Impact of School Start Time Delays and Learning Modality on Sleep Timing and Duration During COVID-19

OBJECTIVES

To assess the impact of a school start time (SST) delay on adolescent sleep health during the COVID-19 pandemic, and whether there were differences by learning modality.

METHODS

Data were collected from a longitudinal study evaluating sleep, education, and health among high school students in Georgia in 2020. Paired t-tests and multivariable linear regression analyses were conducted to examine changes in sleep duration and timing among 9th grade students (n = 134) and their association with the learning modality (remote vs. in-person learner).

RESULTS

Students' school day wake times were 1.5 hours later, school night sleep duration was 1.2 hours longer, and social jetlag was 0.9 hours shorter after the school start time delay (all P < .05). The learning modality was a significant predictor of changes in sleep timing but was not associated with changes in sleep duration.

CONCLUSIONS

Delayed school start time was associated with positive changes in adolescent sleep health during the COVID-19 pandemic. Sleep timing was affected by the learning modality, however in-person and virtual students had similar gains in sleep duration. Learning modality may be more beneficial for adolescents with early school start times to promote healthier sleep habits.

The Association of Sleep Duration with Grade Point Averages and Absences Among 9th Graders in Georgia, USA

This study aimed to examine the association between sleep measures (self-reported sleep duration and weekend catch-up sleep) and grade point average (GPA) and absences among 9th grade students from two racially and economically diverse high schools in a semi-rural county of north-central Georgia. Linear and Poisson regression models estimated the association between sleep measures and GPA and absences (separately), respectively. Analyses adjusted for gender, race/ethnicity, free/reduced-price school lunch status, and parental education. Sleep duration was significantly associated with both GPA and absences, such that for every one additional hour of sleep, GPA increased by 0.8 percentage points (b=0.8, 95% CI:0.1,1.5) while the number of absences was lower by 6% (b=-0.05; OR=0.94, 95% CI:0.91,0.98). Weekend catch-up sleep was also significantly and positively associated with absences (b=0.04; OR=1.04, 95% CI; 1.02, 1.07). Increasing sleep may be a strategy to improve GPA and reduce absences among teenagers. Future research should identify effective measures to lengthen sleep.

The Impact of COVID-19 on Education Experiences of High School Students in Semi-Rural Georgia

BACKGROUND

This study examines the consequences of COVID-19 pandemic on academic and career concerns of high school students; relationship between attendance and grades with educational concerns; and association between student perception of the pandemic and decision to attend school virtually or in-person.

METHODS

Diverse students in grades 9-12 from two public high schools in semi-rural Georgia (n = 666) completed a survey shortly after school closures. Survey results were linked to academic and demographic data. Analyses were examined for differences by demographic and education measures.

RESULTS

Overall, 60% expressed academic worry and reported obstacles to virtual learning. Hispanic students expressed more worry and less confidence while black students reported less worry than peers. Females indicated more worry than males. Grade 12 students reported greater academic and career worry than younger students. Students eligible for free and reduced lunch expressed more worry and obstacles than peers. Non-honors and low-attendance students reported more worry about grades and graduation than their counterparts.

CONCLUSIONS

Results can inform targeted multi-level interventions to reduce the pandemic's effects on learning and ensure healthy trajectories for development across demographics. Stakeholders must take proactive measures to recover from academic loss to ensure our youth's healthy development.

Adolescent sleep duration and timing during early COVID-19 school closures

Objectives

To investigate adolescent sleep parameters and predictors during COVID-19-related school closures.

Methods

Original data were analyzed from a cross-sectional online survey of 590 teens in grades 6-12 attending school remotely in 35 US states, in May/June 2020.

Results

Students reported waking up 2.1-2.9 hours later during school closures and averaged 7.9-8.7 hours of sleep and 8.6-9.5 hours in bed on school nights. Compared to middle schoolers, high school students had later bed and wake times, accompanied by spending less time in bed and less time sleeping. The delay in wake time after school closures was also longer for high school students than for middle schoolers. Students with later class start times went to bed later, but also woke up later, slept longer, and spent more time in bed. When comparing intraindividual sleep before and after school closures, later class start times resulted in greater delays in wake time and greater odds of increased sleep duration. In addition, parent-set bedtimes were associated with earlier bedtimes and longer sleep duration during school closures.

Conclusions

As a result of COVID-19-related school closures and remote instruction, more middle and high school students achieved recommended amounts of sleep, primarily by waking up later in the morning. This study supports previous evidence that morning start schedule affects adolescent sleep behaviors. The implications of this study extend beyond COVID-19 school closures; adolescent sleep health improves with later school start times and fewer scheduled morning activities.

COVID-19 Knowledge and Behavior Change among High School Students in Semi-Rural Georgia

BACKGROUND

Many schools transitioned online to reduce viral spread and promote social distancing amid the COVID-19 pandemic. Remote learning may impact health behaviors and coping strategies among adolescents, including reduced physical activity and increased screen time. Social media and peers provide information about the pandemic to adolescents and may influence prevention behaviors. This study aims to assess adolescent knowledge and information about COVID-19, determine students' behavior change, and identify sources students turn to for information on the pandemic.

METHODS

Students from 2 high schools in semi-rural Georgia participated in a cross-sectional online survey in March 2020 (N = 761).

RESULTS

Common sources for COVID-19 news were peers (80%) and social media (58%). Few adolescents (3%) indicated that teenagers were at higher risk of severe illness due to COVID-19. Responses reveal adolescents understand social distancing and many are participating in prevention behaviors, including handwashing (87%) and staying at home as much as possible (87%). Most respondents reported increases in screen time outside of class (82%).

CONCLUSIONS

Findings suggest adolescents are obtaining COVID-19 knowledge from various sources, including social media, with varying degrees of implementation of prevention practices. Increases in screen time and reduced physical activity may impact long-term health among adolescents.

Impact of COVID-19 Pandemic on the Mental Health of Students From 2 Semi-Rural High Schools in Georgia

BACKGROUND

Concerns have been raised about the consequences of the COVID-19 pandemic on the mental health of adolescents during an important developmental and social period in their lives. This study examines the mental health impact of the pandemic on high school students shortly after closure of public schools in spring 2020, and whether this impact varies by sex, race/ethnicity, socioeconomic status (SES), and grade level.

METHODS

A cross-sectional, one-time online survey was completed by racial/ethnically and socioeconomically diverse students in 9th through 12th grade at 2 semi-rural Georgia public high schools (N = 761).

RESULTS

Overall, almost one-fourth of high school students were extremely or very worried about the pandemic, with higher rates of worry among students who are racial/ethnic minorities, lower SES, female and in older grades. Results indicated a concerning impact on the stress, anxiety, depression, and loneliness that students are feeling, particularly among girls and those in older grades. Students of color and low SES, who are already disproportionately impacted by COVID-19, are also more likely to experience mental health challenges.

CONCLUSIONS

Findings highlight important demographic differences of the impact of COVID-19 on the mental health of high school students and have implications for schools with addressing these needs.

