Effects of diacetyl monoxime (DAM) on slow and fast action potentials of young and old embryonic chick hearts and rabbit hearts

The effects of the negative inotropic agent, diacetyl monoxime (DAM), were studied on cardiac action potentials in young (3 day old) and old (13-18 day old) embryonic chick hearts and in rabbit papillary muscles. DAM (5-10 mM) decreased the action potential (AP) amplitude, the maximum rate of rise (Vmax), and AP duration of slow and fast APs in both young and old embryonic chick hearts. Concomitantly with these changes, the spontaneous beating rate was increased in young hearts. At 20 mM DAM, the maximum diastolic potential (MDP) was decreased. In fast APs of old embryonic chick hearts, the most prominent effect of DAM was a shortening of AP duration, the effects on AP amplitude, Vmax, and resting potential being less. In rabbit papillary muscles, DAM had little effect on the slow AP parameters except for shortening of AP duration. The onset of the DAM effects was rapid, the peak effect being attained by 5 min in all preparations. Reversal of the effects of DAM upon washout were fast, complete recovery being attained within 10 min. We conclude that DAM suppressed the slow Na+ current present in young chick embryonic hearts. The effects of DAM, up to 10 mM, on Vmax of the fast Na+-dependent APs in old embryonic chick hearts was less, and there was no effect on rabbit hearts. DAM suppressed the slow Ca2+-dependent APs in old embryonic chick hearts but not in rabbit papillary muscles, suggesting that the effect of DAM on the Ca2+ slow channels depends on the species.(ABSTRACT TRUNCATED AT 250 WORDS)     

Slow Ca2+ channels neither contribute to upstroke of action potential nor to pacemaker potential in spontaneously active freshly isolated three day embryonic chick ventricle

Contribution of slow Ca2+ channels to the upstroke of action potential (AP) and pacemaker potential was studied by observing the effects of Ca2+ channel activators- high [Ca2+]0, Bay-K-8644, isoproterenol, forskolin and dibutyryl-cAMP on spontaneous AP of freshly isolated 3 day embryonic chick ventricle (3 day ECV). The spontaneous APs showed maximal upstroke velocity (+Vmax), maximum diastolic potential (MDP), overshoot (Eov) and AP duration at -20 mv (APD20) of 42.60 +/- 2.40 V/sec, -59.05 +/- 0.95 my, 16.30 +/- 0.53 mv and 70.32 +/- 4.60 msec, respectively (an average value of 35 preparations). Bay-K-8644 (0.1-0.8 microM), isoproterenol (5-10 pM) and forskolin (0.1-2.0 microM) induced a concentration-dependent increase in APD20 and Eov without affecting +Vmax. Dibutyryl-cAMP (1 microM) also enhanced the APD20 and Eov and had no effect on +Vmax. Elevation of [Ca2+]0 from 0.6 mM to 9.6 mM caused a concentration-dependent increase in APD20 and Eov leaving +Vmax unaltered. Elevated [Ca2+] and the other Ca2+ channel activators had no significant effect on MDP in above concentration range. Increase in APD20 and Eov could be explained at least by activation of slow Ca2+ channels but the lack of any change in +Vmax clearly suggests that the slow Ca2+ channels do not contribute to the upstroke of AP. All these interventions reduced the rate of spontaneous firing without any noticeable effect on MDP. This finding shows that the slow Ca2+ channels also do not contribute directly to the generation of pacemaker potential in spontaneously active freshly isolated 3 day ECV.     

Recovery of the slow action potential is hastened by the calcium slow channel agonist, Bay-K-8644

Effects of the positive inotropic drug, Bay-K-8644, were studied on the slow action potential (AP) parameters and diastolic recovery of Vmax in K+ (22 mM)-depolarized rabbit papillary muscles. Bay-K-8644 (10(-6) M) increased the amplitude, maximum rate of rise (Vmax) and duration of the slow APs. Diastolic recovery of Vmax, examined by a paired-pulse protocol, was approximated by a single exponential function, both in control and in drug-treated muscles. The time constant of the recovery for drug-treated preparations was 171 +/- 20 ms (n = 9), and was significantly smaller than that for control: 414 +/- 45 ms (n = 12) (P less than 0.001). The diastolic intervals which allow 90% recovery of Vmax (T90%) were: 752 +/- 106 ms (n = 12) for control and 364 +/- 53 ms (n = 9) in the presence of drug, the latter being significantly shorter (P less than 0.01). The extent of the reductions in Vmax, at driving frequencies higher than 0.5 Hz, was minimal in the presence of the drug compared to the control. It was concluded that Bay-K-8644 not only enhanced the slow inward current, but also accelerated the reactivation process of the slow inward current and Ca2+ slow channel.     

