Evidence for inhibition by ICS 205-930 and stimulation by BRL 34915 of K+ conductance in cardiac muscle

Abolition of drug-induced early afterdepolarizations by potassium channel activators in guinea-pig Purkinje fibres

1. Drug-induced early afterdepolarizations (EAD) are considered to be the underlying mechanism of the polymorphic ventricular dysrhythmia torsades de pointes. Sotalol and disopyramide are well known to generate EAD. Therefore, it was of interest to study the effects of potassium channel activators, such as nicorandil, pinacidil and lemakalim, on those drug-induced EAD in spontaneously beating guinea-pig Purkinje fibres using the intracellular microelectrode technique. 2. Early afterdepolarizations induced by sotalol at concentrations of 80 and 160 micromol/L could be completely abolished by nicorandil at concentrations between 50 and 500 micromol/L. The extracellular K+ concentration was 2.7 mmol/L. 3. Disopyramide-induced EAD at concentrations of 10, 20 and 30 micromol/L in a Tyrode's solution containing 1.35 mmol/L K+ and these EAD were abolished by pinacidil (30 and 100 micromol/L) and lemakalim (10 and 30 micromol/L). 4. Early afterdepolarizations could be regenerated by superfusion of Purkinje fibres with K+ channel activator-free Tyrode's solution containing either sotalol or disopyramide. 5. Our results demonstrate that drug-induced EAD can be abolished by K+ channel activators and, therefore, may provide anti-arrhythmic effects in heart diseases.

Single-blind study of the effects of intravenous dolasetron mesylate versus ondansetron on electrocardiographic parameters in normal volunteers

A single-blind, randomized, five-way cross-over, safety and tolerability trial was conducted to determine whether intravenous (i.v.) dolasetron mesylate at varying single doses induces changes in ECG intervals in healthy volunteers and to compare these changes with a single intravenous dose of ondansetron or placebo. Thirty healthy male volunteers received 1.2, 1.8, and 2.4 mg/kg i.v. dolasetron mesylate, 32 mg i.v. ondansetron, and placebo on 5 separate days. ECGs were recorded at intervals during the 24 h after study drug administration. The changes in ECG intervals observed after dolasetron mesylate or ondansetron were acute, transient, and asymptomatic. Dolasetron mesylate resulted in slight but statistically significant dose-related increases in heart rate (HR) and PR and QRS intervals (between h 0 and 4). A statistically significant increase in QTc interval was detected with both dolasetron mesylate (2.4 mg/kg) and ondansetron. Ondansetron also produced a slight but statistically significant increase in JT interval and a decrease in HR. These changes in ECG intervals were usually observed between h 0 and 4; all parameters returned to baseline within 8 h of treatment. The results demonstrate that both dolasetron mesylate and ondansetron prolong the QTc interval. However, dolasetron mesylate predominantly altered ECG parameters indicative of ventricular depolarization (QRS duration), whereas ondansetron predominantly affected ventricular repolarization as measured by a prolongation in the JT interval. Both dolasetron and ondansetron were well tolerated. The adverse event (AE) rate was 13.3% (4 of 30); all AE were of mild or moderate severity and were distributed across all dose arms.

5-HT3 receptor-independent inhibition of the depolarization-induced 86Rb efflux from human neuroblastoma cells, TE671, by ondansetron

The 5-HT3-receptor antagonist, ondansetron, has been shown to have positive effects in selected in-vivo models of memory impairment and anxiety. The exact mechanisms underlying such bioactivities are unknown. In the present work, an 86Rb efflux bioassay was used to show that ondansetron has a unique ability to block voltage-gated potassium channels in TE671 human neuroblastoma cells. This intrinsic potassium-channel-blocking (KCB) property is relatively weak (IC50 20 microM), but is not shared by other 5-HT3-receptor ligands including zatosetron, MDL 72222, LY 278, 584, zacopride, 1-phenylbiguanide, and ICS 205-930 (tropisetron). Pre-incubation of the target neuroblastoma cells with several 5-HT-receptor ligands including 5-hydroxytryptamine, 8-OH-DPAT, ketanserin, 2-methyl-5-HT, as well as a number of potent 5-HT3 agonists and antagonists and two selective neurotoxins, failed to abolish the KCB action of ondansetron. A preliminary structure-activity relationship analysis indicates that the KCB activity of ondansetron is almost entirely attributable to its structural nucleus, 2,3-dihyro-9-methyl-4(1H)-carbazolone. It is hypothesized that the KCB action of ondansetron is mediated through receptors other than 5-HT3 receptors. The KCB activity of ondansetron may be a significant factor in the in-vivo cognition-enhancing activities of this compound, conceivably due to depolarization of the hippocampal synaptic membranes and a consequent augmentation of neurotransmission.