Gap26, a connexin mimetic peptide, inhibits currents carried by connexin43 hemichannels and gap junction channels

Connexin mimetic peptides corresponding to short conserved extracellular loop sequences of connexins have been used widely as reversible inhibitors of gap junctional intercellular communication. These peptides also block movement of ATP and Ca(2+) across connexin hemichannels, i.e. hexameric channels yet to dock with partners in aligned cells and to generate the gap junction cell-cell conduit. By means of electrophysiology, we compared the effects of Gap26, a mimetic peptide corresponding to a short linear sequence in the first extracellular loop of connexin43, on connexin channel function in HeLa cells expressing connexin43. We demonstrate that Gap26 inhibited electrical coupling in cell pairs mediated by gap junctions after exposure for 30min. In contrast, Gap26 applied to single cells, inhibited hemichannel currents evoked in low Ca(2+) solution with a response time of less than 5min. The results further support the view that the likely primary and direct inhibitory effect of Gap26 is on connexin hemichannels, with gap junctions becoming inhibited later. The mechanism of action of Gap26 in blocking hemichannels and gap junction channels is discussed in the context of their different functions and locations.

Characterization of zebrafish Cx43.4 connexin and its channels

Connexins (Cx) form intercellular junctional channels which are responsible for metabolic and electrical coupling. We report here on the biochemical and immunohistochemical characterization of zebrafish connexin zfCx43.4, an orthologue of mammalian and avian Cx45, and the electrophysiological properties of junctional channels formed by this protein. The investigations were performed on transfected COS-7 cells or HeLa cells. Using site-directed antibodies, zfCx43.4 cDNA (GenBank accession no. X96712) was demonstrated to code for a protein with a M(r) of 45 000. In transfected cells, zfCx43.4 was localized in cell-cell contact areas as expected for a gap junction protein. zfCx43.4 channels were shown to transfer Lucifer Yellow. The multichannel currents were sensitive to the transjunctional voltage (V(j)). Their properties were consistent with a two-state model and yielded the following Boltzmann parameters for negative/positive V(j): V(j,0) = -38.4/41.9 mV; g(j,min) = 0.19/0.18; z = 2.6/2.3. These parameters deviate somewhat from those of zfCx43.4 channels expressed in Xenopus oocytes and from those of Cx45, an orthologue of zfCx43.4, expressed in mammalian cells or Xenopus oocytes. Conceivably, the subtle differences may reflect differences in experimental methods and/or in the expression system. The single channel currents yielded two prominent levels attributable to a main conductance state (gamma(j,main) = 33.2 +/- 1.5 pS) and a residual conductance state (gamma(j,residual) = 11.9 +/- 0.6 pS).

Conductive and Kinetic Properties of Connexin45 Hemichannels Expressed in Transfected HeLa Cells

Human HeLa cells transfected with mouse connexin Cx45 were used to examine the conductive and kinetic properties of Cx45 hemichannels. The experiments were carried out on single cells using a voltage-clamp method. Lowering the [Ca2+]o revealed an extra current. Its sensitivity to extracellular Ca2+ and gap junction channel blockers (18alpha-glycyrrhetinic acid, palmitoleic acid, heptanol), and its absence in non-transfected HeLa cells suggested that it is carried by Cx45 hemichannels. The conductive and kinetic properties of this current, Ihc, were determined adopting a biphasic pulse protocol. Ihc activated at positive Vm and deactivated partially at negative Vm. The analysis of the instantaneous Ihc yielded a linear function ghc,inst = f(Vm) with a hint of a negative slope (ghc,inst: instantaneous conductance). The analysis of the steady-state Ihc revealed a sigmoidal function ghc,ss=f(Vm) best described with the Boltzmann equation: Vm,0= -1.08 mV, ghc,min=0.08 (ghc,ss: steady-state conductance; Vm,0: Vm at which ghc,ss is half-maximally activated; ghc,min: minimal conductance; major charge carriers: K+ and Cl-). The ghc was minimal at negative Vm and maximal at positive Vm. This suggests that Cx45 connexons integrated in gap junction channels are gating with negative voltage. Ihc deactivated exponentially with time, giving rise to single time constants, taud. The function taud = f(Vm) was exponential and increased with positive Vm (taud=7.6 s at Vm=0 mV). The activation of Ihc followed the sum of two exponentials giving rise to the time constants, taua1 and taua2. The function taua1=f(Vm) and taua2 = f(Vm) were bell-shaped and yielded a maximum of congruent with 0.6 s at Vm congruent with -20 mV and congruent with 4.9 s at Vm congruent with 15 mV, respectively. Neither taua1 =f(Vm) nor taua2 = f(Vm) coincided with taud=f(Vm). These findings conflict with the notion that activation and deactivation follow a simple reversible reaction scheme governed by first-order voltage-dependent processes.

Synthesis of the fully phosphorylated GPI anchor pseudohexasaccharide of Toxoplasma gondii

Retrosynthesis of the fully phosphorylated glycosylphosphatidyl inositol (GPI) anchor pseudohexasaccharide 1a led to building blocks 2-6, of which 5 and 6 are known. The formation of pseudodisaccharide building block 2 is based on readily available building block 7, which gave, via derivative 11 and its glycosylation with known donor 12, the desired compound 2. Building block 3, with the required access to all hydroxy groups being permitted, was prepared from mannose in five steps. From a readily available precursor, building block 4 was obtained, which on reaction with 3 gave disaccharide 23. The synthesis of the decisive pseudohexasaccharide intermediate 32 was based on the reaction of 23 with 5, then with 6, and finally with 2. To obtain high stereoselectivity and good yields in the glycosylation reactions, anchimeric assistance was employed. To enable regioselective attachment of the two different phosphorus esters, the 6f-O-silyl group of 32 was first removed and the aminoethyl phosphate residue was attached. Then the MPM group was oxidatively removed, and the second phosphate residue was introduced. Unprotected 1a was then liberated in two steps: treatment with sodium methanolate removed the acetyl protecting groups, and finally, catalytic hydrogenation afforded the desired target molecule, which could be fully structurally assigned.

Influence of dynamic gap junction resistance on impulse propagation in ventricular myocardium: a computer simulation study

The gap junction connecting cardiac myocytes is voltage and time dependent. This simulation study investigated the effects of dynamic gap junctions on both the shape and conduction velocity of a propagating action potential. The dynamic gap junction model is based on that described by Vogel and Weingart (J. Physiol. (Lond.). 1998, 510:177-189) for the voltage- and time-dependent conductance changes measured in cell pairs. The model assumes that the conductive gap junction channels have four conformational states. The gap junction model was used to couple 300 cells in a linear strand with membrane dynamics of the cells defined by the Luo-Rudy I model. The results show that, when the cells are tightly coupled (6700 channels), little change occurs in the gap junction resistance during propagation. Thus, for tight coupling, there are negligible differences in the waveshape and propagation velocity when comparing the dynamic and static gap junction representations. For poor coupling (85 channels), the gap junction resistance increases 33 MOmega during propagation. This transient change in resistance resulted in increased transjunctional conduction delays, changes in action potential upstroke, and block of conduction at a lower junction resting resistance relative to a static gap junction model. The results suggest that the dynamics of the gap junction enhance cellular decoupling as a possible protective mechanism of isolating injured cells from their neighbors.

