Blockade of epidermal growth factor receptor function by bivalent and monovalent fragments of 225 anti-epidermal growth factor receptor monoclonal antibodies

We have previously described anti-epidermal growth factor (EGF) receptor monoclonal antibodies (MAbs) which can block binding of transforming growth factor alpha (TGF-alpha) and EGF to receptors and inhibit activation of receptor tyrosine kinase. Studies with these MAbs involving cell cultures and nude mouse xenografts demonstrated their capacity to inhibit the growth of a variety of tumor cell lines, which express EGF receptors and TGF-alpha and appear to depend upon receptor activation for cell proliferation. To explore the mechanism(s) by which anti-EGF receptor 225 MAb inhibits cell proliferation, we have compared the activity of native 225 MAb with the response to bivalent 225 F(ab')2 and monovalent 225 Fab' fragments. Both native 225 MAb and its fragments could inhibit the binding of 125I-EGF to EGF receptors. Scatchard analysis revealed that the Kd of 225 F(ab')2 is comparable to that of 225 MAb (1 nM), whereas the Kd of 225 Fab' is 5 nM. Both bivalent 225 MAb and 225 F(ab')2 and monovalent 225 Fab' were able to completely inhibit TGF-alpha-induced EGF receptor tyrosine kinase activation, as assayed by autophosphorylation of tyrosine residues of EGF receptors on MCF10A nonmalignant human mammary cells, MDA468 human breast adenocarcinoma cells, and A431 human vulvar squamous carcinoma cells. The bivalent forms of MAb could inhibit proliferation stimulated by endogenous (autocrine) TGF-alpha in cultures of these three cell lines. They also blocked growth stimulation by added exogenous TGF-alpha in cultures of MCF10A cells and the growth-inhibitory effect of exogenous TGF-alpha upon MDA468 and A431 cell cultures. Monovalent 225 Fab' had weaker inhibitory effects upon the proliferation of these cell lines. To determine whether the in vivo antiproliferative activity of anti-EGF receptor MAb can occur without the participation of the Fc portion of MAb, the capacities of 225 F(ab')2 and native 225 MAb to inhibit growth of s.c. A431 cell xenografts were compared. Equimolar amounts of either 225 MAb or 225 F(ab')2 were administered at intervals equivalent to the half-lives of the molecules, to attempt to maintain comparable plasma levels. Both 225 MAb and 225 F(ab')2 inhibited A431 cell xenograft growth in a dose-dependent manner, with a more sustained response in the case of the intact antibody.(ABSTRACT TRUNCATED AT 400 WORDS)

Use-dependent block of Na+ currents by mexiletine at the single channel level in guinea-pig ventricular myocytes

1. The mechanism of use-dependent block of Na+ current by mexiletine was studied at the single channel level in guinea-pig ventricular myocytes by the patch-clamp techniques. All experiments were performed using stimulation protocols to enable us to analyze the strict dependence of changes in channel properties on channel use. 2. In cell-attached patches, bath or pipette application of mexiletine (40 microM) produced a use-dependent reduction of the peak average current without changes in single channel conductance. Null sweeps were increased and the number of openings per sweep decreased with successive pulses, whereas no significant change in the mean open time was detected during the train. 3. Block by mexiletine became greater when pulse duration was extended beyond the period in which channels were open, suggesting that block progressed without channel opening. 4. At near threshold potentials, mexiletine decreased the later occurrence of first openings. Additionally, late openings were reduced in a use-dependent way. 5. We conclude that mexiletine binds to the inactivated closed states of the Na+ channel and then causes a failure of late openings as well as early, which results in null sweeps on subsequent depolarization.

Activation and reactivation of the ATP-sensitive K+ channel of the heart can be modified by drugs

