PorB of the pathogenic Neisseria species belongs to the large family of pore-forming proteins (porins) produced by gram-negative bacteria. PorB is exceptional in that it is capable of translocating vectorially into membranes of infected target cells and functions in the infection process. Here we report on an unexpected similarity between Neisserial PorB and mitochondrial porins. Both porin classes interact with purine nucleoside triphosphates, which down-regulate pore size and cause a shift in voltage dependence and ion selectivity. Patch-clamp analyses indicate that PorB channel activity is tightly regulated in intact epithelial cells. In light of recent findings on the pivotal role of PorB in virulence and the prevention of phagosome lysosome fusion, these data provide important mechanistic clues on the intracellular pathogen accommodation reminiscent of mitochondrial endosymbiosis.

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The addition of progesterone (1-100 mumol/l) to the extracellular fluid bathing rat hepatocytes led to a rapid and fully reversible depolarization of the cell membrane. The progesterone-induced depolarization was paralleled by a decrease of potassium selectivity and an increase of cell membrane resistance and was abolished in the presence of the potassium channel blocker barium. Accordingly, in whole cell recordings, progesterone led to a decrease of the cell membrane conductance. 17 alpha-Hydroxyprogesterone and beta-estradiol were less effective by a factor of 10, whereas cholesterol, corticosterone and hydrocortisone did not significantly alter the potential difference across the cell membrane. In conclusion, acute administration of progesterone depolarized rat hepatocytes by decreasing the potassium conductance of the cell membrane.

Gap junctional coupling was studied in pairs of murine pancreatic acinar cells using the double whole-cell patch-clamp technique. During stable electrical coupling, addition of OAG (1-oleoyl-2-acetyl-sn-glycerol) induced a progressive reduction of the junctional conductance to the detectable limit (approximately 3 pS). Prior to complete electrical uncoupling, various discrete single channel conductances between 20 and 100 pS could be observed. Polymyxin B, a potent inhibitor of the protein kinase C (PKC) system, completely suppressed OAG-stimulated electrical uncoupling. Dialysis of cell pairs with solutions containing PKC, isolated from rat brain, also caused electrical uncoupling. The presence of 0.1 mM dibutyryl cyclic AMP and 5 mM ATP in the pipette solution, which serves to stabilize the junctional conductance, did not suppress the effects of OAG or isolated PKC. We conclude that an increase of protein kinase C activity leads to the closure of gap junction channels, presumably via a PKC-dependent phosphorylation of the junctional peptide, and that this mechanism is dominant over cAMP-dependent upregulatory effects in the experimental time range (less than or equal to 1 hr). A correlation of the observed single channel conductances with the appearance of channel subconductance states or various channel populations is discussed.

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