SHORT COMMUNICATION: EFFECTS OF NIFEDIPINE AND BAY-K-8644 ON THE RELEASE OF CATECHOLAMINES FROM THE DOG ADRENAL GLAND IN RESPONSE TO SPLANCHNIC NERVE STIMULATION

1. The effects of nifedipine and Bay-K-8644 on the release of adrenal catecholamines were examined in anaesthetized dogs. 2. Splanchnic nerve stimulation (SNS) at 1 and 3 Hz produced frequency-dependent increases in adrenaline (ADR) and noradrenaline (NA) output determined from adrenal venous blood. 3. Neither nifedipine (10 and 30 μg/kg, i.v.) nor Bay-K-8644 (10 and 30 μg/kg, i.v.) modified the SNS-induced increases in catecholamine output. Basal catecholamine output tended to be increased and decreased by nifedipine and Bay-K-8644, respectively. 4. Nifedipine produced significant decreases in arterial pressure and renal blood flow rate. Bay-K-8644 produced a significant increase in arterial pressure associated with a decrease in renal blood flow rate. 5. These results suggest that dihydropyridine-sensitive calcium channels do not play a major role in adrenal catecholamine release evoked by SNS.     

Interaction between Bay-K-8644 and various relaxant agents on K+-depolarized contracture of guinea pig taenia coli

1. Interaction between Bay-K-8644 and nifedipine, D600, diltiazem, papaverine or W-7 on a K+-contracture of the guinea pig taenia coli have been investigated. 2. These relaxing drugs except Bay-K-8644 inhibited the K+-contracture in a concentration-dependent manner. 3. Complete inhibition by nifedipine (1 x 10(-7) M) of the contracture was absolutely antagonized by Bay-K-8644 (1 x 10(-6) M), a Ca2+-agonist, an apparent pA2 value of Bay-K-8644 as an antagonist for nifedipine obtained from Schild plot being 7.94 and its slope 0.99. 4. Marked inhibitions by D600 (1 x 10(-5) M), diltiazem (1 x 10(-5) M), papaverine (1 x 10(-4) M) and W-7 (1 x 10(-4) M) were unaffected but mild inhibitions by around IC50 of these relaxing agents, except W-7, were fully antagonized by Bay-K-8644. 5. These results suggest that Bay-K-8644 is a competitive antagonist for nifedipine, but not for other agents, on common Ca2+ channel sites.     

Two pharmacological types of cardiac slow Na+ channels as distinguished by verapamil

Old chick embryonic hearts were either minced and placed into explant cultured or disaggregated and placed into cell culture. TTX insensitive slow Na⁺ channels were acquired which were not blocked by 1 mM Mn²⁺. Verapamil also failed to block these channels. In contrast, fresh young (3 day old) embryonic hearts left intact possess naturally occurring slow Na⁺ channels which are blocked by verapamil. These findings suggest that there are two pharmacological types of slow Na⁺ channels; verapamil blocks one type but not the other.     

Effects of the calmodulin inhibitor, trifluoperazine, on membrane potentials and slow action potentials of cultured heart cells

The effects of an inhibitor of calmodulin, trifluoperazine (TFP), were determined on the electrical activity of cultured cell reaggregates derived from chick embryonic hearts (15-day-old). The cells exhibited naturally occurring slowly rising action potentials (APs) having a maximum rate of rise (+Vmax) of less than 35 V/s. After superfusion with 100 microM TFP, the maximal diastolic potential (MDP) decreased, within 30 min, from -66.0 to -55.5 mV. The frequency of discharge decreased, and there was also a decrease in AP amplitude and in +Vmax (from 10.0 to 4.9 V/s). By 90 min, all spontaneous activity had stopped, and the resting potential was about -10 mV. Input resistance increased, consistent with a decrease in K+ conductance. Hyperpolarization by current pulses did not allow the production of APs upon electrical stimulation, suggesting that the TFP blocks slow inward current (Isi). No recovery occurred upon washout (up to 48 h). Higher concentrations of TFP (200-500 microM), or injection of the inhibitor intracellularly be means of phosphatidylcholine liposomes, accelerated the time course of the blockade (e.g. within 15 min). In fresh (non-cultured) chick ventricle with fast-rising APs, TFP (400 microM) caused excitation-contraction uncoupling within 10 min, presumably by blocking the slow Ca2+ channels; the the fast APs were depressed (+Vmax) within 45 min, before any depolarization occurred. The cells became completely depolarized (Em congruent to -4 mV) by 195 min; hyperpolarization by current pulses did not allow the production of APs, suggesting that the fast Na+ channels were blocked.(ABSTRACT TRUNCATED AT 250 WORDS)     