Block of the delayed rectifier current (IK) by the 5-HT3 antagonists ondansetron and granisetron in feline ventricular myocytes

1. We investigated the effects of two 5-HT3 antagonists, ondansetron and granisetron, on the action potential duration (APD) and the delayed rectifier current (IK) of feline isolated ventricular myocytes. Whole-cell current and action potential recordings were performed at 37 degrees C with the patch clamp technique. 2. Ondansetron and granisetron blocked IK with a KD of 1.7 +/- 1.0 and 4.3 +/- 1.7 microM, respectively. At a higher concentration (30 microM), both drugs blocked the inward rectifier (IKl). 3. The block of IK was dependent on channel activation. Both drugs slowed the decay of IK tail currents and produced a crossover with the pre-drug current trace. These results are consistent with block and unblock from the open state of the channel. 4. Granisetron showed an intrinsic voltage-dependence as the block increased with depolarization. The equivalent voltage-dependency of block (delta) was 0.10 +/- 0.04, suggesting that granisetron blocks from the intracellular side at a binding site located 10% across the transmembrane electrical field. 5. Ondansetron (1 microM) and granisetron (3 microM) prolonged APD by about 30% at 0.5 Hz. The prolongation of APD by ondansetron was abolished at faster frequencies (3 Hz) showing reverse rate dependence. 6. In conclusion, the 5-HT3 antagonists, ondansetron and granisetron, are open state blockers of the ventricular delayed rectifier and show a clear class III action.

Constipation evoked by 5-HT3-receptor antagonism: evidence for heterogeneous efficacy among different antagonists in guinea-pigs

The abilities of selective 5-HT3-receptor antagonists to evoke constipation were examined in conscious guinea-pigs and in preparations of guinea-pig isolated colon. Compared with vehicle-treated guinea-pigs, acute doses of granisetron (0.1, 1 and 10 mg kg-1, i.p.) and tropisetron (10 mg kg-1, i.p., but not 1 and 0.1 mg kg-1, i.p.) significantly (P < 0.05) reduced the total number of faecal pellets excreted during a 12-h observation period. By contrast, BRL 46470 (0.1-10 mg kg-1, i.p.) had no significant effect on the incidence of defecation. Mid-to-distal lengths of guinea-pig isolated colon spontaneously expelled faecal pellets. Granisetron (0.1 and 1 microM) and tropisetron (1 microM) reduced or prevented the rate at which they were spontaneously expelled. Morphine (0.1 microM) and clonidine (10 nM) also showed faecal pellet transit time. Naloxone (0.1 microM) had no effects alone, but reversed the actions of granisetron, morphine and clonidine. BRL 46470 (1 microM) had no significant effect on the transit of faecal pellets in guinea-pig isolated colon. In segments of guinea-pig isolated colon which did not contain faecal pellets, granisetron, tropisetron and BRL 46470 antagonized the ability of 5-HT to evoke cholinergically-mediated contractions of the longitudinal muscle. The respective pA2 values and slopes of the Schild plots were 8.5 +/- 0.05, slope 1.06 +/- 0.03; 8.5 +/- 0.1, slope 0.91 +/- 0.04; and 7.9 +/- 0.1, slope 0.93 +/- 0.05. Our experiments suggest that not all 5-HT3-receptor antagonists are the same.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of SB 204070, a highly potent and selective 5-HT4 receptor antagonist, on guinea-pig distal colon

1. The pharmacology of a novel 5-HT4 receptor antagonist, SB 204070 has been evaluated in the guinea-pig isolated distal colon longitudinal muscle-myenteric plexus (LMMP). 2. SB 204070 is a highly potent antagonist of 5-HT-evoked cholinergically-mediated contractions in the guinea-pig distal colon. Low concentrations (10-100 pM) produced a shift to the right of the curve (apparent pA2 10.8 +/- 0.1) with no significant effect on the maximum response. With higher concentrations of SB 204070 (300 pM and above), the maximum response to 5-HT was reduced. 3. When tested against the partial 5-HT4 receptor agonist, BIMU 1, SB 204070 was active at similar low concentrations (10 pM and above) but produced a reduction in maximum, with no prior shift to the right of the curve, at all concentrations tested (10-300 pM). 4. The antagonism seen with SB 204070 is unlikely to be due to a non-selective effect since high concentrations (10 nM and 1 microM) of the compound had no effect on cholinergically-mediated contractions evoked by the nicotinic receptor agonist, DMPP, in the same preparation. SB 204070 is unlikely to be an irreversible antagonist since the effects of the compound could be reversed upon washing of the tissue. 5. Radioligand binding studies show that SB 204070 has a greater that 5000 fold selectivity for the 5-HT4 receptor over 5-HT1A, 5-HT1D, 5-HT1E, 5-HT2A, 5-HT2C, 5-HT3, GABAA, BDZ, TBPS, A1 adenosine receptors, alpha 1, alpha 2, beta 1, beta 2 adrenoceptors and D1, D2 and D3 dopamine receptors. 6. SB 204070 is a highly potent, highly selective 5-HT4 receptor antagonist and as such is an important new tool in evaluating the functional role of the 5-HT4 receptor.

