Effects of general anesthetics on current flow across membranes in guinea pig myocytes

Anesthetics and Cell-Cell Communication: Potential Ca2+-Calmodulin Role in Gap Junction Channel Gating by Heptanol, Halothane and Isoflurane

Cell–cell communication via gap junction channels is known to be inhibited by the anesthetics heptanol, halothane and isoflurane; however, despite numerous studies, the mechanism of gap junction channel gating by anesthetics is still poorly understood. In the early nineties, we reported that gating by anesthetics is strongly potentiated by caffeine and theophylline and inhibited by 4-Aminopyridine. Neither Ca²⁺ channel blockers nor 3-isobutyl-1-methylxanthine (IBMX), forskolin, CPT-cAMP, 8Br-cGMP, adenosine, phorbol ester or H7 had significant effects on gating by anesthetics. In our publication, we concluded that neither cytosolic Ca²⁺_i nor pH_i were involved, and suggested a direct effect of anesthetics on gap junction channel proteins. However, while a direct effect cannot be excluded, based on the potentiating effect of caffeine and theophylline added to anesthetics and data published over the past three decades, we are now reconsidering our earlier interpretation and propose an alternative hypothesis that uncoupling by heptanol, halothane and isoflurane may actually result from a rise in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and consequential activation of calmodulin linked to gap junction proteins.

Heterogeneities in Ventricular Conduction Following Treatment with Heptanol: A Multi-Electrode Array Study in Langendorff-Perfused Mouse Hearts

Background: Previous studies have associated slowed ventricular conduction with the arrhythmogenesis mediated by the gap junction and sodium channel inhibitor heptanol in mouse hearts. However, they did not study the propagation patterns that might contribute to the arrhythmic substrate. This study used a multi-electrode array mapping technique to further investigate different conduction abnormalities in Langendorff-perfused mouse hearts exposed to 0.1 or 2 mM heptanol. Methods: Recordings were made from the left ventricular epicardium using multi-electrode arrays in spontaneously beating hearts during right ventricular 8 Hz pacing or S1S2 pacing. Results: In spontaneously beating hearts, heptanol at 0.1 and 2 mM significantly reduced the heart rate from 314 ± 25 to 189 ± 24 and 157 ± 7 bpm, respectively (ANOVA, p < 0.05 and p < 0.001). During regular 8 Hz pacing, the mean LATs were increased by 0.1 and 2 mM heptanol from 7.1 ± 2.2 ms to 19.9 ± 5.0 ms (p < 0.05) and 18.4 ± 5.7 ms (p < 0.05). The standard deviation of the mean LATs was increased from 2.5 ± 0.8 ms to 10.3 ± 4.0 ms and 8.0 ± 2.5 ms (p < 0.05), and the median of phase differences was increased from 1.7 ± 1.1 ms to 13.9 ± 7.8 ms and 12.1 ± 5.0 ms by 0.1 and 2 mM heptanol (p < 0.05). P5 took a value of 0.2 ± 0.1 ms and was not significantly altered by heptanol at 0.1 or 2 mM (1.1 ± 0.9 ms and 0.9 ± 0.5 ms, p > 0.05). P50 was increased from 7.3 ± 2.7 ms to 24.0 ± 12.0 ms by 0.1 mM heptanol and then to 22.5 ± 7.5 ms by 2 mM heptanol (p < 0.05). P95 was increased from 1.7 ± 1.1 ms to 13.9 ± 7.8 ms by 0.1 mM heptanol and to 12.1 ± 5.0 ms by 2 mM heptanol (p < 0.05). These changes led to increases in the absolute inhomogeneity in conduction (P5−95) from 7.1 ± 2.6 ms to 31.4 ± 11.3 ms, 2 mM: 21.6 ± 7.2 ms, respectively (p < 0.05). The inhomogeneity index (P5−95/P50) was significantly reduced from 3.7 ± 1.2 to 3.1 ± 0.8 by 0.1 mM and then to 3.3 ± 0.9 by 2 mM heptanol (p < 0.05). Conclusion: Increased activation latencies, reduced CVs, and the increased inhomogeneity index of conduction were associated with both spontaneous and induced ventricular arrhythmias.

Myofibroblast modulation of cardiac myocyte structure and function

After myocardial infarction, resident fibroblasts (Fb) differentiate towards myofibroblasts (MyoFb), generating the scar tissue and the interstitial fibrosis seen in the adjacent myocardium. Fb and MyoFb have the potential to interact with cardiac myocytes (CMs) but insight into the phenotype-specific role and mode of interaction is still incomplete. Our objectives are to further define the modulation of CMs by MyoFbs compared to Fbs, as well as the role of direct contact through gap junctions vs. soluble mediators, using Fbs and CMs from pig left ventricle. Fbs were treated to maintain an undifferentiated state (SD-208) or to attain full differentiation to MyoFb (TGF-β1). Fbs and MyoFbs were co-cultured with CMs, with the possibility of direct contact or separated by a Thincert membrane. Only in direct co-culture, both Fbs and MyoFbs were able to decrease CM viability after 2 days. Only MyoFbs induced significant distal spreading of CMs in both direct and indirect co-culture. MyoFbs, but not Fbs, readily made connections with CMs in direct co-culture and connexin 43 expression in MyoFb was higher than in Fb. When coupled to CMs, MyoFbs reduced the CM action potential duration and hyperpolarized the CM resting membrane potential. Uncoupling reversed these effects. In conclusion, MyoFbs, but not Fbs, alter the CM structural phenotype. MyoFbs, but not Fbs, are likely to electrically connect to CMs and thereby modulate the CM membrane potential. These data provide further support for an active role of MyoFbs in the arrhythmogenic substrate after cardiac remodelling.

Effect of heptanol and ethanol on excitation wave propagation in a neonatal rat ventricular myocyte monolayer

In this work, the action of heptanol and ethanol was investigated in a two-dimensional (2D) model of cardiac tissue: the neonatal rat ventricular myocyte monolayer. Heptanol is known in electrophysiology as a gap junction uncoupler but may also inhibit voltage-gated ionic channels. Ethanol is often associated with the occurrence of arrhythmias. These substances influence sodium, calcium, and potassium channels, but the complete mechanism of action of heptanol and ethanol remains unknown. The optical mapping method was used to measure conduction velocities (CVs) in concentrations of 0.05-1.8 mM heptanol and 17-1342 mM ethanol. Heptanol was shown to slow the excitation wave significantly, and a mechanism that involves a simultaneous action on cell coupling and activation threshold was suggested. Whole-cell patch-clamp experiments showed inhibition of sodium and calcium currents at a concentration of 0.5 mM heptanol. Computer modeling was used to estimate the relative contribution of the cell uncoupling and activation threshold increase caused by heptanol. Unlike heptanol, ethanol slightly influenced the CV at clinically relevant concentrations. Additionally, the critical concentrations for re-entry formation in ethanol were determined.

Coupling an HCN2-expressing cell to a myocyte creates a two-cell pacing unit

We examined whether coupling of a ventricular myocyte to a non-myocyte cell expressing HCN2 could create a two-cell syncytium capable of generating sustained pacing. Three non-myocyte cell types were transfected with the mHCN2 gene and used as sources of mHCN2-induced currents. They were human mesenchymal stem cells and HEK293 cells, both of which express connexin43 (Cx43), and HeLa cells transfected with Cx43. Cell-cell coupling between heterologous pairs increased with time in co-culture, and hyperpolarization of the myocyte induced HCN2 currents, indicating current transfer from the mHCN2-expressing cell to the myocyte via gap junctions. The magnitude of the HCN2 currents recorded in myocytes increased with increasing junctional conductance. Once a critical level of electrical cell-cell coupling between myocytes and mHCN2 transfected cells was exceeded spontaneous action potentials were generated at frequencies of approximately 0.6 to 1.7 Hz (1.09 +/- 0.05 Hz). Addition of carbenoxolone (200 microM), a gap junction channel blocker, to the media stopped spontaneous activity in heterologous cell pairs. Carbenoxolone washout restored activity. Blockade of HCN2 currents by 100 microM 9-amino-1,2,3,4-tetrahydroacridine (THA) stopped spontaneous activity and subsequent washout restored it. Neither THA nor carbenoxolone affected electrically stimulated action potentials in isolated single myocytes. In summary, the inward current evoked in the genetically engineered (HCN2-expressing) cell was delivered to the cardiac myocyte via gap junctions and generated action potentials such that the cell pair could function as a pacemaker unit. This finding lays the groundwork for understanding cell-based biological pacemakers in vivo once an understanding of delivery and target cell geometry is defined.