Possible increases in potassium conductance by apamin in mammalian ventricular papillary muscles: a comparison with the effects on enzymatically isolated ventricular cells

Apamin, a bee venom polypeptide, is reported to block the Ca2+-dependent K+ channel in smooth muscle, hepatocyte, and neuroblastoma cells. In embryonic chick hearts, it was found to block the Ca2+ channel. We report here that apamin (10(-9)-10(-7) M) hyperpolarizes the resting membrane potential and shortens the duration of the action potential (AP) in the fast response of adult guinea pig ventricular papillary muscles. This peptide also depresses the isoproterenol or Ba2+-induced slow response APs in the presence of high K+ (21.6 mM) Tyrode solution, without affecting the resting potential. The most striking effect of apamin on the slow response was to shorten the duration of AP with only slight decreases in the maximal rate of increase (Vmax) of the AP, a nonlinear measure of Ca2+ currents. These findings suggest that apamin increases membrane K+ conductance in the mammalian ventricular myocardium. However, in enzymatically isolated single ventricular cells and at wide range of concentrations (10(-7)-10(-11) M), apamin did not affect the AP configuration and did not alter the membrane Ca2+ or K+ current, perhaps because of a loss of apamin sensitivity secondary to enzymatic digestion of the tissue with collagenase.     

Bay K 8644 enhances Ca2+ channel activities in embryonic chick heart cells without prolongation of open times.

The effects of the Ca2+ channel agonist, Bay K 8644, on the slow (L-type) Ca2+ channels was examined in young (3-day-old) embryonic chick heart cells, which naturally exhibit long-lasting openings. Bay K 8644 (5 microM) increased (a) the peak amplitude of the ensemble-averaged current by 3.9 +/- 0.9-fold (mean +/- S.E.) and (b) the maximal number of simultaneous opening from 2.6 +/- 0.4 to 4.4 +/- 0.9. Bay K 8644 had no effect on the unitary conductance (27 pS in control), and relatively little effect in the open-close kinetic analysis. The mean open times were 4.2 ms and 5.2 ms, in control and Bay K 8644, respectively. These results suggest that the agonistic effect of Bay K 8644 involves a mechanism other than open-time prolongation, such as activation of silent channels.     

Mechanism of the histamine-induced positive inotropic action in cardiac muscle.

Histamine, a positive inotropic agent which elevates cyclic AMP, was tested for ability to induce Ca2+ channels in 3 preparations of embryonic (16-day-old) chick ventricular myocardial cells whose fast Na+ channels were blocked by tetrodotoxin or voltage inactivated in 25 mM K+. In such inexcitable cells, histamine (10(-6)M) rapidly (1-3 min) induced slowly rising, overshooting, plateau-like responses accompanied by contractions. Mn2+, D600, or H2-receptor blocking agents abolished the slow responses. These results suggest that the positive inotropic action of histamine, like that of catecholamines, is mediated by an increased availability of slow Ca2+ channels.     

兴奋性氨基酸及尼莫地平对培养大鼠脑皮层神经元钙电流的影响

AIM: To study the effect of excitatory amino acid (EAA) and calcium channel blocker on neuronal calcium channels. METHODS: With path-clamp technique (whole-cell recording), the effects of Bay-K-8644, cesium glutamate, potassium aspartate, and nimodipine (Nim) on calcium currents (ICa) in cultured cortical neurons of neonatal rats were studied. RESULTS: ICa was raised obviously by Bay-K-8644 and glutamate. ICa was raised concentration-dependently by aspartate (0.5, 5, 50 mmol.L-1), with increasing rates 15% +/- 3%, 37% +/- 3%, and 53% +/- 6%, respectively. The inhibition of ICa was obvious while adding Nim in the bath solution. With Nim 10 mumol.L-1, the inhibitory rate was 46% +/- 4%. CONCLUSION: EAA had increasing effects on neuronal calcium currents and Nim inhibited Ca2+ influx in neurons.     

