On the electrotonic coupling mechanism of crayfish segmented axons: temperature dependence of junctional conductance

Humoral factors reduce gap junction sensitivity to cytoplasmic pH. I. Organ ablation studies

The sensitivity of gap junctions connecting crayfish lateral axons to uncoupling by axoplasmic acidification was studied after altering the hormonal balance of animals by 1) ablation of eyestalks or sinus glands or 2) inducing long-lasting defensive posturing behavior (stress). Internal pH (pHi) was measured with microelectrodes, and junctional resistance (Rj) was calculated from input and transfer resistances. In isolated nerve cords from intact animals, the maximal Rj (Rjmax) reached after acidification varied diurnally (Rjmax approximately 10 and 0.6 M omega at 0900 and 1800 h, respectively). Basal Rj (20-30 k omega) did not change during the 24-h period. Organ ablation (eyestalks or sinus glands) or stress rendered gap junctions less sensitive to uncoupling by low pHi within 1 h or 2 days; recovery toward control values had different time courses. The reduced pH sensitivity of crayfish junctions seen after eyestalk ablation is attributable to stress in its early phase (lasting 1-2 days) and to ablation of the endocrine organs in its late phase (2-7 days). No striking structural differences accompanied these changes, indicating that the altered properties are not due to major changes in gap junction expression.

Protein phosphorylation and hydrogen ions modulate calcium-induced closure of gap junction channels

The regulation of the cell-to-cell pathway formed by gap junctions seems to involve the interaction of the junctional channels with either calcium or hydrogen ions, as well as protein phosphorylation and calmodulin. These mechanisms of junctional regulation have been considered to act independently on specific sites of the gap junction protein; however, the possibility that they may be interrelated has not been adequately explored mainly due to the difficulties involved in simultaneous measurement of intracellular cations and protein phosphorylation. To further understanding of mechanisms regulating gap junctions, we have internally perfused coupled lateral axons from crayfish with solutions containing different calcium and hydrogen concentrations under conditions favoring phosphorylation, while monitoring the junctional conductance. We found that calcium ions regulate cell communication probably through a direct interaction with the channel protein. Phosphorylation and low pH do not alter junctional conductance themselves, but appear only to modulate the effects of calcium, possibly by altering the affinity of the channel for calcium. We propose that a combination of free intracellular calcium and protein phosphorylation form an important physiological mechanism regulating intercellular communication.

The effect of temperature on electrical interactions between antidromically stimulated frog motoneurons and dorsal root afferent axons

The effect of temperature on electrical interactions between antidromically stimulated motoneurons and dorsal root afferents was studied in the isolated and hemisected spinal cord of the frog, superfused with Ringer in which Ca2+ was equimolarly replaced by Co2+ or Mn2+ to suppress chemical synaptic transmission. Suction electrodes were used for stimulating and/or recording from dorsal and ventral roots from segments IX or X. Intrafibre recordings from sensory fibres were made at their point of entry into the spinal cord. Supramaximal ventral root stimuli elicited two distinct responses in the segmental dorsal root. First a brief short-latency depolarizing potential. Second, at temperatures below 11 degrees C, a second depolarizing root potential appeared following the short-latency depolarizing potential-I. Amplitude and duration of short-latency depolarizing potential-II reversibly increased as the bath temperature was decreased, reaching a maximum at 3 degrees C. Between 8 and 3 degrees C, short-latency depolarizing potential-II increased in amplitude by 20%/degrees C. In contrast short-latency depolarizing potential-I did not show substantial changes with temperature. The short-latency depolarizing potential-II, unlike short-latency depolarizing potential-I showed stepped fluctuations in amplitude, and appeared to be composed of unitary events. Intrafibre records revealed that the unitary events corresponded to action potentials on individual dorsal root fibres. Double shocks applied to the ventral root, at constant bath temperatures (below 11 degrees C), revealed facilitation of the short-latency depolarizing potential-II, which was maximal between 50 and 80 ms and lasted about 200 ms. Neither the antidromic motoneurone field potential nor the short-latency depolarizing potential-I were facilitated.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural characteristics of gap junctions. I. Channel number in coupled and uncoupled conditions