Activation and reactivation of the ATP-sensitive K+ channel (IK.ATP) were studied with the patch-clamp technique in guinea-pig ventricular myocytes. The K+ channel openers, nicorandil and pinacidil, activated IK.ATP in an internal ATP-dependent manner. Both drugs increased the open probability of IK.ATP without changing the channel conductance. They prolonged lifetimes of bursts and shortened interburst intervals without influencing the fast gating within bursts. These effects were the opposite of those of internal ATP. However, the interaction between ATP and either nicorandil or pinacidil appeared not to be simple competition. We found that three carbonyl compounds--3,4-dihydroxybenzaldehyde, 2,3-dihydroxybenzaldehyde, and 2,4-dihydroxyacetophenone--could activate IK.ATP through an intracellular mechanism that was dependent upon the presence of ADP and Mg2+. It has been suggested that these three carbonyl compounds bind covalently to proteins to form a Schiff base, which may be responsible for their effects upon IK.ATP. Internal application of the proteolytic enzyme trypsin prevented both the spontaneous and Ca(2+)-induced rundown of the KK.ATP channel. Tryptic digestion did not change either the channel's sensitivity to inhibition by ATP nor the fast gating kinetics of IK.ATP. Internal application of an exopeptidase, carboxypeptidase A, but not leu-aminopeptidase, prevented the spontaneous and Ca(2+)-induced rundown of the IK/ATP channel, effects similar to those of trypsin treatment. These results suggest that the target site of trypsin digestion may be located on the carboxy (C)-terminal of the channel proteins or associated regulatory units.

Post-repolarization block of cardiac sodium channels by saxitoxin

Phasic block of rat cardiac Na+ current by saxitoxin was assessed using pulse trains and two-pulse voltage clamp protocols, and the results were fit to several kinetic models. For brief depolarizations (5 to 50 ms) the depolarization duration did not affect the rate of development or the amplitude of phasic block for pulse trains. The pulse train data were well described by a recurrence relation based upon the guarded receptor model, and it provided rate constants that accurately predicted first-pulse block as well as recovery time constants in response to two-pulse protocols. However, the amplitudes and rates of phasic block development at rapid rates (> 5 Hz) were less than the model predicted. For two pulse protocols with a short (10 ms) conditioning step to -30 mV, block developed only after repolarization to -150 mV and then recovered as the interpulse interval was increased. This suggested that phasic block under these conditions was caused by binding with increased affinity to a state that exists transiently after repolarization to -150 mV. This "post-repolarization block" was fit to a three-state model consisting of a transient state with high affinity for the toxin, the toxin bound state, and the ultimate resting state of the channel. This model accounted for the biphasic post-repolarization block development and recovery observed in two-pulse protocols, and it more accurately described phasic block in pulse trains. The transient state after repolarization was predicted to have a dwell time of 570 ms, an on rate for saxitoxin of 16 s-1 micro M-1, and an off rate of 0.2 s-1 (KD = 12 nM). These results and the proposed model suggest a novel variation on phasic block mechanisms and suggest a long-lived transient Na+ channel conformation during recovery.

Modification of the adenosine 5'-triphosphate-sensitive K+ channel by trypsin in guinea-pig ventricular myocytes

1. The adenosine 5'-triphosphate (ATP)-sensitive K+ channel current was recorded in guinea-pig ventricular myocytes using the patch clamp technique with inside-out patch configuration. Modification of the channel activity by intracellular application of an endoprotease trypsin was studied, and was related to a possible model of regulation of this channel. 2. Maximal ATP-sensitive K+ channel activity was observed immediately upon formation of inside-out patches in the ATP-free internal solution, thereafter activity declined both spontaneously and gradually with time; a phenomenon known as rundown. When trypsin (1 mg/ml) was applied to the intracellular side of the membrane upon formation of inside-out patches, spontaneous run-down did not occur, and this trypsin action was irreversible. Neither trypsin (1 mg/ml) applied with trypsin inhibitor (0.25 mg/ml) nor heat-denatured trypsin (1 mg/ml) could mimic this effect. When trypsin was applied to the patches after run-down, channels were reactivated at approximately 13 min. 3. Treatment with trypsin did not affect unitary current amplitude, channel gating kinetics, or sensitivity to intracellular ATP. 4. Intracellularly applied Ca2+ induced run-down of channel activity in a dose-dependent manner. In membrane patches that were treated with trypsin (1 mg/ml) for 20 min, intracellularly applied Ca2+ up to 1 mM did not induce run-down of channel activity. 5. Intracellular application of an exopeptidase, carboxypeptidase A (1 mg/ml), but not Leu-aminopeptidase (0.5 mg/ml), prevented spontaneous or Ca(2+)-induced run-down of channel activity. 6. As postulated for several other channels, such as Na+ and Ca2+ channels, there may be a possible 'chemical gate' that is responsible for run-down of this channel activity. Application of trypsin might somehow modify this 'chemical gate', resulting in prevention of spontaneous or Ca(2+)-induced run-down. This target site for trypsin may be situated on the carboxy-terminus of the channel proteins, or of associated regulatory units. Because ATP sensitivity remained intact after trypsin treatment, the trypsin-selective site for channel inhibition is not related physically to the ATP binding site.