Effects of calcium channel blockers on ouabain-induced oscillatory afterpotentials in organ-cultured young embryonic chick hearts

Ouabain induces oscillatory afterpotentials (OAPs) in organ-cultured young (3 day old) embryonic chick hearts. Since increased [Ca]i resulting from an inhibition of the Na pump by ouabain triggers oscillatory movements of Ca2+ (i.e. OAPs) intracellularly, Ca2+ influx through the cell membrane, which tends to increase [Ca]i, may be important in developing the OAPs. Therefore, in the present experiments, effects of calcium channel blockers on ouabain-induced OAPs in organ-cultured 3 day old embryonic chick hearts were examined. Automaticity was suppressed by elevating [K]o to 6 mM. To induce the OAPs, the preparations were stimulated (0.5 Hz) in the presence of ouabain (2.5-6.3 microM). The calcium channel blockers (10 microM) depressed the OAPs in the following order of potency: bepridil greater than verapamil greater than nifedipine greater than diltiazem. This order of potency of the calcium channel blockers in depressing the OAPs was the same as that for drug penetration into the cells, but different from that for depressing slow action potentials: nifedipine greater than diltiazem greater than verapamil greater than bepridil (our previous findings). These results suggest that an intracellular site of action of the calcium channel blockers is important for depression of the OAPs, and suppression of the slow inward Ca2+ current cannot be the sole mechanism for suppression of the OAPs by these drugs.     

Activators of Sodium,Calcium and Potassium Channels Modulate the Cardiac Effects of Quinidine in vitro

Abstract: Quinidine (25.5 μmol/l) reduced the beating frequency of isolated right guinea-pig atria, caused a negative inotropic effect in papillary muscles and slightly raised the contractile force of left atria. The functional refractory period of both tissues was prolonged. A 20% increase of the extracellular sodium concentration did not reverse the effects of quinidine. The Na-channel activator BDF 9148 (1 μmol/l) and the Ca-channel agonist Bay-K-8644 (0.5 μmol/l) further increased the contractile force and caused an additional prolongation of the functional refractory period in quinidine-pretreated atria. Only Bay-K-8644 was able to reverse the negative inotropic effect of quinidine in papillary muscles. The influence of Bay-K-8644 on the contractile force in quinidine-pretreated muscles was not attenuated by lemacalim (3 μmol/l), an activator of ATP-dependent potassium channels, but the duration of the functional refractory period was significantly reduced. These results suggest that a combination of a calcium channel activator and a potassium channel opener might be able to improve the treatment of quinidine intoxications.     

Local anesthethic blockade of Ca-mediated action potentials in cardiac muscle

The effects of local anesthetic agents (lidocaine, procaine, cocaine) and diphenylhydantoin (DPH) were studied on the slow electrical responses induced by isoproterenol or caffeine in cardiac muscle preparations rendered inexcitabe by tetrodotoxin (TTX) or by partial depolarization with elevated K⁺ (26 mM). In such inexcitable cells, we previously demonstrated that addition of some positive inotropic agents, such as catecholamines, histamine, and methylxanthines, rapidly increase the number of available slow Ca²⁺Na⁺ channels, thus allowing slowly rising electrical responses resembling the plateau component of the cardiac action potential. In embryonic chick (16–20-day-old) myocardial cells (ventricular) studied as intact perfused hearts or as reaggregated cell cultures of trypsin-dispersed cells, high concentrations (10^?3 M) of all the above drugs blocked the induced slow responses with their associated contractions; low concentrations (10^?5 M) of these agents reduce the maximal rate of rise (+V?max) of the slow responses and depressed the contractions. For comparison with their effects on the slow response, the actions of these drugs on the normal action potential were also studied. As with the slow response, all of these drugs depressed the rate of rise of the action potential (10^?4 M) or blocked it at higher concentrations (10^?3 M); in contrast, low concentrations (10^?5 M) of lidocaine and DPH increased +V?max. These findings suggest that local anesthetics, which interact with the lipid phase of the cell membrane, lead to blockade of the slow Ca²⁺Na⁺ channels as well as of the fast Na⁺ channels in the myocardial sarcolemma.     