Electrophysiologic and anticholinergic effects of pirmenol enantiomers in guinea-pig myocardium

Since it has been reported that several class I drugs stereoselectively block sodium channels, potassium channels and muscarinic receptors in cardiac tissues, electrophysiologic and anticholinergic effects of enantiomers of pirmenol, a class I antiarrhythmic drug, were examined. Both (+) and (-) pirmenol depressed the maximum upstroke velocity (Vmax) of the action potential in a concentration-dependent manner in guinea-pig papillary muscles driven at 1.0 Hz, and there was no significant difference in the potency of the class I effect between the enantiomers. The onset rates of use-dependent block (UDB) of Vmax at 2.0 Hz for 10 mumol/l (+) and (-) pirmenol were 0.30 +/- 0.03 and 0.29 +/- 0.01 per action potential, and the recovery time constants from UDB for (+) and (-) pirmenol were 27.0 +/- 2.7 and 27.7 +/- 1.9 s, respectively, indicating no difference in the binding and unbinding kinetics to the sodium channel between the enantiomers. Both (+) pirmenol and (-) pirmenol prolonged action potential duration (APD) at low concentrations (1-10 mumol/l) and shortened it at high concentrations (30-100 mumol/l). Again, there was little difference with respect to the effects on APD between the enantiomers. However, in the isolated guinea-pig left atria (-) pirmenol more potently antagonized the negative inotropic effect of carbachol than (+) pirmenol, and the pA2 values for (+) and (-) pirmenol were 6.41 and 6.71, respectively. The functional study was supported by the radioligand binding experiments using [3H]N-methylscopolamine ([3H]NMS) in guinea-pig left atrial membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Baroreflex and beta-adrenoceptor function are diminished in rat cardiac hypertrophy due to volume overload

We investigated whether cardiac hypertrophy induced by volume loading influences baroreflex sensitivity. Aortic insufficiency (AI) was induced in male Wistar rats by graded disruption of the aortic valve, which, after 2 weeks, resulted in a 30% increase in heart/body weight or left-ventricular/body weight ratio compared with control animals. Baroreflex sensitivity was assessed in conscious animals by measuring the heart rate (HR) responses to the changes in mean arterial pressure (MAP) induced by phenylephrine and nitroprusside sodium at 2 weeks. The slopes of the HR vs MAP plots obtained with phenylephrine and nitroprusside decreased significantly with increasing heart weight/body weight ratio (correlation coefficient r = 0.625 and 0.526, respectively). In isolated right atria from AI animals baseline rate was higher, and the isoproterenol effect on sinus rate was significantly smaller than in atria from control animals, indicating a dysfunction of the beta-adrenoceptor pathway. The data show that baroreflex dysfunction associated with a down-regulation of the beta-adrenoceptor pathway of the sinus node develops simultaneously with volume overload-induced hypertrophy in the absence of overt heart failure.

The 5-HT4 receptor subtype inhibits K+ current in colliculi neurones via activation of a cyclic AMP-dependent protein kinase

1. The aim of the present study was to examine the effect of 5-hydroxytryptamine (5-HT) on K+ current in primary culture of mouse colliculi neurones and to identify the 5-HT receptor subtype that could be involved in this effect. 2. The voltage-activated K+ current of the neurones was partially blocked by 8-bromo adenosine 3':5'-cyclic monophosphate (8-bromo-cyclic AMP). This effect was mimicked by 5-HT and the action of 5-HT could be antagonized by H7, a non specific protein kinase inhibitor, and by PKI, the specific cyclic AMP-dependent protein kinase blocker. 3. A similar cyclic AMP-dependent blockade of the K+ current was found with renzapride (BRL 24,924) and other 5-HT4 receptor agonists such as cisapride, BIMU 8, zacopride and 5-methoxytryptamine (5-MeOT). ICS 205,930, the classical 5-HT4 receptor blocker, could not be used in this study because it inhibited the studied K+ current by itself. However, the novel 5-HT4 receptor antagonist, DAU 6285 blocked the effects of 5-HT and renzapride on the K+ current. 4. The current was insensitive to the 5-HT1 and 5-HT3 receptor agonists (8-hydroxy-2-(di-n-propylamino) tetralin, RU 24,969, carboxamidotryptamine, 2-CH3-5-HT) as well as to 5-HT1, 5-HT2 and 5-HT3 antagonists (methiothepin, ketanserin, ondansetron [GR 38,032]). Moreover, these antagonists did not affect the actions of the tested 5-HT4 receptor agonists. 5. The present results show that part of the voltage-activated K+ current in mouse colliculi neurones is cyclic AMP-sensitive and the blockade of the current by 5-HT involves the 5-HT4 receptor subtype.The putative implication of 5-HT4 receptors in neuronal plasticity, via a blockade of K+ channels, is discussed.