Reducing the cyclic variations of ultrasonic integrated backscatters and myocardial electrical synchronism by reversibly blocking intercellular communications with heptanol

The purpose of this study is to provide direct evidence for the role of intercellular communications in electrical synchronization and mechanical function of myocardium. We used heptanol, a reversible inhibitor of gap junctions, at low (0.16 mM) and high (0.5 mM) concentration as perfusate for 18 Langendorff-perfused rabbit hearts to study its effects on myocardial electrical and mechanical functions. Optical mapping was performed to measure conduction velocity (CV) and action potential duration (APD). Ultrasonic integrated backscatter and Doppler tissue imaging (DTI) were used to evaluate the intrinsic and global myocardial contractile performance. The CV decreased during low-dose heptanol infusion and became much slower at high dose (high dose vs. baseline, 50.8 +/- 10.2 cm/s vs. 69.3 +/- 8.8 cm/s, p < 0.001). After washout of heptanol, CV completely recovered. The alterations of APD by heptanol infusion were similar to CV. The APD dispersion, standard deviation of APD(80), was increased after heptanol infusion (low dose vs. baseline, 5.9 +/- 1.1 ms vs. 4.3 +/- 1.1 ms, p = 0.004; high dose, 6.0 +/- 1.3 ms, vs. baseline, p = 0.035). However, washout did not restore the APD dispersion which became even larger after washout (13.6 +/- 1.9 ms vs. high dose and baseline, both p < 0.001). Regarding contractile function, heptanol treatment resulted in a progressive decrease of cardiac cycle-dependent variations of integrated backscatter (CVIBS; low dose vs. baseline, 6.1 +/- 1.7 dB vs. 7.2 +/- 1.8 dB, p = 0.007; high dose 1.7 +/- 0.3 dB vs. baseline, p < 0.001) and peak systolic strain rate (low dose vs. baseline, -1.5 +/- 0.6 1/s vs. -1.9 +/- 0.6 1/s, p = 0.014; high dose -0.4 +/- 0.2 1/s; vs. baseline, p < 0.001). That both CVIBS and strain rate incompletely recovered after heptanol washout may be attributed to the increased APD dispersion. In conclusion, uncoupling of gap junctions resulted in slowing CV, increased repolarization heterogeneity, reduced CVIBS and impaired myocardial contractility. There was a reversible dose-response relationship between the myocardial electromechanical functions and gap junction coupling.

Evidence of intercellular coupling between co-cultured adult rabbit ventricular myocytes and myofibroblasts

Intercellular coupling between ventricular myocytes and myofibroblasts was studied by co-culturing adult rabbit ventricular myocytes with previously prepared layers of cardiac myofibroblasts. Intercellular coupling was examined by: (i) tracking the movement of the fluorescent dye calcein; (ii) immunostaining for connexin 43 (Cx43); and (iii) measurement of intracellular [Ca2+] ([Ca2+]i). The effects of stimulating ventricular myocytes on the underlying myofibroblasts was examined by confocal measurements of [Ca2+]i using fluo-3. When ventricular myocytes were preloaded with calcein and co-cultured with myofibroblasts for 24 h, calcein fluorescence was detected in 52+/-4% (n=8 co-cultures) of surrounding myofibroblasts. Treatment with the gap junction uncoupler heptanol significantly reduced the movement of calcein (12+/-3%, n=6 co-cultures). Immunostaining showed expression of Cx43 in co-cultured myofibroblasts and myocytes. Field stimulation of ventricular myocytes co-cultured with myofibroblasts increased myofibroblast [Ca2+]i, no response was observed after treatment with heptanol or stimulation of fibroblasts in the absence of ventricular myocytes. Action potential parameters of ventricular myocytes in co-culture were similar to control values. However, application of the hormone sphingosine-1-phosphate (S-1-P) to the co-culture caused a depolarization of ventricular myocytes to approximately -20 mV. Sphingosine-1-phosphate had no effect on ventricular myocytes alone. Voltage-clamp measurements of isolated myofibroblasts indicated that S-1-P activated a significant quasi-linear current with a reversal potential of approximately -40 mV. In conclusion, this study shows that stimulation of the ventricular myocyte influences the intracellular Ca2+ of the linked myofibroblast via connexons. These intercellular links also allow the myofibroblasts to influence the electrical activity of the myocyte. This work indicates the nature of the gap junction-mediated bi-directional interactions that occur between ventricular myocyte and myofibroblast.

Genesis of ectopic waves: role of coupling, automaticity, and heterogeneity

Many arrhythmias are believed to be triggered by ectopic sources arising from the border of the ischemic tissue. However, the development of ectopic activity from individual sources to a larger mass of cardiac tissue remains poorly understood. To address this critical issue, we used monolayers of neonatal rat cardiomyocytes to create conditions that promoted progression of ectopic activity from single cells to the network that consisted of hundreds of cells. To explain complex spatiotemporal patterns observed in these experiments we introduced a new theoretical framework. The framework's main feature is a parameter space diagram, which uses cell automaticity and coupling as two coordinates. The diagram allows one to depict network behavior, quantitatively address the heterogeneity factor, and evaluate transitions between different regimes. The well-organized wave trains were observed at moderate and high cell coupling values and network heterogeneity was found to be qualitatively unimportant for these regimes. In contrast, at lower values of coupling, spontaneous ectopic activity led to the appearance of fragmented ectopic waves. For these regimes, network heterogeneity played an essential role. The ectopic waves occasionally gave rise to spiral activity in two different regions within the parameter space via two distinct mechanisms. Together, our results suggest that localized ectopic waves represent an essential step in the progression of ectopic activity. These studies add to the understanding of initiation and progression of arrhythmias and can be applied to other phenomena that deal with assemblies of coupled oscillators.

Endocochlear potential and endolymphatic K+ changes induced by gap junction blockers

OBJECTIVE

To examine the effects of gap junction blockers on the endocochlear potential (EP) and endolymphatic potassium concentration ([K(+)](e)).

MATERIAL AND METHODS

The EP and [K(+)](e) were monitored using double-barreled ion-selective microelectrodes in the second turn of the guinea pig cochlea during perilymphatic perfusion.

RESULTS

When the perilymphatic scalae of the cochlea were perfused with artificial perilymph containing 10 mM n-heptanol the EP was decreased by -8.8+/-1.4 mV (n=10), and this was accompanied by a decline in the [K(+)](e) of -6.7+/-2.1 mM (n=6). Perilymphatic application of 10 mM hexanol also produced declines in both the EP and [K(+)](e). In control studies, perilymphatic perfusion with 10 mM ethanol showed no remarkable changes in either the EP or [K(+)](e). Anoxia during perfusion with heptanol resulted in the generation of a negative EP, similar to the situation in controls.

CONCLUSIONS

A decline in the EP together with a lowering of [K(+)](e) induced by long-chain n-alkanols, which act as gap junction blockers, may be explained by an interruption in potassium ion transport related to a gap junction dysfunction.

Electrical propagation in synthetic ventricular myocyte strands from germline connexin43 knockout mice

To characterize the role of connexin43 (Cx43) as a determinant of cardiac propagation, we synthesized strands and pairs of ventricular myocytes from germline Cx43-/- mice. The amount of Cx43, Cx45, and Cx40 in gap junctions was analyzed by immunohistochemistry and confocal microscopy. Intercellular electrical conductance, gj, was measured by the dual-voltage clamp technique (DVC), and electrical propagation was assessed by multisite optical mapping of transmembrane potential using a voltage-sensitive dye. Compared with wild-type (Cx43+/+) strands, immunoreactive signal for Cx43 was reduced by 46% in Cx43+/- strands and was absent in Cx43-/- strands. Cx45 signal was reduced by 46% in Cx43+/- strands and to the limit of detection in Cx43-/- strands, but total Cx45 protein levels measured in immunoblots of whole cell homogenates were equivalent in all genotypes. Cx40 was detected in 2% of myocytes. Intercellular conductance, gj, was reduced by 32% in Cx43+/- cell pairs and by 96% in Cx43-/- cell pairs. The symmetrical dependence of gj on transjunctional voltage and properties of single-channel recordings indicated that Cx45 was the only remaining connexin in Cx43-/- cells. Propagation in Cx43-/- strands was very slow (2.1 cm/s versus 52 cm/s in Cx43+/+) and highly discontinuous, with simultaneous excitation within and long conduction delays (2 to 3 ms) between individual cells. Propagation was abolished by 1 mmol/L heptanol, indicating residual junctional coupling. In summary, knockout of Cx43 in ventricular myocytes leads to very slow conduction dependent on the presence of Cx45. Electrical field effect transmission does not contribute to propagation in synthetic strands.