Intracellular domains of mouse connexin26 and -30 affect diffusional and electrical properties of gap junction channels

To evaluate the influence of intracellular domains of connexin (Cx) on channel transfer properties, we analyzed mouse connexin (Cx) Cx26 and Cx30, which show the most similar amino acid sequence identities within the family of gap junction proteins. These connexin genes are tightly linked on mouse chromosome 14. Functional studies were performed on transfected HeLa cells stably expressing both mouse connexins. When we examined homotypic intercellular transfer of microinjected neurobiotin and Lucifer yellow, we found that gap junctions in Cx30-transfected cells, in contrast to Cx26 cells, were impermeable to Lucifer yellow. Furthermore, we observed heterotypic transfer of neurobiotin between Cx30-transfectants and HeLa cells expressing mouse Cx30.3, Cx40, Cx43 or Cx45, but not between Cx26 transfectants and HeLa cells of the latter group. The main differences in amino acid sequence between Cx26 and Cx30 are located in the presumptive cytoplasmic loop and C-terminal region of these integral membrane proteins. By exchanging one or both of these domains, using PCR-based mutagenesis, we constructed Cx26/30 chimeric cDNAs, which were also expressed in HeLa cells after transfection. Homotypic intercellular transfer of injected Lucifer yellow was observed exclusively with those chimeric constructs that coded for both cytoplasmic domains of Cx26 in the Cx30 backbone polypeptide chain. In contrast, cells transfected with a construct that coded for the Cx26 backbone with the Cx30 cytoplasmic loop and C-terminal region did not show transfer of Lucifer yellow. Thus, Lucifer yellow transfer can be conferred onto chimeric Cx30 channels by exchanging the cytoplasmic loop and the C-terminal region of these connexins. In turn, the cytoplasmic loop and C-terminal domain of Cx30 prevent Lucifer yellow transfer when swapped with the corresponding domains of Cx26. In chimeric Cx30/Cx26 channels where the cytoplasmic loop and C-terminal domains had been exchanged, the unitary channel conductance was intermediate between those of the parental channels. Moreover, the voltage sensitivity was slightly reduced. This suggests that these cytoplasmic domains interfere directly or indirectly with the diffusivity, the conductance and voltage gating of the channels.

Co-operativity between mouse connexin30 gap junction channels

HeLa cells stably transfected with mouse cDNA coding for connexin30 (Cx30) were used to study the electrical properties of gap junction channels. The experiments involved the measurement of intercellular currents (Ij) from cell pairs using dual whole-cell recording with the patch-clamp method. The aim was to compare Ij from cell pairs whose gap junctions consisted of a single channel and cell pairs whose gap junctions consisted of many channels. We found that both the ensemble average currents gained from single-channel records and the currents obtained from multichannel records inactivated exponentially with time. However, the former inactivated significantly slower than the latter. At ajunctional voltage (Vj) of 50 mV, the time constants of inactivation (tau(i)) were 8.1 s and 1.6 s, respectively. Moreover, the ratio tau(i)(single-channel)/tau(i)(multichannel) turned out to be voltage sensitive, i.e. it decreased with increasing V(j) These observations suggest that the operation of Cx30 gap junction channels in the multichannel configuration involves co-operative interactions.

The kinetics of gap junction currents are sensitive to the ionic composition of the pipette solution

Myocytes were isolated from neonatal rat hearts using an enzymatic procedure. Cell pairs were used to control the junctional voltage, V(j), and to measure the transjunctional current, I(j), using the dual voltage-clamp method. V(j) gradients provoked I(j) signals with voltage-dependent inactivation. During voltage pulses, I(j) remained virtually constant at ¿V(j)¿ <40 mV. At ¿V(j)¿>40 mV, it inactivated with time to a residual level. The inactivation followed a single exponential. The time constant of I(j) inactivation, taui, and the size of I(j) at steady state, I(j,ss), were both sensitive to the ions in the pipette solution. I(j,ss) was smaller in the presence of tetraethylammonium aspartate (TEA+ aspartate-) than KC1, while taui was smaller in the presence of KC1 than TEA+ aspartate-. The modification of I(j,ss) is readily explained by a change in the residual conductance of the gap junction channels, gammaj,residual x The alterations in taui are correlated with a change in beta, the rate constant that describes the transition of the channel from the main state to the residual state. Pipette solutions may affect the kinetics of gap junction currents by altering the conductive and/or kinetic parameters. Computer simulations revealed a substantial influence of the latter, but only a marginal effect of the former. Conceivably, ions of the pipette solution may affect the kinetics of gap junction channels by screening surface charges of the channel wall.

Electrical properties of gap junction hemichannels identified in transfected HeLa cells

Human HeLa cells transfected with mouse connexin Cx30, Cx46 or Cx50 were used to study the electrical properties of gap junction hemichannels. With no extracellular Ca2+, whole-cell recording revealed currents arising from hemichannels. Multichannel currents showed a time-dependent inactivation sensitive to voltage, Vm. Plots of the instantaneous conductance, ghc,inst, versus Vm were constant; plots of the steady-state conductance, ghc,ss, versus Vm were bell-shaped. Single-channel currents showed two conductances, gammahc,main and gammahc,residual, the latter approximately or approximately equals=1/6 of the former. Single-channel currents exhibited fast transitions (1-2 ms) between the main state and residual state. Late during wash-in and early during wash-out of 2 mM heptanol, single-hemichannel currents showed slow transitions between an open state and closed state. The open channel probability, Po, was Vm-dependent. It declined from approximately =1 at Vm= 0 mV to 0 at large Vm of either polarity. Hemichannel currents showed a voltage-dependent gammahc,main, i.e., it increased/decreased with hyperpolarization/depolarization. Extrapolation to Vm=0 mV led to a gammahc,main of 283, 250 and 352 pS for Cx30, Cx46 and Cx50, respectively. The hemichannels possess two gating mechanisms. Gating with positive voltage reflects Vj-gating of gap junction channels, gating with negative voltage reflects a property inherent to hemichannels, i.e., Vm or "loop" gating. We conclude that Cx30, Cx46 and Cx50 form voltage-sensitive hemichannels in single cells which are closed under physiological conditions.

Formation of heterotypic gap junction channels by connexins 40 and 43

Gap junctions formed between transfected cells expressing connexin (Cx) 40 and Cx43 (Cx43-RIN, Cx40-HeLa, and Cx43-HeLa) revealed a relationship, g(j)=f(V(j)), at steady state, that is typified by a nonsymmetrical behavior similar to that previously reported for other heterotypic channels (gap junction conductance [g(j)]; transjunctional voltage [V(j)]). The unitary conductance of the channels was sensitive to the polarity of V(j). A main state conductance of 61 pS was found when the Cx43 cell was stepped positively or the Cx40 cell negatively (V(j)=70 mV); the reverse polarities yielded a conductance of 100 pS. These heterotypic channels were permeable to carboxyfluorescein. In addition, two other heterotypic forms are illustrated to demonstrate that endogenous Cx45 expression cannot explain the results. The demonstration of heterotypic Cx40-Cx43 channels may have implications for the propagation of the electrical impulse in heart. For example, they may contribute to the slowing of the impulse propagation through the junctions between Purkinje fibers and ventricular muscle.

Voltage gating of Cx43 gap junction channels involves fast and slow current transitions

RIN cells transfected with mouse cDNA coding for connexin43 (Cx43) were used to further examine the electrical properties of single gap junction channels. The experiments involved measuring intercellular currents from cell pairs using dual whole-cell recording with the patch-clamp method. We found that the single-channel currents exhibit two types of transitions and several conductance states. Besides fast transitions between the main open state and the residual state, the channels underwent slow transitions between an open state (i.e. main open state or residual state) and a closed state. The fast transitions lasted less than 2 ms, the slow ones ranged from 3.5 to 145 ms. The incidence of slow transitions increased with increasing transjunctional voltage. These observations are consistent with the notion that Cx43 gap junction channels possess more than one mechanism of voltage gating.