Complex-valued error diffusion for off-axis computer-generated holograms

Error diffusion has been proven to be a valuable tool in the calculation of computer-generated holograms. Real-valued error diffusion has been used to calculate the transmission functions for real-valued holograms with off-axis reconstructions. We demonstrate the use of a complex-valued error-diffusion algorithm on real-valued hologram data in order to achieve larger flexibility in the shaping of the noise spectrum.

Intracellular H+ and Ca2+ modulation of trypsin-modified ATP-sensitive K+ channels in rabbit ventricular myocytes

ATP-sensitive K+ current (IK.ATP) channels are thought to play a role in the K+ efflux observed in cardiac ischemia. Intracellular acidosis is a prominent early effect in ischemia; therefore, the effects of acidosis on IK.ATP may have certain pathophysiological implications. Increased intracellular proton concentration (pHi) is known to regulate IK.ATP in frog skeletal muscle by increasing open probability. The pHi effect on IK.ATP is not clearly understood in heart because, unlike frog skeletal muscle, low pHi causes IK.ATP run-down in inside-out patches. This would tend to mask any opening effect of low pHi if it exists. Trypsin modification of IK.ATP has recently been shown to prevent run-down in inside-out patches. We used single channel recordings in inside-out patches to study IK.ATP after exposure to trypsin. After trypsin treatment, the open probability of IK.ATP was not sensitive to pHi in the absence of ATP. In the presence of ATP, however, a decrease in pHi consistently increased the open probability of trypsin-modified IK.ATP by reducing ATP inhibition. In the absence of ATP the mean open probability was 0.43 +/- 0.07 at pHi 7.4, and 0.5 mM ATP decreased the mean open probability to 0.03 +/- 0.04 at pHi 7.4, but mean open probability was significantly increased to 0.20 +/- 0.07 at pHi 6.3 (n = 7, p < 0.01). Ca2+ did not affect the activity of trypsin-modified IK.ATP in either the absence or presence of ATP at pHi 7.4. However, Ca2+ was able to antagonize the low pHi effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracranial longitudinal splitting of facial nerve: a new approach for hemifacial spasm

A new surgical technique for the treatment of hemifacial spasm consisting of longitudinal splitting of the facial nerve in the cerebellopontine angle is described. Thirty-three cases have been treated with good results. Follow-up of 20 cases for 1 year or more showed no recurrence or facial paresis. One case required a second operation. The novelty of this approach lies in its effectiveness in relieving the symptoms without running the risk of overmanipulating the aberrant artery.

Properties of veratridine-modified single Na+ channels in guinea pig ventricular myocytes

Modification of single Na+ channels by the alkaloid neurotoxin veratridine was investigated in guinea pig ventricular myocytes using the cell-attached configuration of the patch-clamp technique. Pipette application of veratridine (50 microM) induced long-lasting openings with two different single-channel conductances of 7.6 and 3.0 pS, in addition to normal type of short openings with a single-channel conductance of 16 pS. The veratridine-modified high- and low-conductance channels appeared commonly, and they could coexist with the normal one in the same patch. The open-time distributions for the high- and low-conductance channels could be fitted by a single exponential. The mean open time for the high- and low-conductance events ranged between 19.1 ms at -120 mV and 86.0 ms at -10 mV and between 4.5 ms at -120 mV and 16.2 ms at -10 mV, respectively. The closed-time distributions for the two conductance channels consisted of at least two components, and their values and voltage dependence were similar. External Ca2+ block resulted in an apparent reduction of unitary current amplitudes with a similar voltage dependence and affinity for Ca2+ in the high- and low-conductance channels. However, the low-conductance channel was more resistant to tetrodotoxin than the high one. The probability of simultaneous occurrence of the high and low events was equal to the product of the probabilities of occurrence of the high event times that of the low event. Furthermore, we observed modified channel openings after a normal opening for the two conductance channels and a modified one turning into a normal one for the high-conductance channel. It is concluded that veratridine induces the two different types of modified Na+ channels in cardiac myocytes and these are correlated with normal openings.