Enhanced effect of gap junction uncouplers on macroscopic electrical properties of reperfused myocardium

Transient inhibition of gap junction (GJ)-mediated communication with heptanol during myocardial reperfusion limits infarct size. However, inhibition of cell coupling in normal myocardium may be arrhythmogenic. The purpose of this study was to test the hypothesis that the consequences of GJ inhibition may be magnified in reperfused myocardium compared with normal tissue, thus allowing the inhibition of GJs in reperfused tissue while only minimally modifying overall macroscopic cell coupling in normal myocardium. Concentration-response curves were defined for the effects of heptanol, 18alpha-glycyrrhetinic acid, halothane, and palmitoleic acid on conduction velocity, tissue electrical impedance, developed tension and lactate dehydrogenase (LDH) release in normoxically perfused rat hearts (n= 17). Concentrations lacking significant effects on tissue impedance were added during the initial 15 min of reperfusion in hearts submitted to 60 min (n= 43) or 30 min (n= 35) of ischaemia. These concentrations markedly increased myocardial electrical impedance (resistivity and phase angle) in myocardium reperfused after either 30 or 60 min of ischaemia, and reduced reperfusion-induced LDH release after 1 h of ischaemia by 83.6, 57.9, 51.7 and 52.5% for heptanol, 18alpha-glycyrrhetinic acid, halothane and palmitoleic acid, respectively. LDH release was minimal in hearts submitted to 30 min of ischaemia, independently of group allocation. In conclusion, the present results strongly support the hypothesis that intercellular communication in postischaemic myocardium may be effectively reduced by concentrations of GJ inhibitors affecting only minimally overall electrical impedance in normal myocardium. Reduction of cell coupling during initial reperfusion was consistently associated with attenuated lethal reperfusion injury.

Besides affecting intracellular calcium signaling, 2-APB reversibly blocks gap junctional coupling in confluent monolayers, thereby allowing measurement of single-cell membrane currents in undissociated cells

2-aminoethoxydiphenyl borate (2-APB) has been widely used as a blocker of the IP3 receptor and TRP channels, including store-operated calcium channels. We now show in monolayers of normal rat kidney cells (NRK/49F) that 2-APB completely and reversibly blocks gap junctional intercellular communication at concentrations similar to that required for inhibition of PGF2alpha-induced increases in intracellular calcium. Gap junctional conductances between NRK cells were estimated with single-electrode patch-clamp measurements and were fully blocked by 2-APB (50 microM), when applied extracellularly but not via the patch pipette. Half maximal inhibition (IC50) of electrical coupling in NRK cells was achieved at 5.7 microM. Similar results were obtained for human embryonic kidney epithelial cells (HEK293/tsA201) with an IC50 of 10.3 microM. Using 2-APB as an electrical uncoupler of monolayer cells, we could thus measure inward rectifier potassium, L-type calcium, and calcium-dependent chloride membrane currents in confluent NRK monolayers, with properties similar to those in dissociated NRK cells in the absence of 2-APB. The electrical uncoupling action described here is a new 2-APB property that promises to provide a powerful pharmacological tool to study single-cell properties in cultured confluent monolayers and intact tissues by electrical and chemical uncoupling of the cells without the need of prior dissociation.

Localized injury in cardiomyocyte network: a new experimental model of ischemia-reperfusion arrhythmias

We developed a new experimental approach to study the effects of local injury in a multicellular preparation and tested the ability of the method to induce reperfusion arrhythmias in cardiomyocyte monolayers. A small region of injury was created using geometrically defined flows of control and ischemia-like solutions. Calcium transients were acquired simultaneously from injured, control, and border zone cells using fluo 4. Superfusion with the injury solution rapidly diminished the amplitude of calcium transients within the injury zone, followed by cessation of cell beating. Reperfusion caused an immediate tachyarrhythmic response in approximately 17% of experiments, with a wave front propagating from a single cell or small cell cluster within the former injury zone. Inclusion of a gap junction uncoupler (1 mM heptanol) in the injury solution narrowed the functional border and sharply increased the number of ectopic foci and the incidence of reperfusion arrhythmias. The model holds a potential to reveal both micro- and macroscopic features of propagation, conduction, and cell coupling in the normal and diseased myocardium and to serve as a new tool to test antiarrhythmic protocols in vitro.

Psoriasis and altered calcium metabolism: downregulated capacitative calcium influx and defective calcium-mediated cell signaling in cultured psoriatic keratinocytes

Intracellular calcium plays an important part in the regulation of proliferation and differentiation of keratinocytes. Detached from their in vivo environment, cultured psoriatic keratinocytes were investigated by monitoring free intracellular calcium concentration, which was measured using fura-2/AM as a calcium-sensitive probe. The mean increase in intracellular calcium of psoriatic keratinocytes was significantly reduced compared with control keratinocytes when intracellular calcium stores were mobilized from endoplasmic reticulum with thapsigargin. This finding suggests defective capacitative calcium influx of psoriatic cells. Intracellular calcium stores were similar in psoriatic and control keratinocytes, when extracellular calcium was chelated with ethyleneglycol-bis(beta-aminoethyl ether)-N,N,N',N',-tetraacetic acid and intracellular calcium was depleted with thapsigargin. Mechanical wounding of keratinocyte monolayer resulted in a significantly reduced rise in intracellular calcium of psoriatic cells in low (< 0.1 mM) and high (1.8 mM) extracellular calcium suggesting defective intercellular coupling of psoriatic keratinocytes. Blocking of gap-junctions with heptanol in wounded keratinocytes did not affect the intracellular calcium response in psoriatic keratinocytes in contrast to healthy keratinocytes. Adding adenosine triphosphate to culture medium resulted in a more pronounced intracellular calcium increase than thapsigargin in psoriatic keratinocytes, suggesting that inositol triphosphate-mediated, P2-purinergic signaling was enhanced in these cells. Moreover, psoriatic keratinocytes maintained their defective responses up to at least fifth passage suggesting that psoriatic keratinocytes have an inborn error in calcium metabolism, rather than a localized defect in response to altered extracellular calcium gradient observed in vivo.

Influence of cell number on the characteristics and synchrony of Ca2+ oscillations in clusters of mouse pancreatic islet cells

1. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in single cells and cell clusters of different sizes prepared from mouse pancreatic islets. 2. During stimulation with 15 mM glucose, 20 % of isolated cells were inert, whereas 80 % showed [Ca2+]i oscillations of variable amplitude, duration and frequency. Spectral analysis identified a major frequency of 0.14 min-1 and a less prominent one of 0.27 min-1. 3. In contrast, practically all clusters (2-50 cells) responded to glucose, and no inert cells were identified within the clusters. As compared to single cells, mean [Ca2+]i was more elevated, [Ca2+]i oscillations were more regular and their major frequency was slightly higher (but reached a plateau at approximately 0.25 min-1). In some cells and clusters, faster oscillations occurred on top of the slow ones, between them or randomly. 4. Image analysis revealed that the regular [Ca2+]i oscillations were well synchronized between all cells of the clusters. Even when the Ca2+ response was irregular, slow and fast [Ca2+]i oscillations induced by glucose were also synchronous in all cells. 5. In contrast, [Ca2+]i oscillations resulting from mobilization of intracellular Ca2+ by acetylcholine were restricted to certain cells only and were not synchronized. 6. Heptanol and 18alpha-glycyrrhetinic acid, two agents widely used to block gap junctions, altered glucose-induced Ca2+ oscillations, but control experiments showed that they also exerted effects other than a selective uncoupling of the cells. 7. The results support theoretical models predicting an increased regularity of glucose-dependent oscillatory events in clusters as compared to isolated islet cells, but contradict the proposal that the frequency of the oscillations increases with the number of coupled cells. Islet cell clusters function better as electrical than biochemical syncytia. This may explain the co-ordination of [Ca2+]i oscillations driven by depolarization-dependent Ca2+ influx during glucose stimulation.

