rb-efflux which was completely blocked by 1 microM glibenclamide. Isolated membranes showed dose-dependent saturable binding of [3H]glibenclamide to both high (Kd = 1.12 nM) and low (Kd = 136 nM) affinity binding sites. We estimate about 5000 high-affinity binding sites per cell. [3H]-glibenclamide binding was inhibited by tolbutamide (IC50 = 125 microM) but was not affected by diazoxide. ADP (0.5 or 1.0 mM) markedly reduced binding; other nucleotides tested were ineffective.