Clinical pharmacology of potassium channel openers

Opening of K+ channels in cell membranes with resulting increase in K+ conductance, shifts the membrane potential in a hyperpolarizing direction towards the K+ equilibrium potential. Hyperpolarization reduces the opening probability of ion channels involved in membrane depolarization and excitation is reduced. K+ channel openers are believed to hyperpolarize smooth muscle cells by a direct action on the cell membrane. The best known members of the group are cromakalim, nicorandil and pinacidil, but several new compounds are being evaluated. In addition, it has recently been shown that also clinically well-known drugs like, e.g. diazoxide and minoxidil exhibit K+ channel opening properties. Nicorandil and new compounds containing nitro groups have a dual mechanism of action, also activating guanylate cyclase, an effect that contributes to their cardiovascular effect profile. K+ channel openers have a wide range of effects. Some of their properties and actions are summarized, and their present applications and/or potential for future application, in e.g. hypertension, angina pectoris, asthma, bladder instability, and several other disorders are discussed. It is concluded that K+ channel openning represents an interesting pharmacological principle with many potential clinical applications. However, most available drugs do not seem to have a sufficient tissue selectivity to be useful therapeutic alternatives. Before the potential of the new members of the group on clinical trials can be properly evaluated, clinical experiences are needed.

Modulation of intracellular calcium by potassium channel openers in vascular muscle

We investigated two putative K+ channel openers, pinacidil and BRL34915 (cromakalim), and demonstrated their vasorelaxant effectiveness on rat artery contractions induced by K+, tetraethylammonium (TEA), or norepinephrine. The K+ channel opener-induced decrease in tension was rapid, even when tension was stimulated by 100 mmol/l K+. Measurements of intracellular free Ca++ (activity) by ultra-high sensitivity digital imaging microscopy was carried out by briefly loaded fura 2 (fluorescence ratio) quantitation in isolated, contracting cells of rat azygos vein. Submicron resolution was achieved by measuring cytoplasmic Ca(++)-sensitive fluorescence at each pixel, and size and intensity of areas with high Ca++ concentrations, called hot spots, were determined by a computer-generated, 3 lambda algorithm. Hot spots, which most likely represent the sites of Ca++ release and re-uptake by Ca(++)-regulatory organelles, increased in size and intensity upon addition of K+ or norepinephrine, reaching an early peak prior to the whole cell average peak in cytoplasmic Ca++ activity. Both norepinephrine and K(+)-induced stimulation resulted in Ca++ activity increases that were primarily due to Ca++ release from storage sites. Reduction of free Ca++ activity to resting or lower levels occurred upon addition of pinacidil or cromakalim. Intracellular Ca++ decreases due to K+ channel openers appeared abruptly beginning at the central portions of the cells, resulting in a pronounced early drop in central Ca++ activity while elevated Ca++ levels persisted at the periphery. While this late stage residual of peripheral Ca++ appears to be a significant step in the vascular muscle relaxant action of both K+ channel opener drugs, the level of Ca++ at peripheral sites was greater in response to pinacidil than to cromakalim. The results of this study suggest that in addition to increasing K+ conductance, pinacidil and cromakalim cause 1) decreased Ca++ activity in central regions of the myocytes, and 2) a shift in Ca++ distribution to primarily subsarcolemmal sites. These observations lead us to hypothesize separate control of peripheral and central Ca++ activity within a vascular muscle cell, with Ca++ redistribution that can be altered by vasorelaxants. We suggest that intracellular Ca++ redistribution may contribute the membrane potential-independent part of the vasorelaxant action of the K+ channel openers.