Na(+)/Ca(2+) exchanger inhibition exerts a positive inotropic effect in the rat heart, but fails to influence the contractility of the rabbit heart

BACKGROUND AND PURPOSE: The Na(+)/Ca(2+) exchanger (NCX) may play a key role in myocardial contractility. The operation of the NCX is affected by the action potential (AP) configuration and the intracellular Na(+) concentration. This study examined the effect of selective NCX inhibition by 0.1, 0.3 and 1.0 microM SEA0400 on the myocardial contractility in the setting of different AP configurations and different intracellular Na(+) concentrations in rabbit and rat hearts. EXPERIMENTAL APPROACH: The concentration-dependent effects of SEA0400 on I(Na/Ca) were studied in rat and rabbit ventricular cardiomyocytes using a patch clamp technique. Starling curves were constructed for isolated, Langendorff-perfused rat and rabbit hearts. The cardiac sarcolemmal NCX protein densities of both species were compared by immunohistochemistry. KEY RESULTS: SEA0400 inhibited I(Na/Ca) with similar efficacy in the two species; there was no difference between the inhibitions of the forward or reverse mode of the NCX in either species. SEA0400 increased the systolic and the developed pressure in the rat heart in a concentration-dependent manner, for example, 1.0 microM SEA0400 increased the maximum systolic pressures by 12% relative to the control, whereas it failed to alter the contractility in the rabbit heart. No interspecies difference was found in the cardiac sarcolemmal NCX protein densities. CONCLUSIONS AND IMPLICATIONS: NCX inhibition exerted a positive inotropic effect in the rat heart, but it did not influence the contractility of the rabbit heart. This implies that the AP configuration and the intracellular Na(+) concentration may play an important role in the contractility response to NCX inhibition.     

Trapidil stimulation of slow Ca2+ current in cardiac muscle

The effect of trapidil, a coronary vasodilator and positive inotropic agent (associated with elevated tissue cyclic AMP levels due to phosphodiesterase inhibition), was examined on the electrophysiological properties of cardiac muscle. Specifically, the trapidil was tested for its ability to induce slow action potentials (APs), and to affect the maximum upstroke velocity (+Vmax) of the slow APs in the ventricular myocardial cells of isolated perfused chick hearts. The effect of trapidil on the contractions accompanying the slow APs and on the tissue cyclic AMP levels was also examined. To study the slow channels exclusively, the fast Na+ channels were voltage-inactivated by elevated (25 mM) K+. In this condition of functional removal of the fast channels, the hearts could not be excited even by intense electrical stimulation. It was found that trapidil (10(-4)--10(-3) M) induced slow APs accompanied by contractions. Elevation of the trapidil concentration produced dose-dependent increases in +Vmax, dT/dt (first derivative of developed tension) and cyclic AMP. These effects of trapidil were not affected by propranolol, suggesting that they were not mediated by beta-adrenergic receptors. These results support the hypothesis that intracellular cyclic AMP levels regulate the number of available slow channels, thereby controlling contractile force in the heart muscle via the Ca2+ influx mediated by slow channels.     

Pertussis toxin-sensitive G-protein activation does not influence the response to Bay K 8644 in embryonic chick myocytes.

Pretreatment of embryonic chick cardiac myocytes with pertussis toxin (1-2.5 micrograms ml-1 for 22 h) abolished the negative chronotropic effects of carbachol but not the positive inotropic effects of Bay K 8644. Neither guanosine 5'-O-3-thiotriphosphate (GTP gamma S 500 microM intracellularly), nor pertussis toxin (0.5-1 microgram ml-1 for 22 h) modified the agonist effects of Bay K 8644 on calcium channel currents recorded under whole-cell voltage clamp. These findings indicate that pertussis-sensitive guanine nucleotide binding (G)-proteins does not modulate the effects of calcium channel activators in embryonic chick cardiac myocytes.     

Voltage-dependent effects of YC-170, a dihydropyridine calcium channel modulator,in cardiovascular tissues