Maturation of neuromuscular transmission during early development in zebrafish

We have examined the rapid development of synaptic transmission at the neuromuscular junction (NMJ) in zebrafish embryos and larvae by patch-clamp recording of spontaneous miniature endplate currents (mEPCs) and single acetylcholine receptor (AChR) channels. Embryonic (24-36 h) mEPCs recorded in vivo were small in amplitude (<50 pA). The rate of mEPCs increased in larvae (3.5-fold increase measured by 6 days), and these mEPCs were mostly of larger amplitude (10-fold on average) with (</=5-fold) faster kinetics. Intracellular labeling with Lucifer yellow indicated extensive coupling between muscle cells in both embryos and larvae (</=10 days). Blocking acetylcholinesterase (AChE) with eserine had no effect on mEPC kinetics in embryos at 1 day and only partially slowed (by approximately 1/2) the decay rate in larvae at 6 days. In acutely dissociated muscle cells, we observed the same two types of AChR with conductances of 45 and 60 pS and with similar, brief (<0.5 ms) mean open times in both embryos and larvae. We conclude that AChR properties are set early during development at these early stages; functional maturation of the NMJ is only partly shaped by expression of AChE and may also depend on postsynaptic AChR clustering and presynaptic maturation.

Isolated epithelial cells from amphibian urinary bladder express functional gap junctional hemichannels

Exposure of the urinary bladder epithelium of Necturus maculosus (NUB) to protease and collagenase yields approximately 50% isolated polarized cells. These cells express a membrane current slowly activated by depolarization or by removal of external divalent cations. The biophysical and pharmacological properties of the current are largely consistent with those of gap junctional hemichannels. After removal of divalent cations, the cells can also be loaded with 5(6)-carboxyfluorescein, a hydrophilic fluorescent anionic dye, and exposure to dye reduces the current in a manner dependent on membrane voltage and side of application. In contrast, Necturus gallbladder (NGB) cells exhibit no membrane conductance attributable to gap junctional hemichannels, although previous studies reveal the persistence of gap junction plaques on the plasma membrane. We conclude that functional gap junctional hemichannels can be expressed on the surface of certain isolated epithelial cells and that this is not a necessary consequence of the isolation procedure. These structures may contribute to cell damage under pathological conditions involving cell detachment.

Slow conduction in cardiac tissue, I: effects of a reduction of excitability versus a reduction of electrical coupling on microconduction

It was the aim of this study to characterize the spread of activation at the cellular level in cardiac tissue during conduction slowing, a key element of reentrant arrhythmias; therefore, activation patterns were assessed at high spatiotemporal resolution in narrow (70 to 80 microm) and wide (230 to 270 microm) linear strands of cultured neonatal rat ventricular myocytes, using multiple site optical recording of transmembrane voltage. Slow conduction was induced by graded elevation of [K+]o, by applying tetrodotoxin, or by exposing the preparations to the gap junctional uncouplers palmitoleic acid or 1-octanol. The main findings of the study are 4-fold: (1) gap junctional uncoupling reduced conduction velocity (range, 37 to 47 cm/s under control conditions) to a substantially larger extent before block (</=1 cm/s; ultra-slow conduction) than did a reduction of excitability (range, approximately 10 to 15 cm/s); (2) activation wavefronts during uncoupling meandered within the boundaries of the preparations, resulting in a pronounced additional slowing of conduction in wide cell strands; (3) at the cellular level, propagation during uncoupling-induced ultra-slow conduction was sustained by sequentially activated tissue patches, each of which consisted of a few cells being activated simultaneously; and (4) depending on the uncoupler used, maximal action potential upstroke velocities during ultra-slow conduction were either slightly (palmitoleic acid) or highly (1-octanol) depressed. Thus, depolarizing inward currents, the spatial pattern and degree of gap junctional coupling, and geometrical factors all contribute in a concerted manner to conduction slowing, which, at its extreme (0.25 cm/s measured over 1 mm), can reach values low enough to permit, theoretically, reentrant excitation to occur in minuscule areas of cardiac tissue (<<1 mm2).

Cardiovascular effects of alcohol

The ingestion of one or two alcoholic drinks can affect heart rate, blood pressure, cardiac output, myocardial contractility, and regional blood flow. These actions generally are not clinically important. In the presence of cardiovascular disease, however, even such small quantities of alcohol might result in transient unfavorable hemodynamic changes. Moreover, alcohol abuse can produce cardiac arrhythmias, hypertension, cardiomyopathy, stroke, and even sudden death. In contrast, moderate alcohol use produces changes that have an overall favorable effect on atherosclerotic-related vascular diseases. Because cardiovascular disease due to atherosclerosis is the leading cause of death in Western society, this desirable effect of alcohol use outweighs its detrimental actions, resulting in favorable findings in population studies. Nevertheless, the body of evidence argues against encouraging alcohol use for its cardiovascular effects.

A 17mer peptide interferes with acidification-induced uncoupling of connexin43

Structure/function analysis shows that the carboxyl terminal (CT) domain of connexin43 (Cx43) is essential for the chemical regulation of cell-cell communication. Of particular interest is the region between amino acids 260 and 300. Structural preservation of this region is essential for acidification-induced uncoupling (ie, pH gating). In this study, we report data showing that a 17mer peptide of the same sequence as amino acids 271 to 287 of Cx43 (CSSPTAPLSPMSPPGYK) can prevent pH gating of Cx43-expressing oocytes. Experiments were carried out in pairs of Xenopus oocytes previously injected with connexin38 antisense and expressing wild-type Cx43. Junctional conductance was measured electrophysiologically. pHi was determined from the light emission of the proton-sensitive dye dextran-seminaphthorhodafluor. Intracellular acidification was induced by superfusion with a bicarbonate-buffered solution gassed with a progressively increasing concentration of CO2. Injection of water alone into both oocytes of a Cx43-expressing pair or injection of a peptide from region 321 to 337 of Cx43 did not modify pH sensitivity. However, injection of a polypeptide corresponding to amino acids 241 to 382 of Cx43 interfered with the ability of gap junctions to close on acidification. Similar results were obtained when a 17mer peptide (region 271 to 287) was injected into both oocytes of the pair. Normal Cx43 pH gating was observed if (1) the amino acid sequence of the 17mer peptide was scrambled or (2) the N and the C ends of the 17mer peptide were not included in the sequence. This is the first demonstration of a molecule that can interfere with the chemical regulation of connexin channels in a cell pair. The data may lead to the development of small molecules that can be used in Cx43-expressing multicellular preparations to study the role of gap junction regulation in normal as well as diseased states.

The effects of halothane on single human neuronal L-type calcium channels

We investigated halothane's effects on the function of L-type Ca2+ channels in a human neuronal cell line, SH-SY5Y, by using the cell-attached patch voltage clamp configuration and Ba2+ as the charge carrier. In multiple-channel patches, halothane decreased the peak and persistent Ba2+ currents, accelerated the rate of inactivation, and slowed the rate of activation. Single-channel analysis showed that halothane (0.14-1.26 mM) increased the latency time for the first channel opening, increased the lifetime of nonconducting events, increased the proportion of short-lived open events, decreased the lifetime of the two open populations, and increased the percentage of current traces without channel activity. All of the observed halothane effects contribute to the halothane-induced decrease in macroscopic Ba2+ currents. The halothane concentration producing 50% reduction (IC50) of the peak Ba2+ current was 0.80 mM (approximately 1.9 hypothetical minimum alveolar anesthetic concentration [H-MAC] at 28 degrees C) and of the persistent Ba2+ current was 0.69 mM (approximately 1.7 H-MAC). The halothane effects did not always occur together, and the Hill slope of 1.6 suggested the presence of more than one interaction site or of more than one population of L-type Ca2+ channels. Halothane reduces L-type Ca2+ channel currents in human neuronal cells primarily through the stabilization of nonconducting states such as closed (before and after channel opening) and inactivated states.

IMPLICATIONS

Calcium is a signaling molecule in neurons. We measured the effect of halothane on Ba2+ (a Ca2+ surrogate) movement into a human neuron-like cell electronically. Ba2+ entry through the L-type channel was depressed. Halothane decreased the likelihood of the channel opening and enhanced the rate at which the channel closed and inactivated. These actions of halothane are probably related to its anesthetic action.

