Capsaicin (Cap) is a pungent extract of the Capsicum pepper family, which activates nociceptive primary sensory neurons. Inward current and membrane potential responses of cultured neonatal rat dorsal root ganglion neurons to capsaicin were examined using whole-cell and perforated patch recording methods. The responses exhibited strong desensitization operationally classified as acute (diminished response during constant Cap exposure) and tachyphylaxis (diminished response to successive applications of Cap). Both acute desensitization and tachyphylaxis were greatly diminished by reductions in external Ca2+ concentration. Furthermore, chelation of intracellular Ca2+ by addition of either EGTA or bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid to the patch pipette attenuated both forms of desensitization even in normal Ca2+. Release of intracellular Ca2+ by caffeine triggered acute desensitization in the absence of extracellular Ca2+, and barium was found to effectively substitute for calcium in supporting desensitization. Cap activated inward current at an ED50 of 728 nM, exhibiting cooperativity (Hill coefficient, 2.2); however, both forms of desensitization were only weakly dependent on [Cap], suggesting a dissociation between activation of Cap-sensitive channels and desensitization. Removal of ATP and GTP from the intracellular solutions resulted in nearly complete tachyphylaxis even with intracellular Ca2+ buffered to low levels, whereas changes in nucleotide levels did not significantly alter the acute form of desensitization. These data suggest a key role for intracellular Ca2+ in desensitization of Cap responses, perhaps through Ca2+-dependent dephosphorylation at a locus that normally sustains Cap responsiveness via ATP-dependent phosphorylation. It also seems that the signaling mechanisms underlying the two forms of desensitization are not identical in detail.

A B S T R A C TThe open-channel conductance properties of a voltage-gated channel from sarcoplasmic reticulum were studied in planar phospholipid membranes. The channel is ideally selective for K + over C1-and for K + over Ca ++. In symmetrical 1 M solutions, the single-channel conductance (in pmho) falls in the order: K + (214) > NH~-(157) > Rb § (125) > Na § (72) > Li + (8.1) > Cs + (<3). In neutral bilayers, the channel conductance saturates with ion activity according to a rectangular hyperbolic relation, with half-saturation activities of 54 mM for K + and 34 mM for Na +. Under symmetrical salt conditions, the K+:Na + channel conductance ratio increases with salt activity, but the permeability ratio, measured by single-channel bi-ionic potentials, is constant between 20 mM and 2.5 M salt; the permeability ratio is equal to the conductance ratio in the limit of Iow-sah concentration. The channel conductance varies <5% in the voltage range -100 to +70 mV. The maximum conductance ofK § and Na + is only weakly temperature dependent (AH:[: = 4.6 and 5.3 kcal/mol, respectively), but that of Li + varies strongly with temperature (~-/:~ = 13 kcal/mol). The channel's K + conductance is blocked asymmetrically by Cs +, and this block is competitive with K +. The results are consistent with an Eyring-type model of the conduction process in which the ion must traverse three kinetic barriers as it permeates the channel. The data conform to L~iuger's (1973. Biochim. Biophys. Acta. 311:423-441) predictions for a "pure" single-ion channel.A description of channel-mediated ion conductance requires the study of two separate processes: the formation of the conducting channel (the gating process) and the movement of current-carrying ions through the open channel (the conduction process). In the previous paper (Labarca et al., 1980) we described the gating properties of a K+-selective channel from sarcoplasmic reticulum (SR) of mammalian skeletal muscle. We now wish to describe this channel's conduction process. Earlier work (Miller, 1978) showed that SR vesicles can be made to donate ion-conducting channels to a planar phospholipid bilayer membrane. These channels display selectivity for monovalent cations, but this selectivity was not quantified previously, and no attempt was made to develop a picture of the conduction process. We now intend to do J. GEN. PnvsloL. 9 The Rockefeller University Press

