SUMMARY1. The effect of changes in the pH of the extracellular solution on the membrane conductance of frog sartorius and toe muscle fibres was measured with intracellular micro-electrodes.2. In Ringer solution the membrane conductance was found to be highly sensitive to changes in pH between 5 0 and 9 8. In alkaline solution the conductance rose; in acid solution it fell. 3. After replacement of chloride by the relatively impermeant methylsulphate ion the membrane conductance showed little change when pH was altered. It is concluded that chloride is the ion species principally concerned in the pH sensitivity of the resting membrane conductance.4. The relation between pH and the chloride conductance was sigmoid, with the steepest part of the curve lying in the region of neutrality.5. The membrane conductance of muscles equilibrated in a 100 mm-K 216 mM-Cl solution was also sensitive to changes of extracellular pH. As in Ringer solution, the membrane conductance rose in alkaline and fell in acid solutions in a sigmoid fashion.6. Sartorius muscles in isotonic potassium methylsulphate solution showed no change in membrane conductance at different pH values.7. In chloride-free solution a fall in pH tended to cause depolarization; a rise in pH had the opposite effect.8. In Ringer solution the initial effect of a rise in pH was usually a transient depolarization. The indication is that the intracellular concentration of chloride ions may be slightly in excess of that which corresponds to the resting potential. The long-term effects of changes in pH on the membrane potential in Ringer solution were in the same direction as in the absence of chloride.9. The transient potential changes produced on addition and withdrawal of -chloride ions were found to be larger in alkaline solutions than in acid solutions. This is further evidence for a higher chloride permeability in alkaline solutions.

Antibodies to the major protein of rat liver gap junctions, molecular weight 27,000 (27K), have been microinjected into one identified cell of 8-cell stage Xenopus embryos. This treatment selectively disrupts both dye transfer and electrical coupling between the progeny cells. These results provide evidence that the 27K protein is an integral component of the cell-to-cell junctional channel. The disruption of junctional communication at early stages results in specific developmental defects, suggesting that blocking intercellular communication can have a pronounced influence on embryonic development.

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1. Gap junction formation was compared in the absence and presence of small peptides containing extracellular loop sequences of gap junction (connexin) proteins by measuring the time taken for pairs of spontaneously beating embryonic chick heart myoballs to synchronize beat rates. Test peptides were derived from connexin 32. Non-homologous peptides were used as controls. Control pairs took 42 + 0o5 min (mean + S.E.M.; n = 1088) to synchronize. 2. Connexins 32 and 43, but not 26, were detected in gap junction plaques. The density and distribution of connexin immunolabelling varied between myoballs. 3. Peptides containing conserved motifs from extracellular loops 1 and 2 delayed gap junction formation. The steep portion of the dose-response relation lay between 30 and 300 /M peptide. 4. In loop 1, the conserved motifs QPG and SHVR were identified as being involved in junction formation. In loop 2, the conserved SRPTEK motif was important. The ability of peptides containing the SRPTEK motif to interfere with the formation of gap junctions was enhanced by amino acids from the putative membrane-spanning region. 5. Peptides from loop 1 and loop 2 were equivalently effective; there was no synergism between them. 6. The inclusion of conserved cysteines in test peptides did not make them more effective in the competition assay.

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