Voltage-dependent effects of YC-170, a putative calcium channel activator, were examined and compared with those of Bay K 8644 in isolated guinea-pig cardiac tissues and rabbit aortae. In guinea-pig left atria superfused with a normal bathing solution (4 mmol/l K+), both YC-170 (10 mumol/l) and Bay K 8644 (1 mumol/l) produced a positive inotropic action accompanied by a prolongation of action potential durations (APDs). In normally-polarized guinea-pig papillary muscles Bay K 8644 increased force of contraction (fc) and APDs. However, YC-170 failed to increase fc in spite of a slight prolongation of APDs. In papillary muscles partially depolarized by 25 mmol/l K+ solution, Bay K 8644 enhanced the electrically-induced slow action potentials and contractile force. In contrast with Bay K 8644, YC-170 significantly depressed the slow action potentials and decreased fc. YC-170 also showed the depressant action on the slow action potentials induced by isoproterenol (0.1 mumol/l), histamine (3 mumol/l) and tetraethylammonium (10 mmol/l) plus high Ca2+ (4 mmol/l). In sinoatrial node cells of guinea-pig right atria Bay K 8644 produced a positive chronotropic action with increases in the maximum rate of rise (Vmax) and action potential amplitude (APA), whereas YC-170 produced a negative chronotropic action with decreases in Vmax and APA. In the rabbit aortic strips preincubated with bathing solution containing various concentrations of K+ (15, 20, 30 and 40 mmol/l), Bay K 8644 produced concentration-dependent contractions in a range of concentrations up to 0.3 mumol/l. However, when the concentration exceeded 1 mumol/l, Bay K 8644 caused a slight relaxation, irrespective of the K+ concentrations of bathing solution. YC-170 in concentrations of 10 and 30 mumol/l contracted the aortic strips placed in 5.9 or 15 mmol/l K+ bathing solution, but caused relaxation in 30 or 40 mmol/l K+ bathing solution. These results suggest that YC-170 is a dihydropyridine calcium channel modulator which behaves as a Ca2+ channel agonist in tissues of high membrane potentials, but as a Ca2+ channel antagonist in tissues of low membrane potentials.     

EFFECTS OF GENERAL ANAESTHETICS ON THE ACTIVITY OF THE Na,K-ATPASE OF CANINE RENAL MEDULLA

Several previous studies have reported inhibition of Na,K-ATPase activity by chlorpromazine, phenobarbital and pentobarbital, thiopental, and monoketones. The purpose of this study is to investigate the influences of other general anaesthetics on Na,K-ATPase activity. The ATPase activity of Na,K-ATPase-enriched membranes from canine renal medulla was determined at 37 degrees C in the absence and in the presence of hexanol, diethylether, halothane, and propofol. The influence of hexanol on stimulation of Na,K-ATPase activity by Na+ and K+ was investigated. Hexanol, diethylether, halothane, and propofol inhibited the activity at 37 degrees C of the Na,K-ATPase of canine renal medulla. The IC50 values at 37 degrees C were: hexanol, 12.3 mM; diethylether, 170 mM; halothane, 7.35 mM; propofol, 0.127 mM. Hexanol increased the K0.5 of the Na,K-ATPase for K+ at 37 degrees C, but did not affect the K0.5 for Na+. At lower [K+] hexanol was a more potent inhibitor than at higher [K+].     

Interaction of palmitoyl carnitine with calcium antagonists in myocytes.

1. Beating of aggregates of embryonic chick myocytes, in primary culture, was quantified by use of a motion-detector and video-recorder technique. Interactions of palmitoyl carnitine, a putative endogenous ligand at Ca2+ channels, with calcium antagonists were investigated. 2. Bay K 8644 (1-100 nM) and palmitoyl carnitine (0.2-30 microM) increased edge movement of the aggregates; beats fused so that there was an increase in baseline 'tone'. The concentrations required to produce a 50% increase in edge movement were 2.5 nM for Bay K 8644 and 2 microM for palmitoyl carnitine. Higher concentrations (20-30 microM) of palmitoyl carnitine caused tachycardia of abrupt onset but resulted in cessation of beating. The effects of palmitoyl carnitine were not stereo-selective in that the (+)- and (-)-isomers were equieffective. Lysophosphatidyl choline (LPC) had no effect in concentrations up to 10 microM but higher concentrations caused tachycardia followed by cessation of beating. High concentrations of both palmitoyl carnitine and LPC (100 microM) caused break-up of the aggregates, presumably as a result of detergent effects. 3. Palmitoyl carnitine (1-100 microM) reversed the inhibitory effects of nisoldipine (0.3 microM), diltiazem (10 microM) and verapamil (1 microM). Ouabain was ineffective in reversing the effects of nisoldipine, differentiating the effects of palmitoyl carnitine from those of Na+/K+ ATPase inhibition. In contrast, palmitoyl carnitine did not reverse the inhibitory effects of pimozide (2 microM) or lidoflazine (7 microM); palmitoyl carnitine showed a similar profile to Bay K 8644 in this respect.(ABSTRACT TRUNCATED AT 250 WORDS)