Biophysical properties of mouse connexin30 gap junction channels studied in transfected human HeLa cells

1. Human HeLa cells expressing mouse connexin30 (Cx30) were used to study the electrical properties of Cx30 gap junction channels. Experiments were performed on cell pairs with the dual voltage-clamp method. 2. The gap junction conductance (gj) at steady state showed a bell-shaped dependence on junctional voltage (Vj; Boltzmann fit: Vj,0 = 27 mV, gj,min = 0.15, z = 4). The instantaneous gj decreased slightly with increasing Vj. 3. The gap junction currents (Ij) declined with time following a single exponential. The time constants of Ij inactivation (taui) decreased with increasing Vj. 4. Single channels exhibited a main state, a residual state and a closed state. The conductances gammaj,main and gammaj,residual were 179 and 48 pS, respectively (pipette solution, potassium aspartate; temperature, 36-37 degrees C; extrapolated to Vj = 0 mV). 5. The conductances gammaj,residual and gammaj,main showed a slight Vj dependence and were sensitive to temperature (Q10 values of 1.28 and 1.16, respectively). 6. Current transitions between open states (i.e. main state, substates, residual state) were fast (< 2 ms), while those between an open state and the closed state were slow (12 ms). 7. The open channel probability (Po) at steady state decreased from 1 to 0 with increasing Vj (Boltzmann fit: Vj,0 = 37 mV; z = 3). 8. Histograms of channel open times implied the presence of a single main state; histograms of channel closed times suggested the existence of two closed states (i.e. residual states). 9. We conclude that Cx30 channels are controlled by two types of gates, a fast one responsible for Vj gating involving transitions between open states (i.e. residual state, main state), and a slow one correlated with chemical gating involving transitions between the closed state and an open state.

Electrophysiological properties of gap junction channels in hepatocytes isolated from connexin32-deficient and wild-type mice

Hepatocytes were isolated from wild-type and connexin32-deficient (Cx32-deficient) mice. Pairs of cells were chosen to study the electrical properties of gap junction channels using the dual voltage-clamp method. The total gap junction currents revealed that Cx32-deficient hepatocytes express one type of connexin (Cx26) and wild-type hepatocytes express two types of connexins (Cx26 and Cx32). The unitary gap junction currents suggest that Cx32-deficient cells have homotypic channels (Cx26-Cx26) while wild-type cells form homotypic (Cx26-Cx26, Cx32-Cx32) and heterotypic channels (Cx26-Cx32). Homotypic channels exhibited a main conductance and a residual conductance, both virtually insensitive to gap junction voltage (Vj) (Cx32-Cx32: gammaj,main=31 pS, gammaj,residual=9 pS; Cx26-Cx26: gammaj,main=102 pS, gammaj,residual=17 pS). Residual states were regularly seen in Cx32-Cx32 channels, but rarely in Cx26-Cx26 channels. Heterotypic channels showed a main conductance and a residual conductance. The former was sensitive to Vj (average gammaj,main=52 pS). The electrophysiological data suggest that Cx32 hemichannels are more abundant than Cx26 hemichannels in prenatal (ratio 4:1) and adult wild-type hepatocytes (ratio 23:1) and that the total number of gap junction channels is larger in prenatal cells than in adult cells. The diversity of the relationship gj, ss/gj,inst=f(Vj) (gj,ss: gap junction conductance at steady state; gj,inst: instantaneous gap junction conductance; Vj: transjunctional voltage) seen in wild-type cells suggests that the ratio Cx26/Cx32 hemichannels is variable among hepatocytes. A comparison of total and unitary conductances implies that Cx26 hemichannels are down-regulated in Cx32-deficient cells and that docking between Cx26 and Cx32 hemichannels occurs randomly. While the gap junction currents are compatible with homotypic and heterotypic channels, the presence of heteromeric channels cannot be excluded.

Mathematical model of vertebrate gap junctions derived from electrical measurements on homotypic and heterotypic channels

1. A mathematical model has been developed which describes the conductive and kinetic properties of homotypic and heterotypic gap junction channels of vertebrates. 2. The model consists of two submodels connected in series. Each submodel simulates a hemichannel and consists of two conductances corresponding to a high (H) and low (L) conductance state and a switch, which simulates the voltage-dependent channel gating. 3. It has been assumed that the conductances of the high state and low state vary exponentially with the voltage across the hemichannel. 4. The parameters of the exponentials can be derived from data of heterotypic or homotypic channels. As a result, the behaviour of heterotypic channels can be predicted from homotypic channel data and vice versa. 5. The two switches of a channel are governed by the voltage drop across the respective hemichannel. The switches of a channel work independently, thus giving rise to four conformational states, i.e. HH, LH, HL and LL. 6. The computations show that the dogma of a constant conductance for homotypic channels results from the limited physiological range of transjunctional voltages (Vj) and the kinetic properties of the channel, so a new fitting procedure is presented. 7. Simulation of the kinetic properties at the multichannel level revealed current time courses which are consistent with a contingent gating. 8. The calculations have also shown that the channel state LL is rare and of short duration, and hence easy to miss experimentally. 9. The design of the model has been kept flexible. It can be easily expanded to include additional features, such as channel substates or a closed state.

Long-chain n-alkanols and arachidonic acid interfere with the Vm-sensitive gating mechanism of gap junction channels

Experiments were carried out on preformed cell pairs and induced cell pairs of an insect cell line (mosquito Aedes albopictus, clone C6/36). The coupling conductance, gj, was determined with the dual voltage-clamp method. Exposure of preformed cell pairs to lipophilic agents, such as long-chain n-alkanols (n-hexanol, n-heptanol, n-octanol, n-nonanol, n-decanol) or arachidonic acid, provoked a decrease in gj. Hyperpolarization of both cells led to a recovery of gj. Systematic studies revealed that this phenomenon is caused by a shift of the sigmoidal relationship gj(ss) = f(Vm) towards more negative values of Vm (where gj(ss) = conductance at steady-state; Vm = membrane potential). The shift was dose dependent, it developed with time and was reversible. The longer the hydrocarbon chain of n-alkanols, the lower was the concentration required to produce a given shift. Besides shifting the function gj(ss) = f(Vm), arachidonic acid decreased the maximal conductance, gj(max). Single-channel records gained from induced cell pairs revealed that the lipophilic agents interfere with the Vm-sensitive slow channel gating mechanism. Application provoked slow current transitions (transition time: 5-40 ms) between an open state of the channel (i.e. main state or residual state) and the closed state; subsequently, fast channel transitions (transition time: < 2 ms) involving the main state and the residual state ceased completely. Hyperpolarization of Vm or washout of the lipophilic agents gave rise to the inverse sequence of events. The single-channel conductances gammaj(main state) and gammaj(residual state) were not affected by n-heptanol. We conclude that long-chain n-alkanols and arachidonic acid interact with the Vm-sensitive gating mechanism.