Cell-cell coupling between CO2-excited neurons in the dorsal medulla oblongata

Anatomically coupled neurons (17 of 137) and non-coupled neurons (120 of 137), in and near the nucleus tractus solitarius and dorsal motor nucleus (i.e. solitary complex), were studied by rapid perforated patch recording in slices (rat, 150-350 microm thick, postnatal day 0-21) before, during and after exposure to hypercapnic acidosis. Anatomical coupling refers to the intercellular transfer of Lucifer Yellow and Biocytin into adjoining neurons, presumably via gap junctions [see Dean et al. (1997) Neuroscience 80, 21-40]. Eighty-six per cent of the anatomically coupled neurons (12 of 14) were depolarized by hypercapnic acidosis, a response referred to as CO2 excitation or CO2 chemosensitivity. In all, 28% (12 of 43) of the CO2-excited neurons were anatomically coupled to at least one other neuron. None (0 of 39) of the CO2-inhibited neurons were anatomically coupled, and only 4% (two of 46) of the CO2-insensitive neurons were anatomically coupled. Increasing the fractional concentration of CO2 from five to 10 and 15% in constant bicarbonate (26 mM) decreased intracellular pH (control 7.3 7.4, 22-25 degrees C) by approximately 1.0 and 1.5 pH units, respectively, as measured using the pH-sensitive fluorescent dye, 2',7'-bis (2-carboxyethyl)-5,6-carboxyfluorescein. Nine of the anatomically coupled neurons (six CO2-excited, one CO2-insensitive and two unidentified) exhibited spontaneous electrotonic postsynaptic potential-like activity, suggesting that they were also electrotonically coupled. During hypercapnic acidosis, the amplitudes of electrotonic postsynaptic potentials were unchanged, concomitant with small changes in input resistance. The frequency of electrotonic postsynaptic potentials increased during hypercapnic acidosis in many anatomically coupled neurons (eight of nine), indicating that both neurons of the coupled pair were stimulated. Cell-cell coupling occurred preferentially in and between CO2-excited neurons of the solitary complex. Further, CO2-excited neurons were not electrotonically uncoupled during intracellular acidosis, in contrast to the effect that decreased intracellular pH has on many other types of coupled cells. It was not determined whether anatomical coupling was affected by hypercapnic acidosis since dye mixture was always administered under normocapnic conditions. The high correlation between anatomical coupling, electrotonic coupling activity and CO2-induced depolarization suggests that cell-cell coupling is an important electroanatomical feature in CO2-excited neurons of the solitary complex. CO2-excited neurons have been hypothesized to function in central chemoreception for the cardiorespiratory control systems, suggesting that cell cell coupling may contribute in part to central chemoreception of CO2 and H+.

Effect of heptanol on the short circuit currents of cornea and ciliary body demonstrates rate limiting role of heterocellular gap junctions in active ciliary body transport

Rabbit ciliary body and cornea were mounted in Ussing-type chambers in Tyrode's under voltage clamp and the effects of heptanol, a gap junction inhibitor, on the short circuit current generated by each of the respective epithelia were determined. Studies were carried out either in control conditions or following amphotericin B permeabilization of either the basolateral membrane of the nonpigmented epithelium of the ciliary body or the apical membrane of the corneal epithelium, respectively. Previous studies have shown that, following these permeabilizations, short circuit currents are established, reflecting aqueous (or tear)-to-serosa Na+ fluxes, and that Na+ translocation through gap junctions connecting the individual layers of these tissues constitutes the major rate limiting step. Heptanol inhibited most of the short circuit current of the amphotericin B-modified ciliary body and cornea and of the unmodified ciliary body epithelium (control). In all these cases, the apparent IC50 was about 0.8 M. In the unmodified corneal epithelium, where ion translocation across the apical membrane constitutes the main rate limiting step for active secretion, 0.4 or 0.8 mM heptanol induced short circuit current increases; partial inhibition was observed only at high concentrations known to cause maximal inhibition of junctional permeability. Heptanol also enhanced the volume regulatory decrease of cultured human NPE cells, a process dependent on cell swelling-induced stimulation of Cl- and K+ permeabilities. Combined with our previous results demonstrating the lack of heptanol effects on other epithelial functions, these data suggest that the effect of heptanol on the active ciliary body transepithelial transport is primarily due to inhibition of the nonpigmented-pigmented junctional path and that this path is a potential site of rate limitation for the secretory process.

Triggered propagated contractions in rat cardiac trabeculae. Inhibition by octanol and heptanol

We studied the role of Ca2+ diffusion through gap junctions (GJs) in triggering and propagation of damage-induced contractions in cardiac muscle (TPCs) by evaluating effects of the GJ blockers octanol and heptanol (O&H) on TPCs. TPCs were elicited in trabeculae from rat right ventricle superfused with Krebs-Henseleit solution at 20 degrees C and 0.7 to 1.75 mmol/L [Ca2+]o. Force was measured with a silicon strain gauge; sarcomere length, by laser diffraction techniques. O&H (3 to 300 mumol/L) decreased force, propagation velocity, and triggering rate of TPCs in a dose-dependent manner. At 300 mumol/L, O&H decreased TPC force to 21.3% and 25.7%, propagation velocity to 15.4% and 13.0%, and triggering rate to 26.5% and 25.7%. At 300 mumol/L. O&H decreased twitch force to 79.0% and 77.8% and reduced time to 90% relaxation by 10% to 15%. Above 1 mmol/L, O&H abolished twitch force and TPCs. Image analysis of spread of the fluorescence profile of microinjected fura 2 salt revealed an effective diffusion coefficient for fura 2 of 21.0 +/- 3.3 microns2/s, which decreased to 12.6+/-1.5 and 7.07 +/- 0.7 microns2/s after 1 and 3 hours of exposure, respectively, to 100 mumol/L octanol, with a time constant of decline of 1.5+/-0.5 hours. These results are consistent with the hypothesis that propagation of TPCs is due to Ca(2+)-induced Ca2+ release mediated by Ca2+ diffusion from cell to cell through GJs. Reduction of propagation velocity reduces the number of activated sarcomeres in the TPC, which reduces TPC force. O&H slow triggering of TPCs, presumably by blocking Ca2+ diffusion from myocytes within damaged areas to adjacent normal cells.

Halothane and isoflurane preferentially depress a slowly inactivating component of Ca2+ channel current in guinea-pig myocytes

1. The effects of the inhalational anaesthetics halothane and isoflurane on the high-voltage-activated Ca2+ channels were determined in isolated guinea-pig ventricular myocytes using the patch-clamp technique. 2. Recording solutions were equilibrated with inhalational anaesthetic vapour delivered from a calibrated vaporizer set at clinically relevant ranges of partial pressure. Anaesthetic concentrations in solution were determined using gas chromatography. 3. Halothane (0.9 mM in solution) and isoflurane (0.8 mM in solution) decreased peak whole-cell CA2+ current (ICa) by approximately 40 and approximately 20%, respectively, while increasing the apparent rate of inactivation. 4. The sum of fast and slow exponential decay functions was required to fit the inactivation phase of ICa. The anaesthetics preferentially affected the slow component of inactivation while also increasing the rate of slow inactivation. The physiological significance of these effects was addressed by examining ICa evoked by a ventricular action potential waveform. 5. Measurement of the current carried by Ba2+ through Ca2+ channels (IBa) permitted the isolation of the slow component of inactivation. Halothane and isoflurane diminished peak IBa at 0 mV by approximately 45 and approximately 20% respectively, with similar changes in rate and magnitude of the slowly inactivating component as with ICa. 6. Cell-attached patch-clamp measurements of Ca2+ channel activity revealed that halothane did not alter single-channel conductance. Instead, the anaesthetic reduced channel open probability to the same extent as observed during the whole-cell recording, an effect partially due to an increase in null sweeps. In patches with a single channel present, the open-time distribution, fitted by a single exponential, showed a decrease in mean open time. The closed-time distribution, fitted by the sum of slow and fast exponential components, revealed an anaesthetic-induced increase in the duration of the slow component with no effect on the fast component. Results are presented in terms of a channel-gating model, and model predictions are examined with a computer simulation.