Neisseria gonorrhoeae has two porins, PIA and PIB, whose genes (porA and porB, respectively) are alleles of a single por locus. We recently demonstrated that penB mutations at positions 120 and 121 in PIB, which are presumed to reside in loop 3 that forms the pore constriction zone, confer intermediate-level resistance to penicillin and tetracycline (M. Olesky, M. Hobbs, and R. A. Nicholas, Antimicrob. Agents Chemother. 46: 2811-2820, 2002). In the present study, we investigated the electrophysiological properties as well as solute and antibiotic permeation rates of recombinant PIB proteins containing penB mutations (G120K, G120D/A121D, G120P/A121P, and G120R/A121H). In planar lipid bilayers, the predominant conducting state of each porin variant was 30 to 40% of the wild type, even though the anion selectivity and maximum channel conductance of each PIB variant was similar to that of the wild type. Liposome-swelling experiments revealed no significant differences in the permeation of sugars or ␤-lactam antibiotics through the wild type or PIB variants. Although these results are seemingly contradictory with the ability of these variants to increase antibiotic resistance, they are consistent with MIC data showing that these porin mutations confer resistance only in strains containing an mtrR mutation, which increases expression of the MtrC-MtrD-MtrE efflux pump. Moreover, both the mtrR and penB mutations were required to decrease in vivo permeation rates below those observed in the parental strain containing either mtrR or porin mutations alone. Thus, these data demonstrate a novel mechanism of porin-mediated resistance in which mutations in PIB have no affect on antibiotic permeation alone but instead act synergistically with the MtrC-MtrD-MtrE efflux pump in the development of antibiotic resistance in gonococci.In the 1970s and early 1980s, Neisseria gonorrhoeae, the etiological agent of the sexually transmitted infection gonorrhea, gradually became more resistant to penicillin and tetracycline until these antibiotics were discontinued as a first-line therapy. Resistance to these antibiotics can be either plasmid mediated or chromosomally mediated. Plasmid-mediated resistance to penicillin or tetracycline involves expression of a TEM-1-like ␤-lactamase (8, 30) or the TetM protein (25), respectively. In contrast, chromosomally mediated resistant N. gonorrhoeae (CMRNG) strains arise from cumulative mutations in endogenous genes or loci that gradually increase resistance until treatment failure occurs (3).Four genes or loci (penA, mtrR, penB, and ponA) are known to be involved in high-level penicillin resistance (MICs, Ն2 g/ml) in CMRNG strains, while the existence of a fifth resistance gene is inferred but not yet identified (3, 9, 32). penA (38, 39) and ponA (32, 33) encode alterations in the two essential penicillin-binding proteins (PBP 2 and PBP 1, respectively) that decrease their rates of acylation by ␤-lactam antibiotics. A single-base-pair deletion in the mtrR promoter abolishes transcription of the mtrR...

The malaria parasite, Plasmodium falciparum, requires large amounts of nutrients to sustain its rapid growth within the human red blood cell. A recently identified ion channel on the surface of the intraerythrocytic parasite may provide direct access to these nutrients in the red blood cell cytosol. Evidence supporting this role was obtained by incorporating this channel into planar lipid bilayers. In bilayers, this channel has conductance and gating properties identical to the in situ channel, passes soluble macromolecules of up to 1400 Da, and functions as a high capacity, low affinity molecular sieve. These properties, remarkably similar to those of a pore on Toxoplasma gondii (another protozoan parasite causing human disease), suggest a novel class of channels used by these intracellular parasites to acquire nutrients from host cytosol.The intraerythrocytic malaria parasite acquires nutrients by endocytosis of red blood cell (RBC) cytosol (1), diffusion through a membranous duct (2), and͞or direct access to RBC cytosol through a nutrient-permeable ion channel (3) on the parasitophorous vacuole membrane (PVM), the outer of two membranes that surround the parasite. This ion channel, identified by patch clamp of erythrocyte-free parasites, is present in high density and is permeable to amino acids and monosaccharides (3).The physiological role of this PVM ion channel remains controversial for several reasons. First, patch clamp experiments using intact parasites (3) could not distinguish whether this channel spans only the PVM or both the PVM and the parasite plasma membrane, an underlying apposed membrane not accessible to the patch clamp pipette. This uncertainty has significant implications for how nutrients move from RBC cytosol into the parasite (4, 5); a channel spanning both membranes would obviate the need for additional transport pathways in the parasite plasma membrane but would deplete known Na ϩ and K ϩ gradients between parasite and RBC cytosols (6).Second, patch clamp of the small intracellular parasite (ϳ3 m in diameter) is technically difficult and requires high concentrations of divalent cations (Ca 2ϩ or Mg 2ϩ ), which may alter channel function. Although those experiments demonstrated that the channel is open Ͼ98% of the time at the PVM's resting membrane potential (3), open probabilities with more physiological solutions could not be determined. Indeed, the RBC's Ca 2ϩ -activated K ϩ channel is closed under physiological conditions but has a high open probability at micromolar Ca 2ϩ concentrations (7).Finally, the on-cell patch clamp configuration does not allow detailed biophysical characterization of the channel. For example, it could not determine the channel's affinity for permeating nutrients because it required near-isotonic pipette solutions. Furthermore, size restrictions on permeant molecules could not be examined.To resolve these issues, we have incorporated this ion channel into planar lipid bilayers. In this method, channels are studied in a single bilayer without the sec...