Gap junctions between crayfish lateral axons were studied by combining anatomical and electrophysiological measurements to determine structural changes associated during uncoupling by axoplasmic acidification. In basal conditions, the junctional resistance, Rj, was approximately 60-80 k omega and the synapses appeared as two adhering membranes; 18-20-nm overall thickness, containing transverse densities (channels) spanning both membranes and the narrow extracellular gap (4-6 nm). In freeze-fracture replicas, the synapses contained greater than 3 X 10(3) gap junction plaques having a total of approximately 3.5 X 10(5) intramembrane particles. "Single" gap junction particles represented approximately 10% of the total number of gap junction particles present in the synapse. Therefore, in basal conditions, most of the gap junction particles were organized in plaques. Moreover, correlations of the total number of gap junction particles with Rj suggested that most of the junctional particles in plaques corresponded to conducting channels. Upon acidification of the axoplasm to pH 6.7-6.8, the junctional resistance increased to approximately 300 k omega and action potentials failed to propagate across the septum. Morphological measurements showed that the total number of gap junction particles in plaques decreased approximately 11-fold to 3.1 X 10(4) whereas the number of single particles dispersed in the axolemmae increased significantly. Thin sections of these synapses showed that the width of the extracellular gap increased from 4-6 nm in basal conditions to 10-20 nm under conditions where axoplasmic pH was 6.7-6.8. These observations suggest that single gap junction particles dispersed in the synapse most likely represent hemi-channels produced by the dissasembly of channels previously arranged in plaques.

Protein transport between crayfish lateral giant axons

Segmental lateral giant axons (SLGAs) in crayfish were used to determine whether functionally intact proteins can move between axons under physiological conditions. Horseradish peroxidase (HRP) was chosen as the tracer protein because its localization requires intact enzymatic activity and because it can be localized in living cells using a non-cytotoxic procedure. Following iontophoretic injection of HRP in a single SLGA, HRP often transferred to adjacent SLGAs. HRP transferred from an injected SLGA to a caudal SLGA with greater frequency than HRP transferred to a rostral SLGA. When HRP transferred between SLGAs, it was ultrastructurally associated with vesicles on both sides of septate junctions between adjacent SLGAs and was also seen in the perijunctional extracellular space. There was no difference between the electrical resistance of synapses at which HRP transferred and those synapses where HRP did not transfer. HRP transfer was significantly reduced when axons were bathed in reduced calcium saline. These and other observations indicate that axon-to-axon transport in this system is accomplished by exocytosis of HRP from injected axons followed by its endocytotic uptake by adjacent, non-injected axons. Similar transfer of endogenous proteins may contribute to the long-term survival for months to years of distal stumps of severed SLGAs.

Computer simulation of action potential propagation in septated nerve fibers

The nonlinear, core-conductor model of action potential propagation down axisymmetric nerve fibers is adapted for an implicit, numerical simulation by computer solution of the differential equations. The calculation allows a septum to be inserted in the model fiber; the thin, passive septum is characterized by series resistance Rsz and shunt resistance Rss to the grounded bath. If Rsz is too large or Rss too small, the signal fails to propagate through the septum. Plots of the action potential profiles for various axial positions are obtained and show distortions due to the presence of the septum. A simple linear model, developed from these simulations, relates propagation delay through the septum and the preseptal risetime to Rsz and Rss. This model agrees with the simulations for a wide range of parameters and allows estimation of Rsz and Rss from measured propagation delays at the septum. Plots of the axial current as a function of both time and position demonstrate how the presence of the septum can cause prominent local reversals of the current. This result, not previously described, suggests that extracellular magnetic measurements of cellular action currents could be useful in the biophysical study of septated fibers.

Variation of gap junction sensitivity to H ions with time of day

Conductance of gap junctions between segments of the lateral axons of the crayfish nerve cord is reduced by cytoplasmic acidification. Simultaneous measurements of axoplasmic pH and junctional conductance at 09.00 and 12.00 h revealed a shift of about 0.25 pH units in the apparent pK of the crayfish electrotonic synapse. These findings indicate that gating mechanisms of gap junctions are not fixed properties, but may rather be modulated by other, possibly humoral, factors.

A transmission delay and the effect of temperature at the triadic junction of skeletal muscle

The coupling process at the triadic junctions in skeletal muscle fibres is characterized by a significant latency between the depolarization of the transverse tubular membrane and the release of Ca from the sarcoplasmic reticulum. This time interval, the triadic delay, is sufficiently long to allow for the participation of a chemical process. The strong temperature dependence of the triadic delay (Q10 near 2.7) suggests that a sequence of chemical steps may link the electrical signal in the T-tubules to the opening of Ca channels in the terminal cisternae of the sarcoplasmic reticulum.