Connexin43 gap junctions exhibit asymmetrical gating properties

A communication-deficient cell line (RIN cells, derived from a rat islet tumour), stably transfected with cDNA coding for rat connexin43 (Cx43), was chosen to further assess the mechanism of voltage gating of Cx43 gap junction channels. The experiments were carried out on preformed cell pairs using a dual whole-cell, voltage-clamp method. The junctional current, Ij, revealed a time- and voltage-dependent inactivation at transjunctional voltages Vj>+/-40 mV. When an asymmetrical pulse protocol was used (in cell 1 the holding potential was maintained, in cell 2 it was altered to establish a variable Vj), the channels exhibited an asymmetrical gating behaviour: Vj,0=-73.7 mV and 65.1 mV for negative and positive Vj, respectively (Vj at which Ij is half-maximally inactivated); gj(min)=0.34 and 0.29 (normalized minimal conductance); tau = 350 ms and 80 ms at Vj=100 mV (time constant of Ij inactivation). Hence, these parameters were more sensitive to positive Vj values. When a symmetrical pulse protocol was used (the holding potentials in cell 1 and cell 2 were altered simultaneously in steps of equal amplitude but of opposite polarity), the Vj -dependent asymmetries were absent: Vj,0=-60.5 and 59.5; gj (min)=0.27 and 0.29; tau =64 ms and 47 ms at 100mV. Putative explanations for these observations are discussed. A possibility is that the number of channels alters with the polarity of Vj.

Rapid onset and calcium independence of the gap junction uncoupling induced by heptanol in cultured heart cells

The kinetics of the reversible interruption of gap junction communication by the aliphatic alcohol heptanol and the possible mediation of an increase of the cytosolic Ca2+ concentration have been investigated in pairs of myocytes dissociated from neonatal rat ventricles and cultured for 2-3 days. Junctional communication was estimated by measuring either the cell-to-cell electrical conductance with a double whole-cell voltage-clamp method, or the rate constant of dye diffusion with the fluorescence recovery after photo-bleaching (gap FRAP) technique. Electrical coupling was seen to be abruptly interrupted (in less than 0.5 s) by heptanol (1-3 mM). The cytosolic Ca2+ concentration was not affected, even at a saturating heptanol concentration. Heptanol removal allowed a gradual re-opening of gap junctional channels, as shown by the recovery curve of the cell-to-cell conductance, which is 90% complete within 90 s. These data are consistent with a direct interaction of heptanol with channel proteins or with their lipid environment.

Dynamics of circus movement re-entry across canine Purkinje fibre-muscle junctions

1. To determine the cellular electrophysiological mechanisms for unidirectional conduction block and re-entrant excitation, single cycles of circus movement re-entry were induced in canine Purkinje fibre-papillary muscle preparations containing two Purkinje fibre-muscle junctions (PMJs). The preparations were mounted in a partitioned tissue bath that permitted independent superfusion of each PMJ. The pre-existing dispersion of refractoriness between PMJs was accentuated by superfusing PMJ1 with normal Tyrode solution or Tyrode solution containing 6-8 mM KCl and superfusing PMJ2 with Tyrode solution containing 0.5 mM heptanol and 4-10 mM KCl. 2. Premature stimuli delivered to the Purkinje fibre induced unidirectional anterograde conduction block at PMJ2. Conduction proceeded from Purkinje cells to papillary muscle at PMJ1 and from papillary muscle retrogradely across the previously blocked PMJ2. 3. The difference in refractory periods between the two PMJs defined a range of premature coupling intervals within which re-entry was inducible. Conduction block at the PMJ occurred in papillary muscle at short coupling intervals and in the Purkinje fibre at longer intervals. 4. Once initiated, re-entry could be reset or annihilated by properly timed subthreshold current pulses delivered to cells at the PMJ. 5. To define better the mechanisms for conduction block and re-entry, an analytical model was developed using non-linear regression analysis to derive equations from the experimental results. Varying parameters within the constraints of the model reproduced the key features of the rate-dependent conduction block observed experimentally. Critical elements of the model included the induction of significant activation delays at short diastolic intervals and a reduction in the rate of action potential duration restitution after exposure to heptanol. 6. These results help to establish the conditions necessary for induction of one-dimensional circus movement re-entry and to define the roles of non-linearities of activation delay and excitability in the dynamics of conduction block at the PMJ.

The effects of propofol and enflurane on single calcium channel currents of guinea-pig isolated ventricular myocytes

1. The effects of the anaesthetics, propofol (100 microM) and enflurane (3%, 1.46 mM), on single L type calcium channel currents were investigated in single myocytes isolated from guinea-pig ventricles. Channel activity was recorded from membrane patches by use of the 'cell-attached' patch-clamp technique (pipette solution containing 110 mM BaCl2, 5 microM Bay K 8644, 5 microM HEPES, pH 7.4; temperature 36 degrees C). 2. Channel conductance was calculated from the slope of the relationship between single channel current and membrane potential during step depolarizations to activate the channel over a range of approximately -20 to +20 mV. Neither propofol (6 cells) nor enflurane (7 cells) caused any significant reduction in channel conductance. 3. Both propofol (7 cells) and enflurane (9 cells) decreased the probability of the channel being open during depolarizations to +10 mV (measured from histograms of the fraction of time spent by the channel at different current levels, taking areas under the Gaussian curves fitted to the open and closed components of the distributions to represent the proportion of time spent in the two states). 4. A fraction of the current traces showed no detectable channel openings in response to step depolarizations to +10 mV. Both propofol and enflurane significantly increased the fraction of silent traces. 5. Transitions across a threshold halfway between the open and closed levels were used to define periods spent in the open and closed states. Both propofol (7 cells) and enflurane (9 cells) reduced the mean open times and increased the mean closed times of the calcium channel. 6. Histograms were plotted showing the distributions of times spent by the channels in the open and closed states. Two exponentials were fitted to the open and closed time distributions. Both propofol (7 cells) and enflurane (9 cells) shortened both time constants fitted to the open times and lengthened both time constants fitted to the closed times.7. It is concluded that both propofol and enflurane appear to alter the kinetics of opening and closing of calcium channels to favour shut channels without altering channel conductance. This effect would be expected to result in a reduction of the macroscopic calcium current and thus contribute to the negative inotropic action of these anaesthetics.

Electrophysiologic interaction between class I antiarrhythmic drugs and volatile anesthetics in depressant effects on ventricular activation in a canine myocardial infarction model

Previous studies showed that volatile anesthetics depressed ventricular delayed activation in a canine myocardial infarction model. It is well known that class I antiarrhythmic drugs depress the ventricular activation in the infarcted myocardium. In the present study, we examined the electrophysiologic interaction between volatile anesthetics (sevoflurane, isoflurane) and class I antiarrhythmic drugs (lidocaine, procainamide) in effects on the ventricular delayed activation in a canine myocardial infarction model. The conduction time of the premature stimulation-induced ventricular excitation was measured in both normal and infarcted zones of the ventricle. An interval from the premature stimulus artifact to the epicardial activation was measured on bipolar electrograms as an index of conduction time, i.e., activation time. In the infarcted zone, the volatile anesthetics and class I antiarrhythmic drugs prolonged the activation time in the infarcted zone, and the combination of the volatile anesthetics and the class I antiarrhythmic drugs markedly prolonged the activation time or blocked the delayed activation. In the normal zone, a similar synergistic interaction was observed, but the effect of these drugs was less compared with that in the infarcted zone. From these results, possible mechanisms to explain the synergistic interaction were discussed.

Interaction of anesthetics and catecholamines on conduction in the canine His-Purkinje system

The findings in papillary muscles that epinephrine facilitates conduction at Purkinje fiber-muscle junctions and in the endocardium are consistent with older observations that activation of myocardial beta-adrenergic receptors speeds conduction and activation in the heart and thereby increases the synergy of contraction (46,47). The cellular mechanism underlying this action is probably increased cell-to-cell coupling between muscle fibers secondary to elevation of cyclic AMP (19,48). However, the findings that epinephrine alone or with halothane transiently slows conduction in the Purkinje layer while simultaneously improving conduction across Purkinje-muscle junctions and in the endocardium may represent proarrhythmic actions. These actions could facilitate arrhythmogenesis by transiently increasing regional differences of activation and repolarization times in the conduction system and myocardium and thereby increasing vulnerability to induction of reentry by premature impulses. Such a proarrhythmic effect could explain an older observation that low-dose norepinephrine infusions decrease the threshold for induction of fibrillation by two premature beats in pentobarbital-anesthetized animals (49). The cellular basis underlying the different responses of Purkinje fibers and the endocardial muscle layer to catecholamines, in which velocity decreased and increased, respectively, is not known. Our working hypothesis to explain this action in canine Purkinje fibers is a mechanism involving activation of WB4101-sensitive alpha 1-adrenoceptor, G-protein coupling to phospholipase C and the generation of DAG and IP3 leading to modulation of cell-to-cell coupling, which is potentiated in the presence of partial uncoupling by halothane. The different responses of Purkinje and myocardial fibers are speculated to result from differences in the relative density of this subtype of alpha 1-adrenoceptor, differences in the subcellular effector coupling mechanisms, or differences in the specific connexin proteins forming gap junctions between Purkinje and myocardial fibers (50).