Investigation of the 5-hydroxytryptamine receptor mechanism mediating the short-circuit current response in rat colon

1. 5-Hydroxytryptamine (5-HT) stimulated an increase in short-circuit current (SCC) in rat isolated colonic mucosa with an EC50 value of approximately 4 microM. The purpose of the present study was to investigate the 5-HT receptor mechanism(s) involved in this response. 2. The relatively selective 5-HT receptor agonists 5-carboxamidotryptamine (5-CT) and alpha-methyl-5-HT stimulated SCC and were 6 to 8 times less potent than 5-HT. 2-Methyl-5-HT was inactive both as an agonist and an antagonist. 3. The following compounds produced no significant inhibition of the SCC response to 5-HT: ketanserin (1 microM), methysergide (1 microM), methiothepin (0.3 microM), GR38032F (0.3 microM), tetrodotoxin (0.3 microM) and sulpiride (1 microM). 4. Both metoclopramide (3 and 10 microM) and cisapride (0.1 and 1 microM) inhibited the SCC responses to 5-HT in a concentration-related manner, and the higher doses similarly inhibited the responses to 5-CT. With both agonists the inhibitory effects of metoclopramide and cisapride were insurmountable. However, these inhibitory actions appeared to be selective since neither metoclopramide nor cisapride affected the basal SCC or the SCC response to prostaglandin E2. 5. The SCC responses to 5-HT and 5-methoxytryptamine were selectively inhibited by ICS205-930 at 3 microM, and respective pKB values of 6.0 and 6.6 were calculated. 6. It is concluded that 5-HT stimulates an SCC response in rat colon via a receptor mechanism that cannot be clearly identified as 5-HT1-like, 5-HT2 or 5-HT3. This receptor is selectively antagonized by ICS 205-930 and by the benzamides, metoclopramide and cisapride. The 5-HT receptor in rat colon therefore exhibits some of the properties associated with the so-called 5-HT4 receptor.

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.

Suppression of repolarization-related arrhythmias in vitro and in vivo by low-dose potassium channel activators

Marked prolongation of cardiac action potentials and of QT intervals has been associated with early afterdepolarizations and triggered activity in vitro and with ventricular tachycardia in vivo. Because the antihypertensive potassium channel activators pinacidil and cromakalim are known to accelerate repolarization in cardiac tissues, we performed in vitro and in vivo experiments to test the hypothesis that these agents would block the arrhythmogenic effects of delayed repolarization. Early afterdepolarizations and triggered activity were elicited in canine cardiac Purkinje fibers driven at cycle lengths of 4 seconds or more (K0, 2.7 mM) during superfusion with quinidine, cesium, or sematilide, a methylsulfonylamino parasubstituted analogue of procainamide with class III antiarrhythmic activity. The potassium channel activators invariably (17 of 17) abolished this form of abnormal automaticity. This effect was observed at low concentrations that did not alter action potential characteristics at shorter cycle lengths. Intravenous Cs+ (total dose, 4.5 mM/kg) was used to produce ventricular arrhythmias in anesthetized rabbits randomly pretreated in a double-blind fashion with either low-dose pinacidil (0.2 mg/kg) or vehicle. Pinacidil pretreatment resulted in significantly fewer total ventricular ectopic beats (168 +/- 157 versus 582 +/- 448, p less than 0.005) and episodes of ventricular tachycardia (four of nine versus nine of nine, p = 0.057). At this dose, pinacidil did not alter mean blood pressure before Cs+ and maximal hypertensive response after Cs+. In summary, the potassium channel activators pinacidil and cromakalim suppressed triggered activity related to prolonged repolarization at concentrations that did not affect action potential characteristics at normal rates in vitro; pinacidil blunted arrhythmias produced by cesium administration in vivo without lowering blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Action of cromakalim on potassium membrane conductance in isolated heart myocytes of frog

1. The effects of cromakalim on membrane currents were studied at 20 degrees C in frog atrial and ventricular cells in patch clamp recording using the whole cell configuration. 2. When cromakalim (1 microM) was applied in the external medium, a time-independent current was activated in a few minutes. Cromakalim induced a weak increase of inward membrane currents recorded during hyperpolarization and a large increase of outward membrane currents recorded during depolarization. 3. The current voltage relationship of the cromakalim-induced current reversed near EK and rectified in the outward direction. 4. The cromakalim-activated current was inhibited by external application of cesium (20 mM), barium (1.8 mM), tolbutamide (1 mM) and glibenclamide (1 microM). 5. The effects of cromakalim were insensitive to a cytosolic increase in adenosine 5'-triphosphate (ATP, 3-5 mM). Cromakalim had no effects when applied in the cell. 6. Our results confirm that cromakalim activates an IK(ATP)-like conductance and suggest that the effects of the drug are due to an action on the external side of the membrane and are independent of the ATP cell content.

