Associating the mesoscale fiber organization of the tongue with local strain rate during swallowing

The tongue is an intricately configured muscular organ that undergoes a stereotypical set of deformations during the course of normal human swallowing. In order to demonstrate quantitatively the relationship between 3D aligned lingual fiber organization and mechanics during swallowing, the tissue's myoarchitecture and strain rate were imaged before and during the propulsive phase of a 3.0ml water bolus swallow. Mesoscale fiber organization was imaged with high-resolution diffusion tensor imaging (DTI) and multi-voxel myofiber tracts generated along maximum diffusion vectors. Tissue compression/expansion was obtained via lingual pressure-gated phase-contrast (PC) MRI, a method which determines local strain rate as a function of the phase shift occurring along an applied gradient vector. The co-alignment of myofiber tract direction and the localized principal strain rate vectors was obtained by translating the strain rate tensor into the reference frame with the primary axis parallel to the maximum diffusion vector using Mohr's circle, resulting in the generation of fiber-aligned strain rate (FASR). DTI tractography displayed the complete fiber anatomy of the tongue, consisting of a core region of orthogonally aligned fibers encased within a longitudinal sheath, which merge with the externally connected styloglossus, hyoglossus, and genioglossus fibers. FASR images obtained in the mid-sagittal plane demonstrated that bolus propulsion was associated with prominent compressive strain aligned with the genioglossus muscle combined with expansive strain aligned with the verticalis and geniohyoid muscles. These data demonstrate that lingual deformation during swallowing involves complex interactions involving intrinsic and extrinsic muscles, whose contractility is directed by the alignment of mesoscale fiber tracts.     

Characterization of sodium transport in gustatory epithelia from the hamster and rat

The transduction of sodium salts occurs through a variety of mechanisms, including sodium influx through amiloride-sensitive sodium channels, anion-dependent sodium movement through intercellular junctions and unidentified amiloride-insensitive mechanisms. Characterizations of sodium transport in lingual epithelium mounted in Ussing chambers have focused almost exclusively on epithelia containing only fungiform taste buds. In the present study we have investigated sodium transport by measuring NaCl-induced short-circuit current from lingual epithelia containing fungiform, foliate, vallate and palatine taste buds in the hamster and the rat. All areas show measurable sodium transport, yet significant differences were noted between the epithelia from the rat and the hamster and among the different epithelia within a single species in terms of current density, transepithelial resistance and mucosal amiloride sensitivity. In general, epithelia from the anterior tongue were of a lower resistance and transported sodium more effectively than from the posterior tongue. Moreover, fungiform- and vallate-containing epithelia in the rat had a greater current density than did the corresponding tissues in the hamster. Amiloride sensitivity also differed between the rat and the hamster. In the hamster all gustatory areas showed some amiloride sensitivity, while in the rat the vallate-containing epithelia were devoid of amiloride- sensitive sodium transport. The results are consistent with the interpretation that all chemosensitive areas may participate in the detection of salts but the degree of salt transport and the mechanism of transport is variable among different lingual epithelia and different species.      

Noncontact dipole effects on channel permeation. III. Anomalous proton conductance effects in gramicidin

Proton transport on water wires, of interest for many problems in membrane biology, is analyzed in side-chain analogs of gramicidin A channels. In symmetrical 0.1 N HCl solutions, fluorination of channel Trp(11), Trp-(13), or Trp(15) side chains is found to inhibit proton transport, and replacement of one or more Trps with Phe enhances proton transport, the opposite of the effects on K(+) transport in lecithin bilayers. The current-voltage relations are superlinear, indicating that some membrane field-dependent process is rate limiting. The interfacial dipole effects are usually assumed to affect the rate of cation translocation across the channel. For proton conductance, however, water reorientation after proton translocation is anticipated to be rate limiting. We propose that the findings reported here are most readily interpreted as the result of dipole-dipole interactions between channel waters and polar side chains or lipid headgroups. In particular, if reorientation of the water column begins with the water nearest the channel exit, this hypothesis explains the negative impact of fluorination and the positive impact of headgroup dipole on proton conductance.