Heptanol-induced decrease in cardiac gap junctional conductance is mediated by a decrease in the fluidity of membranous cholesterol-rich domains

To assess whether alterations in membrane fluidity of neonatal rat heart cells modulate gap junctional conductance (gj), we compared the effects of 2 mM 1-heptanol and 20 microM 2-(methoxy-ethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)-octanoate (A2C) in a combined fluorescence anisotropy and electrophysiological study. Both substances decreased fluorescence steady-state anisotropy (rss), as assessed with the fluorescent probe 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) by 9.6 +/- 1.1% (mean +/- SEM, n = 5) and 9.8 +/- 0.6% (n = 5), respectively, i.e., both substances increased bulk membrane fluidity. Double whole-cell voltage-clamp experiments showed that 2 mM heptanol uncoupled cell pairs completely (n = 6), whereas 20 microM A2C, which increased bulk membrane fluidity to the same extent, did not affect coupling at all (n = 5). Since gap junction channels are embedded in relatively cholesterol-rich domains of the membrane, we specifically assessed the fluidity of the cholesterol-rich domains with dehydroergosterol (DHE). Using DHE, heptanol increased rss by 14.9 +/- 3.0% (n = 5), i.e., decreased cholesterol domain fluidity, whereas A2C had no effect on rss (-0.4 +/- 6.7%, n = 5). Following an increase of cellular "cholesterol" content (by loading the cells with DHE), 2 mM heptanol did not uncouple cell pairs completely: gj decreased by 80 +/- 20% (range 41-95%, n = 5). The decrease in gj was most probably due to a decrease in the open probability of the gap junction channels, because the unitary conductances of the channels were not changed nor was the number of channels comprising the gap junction. The sensitivity of nonjunctional membrane channels to heptanol was unaltered in cholesterol-enriched myocytes. These results indicate that the fluidity of cholesterol-rich domains is of importance to gap junctional coupling, and that heptanol decreases gj by decreasing the fluidity of cholesterol-rich domains, rather than by increasing the bulk membrane fluidity.

The effect of a range of alcohols on the contraction of guinea-pig ventricular myocytes

Previous results have shown that ethanol and some anaesthetics have a negative inotropic effect on the heart. This study was undertaken to investigate the influence of a range on n-alcohols (with chain lengths from 2 to 8) on contractility in guinea-pig ventricular myocytes. The results demonstrate that the negative inotropic action of alcohols increases dramatically as the chain length increases. The concentration required to reduce the magnitude of contraction to 50% of control (IC50) was 274 mM, 26 mM, 1.4 mM and 235 microM for ethanol, butanol, hexanol and octanol, respectively. The relationship between the logarithm of IC50 and chain length was linear for all the alcohols tested (up to a chain length of 8).

Recent insights pertaining to sarcolemmal phospholipid alterations underlying arrhythmogenesis in the ischemic heart

Myocardial ischemia in vivo is associated with dramatic electrophysiologic alterations that occur within minutes of cessation of coronary flow and are rapidly reversible with reperfusion. This suggests that subtle and reversible biochemical alterations within or near the sarcolemma may contribute to the electrophysiologic derangements. Our studies have concentrated on two amphipathic metabolites, long-chain acylcarnitines and lysophosphatidylcholine (LPC), which have been shown to increase rapidly in ischemic tissue in vivo and to elicit electrophysiologic derangements in normoxic tissue in vitro. Incorporation of these amphiphiles into the sarcolemma at concentrations of 1 to 2 mole%, elicits profound electrophysiologic derangements analogous to those observed in ischemic myocardium in vivo. The pathophysiological effects of the accumulation of these amphiphiles are thought to be mediated by alterations in the biophysical properties of the sarcolemmal membrane, although there is a possibility of a direct effect upon ion channels. Inhibition of carnitine acyltransferase I (CAT-I) in the ischemic cat heart was found to prevent the increase in long-chain acylcarnitines and LPC and to significantly reduce the incidence of malignant arrhythmias including ventricular tachycardia and fibrillation. This review focuses on the electrophysiologic derangements that are observed during early ischemia and presents data supporting the concept that accumulation of these amphiphiles within the sarcolemma contributes to these changes. The potential contribution of these amphiphiles to the increases in extracellular potassium and intracellular calcium are examined. Finally, recent data pertaining to the accumulation of long-chain acylcarnitines on cell-to-cell uncoupling are presented. In addition to the events reviewed here, there are many other alterations that occur during early myocardial ischemia, but the results from multiple studies over the past two decades indicate that the accumulation of these amphiphiles contributes importantly to arrhythmogenesis and that development of specific inhibitors of CAT-I or phospholipase A2 may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease in man.

Temperature dependence of gap junction properties in neonatal rat heart cells

Cell pairs of neonatal rat hearts were used to study the influence of temperature on the electrical properties of gap junctions. A dual voltage-clamp method was adopted, which allowed the voltage gradient between the cells to be controlled and the intercellular current flow to be measured. Cell pairs with normal coupling revealed a positive correlation between the conductance of the junctional membranes, gj, and temperature. Cooling from 37 degrees C to 14 degrees C led to a steeper decrease in gj, cooling from 14 degrees C to -2 degrees C to a shallower decrease (37 degrees C: gj = 48.3 nS; 14 degrees C: gj = 21.4 nS; -2 degrees C: gj = 17.5 nS), corresponding to a temperature coefficient, Q10, of 1.43 and 1.14 respectively. The existence of two Q10 values implies that gj may be controlled by enzymatic reactions. When gj was low, i.e. below 5 nS (conditions: low temperature; treatment with 3 mM heptanol), it showed voltage-dependent gating. This property was not visible when gj was large, i.e. 20-70 nS (conditions: high temperature; normal saline), presumably because of series resistances (pipette resistance). Cell pairs with weak intrinsic coupling and normally coupled cell pairs treated with 3 mM heptanol revealed a positive correlation between the conductance of single gap-junction channels, gamma j, and temperature (37 degrees C: 75.6 pS; -2 degrees C: 19.6 pS), corresponding to a Q10 of 1.41.

Inhibition of gap junctional conductance by long-chain acylcarnitines and their preferential accumulation in junctional sarcolemma during hypoxia

Electrophysiological and biochemical sequelae of myocardial ischemia occur within minutes of the onset of myocardial ischemia in vivo. Both conduction delay and conduction block occur rapidly within the same time interval as the accumulation of long-chain acylcarnitines. In the present study, double whole-cell voltage-clamp procedures were used to assess the influence of long-chain acylcarnitines on gap junctional conductance in isolated pairs of canine ventricular myocytes. Long-chain acylcarnitine (5 microM) decreased gap junctional conductance from 153 to 48 nS in a time-dependent and reversible manner. Although the amplitude of junctional current was reduced by 68%, the current continued to demonstrate a linear current-voltage relation. The extent of endogenous accumulation of long-chain acylcarnitines in junctional regions of the sarcolemma was assessed in isolated myocytes in which endogenous free, short-chain, and long-chain acylcarnitine pools had been equilibrated with [3H]carnitine. Under normoxic conditions, long-chain acylcarnitines were not detectable in junctional sarcolemma of myocytes as assessed using electron microscopic autoradiography. Exposure of myocytes to hypoxia (PO2, < 15 mm Hg) for 10 minutes resulted in the preferential accumulation of endogenous long-chain acylcarnitines in junctional sarcolemma (173 +/- 5 x 10(5) molecules/microns 3), a concentration that was sevenfold greater than that found in nonjunctional sarcolemma. Therefore, endogenous long-chain acylcarnitines accumulate preferentially in junctional regions of the sarcolemma during short intervals of hypoxia. Exogenously supplied long-chain acylcarnitines can markedly decrease cellular coupling in a reversible manner, suggesting that this amphiphile may contribute to the marked slowing in conduction velocity in the ischemic heart in vivo, not only by suppressing the rapid Na+ inward current directly, as has been shown previously, but also by decreasing cellular coupling.