Characterization of responses to cromakalim and pinacidil in smooth and cardiac muscle by use of selective antagonists

1. In dog isolated coronary artery (precontracted with endothelin, 10 nM) cromakalim (0.1-30 microM) and pinacidil (1-30 microM) produced concentration-dependent vasorelaxant responses. The effects of these compounds could be blocked by glibenclamide (3 microM), phentolamine (30 microM) or alinidine (30 microM) to a similar extent, indicating that both agents alter vascular tone through the same mechanism in this preparation. 2. The ability of the antagonists glibenclamide, phentolamine and alinidine to block the response to cromakalim in a number of smooth muscle types from the guniea-pig was determined. Cromakalim (0.1-30 microM) produced concentration-dependent relaxant responses in thoracic aorta (precontracted with endothelin, 30 nM), ileum (precontracted with K+, 25 mM) and trachea (spontaneously contracted). Responses to cromakalim in all tissues could be blocked by the three antagonists. However, significantly higher concentrations of the antagonists were required to block responses in the thoracic aorta than in the ileum or trachea. Given that the rank order of potency of the antagonists was similar in all tissues (i.e. glibenclamide greater than phentolamine = alinidine), this result may suggest vascular K+ channels opened by cromakalim are quantitatively but not qualitatively different in vascular compared with non-vascular smooth muscle. Glibenclamide was approximately 10 times more potent than phentolamine or alinidine. 3. Cromakalim had minimal functional effects on the rat spontaneously beating right atrial (rate) or electrically driven left ventricular strip (force) preparations. Similarly the three antagonists studied failed to alter force generation in the right ventricular strip. However alinidine and phentolamine did produce a dose-related bradycardia in the spontaneously beating right atria. This effect appears to be unrelated to blockade of the K+ channel opened by cromakalim since glibenclamide, the most potent K+ channel antagonist studied, failed to produce the same response. 4. It would appear that the K+ channel opened by cromakalim is present in a number of vascular and non-vascular smooth muscle. Based on the potency of the three antagonists studied, there appears to be little heterogeneity in the process activated by cromakalim in vascular and non-vascular smooth muscle.

Ionic currents contributing to the action potential in single ventricular myocytes of the guinea pig studied with action potential clamp

With the action potential clamp procedure we studied the contribution of various ionic currents to the action potential in single ventricular myocytes. Action potentials were elicited by a current pulse through the suction pipette and recorded by a computer. A representative action potential was then repetitively replayed to the same cell under voltage-clamp conditions. Successive pharmacological blocks of ionic currents allowed for the first time the measurement of the contribution of the L-type calcium current (ICa) and the [Ca2+]i-activated currents as well as the potassium current to the action potential. Experiments using caffeine as a tool to increase calcium release from the sarcoplasmic reticulum supported the idea that INaCa contributes to the plateau during the second half of the action potential and even lasts into diastole, whereas strong elevation of the intracellular [Ca]i during the action potential additionally activated the non-specific cation channel.

The gastrointestinal prokinetic benzamide derivatives are agonists at the non-classical 5-HT receptor (5-HT4) positively coupled to adenylate cyclase in neurons

We have previusly shown that a non-classical 5-hydroxytryptamine (5-HT4) receptor mediates the stimulation of adenylate cyclase activity in mouse embryo colliculi neurons in primary culture. The pharmacological characteristics of this receptor exclude the possibility that it belongs to the known 5-HT1, 5-HT2 or 5-HT3 receptor types. Here we report that this 5-HT receptor can be stimulated by 4-amino-5-chloro-2-methoxy substituted benzamide derivatives. All these compounds have been reported to be potent stimulants of gastrointestinal motility and some of them are 5-HT3 receptor antagonists. The rank order of potency of these substituted benzamide derivatives in stimulating cAMP formation was: cisapride greater than BRL 24924 greater than 5-HT greater than zacopride greater than BRL 20627 greater than metoclopramide. The non-additivity of benzamide and 5-HT activities suggests that 5-HT and the substituted benzamide derivatives act on the same receptor. Only ICS 205930, a recognized 5-HT3 receptor antagonist, competitively antagonized the stimulatory effect of cisapride, zacopride and BRL 24924. However, its pKi (6-6.3) for this new receptor was very different from its pKi for 5-HT3 receptors (pKi = 8-10). Other selective 5-HT3 receptor antagonists with an indole group (BRL 43694 and GR 38032F), with a benzoate group (cocaïne, MDL 72222) or with a piperazine group (quipazine) were ineffective in reversing the stimulatory effect of benzamide derivatives. Exposure of neuronal cells to potent agonists at this receptor such as BRL 24924 rapidly reduces its capacity to stimulate cAMP production.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of potassium channel openers and blockers in canine atrial muscle