Modulation of cardiac gap junctions: the mode of action of arachidonic acid

Myocytes isolated from neonatal rat hearts were grown in culture dishes. Cell pairs were selected to examine the mode of action of arachidonic acid (AA) on gap junctions. The dual voltage-clamp method was used to measure intercellular currents and determine the gap junction conductance, gj. Exposure of cell pairs to 10 microM AA produced reversible uncoupling. Pretreatment with 10 microM POCA (sodium-2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate; which inhibits mitochondrial beta-oxidation) did not prevent AA-dependent uncoupling. Thus, it seems that metabolites of beta-oxidation are not involved in AA-induced impairment of gj. Pre-exposure to 10 microM indomethacin (which blocks the cyclooxygenase pathway of the AA-cascade) had no effect on AA-dependent uncoupling. This suggests that cyclooxygenase products such as prostaglandins or thromboxanes play no role in gj modulation. Exposure to 5 microM NDGA (nordihydroguaiaretic acid; which inhibits the 5-lipoxygenase pathway) or 10 microM ETYA (5,8,11,14-eicosatetrynoic acid: which inhibits the 12- and 15-lipoxygenase pathway) led to a reversible decrease in gj. Pre-treatment with 4-BPB (4-bromophenacyl bromide: which inhibits phospholipase A2) did not prevent the effects on gj by NDGA or ETYA. This renders it unlikely that gj is regulated by eicosanoids. Also, accumulation of endogenous AA cannot be responsible for NDGA- and ETYA-dependent uncoupling. Exposure to 75 microM SKF-525A (inhibits the epoxygenase pathway) reversibly impaired gj. This is consistent with a direct action of SKF-525A on gj, but leaves open the possibility of an involvement of epoxides. The data gathered will be discussed in terms of molecular mechanisms. Due to their amphipathic character. AA, NDGA, ETYA and SKF-525A may interfere with gj by disturbing the lipid-protein interface of the cell membranes and thereby impair gap junction channels.

Conductances and selective permeability of connexin43 gap junction channels examined in neonatal rat heart cells

Myocytes from neonatal rat hearts were used to assess the conductive properties of gap junction channels by means of the dual voltage-clamp method. The experiments were carried out on three types (groups) of preparations: (1) induced cell pairs, (2) preformed cell pairs with few gap junction channels (1 to 3 channels), and (3) preformed cell pairs with many channels (100 to 200 channels) after treatment with uncoupling agents such as SKF-525A (75 micromol/L), heptanol (3 mmol/L), and arachidonic acid (100 micromol/L). In group 1, the first opening of a newly formed channel was slow (20 to 65 ms) and occurred 7 to 25 minutes after physical cell contact. The rate of channel insertion was 1.3 channels/min. Associated with a junctional voltage gradient (Vj), the channels revealed multiple conductances, a main open state [gamma(j)(main state)], several substates [gamma(j)(substates)], and a residual state [gamma(j)(residual state)]. On rare occasions, the channels closed completely. The same phenomena were observed in groups 2 and 3. The existence of gamma(j)(residual state) provides an explanation for the incomplete inactivation of the junctional current (Ij) at large values of Vj in cell pairs with many gap junction channels. The values of gamma(j)(main state) and gamma(j)(residual state) gained from groups 1, 2, and 3 turned out to be comparable and hence were pooled. The fit of the data to a Gaussian distribution revealed a narrow single peak for both conductances. The values of gamma(j) were dependent on the composition of the pipette solution. Solutions were as follows: (1) KCl solution, gamma(j)(main state)=96 pS and gamma(j)(residual state)=23 pS; (2) Cs+ aspartate solution, gamma(j)(main state)=61 pS and gamma(j)(residual state)=12 pS; and (3) tetraethylammonium+ aspartate solution, gamma(j)(main state)=19 pS and gamma(j)(residual state)=3 pS. The respective gamma(j)(main state)-to-gamma(j)(residual state) ratios were 4.2, 5.1, and 6.3. This indicates that the residual state restricts ion permeation more efficiently than does the main state. Transitions of Ij between open states (main open state, substates, and residual state) were fast (<2 ms), and transitions involving the closed state and an open state were slow (15 to 65 ms). This implies the existence of two gating mechanisms. The residual state may be regarded as the ground state of electrical gating controlled by Vj; the closed state, as the ground state of chemical gating.

Biophysical properties of heterotypic gap junctions newly formed between two types of insect cells

1. Cell pairs of the insect cell line Sf9 (Spodoptera frugiperda) were chosen to examine the electrical properties of gap junction channels. The dual voltage-clamp method was used to control the membrane potential of each cell (Vm,1 and Vm,2) and hence the junctional voltage gradient (Vj), and to measure intercellular current. 2. Studies with preformed pairs revealed that the gap junction conductance (gj) is controlled by a Vj- and a Vm-sensitive gate. At steady state, gj = f(Vj) was bell shaped and symmetrical (Boltzmann: Vj.0 = -54 and 55 mV, the normalized minimum conductance at large Vj values (gj,min) = 0.24 and 0.23, z = 5.5 and 6.1 for negative and positive Vj, respectively) and gj = f(Vm) was S shaped (Vm.0 = 13 mV, gj,min = 0, z = 1.5). 3. Single channels exhibited two conductances, a main state (gamma j,main) of 224 pS and a residual state (gamma j,residual) of 42 pS. 4. We conclude that gap junctions in Sf9 cells behave similarly to those in the insect cell line C6/36 (Aedes albopictus). 5. An induced cell pair approach was used to examine heterotypic gap junction channels between Sf9 cells and C3/36 cells. 6. Heterotypic channels showed a gamma j,main of 303 pS and a gamma j,residual of 45 and 64 pS, depending on whether the Sf9 cell or C6/36 cell was positive inside. 7. In heterotypic gap junctions, gj = f(Vj) was bell shaped and asymmetrical (gj was more sensitive to Vj when the C6/36 cell was positive inside) and gj = f(Vm) was S shaped (Vm,0 = 2 mV, gj,min = 0, z = 2.9). 8. We conclude that heterotypic channels possess a Vj- and Vm-sensitive gating mechanism. Vj gating involves two gates, one located in each hemi-channel. Vj gates are operated independently and close when the cytoplasmic aspect is made positive. 9. A comparison of homo- and heterotypic channel data suggests that docking of hemi-channels may affect channel gating, but not channel conductance.

Incompatibility of connexin 40 and 43 Hemichannels in gap junctions between mammalian cells is determined by intracellular domains

Murine connexin 40 (Cx40) and connexin 43 (Cx43) do not form functional heterotypic gap junction channels. This property may contribute to the preferential propagation of action potentials in murine conductive myocardium (expressing Cx40) which is surrounded by working myocardium, expressing Cx43. When mouse Cx40 and Cx43 were individually expressed in cocultured human HeLa cells, no punctate immunofluorescent signals were detected on apposed plasma membranes between different transfectants, using antibodies specific for each connexin, suggesting that Cx40 and Cx43 hemichannels do not dock to each other. We wanted to identify domains in these connexin proteins which are responsible for the incompatibility. Thus, we expressed in HeLa cells several chimeric gene constructs in which different extracellular and intracellular domains of Cx43 had been spliced into the corresponding regions of Cx40. We found that exchange of both extracellular loops (E1 and E2) in this system (Cx40*43E1,2) was required for formation of homotypic and heterotypic conductive channels, although the electrical properties differed from those of Cx40 or Cx43 channels. Thus, the extracellular domains of Cx43 can be directed to form functional homo- and heterotypic channels. Another chimeric construct in which both extracellular domains and the central cytoplasmic loop (E1, E2, and C2) of Cx43 were spliced into Cx40 (Cx40*43E1,2,C2) led to heterotypic coupling only with Cx43 and not with Cx40 transfectants. Thus, the central cytoplasmic loop of Cx43 contributed to selectivity. A third construct, in which only the C-terminal domain (C3) of Cx43 was spliced into Cx40, i.e., Cx40*43C3, showed neither homotypic nor heterotypic coupling with Cx40 and Cx43 transfectants, suggesting that the C-terminal region of Cx43 determined incompatibility.