Electrical coupling between cells of the insect Aedes albopictus

1. Cell pairs of an insect cell line (Aedes albopictus, clone C6/36) were used to study the electrical properties of intercellular junctions. A double voltage-clamp approach was adopted to control the voltage gradient between the cells and measure the intracellular current flow. 2. Determinations of junctional conductance (gj) revealed two types of intercellular contacts, gap junctions and cytoplasmic bridges. Identification occurred by means of functional criteria, i.e. the dependency of gj on (i) junctional membrane potential, (ii) non-junctional membrane potential, and (iii) heptanol. 3. In cell pairs with putative gap junctions, gj was dependent on the junctional membrane potential (Vj). When determined at the beginning of voltage pulses, gj was insensitive to Vj; when determined at the end of 15 s pulses, it depended on Vj in a bell-shaped manner (70% decrease for a change in Vj of +/- 75 mV). 4. These cell pairs also showed a dependency of gj on the non-junctional membrane potential (Vm). When determined immediately after changing the non-junctional membrane potential in both cells, gj was not affected by Vm; when determined 30 s later, gj was modified by Vm in a S-shaped fashion (100% decrease when Vm was depolarized to +50 mV). 5. Exposure to 3 mM-heptanol gave rise to complete and reversible block of gj in cell pairs with putative gap junctions. 6. Cell pairs susceptible to uncoupling by heptanol revealed junctional currents indicative of the operation of gap junction channels. The single-channel conductance, determined at a Vm of -50 to -70 mV, was 133 pS. 7. In the case of putative cytoplasmic bridges, gj was insensitive to the junctional and non-junctional membrane potential. In addition, it was not affected by 3 mM-heptanol. 8. While most cell pairs showed functional properties characteristic of gap junctions or cytoplasmic bridges, few cell pairs exhibited junctional currents compatible with the co-existence of both junctional structures.

Effects of the anesthetics heptanol, halothane and isoflurane on gap junction conductance in crayfish septate axons: a calcium- and hydrogen-independent phenomenon potentiated by caffeine and theophylline, and inhibited by 4-aminopyridine

This study has monitored junctional and nonjunctional resistance, [Ca2+]i and [H+]i, and the effects of various drugs in crayfish septate axons exposed to neutral anesthetics. The uncoupling efficiency of heptanol and halothane is significantly potentiated by caffeine and theophylline. The modest uncoupling effects of isoflurane, described here for the first time, are also enhanced by caffeine. Heptanol causes a decrease in [Ca2+]i and [H+]i both in the presence and absence of either caffeine or theophylline. A similar but transient effect on [Ca2+]i is observed with halothane. 4-Aminopyridine strongly inhibits the uncoupling effects of heptanol. The observed decrease in [Ca2+]i with heptanol and halothane and negative results obtained with different [Ca2+]o, (Ca2+)-channel blockers (nisoldipine and Cd2+) and ryanodine speak against a Ca2+ participation. Negative results obtained with 3-isobutyl-1-methylxanthine, forskolin, CPT-cAMP, 8Br-cGMP, adenosine, phorbol ester and H7, superfused in the presence and absence of caffeine and/or heptanol, indicate that neither the heptanol effects nor their potentiation by caffeine are mediated by cyclic nucleotides, adenosine receptors and kinase C. The data suggest a direct effect of anesthetics, possibly involving both polar and hydrophobic interactions with channel proteins. Xanthines and 4-aminopyridine may participate by influencing polar interactions. The potentiating effect of xanthines on cell-to-cell uncoupling by anesthetics may provide some clues on the nature of cardiac arrhythmias in patients treated with theophylline during halothane anesthesia.

Block of sodium current by heptanol in voltage-clamped canine cardiac Purkinje cells

Heptanol blocks sodium current (INa) in nerve, but its effects on cardiac INa have not been well characterized. Block of INa by heptanol was studied in 16 internally perfused voltage-clamped cardiac Purkinje cells at reduced Na+ (45 mM outside, 0 mM inside). Heptanol block of peak sodium conductance was well described by a single-site binding curve with half block at 1.3 mM (20 degrees C) and showed no "use dependence." With 1.5 mM heptanol, block increased slightly by 0.7%/degrees C from 10 degrees C to 27 degrees C. With 3.0 mM heptanol, steady-state availability shifted by 9.4 +/- 1.3 mV (n = 6) in the hyperpolarizing direction, and steady-state activation shifted by 8.3 +/- 2.2 mV (n = 5) in the depolarizing direction, thus closing off the INa "window current." Heptanol also decreased the time to peak and accelerated the decay of INa. Similar results were found with octanol at lower concentrations. These alcohols have important effects on cardiac INa at concentrations used in studies for cellular uncoupling in heart.

Biophysical properties of gap junctions between freshly dispersed pairs of mouse pancreatic beta cells

Coupling between beta cells through gap junctions has been postulated as a principal mechanism of electrical synchronization of glucose-induced activity throughout the islet of Langerhans. We characterized junctional conductance between isolated pairs of mouse pancreatic beta cells by whole-cell recording with two independent patch-clamp circuits. Most pairs were coupled (67%, n = 155), although the mean junctional conductance (gj) (215 +/- 110 pS) was lower than reported in other tissues. Coupling could be recorded for long periods, up to 40 min. Voltage imposed across the junctional or nonjunctional membranes had no effect on gj. Up to several hours of treatment to increase intracellular cAMP levels did not affect gj. Electrically coupled pairs did not show transfer of the dye Lucifer yellow. Octanol (2 mM) reversibly decreased gj. Lower concentrations of octanol (0.5 mM) and heptanol (0.5 mM) than required to uncouple beta cells decreased voltage-dependent K+ and Ca2+ currents in nonjunctional membranes. Although gj recorded in these experiments would be expected to be provided by current flowing through only a few channels of the unitary conductance previously reported for other gap junctions, no unitary junctional currents were observed even during reversible suppression of gj by octanol. This result suggests either that the single channel conductance of gap junction channels between beta cells is smaller than in other tissues (less than 20 pS) or that the small mean conductance is due to transitions between open and closed states that are too rapid or too slow to be resolved.

Effects of arachidonic acid on the gap junctions of neonatal rat heart cells

Myocytes were isolated from neonatal rat hearts and grown in tissue-culture dishes for 1-2 days. Spontaneously formed cell pairs were used to study the conductance of gap junctions. The experiments involved a double voltage-clamp approach and whole-cell, tight-seal recording. Exposure to arachidonic acid (AA) produced a quasi dose-dependent decrease in junctional conductance, gi (binding constant, Kd = 4 microM; Hill coefficient, n = 0.75). AA-dependent uncoupling was reversible. Addition of 1 mg/ml albumin to the bath solution accelerated the recovery. During control, cell pairs exhibited a gradual decrease in gi (16.4% in 6 min). Exposure to 20 microM 4-bromophenacyl bromide, a phospholipase inhibitor, suppressed the decay in gi (1.8% in 6 min), suggesting that endogenous AA may be involved in spontaneous uncoupling. The effect of AA on gi was specific. Arachidic acid (100 microM) and arachidonamide (10 microM), structural analogues of AA, had no effect on gi. Currents recorded shortly before complete uncoupling caused by AA, or early during recovery from uncoupling, revealed random opening and closing of single channels. The single channel conductance, gamma i, was not affected by the concentration of AA (1 microM - 100 microM). The mean gamma i turned out to be 33.5 pS. The results suggest that AA-dependent uncoupling was caused via decrease in open channel probability, presumably mediated by a direct action on channel proteins.

Large-conductance ion channel measured by whole-cell voltage clamp in single cardiac cells: modulation by beta-adrenergic stimulation and inhibition by octanol

Membrane currents in single cardiac myocytes from adult guinea pigs were studied by means of the patch-clamp technique (whole-cell mode). During spontaneous or caffeine-induced Ca2+ release from the sarcoplasmic reticulum openings of a novel ion channel with large unitary conductance (280 pS) can be recorded. The density of these channels and/or its open-state probability are unusually low. On average in the whole-cell mode simultaneous maximum superposition of only four channels is observed. Opening events of this channel require an intracellular Ca2+ transient. Activation by [Ca2+]i, however, seems to be indirect; maximum opening activity occurs with a delay of several hundred milliseconds after peak [Ca2+]i. Single-channel activity can be enhanced by a cyclic AMP dependent process via beta-adrenergic stimulation of a cell. This can also be mimicked by caffeine, most likely via inhibition of phosphodiesterase. Octanol, an inhibitor of gap-junctional coupling in a variety of tissues, causes a concentration-dependent and reversible decrease in single-channel activity. Unitary conductance is not affected by octanol. The low density of these channels in cardiac membranes and their poor selectivity render any role in normal cardiac electrical activity unlikely. A possible relation of the channel to cardiac gap junctions is discussed.