1. The possibility that the interaction between potassium channel openers, e.g. cromakalim, pinacidil and nicorandil, and some potassium channel blockers involves a common site was investigated in canine atrial muscle. 2. Cromakalim, pinacidil and nicorandil produced a negative inotropic effect, their pD2 (-log EC50) values being 6.11 +/- 0.07, 5.37 +/- 0.09 and 4.55 +/- 0.07, respectively. 3. The potassium channel blockers, tetraethylammonium (TEA), tetrabutylammonium (TBA), 3,4-diaminopyridine (DAP), CsCl and BaCl2 all produced a positive inotropic effect. 4. The concentration-effect curves for the negative inotropic actions of pinacidil were shifted in a parallel way to the right by low concentrations of TEA, TBA or BaCl2. Maximum responses to pinacidil were depressed by higher concentrations of the blockers. An analysis of the non-competitive antagonism by TEA yielded pKA (-log KA) values of 4.00-4.05 for pinacidil. 5. The concentration-effect curves for cromakalim and nicorandil were shifted by TEA similarly to those for pinacidil, and a similar analysis yielded pKA values of 4.47-4.68 for cromakalim and 3.47-3.74 for nicorandil. 6. The KA values of cromakalim, pinacidil and nicorandil were about 10-30 times greater than their EC50 values, indicating that there are non-linear stimulus-effect relationships between the binding of the three potassium channel openers to their binding sites at potassium channels and their negative inotropic effects. 7. The dissociation constants for TEA could also be estimated from pA2 and pKB values for antagonizing competitively and non-competitively the negative inotropic effects of the three potassium channel openers; they were 3.47-3.89, and did not differ between the potassium channel openers. 8. The concentration-effect curves for the three potassium channel openers were not affected by DAP or CsCl. 9. These results suggest the following: (i) quaternary ammonium compounds like TEA and TBA antagonize the negative inotropic effect of cromakalim, pinacidil and nicorandil by binding to potassium channels, thus preventing binding of the channel openers to the same sites or closely related sites in canine right atrial muscles.

Phentolamine and structurally related compounds selectively antagonize the vascular actions of the K+ channel opener, cromromakalim

1. The effects of cromakalim, a novel vasodilator agent believed to open K+ channels, were studied in a range of large and small arteries in vitro. In dog isolated coronary artery, precontracted with U46619 (a thromboxane A2-mimetic), cromakalim caused concentration-dependent relaxation which could be inhibited by phentolamine (10-100 microM). 2. The ability of phentolamine to antagonize cromakalim was selective since it did not affect responses to a number of other vasodilators including isoprenaline, nitroprusside or nicorandil. 3. The effect of phentolamine was not related to its alpha-adrenoceptor blocking actions since other alpha-adrenoceptor antagonists (prazosin 10 microM, rauwolscine 10 microM and phenoxybenzamine 1 microM) failed to influence the action of cromakalim. 4. A number of compounds structurally related to phentolamine were also able to block the vaso-relaxant response to cromakalim in the dog isolated coronary artery. The rank order of potency was alinidine = phentolamine = ST91 greater than tramazoline = naphazoline. Clonidine and tolazoline were inactive. The most potent compounds (alinidine and phentolamine) were effective only at concentrations above 1 microM. 5. Electrophysiological studies, in which resting membrane potential and tension were measured simultaneously, were carried out on rat isolated femoral artery. Phentolamine (30 microM) antagonized both the vasorelaxation and hyperpolarization caused by cromakalim. 6. These results suggest that phentolamine and some structurally related compounds, may inhibit K+ channel opening, an action which would account for their ability to antagonize the actions of cromakalim. Such compounds may prove useful in determining the role of K+ channels in regulating vascular smooth muscle tone in vivo and in vitro.

Substance P and serotonin act synergistically to activate a cation permeability in a neuronal cell line

Both substance P and, to a lesser degree, serotonin activate cation permeability in neuroblastoma x glioma hybrid cells, as determined by measurement of [14C]guanidinium uptake. Serotonin potentiates the action of substance P by shifting the concentration-effect curve of substance P to the left. The EC50 value for the synergistic effect of serotonin was around 0.3 microM. Dopamine and noradrenaline displayed comparable activity, albeit only at 50 and 130 times higher concentrations, respectively. The order of potency of various substance P-analogues was not changed by serotonin, indicating that the specificity of the substance P site on the hybrid cells was not affected by serotonin. Various other neurotransmitters and peptides had no effect on the response of the hybrid cells to substance P. The serotonin receptor interacting with the substance P receptor may be classified as a 5-HT3-receptor since methysergide, cimetidine, and ketanserin were ineffective, but two inhibitors specific for 5-HT3-receptors, ICS 205-930 (3 alpha-tropanyl-1H-indole-3-carboxylic acid ester) and MDL 72222 (1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate), blocked the effect of serotonin at nanomolar concentrations. However, the two serotonin antagonists might also be blocking the ion permeability, since at higher concentrations they fully inhibited the stimulation of guanidinium uptake by substance P or by substance P plus serotonin. The synergism between substance P and serotonin on the hybrid cells offers the opportunity to study the mechanism of interaction of neurotransmitter receptors on a permanent neuronal cell line.