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.

Modification of gap junction conductance by divalent cations and protons in neonatal rat heart cells

Myocytes were isolated from neonatal rat hearts and grown in culture dishes. Pairs of cells were selected to study the effect of divalent cations and protons on the conductance of gap junctions, gj. The experimental approach involved the dual voltage-clamp method and cell dialysis via patch pipette, i.e. gj was monitored while the cytosolic level of Ca2+, Mg2+, Sr2+, Ba2+ or H+ was modified in one of the cells. A dose-dependent decrease in gj developed when pCa of the pipette solution was lowered (range: pCa = 7.7-2.42, equivalent to a [Ca2+] of 20 nM-3.8 mM). The gj/pCa-relationship revealed a Hill coefficient n of 0.87 and a half-maximal concentration pKCa of 3.5. Pretreatment with 3 mM NiCl2 and 1 micron ryanodine to minimize the removal of cytosolic Ca2+ did not significantly affect the response to gj. Similarly, gj was decreased in a dose-dependent fashion when pHi in the pipette solution was lowered (range: pH = 7.2-5.0, corresponding to a [H+] of 63 nM-10 microns). The gj/pH-relationship yielded an n of 0.92 and a pKH of 5.85. Pretreatment with 1 mM amiloride to minimize the extrusion of protons enhanced the effects of pH on gj. Simultaneous alterations in pCa and pH demonstrated an additive type of action of Ca2+ and H+ on gj. This is consistent with the existence of two types of sensors which contribute separately to the functional state of gj. No significant decrease in gj was detectable when the pipette solution contained Mg2+ or Ba2+ (up to 5 mM). Partial uncoupling was observed with pipette solution containing 5 mM Sr2+. We conclude that gj of neonatal and adult cardiomyocytes exhibit different ionic sensitivities. This discrepancy may reflect differences in connexin expression and/or molecular intermediates involved in regulating gj.

Biophysical properties of gap junction channels formed by mouse connexin40 in induced pairs of transfected human HeLa cells

A clone of human HeLa cells stably transfected with mouse connexin40 DNA was used to examine gap junctions. Two separate cells were brought into physical contact with each other ("induced cell pair") to allow insertion of gap junction channels and, hence, formation of a gap junction. The intercellular current flow was measured with a dual voltage-clamp method. This approach enabled us to study the electrical properties of gap junction channels (cell pairs with a single channel) and gap junctions (cell pairs with many channels). We found that single channels exhibited multiple conductances, a main state (gamma j(main state)), several substates (gamma j(substates)), a residual state (gamma j (residual state)), and a closed state (gamma j(closed state)). The gamma j(main state) was 198 pS, and gamma j(residual state) was 36 pS (temperature, 36-37 degrees C; pipette solution, potassium aspartate). Both properties were insensitive to transjunctional voltage, Vj. The transitions between the closed state and an open state (i.e., residual state, substate, or main state) were slow (15-45 ms); those between the residual state and a substate or the main state were fast (1-2 ms). Under steady-state conditions, the open channel probability, Po, decreased in a sigmoidal manner from 1 to 0 (Boltzmann fit: Vj,o = -44 mV; z = 6). The temperature coefficient, Q10, for gamma j(main state) and gamma j(residual state) was 1.2 and 1.3, respectively (p < 0.001; range 15-40 degrees C). This difference suggests interactions between ions and channel structure in case of gamma j(residual state). In cell pairs with many channels, the gap junction conductance at steady state, gj, exhibited a bell-shaped dependency from Vj (Boltzmann fit, negative Vj, Vj,o = -45 mV, gj(min) = 0.24; positive Vj, Vj,o = 49 mV, gj(min) = 0.26; z = 6). We conclude that each channel is controlled by two types of gates, a fast one responsible for Vj gating and involving transitions between open states (i.e., residual state, substates, main state), and a slow one involving transitions between the closed state and an open state.

Heterotypic gap junction channels (connexin26-connexin32) violate the paradigm of unitary conductance

Human HeLa cells transfected with mouse DNA coding for connexin26 (Cx26) or connexin32 (Cx32) were used to examine the properties of heterotypic Cx26-Cx32 gap junction channels. Intercellular current flow was examined in induced cell pairs by means of the dual voltage-clamp method. We found that Cx26-Cx32 channels exhibit voltage-dependent conductances, gamma j: gamma j(main state) increases with increasing positivity at the cytoplasmic aspect of the Cx26 connexon and decreases with increasing negativity (slope: 32 pS/100 mV; gamma j(main state) reaches 48 pS as Vj approaches 0 mV); gamma j(residual state) with a similar Vj-dependence is present when the cytoplasmic end of Cx26 connexon is positive, but absent when it is negative. The single channel data provide an explanation for the asymmetric relationships between the gap junction conductance, gj, and Vj. The results are consistent with the notion that docking of two connexons co-determines the biophysical properties of a gap junction channel.

Voltage-dependent gating of single gap junction channels in an insect cell line

De novo formation of cell pairs was used to examine the gating properties of single gap junction channels. Two separate cells of an insect cell line (clone C6/36, derived from the mosquito Aedes albopictus) were pushed against each other to provoke formation of gap junction channels. A dual voltage-clamp method was used to control the voltage gradient between the cells (Vj) and measure the intercellular current (Ij). The first sign of channel activity was apparent 4.7 min after cell contact. Steady-state coupling reached after 30 min revealed a conductance of 8.7 nS. Channel formation involved no leak between the intra- and extracellular space. The first opening of a newly formed channel was slow (25-28 ms). Each preparation passed through a phase with only one operational gap junction channel. This period was exploited to examine the single channel properties. We found that single channels exhibit several conductance states with different conductances gamma j; a fully open state (gamma j(main state)), several substates (gamma j(substates)), a residual state (gamma j(residual)) and a closed state (gamma j(closed)). The gamma j(main state) was 375 pS, and gamma j(residual) ranged from 30 to 90 pS. The transitions between adjacent substates were 1/7-1/4 of gamma j(main state). Vj had no effect on gamma j(main state), but slightly affected gamma j (residual). The lj transitions involving gamma j(closed) were slow (15-60 ms), whereas those not involving gamma j(closed) were fast (< 2 ms). An increase in Vj led to a decrease in open channel probability. Depolarization of the membrane potential (Vm) increased the incidence of slow transitions leading to gamma j(closed). We conclude that insect gap junctions possess two gates, a fast gate controlled by Vj and giving rise to gamma j(substates) and gamma j(residual), and a slow gate sensitive to Vm and able to close the channel completely.