Aromatic aldehydes and aromatic ketones open ATP-sensitive K+ channels in guinea-pig ventricular myocytes

Patch-clamp techniques were used to study the effects of three carbonyl compounds, 3,4-dihydroxybenzaldehyde, 2,3-dihydroxybenzaldehyde, and 2,4-dihydroxyacetophenone, on the adenosine-5'-triphosphate(ATP)-sensitive K+ channel current (IK.ATP) in guinea-pig ventricular myocytes. 3,4-Dihydroxybenzaldehyde (0.5-1 mM) shortened the action potential duration, and this effect was inhibited by application of a specific blocker of IK.ATP, glibenclamide. The shortening of the action potential duration was shown to be caused by a time-independent outward current. In the cell-attached patch configuration, all three compounds activated a kind of single-channel current, which showed an inward rectification at positive potentials and which had a linear current/voltage relation at negative potentials, having a conductance of 90 pS. The current reversed at about 0 mV in symmetrical K+ concentrations on both sides of the membrane. In excised patches this current was blocked by internal application of ATP. Thus we identified this channel as IK.ATP. The activation effects of two aromatic aldehydes were stronger than that of the aromatic ketone. The effect of these compounds on IK.ATP was not reduced by addition of cysteine (10 mM). In inside-out patches, 3,4-dihydroxybenzaldehyde increased the activity of IK.ATP, which had been blocked by 0.5 mM MgATP in the presence of 0.5 mM ADP, but the activation effect was variable and much weaker than that in the cell-attached configuration, and was completely eliminated in the absence of ADP.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple ionic mechanisms of early afterdepolarizations in isolated ventricular myocytes from guinea-pig hearts

Ionic mechanisms of early afterdepolarization (EAD) induced by the K(+)-free solution or veratridine were studied with guinea-pig ventricular myocytes using the patch-clamp technique of whole-cell and cell-attached patch configurations. In the K(+)-free solution, myocytes exhibited prolonged action potential duration with humps on the final repolarization phase, which eventually turned into EAD starting around -70 mV and induced triggered activity. Application of 0.5 mM Cd2+ inhibited the development of EAD and caused depolarization of maximum diastolic potentials around -30 mV, although Cd2+ did not prevent prolongation of the action potential. Application of 50-100 microM Ni2+ or 30 microM tetrodotoxin had little effects on EAD and diastolic potentials. The background current-voltage relation examined by a ramp voltage clamp showed inhibition of the inward rectifier K+ current, induction of steady inward current between -40 and -10 mV, and increase in the outward tail current upon repolarization in the K(+)-free solution. Cd2+ completely blocked the steady inward current at the plateau level and partially depressed the delayed outward K+ current, while Ni2+ had no effects on the background I-V relation. Tetrodotoxin showed a mild inhibitory effect on the inward component of the background current negative to -50 mV, but left the steady inward current at the plateau level. Therefore, EAD in the K(+)-free condition is mainly formed by decreased inward rectifier K+ current, activation of the L-type Ca2+ current, and time-dependent decay of the delayed outward K+ current upon repolarization. Application of 25-100 microM veratridine caused marked prolongation of action potential with appearance of regenerative EADs. Action potential prolongation and EADs were partially abolished by Cd2+ and completely eliminated by tetrodotoxin. The single channel current recordings showed a decreased current amplitude, and prolonged and delayed openings of the Na+ channel currents by veratridine. Thus, an ensemble average current showed markedly prolonged decay time constant of 609 msec in veratridine from 3.6 msec in the control. These results indicate that veratridine-induced EAD is mainly formed by altered properties of the Na+ channel current and partly by the L-type Ca2+ current due to slowed repolarization. Thus, EAD can be induced by different ionic mechanisms depending on the basal conditions.