The antiarrhythmic properties of beta-adrenoceptor antagonists

The antiarrhythmic properties of beta-adrenoceptor antagonists have long been recognized and, according to the Vaughan Williams classification of antiarrhythmic drugs, these drugs are regarded as a separate entity, namely class II. In this review, Ad IJzerman and Willem Soudijn focus on the nature and the molecular basis of the antiarrhythmic action of beta-adrenoceptor antagonists. Structure-activity relationship studies demonstrate that the antiarrhythmic profile of different beta-adrenoceptor antagonists is not fixed and some compounds have class I or class III properties which may be clinically relevant.

BRL 34915 (cromakalim) activates ATP-sensitive K+ current in cardiac muscle

The mechanism by which the antihypertensive agent BRL 34915 (cromakalim) affects action potential duration (APD) and effective refractory period (ERP) in isolated cardiac muscle was investigated. BRL 34915 (greater than or equal to 3 microM) shortened ERP of ferret (Mustela putorius furo) and guinea pig (Cavia porcellus) papillary muscles in a concentration-dependent fashion. The reduction in ERP resulted from a decrease in APD. ERP and APD of papillary muscles were also reduced during hypoxia produced by bubbling the physiological bathing solution with N2 instead of O2. Reduction of APD during hypoxia has previously been attributed to activation of ATP-sensitive K+ channels in heart. Glyburide, an inhibitor of ATP-sensitive K+ channels, prevented or reversed the shortening of ERP and APD produced by hypoxia and BRL 34915, respectively. These results suggest that BRL 34915 acts by opening ATP-sensitive K+ channels in heart. The actions of BRL 34915 were temperature-dependent, decreasing ERP 64% at 37 degrees C, but having no effect at 22 degrees C. The effect of BRL 34915 on K+ currents was tested directly in voltage-clamped guinea pig ventricular myocytes. As observed with the papillary muscles, BRL 34915 was without effect at 22 degrees C. At 36 degrees C, BRL 34915 (after a delay) increased outward currents positive to, and less so at potentials negative to, the K+ current reversal potential. The normal inwardly rectifying current-voltage relationship for peak K+ currents during 200-msec pulses was changed to one that was nearly ohmic. The current activated by BRL 34915 was blocked by glyburide. The data support the hypothesis that BRL 34915, like hypoxia, activates ATP-sensitive K+ channels in the heart. Based upon the profound temperature sensitivity of BRL 34915 action, this activation may be indirect, perhaps by means of modulation of an enzymatic activity that regulates gating of these channels. BRL 34915 and glyburide will be valuable tools for studying the role of ATP-sensitive K+ channels in normal and abnormal cardiac function.

The negative inotropic effect of nicorandil is independent of cyclic GMP changes: a comparison with pinacidil and cromakalim in canine atrial muscle

1. The negative inotropic effects of nicorandil, a nitrate with K-channel opening properties, have been compared with those of pinacidil, cromakalim and nifedipine, in canine right atrial muscle. 2. Cromakalim, pinacidil and nicorandil all produced a negative inotropic effect. However, even at their maximally effective concentrations, the force of contraction remained at about 10% of control levels, whereas contraction was abolished by nifedipine. 3. The pD2 values for the negative inotropic effects of cromakalim, pinacidil and nicorandil were 5.93, 5.37, and 4.35, respectively. 4. The negative inotropic effects of cromakalim (3 x 10(-5)M), pinacidil (3 x 10(-5) M and 3 x 10(-4) M) and nicorandil (3 x 10(-5) M) were not accompanied by changes in cyclic AMP and cyclic GMP levels, whereas that of 3 x 10(-4) M nicorandil was accompanied by an increase in cyclic GMP but not cyclic AMP concentrations. 5. The negative inotropic effect produced by 3 x 10(-4) M nicorandil was greatly reduced by 10(-2) M tetraethylammonium, whereas the increase in cyclic GMP produced by this concentration of nicorandil was not significantly changed. Sodium nitroprusside (10(-3) M) produced a large increase in cyclic GMP concentrations but had no significant negative inotropic effect. 6. It is concluded that the negative inotropic effects of nicorandil like those of cromakalim and pinacidil do not result from an increase in cyclic GMP concentrations. Instead these effects may be due to their action as K-channel openers.