Early events of Semliki Forest virus-induced cell-cell fusion

Insect cells (Aedes albopictus) were infected with Semliki Forest Virus. Cell-cell fusion was then induced by lowering the extracellular pH. The underlying processes were examined by monitoring the intercellular current flow, Ij. Experimentally, this involved the use of cell pairs in conjunction with a dual voltage-clamp method. This approach allowed us to monitor the kinetics of fusion at high temporal resolution. The fusion process began shortly after acidification (delay: 3-138 sec). Initially, Ij increased in a stepwise manner, later on it developed more gradually. Fusion between two cells reached a steady state within 7-70 sec. The steps in Ij are attributable to the formation of cytoplasmic connections between the cells, presumably involving proteinaceous fusion pores. The mean amplitude of Ij steps corresponds to a conductance of 300 pS, consistent with a pore radius of 1 nm. Cytoplasmic connections developed rapidly, i.e., Ij steps occurred within less than 1 msec. The absence of Ij flickering implies that formation of cytoplasmic connections, and hence SFV induced cell-cell fusion, is irreversible.

Gap junction channels of insects exhibit a residual conductance

Formation of gap junction coupled cell pairs was used to assess the basic properties of single gap junction channels. For this purpose, two single cells (clone C6/36, derived from larvae of an insect, Aedes albopictus) were maneuvered against each other to provoke gap junction channel insertion. Intercellular current flow was measured with a dual voltage-clamp method. Utilizing this approach, we were able to demonstrate that gap junction channels, after formation, do not close completely upon application of a transjunctional voltage gradient, Vj. Instead, they exhibit a residual conductance, gamma j(residual). On average, gamma j(residual) was 64 +/- 4 pS (n = 40). This corresponds to about 1/6 of the conductance of a fully open channel. The existence of gamma j(residual) explains the observation that the conductance of the entire gap junction, gj, decreases only partially at large Vj.

Effects of phorbol ester on gap junctions of neonatal rat heart cells

Myocytes were isolated from the ventricles of neonatal rat hearts and cultured for 1-3 days. Newly formed cell pairs were used to examine the conductance of gap junctions, gj. Measurements were performed using a dual voltage-clamp method in conjunction with a whole-cell, tight-seal recording. Exposure to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA, 100-160 nM) led to a decrease in gj. Single-channel events recorded immediately before complete uncoupling yielded a single-channel conductance, gamma j, of 40.5 pS, implying that TPA affects the channel kinetics rather than gamma j. TPA-induced uncoupling was observed at subphysiological levels of cytosolic Ca2+ (pipette solution = 18 nM), not at physiological levels (pipette solution = 170 nM). The effects of TPA could not be mimicked by 250 microM 1-oleoyl-2-acetyl-glycerol (OAG). Preincubation with TPA (up to 24 h) revealed no changes in gj attributable to down-regulation of protein kinase C, PKC. Pretreatment with PKC inhibitors, staurosporine or PKCI, prevented the TPA-dependent decrease in gj. TPA-dependent uncoupling was not impaired by 4-bromophenacyl bromide, an inhibitor of phospholipase A2, PLA2; conversely, an arachidonic acid-dependent decrease in gj was not prevented by PKCI. This suggests that gj regulation does not involve an interaction between PLA2 and PKC.

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.

Multiple conductance states of newly formed single gap junction channels between insect cells

Two cells of an insect cell line (Aedes albopictus, clone C6/36) were pushed together to form a cell pair while the intercellular current flow was monitored. This approach enabled us to study the formation of gap junction channels and explore their electrical properties. We found that the single channels exhibit multiple conductance states. The conductance of a fully open channel was 365 pS; the subconductance steps were 1/7 to 1/5 of the maximal conductance. The voltage gradient across the junction did not influence the conductance of fully open channels, but affected the dwell time at particular conductance states. The latter provides an explanation for the voltage-dependent conductance of gap junction membranes seen in these cells. The very first channel opening always was slow (15-50 ms), suggesting the involvement of a mechanism different from conventional channel gating.

pH-dependent pore formation in Semliki forest virus-infected Aedes albopictus cells

The plasma membrane properties of Semliki forest virus-infected Aedes cells were studied using whole-cell patch-clamp recording. Cells exhibited a marked increase in membrane conductance, gm (from 0.48 +/- 0.09 nS to 14.2 +/- 10.8 nS) upon exposure to acidic pH (5.6). The membrane potential depolarized from -40.9 +/- 5.2 mV to -2.43 +/- 7.14 mV under these conditions. In uninfected cells, there was no change in gm after lowering the pH. This implies that viral fusion proteins are involved in changing the membrane properties at low pH. The increased gm in infected cells at low pH was not persistent but declined within minutes. Millimolar concentrations of calcium and zinc prevented or reverted the increase in gm. The results suggest that viral proteins, if brought to their fusogenic conformation by exposure to low pH, form unspecific pores in the residing membrane. These pores might play a role in virus entry.

Cytoplasmic bridges and gap junctions in an insect cell line (Aedes albopictus)

Cell pairs of an insect cell line (Aedes albopictus, clone C6/36) were used study simultaneously the diffusional and electrical properties of intercellular junctions. Diffusion studies involved injection of fluorescent molecules into one cell of a cell pair and visual inspection of their intercellular redistribution. Electrical measurements involved a dual voltage clamp method and whole-cell recording with patch pipette. The voltage clamp protocol was aimed at examining the dependency of the junctional conductance, gj, on membrane potential, Vm. Cell pairs exhibiting a voltage-dependent gj were found to allow intercellular diffusion of Lucifer Yellow CH (molecular mass, 443 Da), but not of FITC-dextran (molecular mass, 4,400 Da). This response pattern is consistent with the presence of gap junctions in the intercellular junctions. Cell pairs showing no voltage dependence of gj were found to permit intercellular diffusion of both Lucifer Yellow CH and FITC-dextran (dextran labelled with fluorescein isothiocyanate). This behaviour is compatible with the presence of cytoplasmic bridges connecting the two adjacent cells. Hence, in culture the cells investigated express two kinds of intercellular structures, gap junctions and cytoplasmic bridges.

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 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.

Electrical properties of gap junction channels in guinea-pig ventricular cell pairs revealed by exposure to heptanol

Cell pairs were isolated from adult guinea pig ventricles to study the electrical properties of gap junction channels. The experiments involved a double voltage-clamp approach and whole-cell, tight-seal recording. Heptanol decreased the intracellular current, In, in a dose-dependent fashion. Before complete uncoupling, In showed fluctuations suggesting the operation of gated channels. In the presence of 3 mM heptanol, In showed quantal steps arising from spontaneous opening and closing of single channels. The IV-relationship of the channels was linear (range: +/- 95 mV). Analysis of current records revealed the following single-channel conductances, gamma n: Mean value = 37 pS; median value = 33 pS. gamma n was insensitive to the non-junctional membrane potential (range: -90 to +10 mV). 3 mM ATP4- in the pipette solution had no effects on gamma n, 6 mM ATP4- produced a small decrease, and 6 mM ATP + 0.1 mM cAMP- an increase in gamma n. Channel transitions from closed to open state were variable (range of apparent time constants: 2.5-32 ms; mean: 11 ms).

