The effect of the mutation K448E in the rat GABA transporter rGAT1 was studied using heterologous expression in Xenopus oocytes and voltage clamp. At neutral pH, the transport‐associated current vs. voltage (I–V) relationship of the mutated transporter was different from wild‐type, and the pre‐steady‐state currents were shifted towards more positive potentials. The mutated transporter showed an increased apparent affinity for Na+ (e.g. 62 vs. 152 mm at −60 mV), while the opposite was true for GABA (e.g. 20 vs. 13 μm at −60 mV). In both isoforms changes in [Na+]o shifted the voltage dependence of the pre‐steady‐state and of the transport‐associated currents by similar amounts. In the K448E form, the moved charge and the relaxation time constant were shifted by increasing pH towards positive potentials. The transport‐associated current of the mutated transporter was strongly reduced by alkalinization, while acidification slightly decreased and distorted the shape of the I–V curve. Accordingly, uptake of [3H]GABA was strongly reduced in K448E at pH 9.0. The GABA apparent affinity of the mutated transporter was reduced by alkalinization, while acidification had the opposite result. These observations suggest that protonation of negatively charged residues may regulate the Na+ concentration in the proximity of the transporter. Calculation of the unidirectional rate constants for charge movement shows that, in the K448E form, the inward rate constant is increased at alkaline pH, while the outward rate constant does not change, in agreement with an effect due to mass action law. A possible explanation for the complex effect of pH on the transport‐associated current may be found by combining changes in local [Na+]o with a direct action of pH on GABA concentration or affinity. Our results support the idea that the extracellular loop 5 may participate to form a vestibule to which sodium ions must have access before proceeding to the steps involving charge movement.

The specific transcytosis of polypeptides, demonstrated in the nasal respiratory mucosa of the rabbit, seems to be involved in antigen sampling at the airway entry, since absorption has been shown only to occur if lymphoid aggregates are present beneath the epithelium and to be proportional to aggregate volume. Nanoparticles and many polypeptides besides the two previously tested (i.e. carbocalcitonin (CCT) and adrenocorticotropic hormone) should be transportable, in agreement with the vesicular transcytosis and antigen sampling hypothesis. Thus unidirectional mucosa-submucosa and opposite fluxes (Jms, Jsm) and the corresponding net fluxes (Jnet) of uncoated or polypeptide-coated polystyrene nanospheres (diameter: about 0.5 micrometer) have been measured with the aid of spectrophotometry and quantitative dark-field microscopy. No net transport has been observed for uncoated beads, whereas it has always been shown for polypeptide-coated beads, although to different extents. The selectivity sequence for the polypeptides tested is as follows: BSA congruent with enkephalin << anti-BSA IgG congruent with IgA congruent with CCT congruent with insulin

An active absorption of polypeptides (elcatonin = CCT; adrenocorticotropic hormone) had been previously observed in the nasal respiratory mucosa of the rabbit. Its saturation kinetics and the parallel absence of a net transfer of other non-polypeptidic organic markers excluded the involvement of a simple pinocytosis. This absorption has been now better localized and further characterized. Unidirectional CCT fluxes (determined with radioimmunoassay) have been concomitantly monitored with transepithelial electric potential difference (Vms). Although the mucosae covering the ectoturbinal A and the lower and upper conchae displayed similar Vms, the active CCT transport was only evidenced in the upper concha. In this region cytochalasin B (which by disassembling actin microfilaments prevents the apical formation of vesicles in epithelial cells) and monensin (which prevents the split of the ligand-receptor complex in the endosomes) both eliminated the net CCT absorption, however, also permanently increasing the passive CCT junctional permeability. Aluminum fluoride (which prevents the fusion of endocytic vesicles into endosomes) and colchicine (which disrupts microtubules along which vesicles move in the cytoplasm) also permanently abolished net CCT transport, without affecting, or shortly and transiently affecting, passive permeability. On the whole these results are in favor of an active CCT transport supported by a specific vesicular transport.

The active transcytosis of many different polypeptides (either presented free or adsorbed on latex nanoparticles), found in the respiratory mucosa of the upper nasal concha, has previously been shown to be proportional to the total volume of the lymphoid aggregates present in the tissue. By combining the use of fluorescent nanoparticles, flux measurements, confocal and scanning electron microscopy and conventional histology, it is shown in this paper that: (i) the areas of epithelium overlying lymphoid aggregates are the only transporting polypeptides; (ii) the respiratory epithelium in these areas consists mainly of non-ciliated microvillar cells, with numerous ciliated cells and rare mucous goblet cells at the periphery of the area only; (iii) non-ciliated microvillar cells are distinguishable in cells with well developed finger-like microvilli and cells with an irregularly pleated apical membrane, similar to that of intestinal and bronchial antigen-sampling M-cells; (iv) groups of polypeptide-coated nanospheres are found bound to this latter type of cells, demonstrating that these are the transporting cells, detected at the first stage of the transcytotic cycle.

We have previously demonstrated that polypeptides (elcatonin and ACTH) can be actively absorbed across the rabbit nasal mucosa. In this paper we show that elcatonin is also transported when it is adsorbed onto microspheres (diameter: 0.5 micron). whereas the elcatonin-uncovered microspheres do not display any net transport. Cytochalasin D (0.1 microgram/ml) abolishes the net absorption of elcatonin presented either alone or adsorbed. At the same concentration the inhibitor does not affect cellular active ion transports (and hence metabolism); although it increases intercellular ion and elcatonin permeability, it does not affect intercellular and paracellular permeability of the elcatonin-covered microspheres. Altogether, these results show that polypeptide transport is supported by a specific vesicular transfer inhibited by cytochalasin D by disassembly of the actin cytoskeleton, probably at the apical border of the cell.