Radioiodinated anti-hepatocellular carcinoma (HCC) ferritin. Targeting therapy, tumor imaging and anti-antibody response in HCC patients with hepatic arterial infusion

Radioimmunoimaging and radioimmunotherapy with radioiodinated anti-(hepatocellular carcinoma ferritin) antibody (131I- or 125I-FtAb) have been applied in patients with primary liver cancer. A total of 41 patients with surgically unresectable hepatocellular carcinoma (HCC) and receiving hepatic artery ligation and cannulation during exploratory laparotomy were treated with this regimen by intrahepatic arterial infusion. Compared with the control group, a decline of serum alpha-fetoprotein (65.7% versus 42.9%) and shrinkage of tumor (68.3% versus 33.9%) were observed in the treated group, and a higher second-look resection rate (31.7% versus 5.1%) and longer survival (1-year: 61.0% versus 37.3%, 3-year: 25.0% versus 6.9%) resulted. The administration of antibody through a hepatic arterial catheter (n = 16) was compared with intravenous injection (n = 17) in terms of the tumor-imaging sensitivity in 33 patients with liver cancer. The results indicated that hepatic arterial infusion was superior to intravenous injection. The sensitivity 7 days after the administration was 100% in the i.a. group and 76.5% in the i.v. group, the uptake ratio of tumor to liver being 1.74 +/- 0.57 in the former and 1.34 +/- 0.29 in the latter. Furthermore, intrahepatic arterial infusion revealed a lower anti-antibody detection rate than intravenous injection (0/14 versus 4/11).

Augmentation of cellular immune function during the early phase of zidovudine treatment of AIDS patients

Twenty-five patients with AIDS in AIDS Clinical Trials Group Protocol 002 were treated with either low or high dosages of zidovudine. This resulted in moderate, transient increases by 10 and 20 weeks in lymphocyte blastogenesis and interferon-gamma (IFN-gamma) production in vitro in response to phytohemagglutinin with and without recombinant interleukin-2. Immune responses to cytomegalovirus and herpes simplex virus type 1 antigens were augmented less frequently during therapy. Natural killer (NK) cell lysis of uninfected and human immunodeficiency virus-infected cells was also transiently increased by 10 and 20 weeks. IFN-gamma production, the only immune parameter directly associated with increases in numbers of CD4+ T cells, peaked at 10 weeks of treatment. The limited efficacy of zidovudine treatment in AIDS patients is associated with moderate, temporary increases in nonspecific and herpesvirus-specific T lymphocyte responses and NK cell function.

The catalytic subunit of cyclic AMP-dependent protein kinase directly inhibits sodium channel activities in guinea-pig ventricular myocytes

We investigated the effects of the purified catalytic subunit (C subunit) of the cAMP-dependent protein kinase (A-kinase) on the cardiac Na+ channel currents. Single Na+ channel currents in guinea-pig ventricular myocytes were recorded using the patch clamp technique of the inside-out configuration. Application of C subunit decreased the peak average current and slowed the current decay, effects which were caused by decrease in the open probability of Na+ channels and increase in the first latency, whereas the unitary current amplitude and mean open times were not affected. We conclude that the cardiac Na+ channel is directly modulated by phosphorylation process through A-kinase.

Action of nicorandil on ATP-sensitive K+ channel in guinea-pig ventricular myocytes

1. Patch-clamp techniques were used to study the effects of nicorandil (2-nicotinamiodethyl nitrate) on the adenosine 5'-triphosphate (ATP)-sensitive K+ channel current (IK.ATP) in guinea-pig ventricular myocytes. 2. Nicorandil activated the time-independent outward current. This effect was dependent on intracellular ATP concentration ([ATP]i) showing a larger effect at 2 mM than at 10 mM [ATP]i. The nicorandil-induced outward current was inhibited by application of 0.3 microM glibenclamide. 3. In the inside-out patch configuration, 0.3-1.0 mM nicorandil increased the open-stage probability of IK.ATP without a change in its conductance value (about 90pS). This effect was inhibited by glibenclamide. Analysis of the open and closed time distributions showed that nicorandil had no effect on open and closed distributions shorter than 5 ms. On the other hand, nicorandil increased the life time of bursts and decreased the interburst intervals. 4 The inward rectifier K+ channel current was not influenced by internal application of nicorandil. 5 Therefore, we conclude that IK.ATP is the only K+ current activated by nicorandil, and the main effect of nicorandil is on the kinetics of the IK.ATP bursting behaviour. These actions are similar to that of pinacidil on this preparation.