Lowering of pH does not directly affect the junctional resistance of crayfish lateral axons

The effect of pH was tested on the junction between crayfish lateral axons. By means of a glass capillary inserted into one of the axons, one side of the junction was perfused with solutions of known pH while the junctional resistance, Rj, was monitored. Integrity of the gap junction was checked electron microscopically. Rj remained unchanged when the pH of the perfusate was lowered from 7.1 to 6.0. However, when the pH of the unperfused side of the junction was lowered by substituting acetate for chloride in the external solution, Rj rose, attesting to the integrity of the junction and its capacity to uncouple in the perfused state. We suggest that H+ does not affect the junctional channels directly, but acts through an intermediary which is inactivated or removed by the perfusion.

The temperature dependence of electrical coupling in the organ of Corti

Electrical coupling in an in vitro preparation of the organ of Corti was evaluated during changes in temperature of the bathing media. The effect of cooling the organ from 35 +/- 2 degrees C to 17 +/- 3 degrees C is to reduce membrane potentials, increase input resistance and decrease coupling ratios. Decreases in coupling ratios ranging from 15 to 75% have been observed. The effects are reversible upon warming. Membrane potentials are very susceptible to depolarization caused by cooling. The reduction in coupling is not due to depolarization nor is it dependent upon extracellular Ca2+. It is conceivable, however, that intracellular stores of Ca2+ are released or that intracellular pH is altered.

Effect of several uncouplers of cell-to-cell communication on gap junction morphology in mammalian heart

Electrical conduction in sheep Purkinje fibers has been blocked by three different procedures: (I) 1 mM 2-4-dinitrophenol, (II) 3.5 mM n-Heptan-1-ol (heptanol), and (III) treatment by a hypotonic (120 mOsmoles) Ca2+-free solution for half an hour, followed by return to normal conditions. The gap junction morphology was analyzed quantitatively in freeze-fracture replicas and compared in electrically conducting and nonconducting fibers. It is found that the three uncouplers of cell-to-cell conduction induce consistent and statistically significant alterations of the gap junction structure. The investigated morphological criteria: (a) P-face junctional particle diameter, control value 8.18 +/- 0.70 nm (mean +/- SD), (b) P-face junctional particles center-to-center spacing, control value 10.23 +/- 1.57 nm, and (c) E-face pits spacing, control value 9.45 +/- 0.98 nm, are, respectively, decreased to 7.46 +/- 0.62 nm, 9.25 +/- 1.34 nm and 8.67 +/- 1.13 nm in Purkinje fibers with complete conduction blocks. All three gap junctional dimensions are seen to decline progressively with time from the onset of an uncoupling treatment towards stable minima reached in half an hour. The observed morphological transitions appear related to the electrical uncoupling for the following reasons: partial electrical uncoupling results in values of the gap junctional dimensions that are intermediate between those measured in electrically coupled and uncoupled preparations, and the three morphological indices are seen to increase again towards control values very soon after electrical conduction has been re-established. It is concluded that the junctional channels closure on electrical uncoupling correlates with a measurable (-0.72 +/- 0.01 nm, difference of the means +/- SE) decrease of the junctional particle diameters.

Effect of deuterium oxide on junctional membrane channel permeability

The effect of deuterium oxide on junctional membrane permeability to dichlorofluorescein was examined to determine the mode of transfer of the dye from one cell interior to another in the septate giant axon of earthworm. Dichlorofluorescein was shown to diffuse through the nexus passively and in a hydrated form. Additionally, evidence suggested an alteration of the cell-to-cell channel structure by deuterium/hydrogen exchange. Dichlorofluorescein was rendered impermeant at 6 degrees C in D2O and 4 degrees C in H2O. Action potentials, however, were capable of propagation from cell to cell at 4 degrees C in D2O and H2O. The results are consistent with a hydrophilic channel where solute molecules diffuse through the junction (nexus) in a hydrated form. The temperature blocks are presumably brought about by increasing hydration shells around solute and channel proteins with cooling until the solute is rendered too large to diffuse.

Electrotonic coupling in internally perfused crayfish segmented axons

We have developed a technique for cannulation and internal perfusion of crayfish segmented lateral axons. Experiments on perfused and non-perfused axons lead to the following conclusions: 1. Internally perfused segmented axons behave very similarly to non-perfused axons. 2. The axial electrical resistance of the junctional region is almost as low as a comparable segment of axon. 3. Neither intracellular Ca2+ nor H+ is effective in disrupting the intercellular communication pathway in perfused axons. On the basis of these findings we have formulated a hypothesis for cellular control of intercellular coupling based on the existence of a soluble intermediate for Ca2+ or H+-induced uncoupling. This hypothesis is consistent with data from both internally perfused and non-perfused axons.