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

Myocytes were isolated from adult guinea pig ventricles. Whole cell, tight-seal recording was employed to investigate the electrical properties of the junctional (nexal membrane) and nonjunctional membrane (sarcolemma) under the influence of n-alkanols (heptanol, octanol) and halothane. Studies of cell pairs with a double voltage-clamp approach showed that these agents give rise to a reversible electrical uncoupling. Examination of single myocytes with a single voltage-clamp method showed that these substances modify several sarcolemmal current systems. The slope conductance was reduced over the entire voltage range examined (-90 to +50 mV). The Ca2+ inward current (Isi) showed a decreased amplitude and an accelerated inactivation. The repriming of Isi remained unchanged. The steady-state inactivation of Isi was shifted by 2-3 mV toward more negative potentials. Optical measurements demonstrated an increase in sarcomere spacing at rest and a decrease during peak systolic shortening. The results suggest that n-alkanols and halothane exert their effects on membrane currents via incorporation into the lipid bilayer.

Action potential transfer in cell pairs isolated from adult rat and guinea pig ventricles

An enzymatic procedure was used to obtain ventricular cells from adult rat and guinea pig hearts. Isolated pairs of cells were selected to study the action potential transfer from cell to cell and determine the resistance of the nexal membrane, rn. For this purpose, each cell of a cell pair was connected to a patch pipette so as to enable whole-cell, tight-seal recording. Normal impulse transmission was observed when rn ranged from 5-265 M omega. In these cases, the action potential in both cells occurred virtually simultaneously. An occasional failure in action potential transfer was seen in cell pairs whose rn had increased to 155-375 M omega. In these cases, the impulse transfer across the nexal membrane occurred with considerable delay. Impulse transfer was completely blocked once rn was larger than 780 M omega. Assuming a single connexon conductance of 100 pS, this would mean that more than 13 connexons are necessary to allow impulse transfer from cell to cell. Two single myocytes, gently pushed together, neither showed electrotonic interaction nor impulse transfer, thus rendering unlikely the possibility of an ephaptic signal transmission.

Electrical properties of the gap junctional membrane studied in rat liver cell pairs

Cell pairs isolated from adult rat liver were used to study the electrical properties of gap junctions. Each cell of a cell pair was connected to a suction pipette so as to enable whole cell tight-seal recording. A double voltage-clamp approach was adopted to control the voltage gradient across the gap junction and measure the transjunctional current. The current-voltage relationship of the gap junctional membrane was linear over the voltage range tested (+/- 50 m V). Under control conditions, the resistance of the gap junction, rj, was 15 M omega (n = 27; range, 4.6 to 45.8 M omega), corresponding to a conductance gj of 67 nS. rj was insensitive to the nonjunctional membrane potential, Vm (voltage range,-90 m V to + 40 m V). There was no indication of a time-dependent gating of rj (time range, 20 ms to 10 s). Dialysis with 1 mM CaCl2 produced irreversible electrical uncoupling without affecting the linearity of the relationship Vj/Ij.

Cell-to-cell coupling studied in isolated ventricular cell pairs

Cell pairs isolated from adult rat and guinea pig ventricles were used to study the electrical properties of the nexal membrane. Each cell of a pair was connected to a voltage-clamp system so as to enable whole-cell, tight-seal recording. The current-voltage relationship of the nexal membrane was found to be linear, revealing a resistance rn of 2-4 M omega. rn was insensitive to the sarcolemmal membrane potential (range: -90 to +30 mV), and exerted no time-dependent gating behavior (range: 0.1 to 10 s). Lowering pHi yielded a small increase in rn. Vigorous elevations in [Ca2+]i gave rise to an increase in rn which was associated with a cell shortening. Uncoupling caused by aliphatic alcohols or halothane did not produce cell shortening. Cell pairs were also used to study action potential transfer.

Cell pairs isolated from adult guinea pig and rat hearts: effects of [Ca2+]i on nexal membrane resistance

Cell pairs isolated from adult rat and guinea pig ventricles were used to study the resistance of the nexal membrane, rn. Each cell of a cell pair was connected to a voltage-clamp circuit to obtain simultaneous whole-cell, tight-seal recordings. With this technique, rn was determined under experimental conditions aimed at primarily modifying [Ca2+]i. Moderate changes in [Ca2+]i (produced by trains of depolarizing voltage-clamp pulses activating the slow inward current, or alterations in [Ca2+]o from 0.5 to 10 mM), resulted in no change in rn for normally coupled cell pairs (rn = 5 M omega), but small and reversible changes in slightly uncoupled preparations (rn greater than or equal to 50 M omega). Large increases in rn developed with substantial elevations in [Ca2+]i (secondary to [Na+]o-withdrawal, exposure to strophanthidin in conjunction with isi, or Ca2+-dialysis). Increases in rn brought about via elevation in [Ca2+]i always were accompanied by cell shortening consistent with a sustained contracture. The current-voltage relationship of the nexal membrane was ohmic regardless of whether rn was low (control) or elevated (after increasing [Ca2+]i).

Electrical properties of the nexal membrane studied in rat ventricular cell pairs

Cell pairs isolated from adult rat ventricles were used to characterize the electrical properties of the nexal membrane located between the cells. Each cell of a cell pair was connected to a suction pipette so as to enable whole-cell recordings. A double voltage-clamp method was employed which allowed the voltage gradient across the nexal membrane to be controlled. The current-voltage relationship of the nexal membrane was found to be linear over a broad range of transnexal voltages ( +/- 50 mV). The measurements revealed a mean value for the apparent nexal membrane resistance, rn(app), of 3.4 M omega. Taking into account the contribution of an uncompensated series resistance (access resistance), the effective nexal resistance, rn(eff), amounts to 1.7 M omega, approximately. The nexal membrane resistance was found to be insensitive to the sarcolemmal membrane potential, Vm (voltage range tested: -90 mV to +30 mV). The nexal membrane showed no rectifying property, i.e. it allows impulse transmission in both directions equally well. The connexons of the nexal membrane exhibited no time-dependent gating behaviour (time range investigated: 0.1-10 s).

Electric current flow in cell pairs isolated from adult rat hearts

Cell pairs were isolated from ventricles of adult rat hearts so as to study cell-to-cell coupling. Both cells of each pair were impaled with micro-electrodes connected to balanced bridge circuits. Rectangular current pulses were passed and the resulting voltage deflexions monitored. The data were analysed in terms of a delta configuration of three resistive elements, the resistances of the non-junctional membrane of cell 1 and cell 2 (rm, 1 and rm, 2), and the resistance of the nexal membrane (rn). The nexal membrane resistance was found to be insensitive to voltage gradients across the non-junctional membrane (range examined: -70 to -10 mV) and direction of current flow. The mean value of rn was 2.12 M omega ([K+]o = 12 mM). Taking into account morphological parameters, this corresponds to a specific nexal membrane resistance (Rn) of 0.1 omega cm2. Spontaneous uncoupling in which one cell remained polarized while the other one depolarized was never observed. The current-voltage relationship of the non-junctional membrane was found to be bell-shaped. The specific resistance (Rm) at the resting membrane potential (approximately -50 mV) was 3.2 k omega cm2 ([K+]o = 12 mM). Comparative studies performed on single cells revealed a similar relationship Rm versus Vm. Rm at the resting membrane potential (Vm approximately -50 mV) was 2.5 k omega cm2 ([K+]o = 12 mM). The specific capacitance of the non-junctional membrane (Cm) was determined from experiments on single cells. Cm was found to be independent of Vm (voltage range: -80 to 0 mV). The mean value of Cm was 1.66 microF/cm2 ([K+]o = 12 mM). For comparison, experiments on cell pairs and single cells were also carried out with [K+]o = 4 mM. The values obtained for Rn, Rm and Cm did not deviate significantly from those found with [K+]o = 12 mM.