Depression of delayed outward K+ current by Co2+ in guinea pig ventricular myocytes

Effects of Co2+ on the delayed outward K+ current (IK) in guinea pig ventricular myocytes were studied using the whole cell patch-clamp technique. IK was activated by depolarizing voltage pulses positive to -30 mV and reached half-maximal activation at +24 mV. Co2+ shifted the activation curve to a more depolarized voltage range in a concentration-dependent manner, with a Co2+ concentration at which half-maximal response occurs (IC50) of 8 mM and a saturation value of +38 mV. The voltage dependency of IK gatings showed a shift similar to that of activation. In both cases the shift could be explained by screening of surface potential. The density of total negative surface charges sensed by Co2+ was estimated to be 1 e/225 A2. Co2+ also reduced the fully activated IK [IK(full)], and the dose-response curve had a Hill coefficient of 0.5 and an IC50 of 1 mM at 0 mV. Depression of IK(full) was mainly voltage independent. The single-channel unitary current estimated by fluctuation analysis was approximately 0.1 pA at -30 mV either in the absence or presence of Co2+. Therefore, the depression of IK(full) is due to an equivalent reduction in the number of functional channels. It is concluded that Co2+ depressed IK through multiple mechanisms.

Cytoreduction and sequential resection: a hope for unresectable primary liver cancer

For decades, unresectable primary liver cancer (PLC) determined by operation was incurable. However, a retrospective study of 24 years' materials with unresectable PLC indicated that 5-year survival of unresectable PLC has increased from 0% in 1966-1977 (n = 137) to 16.9% in 1978-1989 (n = 345). This encouraging improvement was mainly a result of cytoreduction therapy followed by sequential resection. Multimodality combination treatment with hepatic artery ligation, plus hepatic artery infusion with chemotherapy, plus radioimmunotherapy (or radiotherapy) yielded the highest sequential resection rate (30.6%) and 5-year survival (28.0%) as compared with double combination and single modality treatment. The 5-year survival of 33 patients receiving sequential resection after cytoreduction therapy was 63.2%. It is suggested that cytoreduction and sequential resection might offer a hope for surgically verified unresectable PLC.

Two components of use-dependent block of Na+ current by disopyramide and lidocaine in guinea pig ventricular myocytes

We studied the kinetics of the use-dependent block of the Na+ current (INa) by disopyramide and lidocaine. INa was recorded from isolated guinea pig ventricular myocytes by using the whole-cell patch-clamp technique. The use-dependent block of INa by disopyramide with 20- and 200-msec depolarizing pulses developed in two exponential functions. The degree of the use-dependent block and the amplitude of the fast (Af) and slow (As) components with the short (20-msec) pulse protocol were comparable to those with the long (200-msec) pulse protocol. When pH was raised from 7.3 to 8.0, disopyramide increased Af without a change in As. At pH 6.5, INa block developed with a single exponential function revealing only the slow component. The fast and slow components of INa block by disopyramide could be explained by binding of the uncharged and charged forms, respectively, to the activated state of the channel. Development of INa block by lidocaine also was expressed by two exponentials at all pulse durations (5-200 msec). As pulse durations were prolonged or holding potentials were depolarized, the degree of the use-dependent block and Af increased. When pH was lowered to 6.5, the short pulse produced only the slow component, whereas the long pulse caused two exponentials with decreased Af and increased As. Internal application of QX-314, a permanently charged lidocaine analogue, produced a single exponential block of INa with a very slow onset rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Interrelation between pinacidil and intracellular ATP concentrations on activation of the ATP-sensitive K+ current in guinea pig ventricular myocytes

The patch-clamp technique was used to study the relation between pinacidil and intracellular ATP concentration [( ATP]i) on the activation of the outward K+ current in guinea pig ventricular myocytes. Pinacidil shortened the action potential duration, exhibiting stronger effect at 2 mM [ATP]i than at 5 mM [ATP]i. Pinacidil at 5 microM or higher concentrations activated the time-independent outward current at potentials positive to -80 mV, and the pinacidil-activated current was suppressed by increasing [ATP]i from 2 to 5 mM. The dose-response curve of pinacidil at different [ATP]i showed a shift to the right and a depression of the maximum response at increased [ATP]i. The pinacidil-induced shortening of the action potential duration and outward current were inhibited by application of 0.3-1.0 microM glibenclamide. In single-channel current recordings, pinacidil activated the intracellular ATP-sensitive K+ channel current without changing the unitary amplitude, and increased open probability of the channel, an effect dependent on [ATP]i. The pinacidil-activated single-channel current was blocked by glibenclamide. These results prove the notion that pinacidil activates the ATP-sensitive K+ channel current, which explains the action potential shortening in cardiac cells after application of pinacidil.