Na channel kinetics during the spontaneous heart beat in embryonic chick ventricle cells.

Using cell-attached and whole-cell recording techniques simultaneously allows the measurement of Na currents during action potentials in beating heart cells. In 7-d chick ventricle, the dynamic reversal potential for Na is 30 mV, which is 20 mV less than the reversal potential in nonbeating cells. This result implies that the spontaneous activity of heart cells raises the Na concentration at the internal face of the membrane to nearly 40 mM. Fitting the Na action currents to the Hodgkin and Huxley equations shows that patches may contain from 50 to 700 Na channels, with an average density of 23 +/- 21 per micron2. Only approximately 2% of the available Na channels are open at the peak of the Na action current. This low probability is a consequence of the channels' continual inactivation during the diastolic depolarization phase.     

High-pressure infrared spectroscopy of ether- and ester-linked phosphatidylcholine aqueous dispersions.

Infrared spectra of aqueous dispersions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and its ether-linked analogue, 1,2-dihexadecyl-sn-glycero-3-phosphocholine (DHPC), were measured in a diamond anvil cell at 28 degrees C as a function of pressure up to 20 kbar. Although these two lipids differ only in the linkages to the saturated hydrocarbon chains, significant differences were observed in their barotropic behavior. Most notable were the magnitudes of the pressure-induced correlation field splittings of the methylene scissoring and rocking modes, and the relative intensities of the corresponding component bands. In the case of the scissoring mode, not only can the correlation field component band be resolved at a lower pressure in DHPC (1.2 kbar, as compared with 2.2 kbar in DPPC), but the initial magnitude of the correlation field splitting in DHPC, particularly less than 9 kbar, is significantly greater than that observed in DPPC. These differences are attributed to the presence of an interdigitated lamellar gel phase in DHPC. At all pressures where the correlation field component band delta'CH2 can be resolved, the relative peak height/intensity ratio R = I delta'/I delta is greater in DPPC than in DHPC, suggesting that this parameter may be useful as a test of interdigitation.     

Mathematical analysis of cell-target encounter rates in two dimensions. The effect of chemotaxis.

The process by which cells encounter their targets is the first step of a number of cell functions involved in the immune response, such as cell-mediated cytotoxicity and phagocytic ingestion of foreign material. In many instances, this encounter may be rate-limiting, and therefore it is important to understand what factors influence the encounter rate. One key aspect of cell-target encounter is the motility behavior of the cell in the vicinity of a target. This movement may be entirely random, or there may be a directed, or chemotactic, component to it. In this paper we focus on the effects of cell motility properties, and particularly the chemotactic directional bias, on the rate of cell-target encounter. Specifically, we derive an expression for the mean encounter time of cells that meet targets in two dimensions as a function of the cells' directional orientation bias. We show that a modest degree of bias can reduce the mean encounter time by orders of magnitude, while nearly perfect directional bias offers little additional benefit. We illustrate the application of these results to a particular example system: alveolar macrophages removing inhaled particles and bacteria from the lung surface.     

Mechanical equilibrium of thick, hollow, liquid membrane cylinders.

The mechanical equilibrium of bilayer membrane cylinders is analyzed. The analysis is motivated by the observation that mechanically formed membrane strands (tethers) can support significant axial loads and that the tether radius varies inversely with the axial force. Previously, thin shell theory has been used to analyze the tether formation process, but this approach is inadequate for describing and predicting the equilibrium state of the tether itself. In the present work the membrane is modeled as two adjacent, thick, anisotropic liquid shells. The analysis predicts an inverse relationship between axial force and tether radius, which is consistent with experimental observation. The area expansivity modulus and bending stiffness of the tether membrane are calculated using previously measured values of tether radii. These calculated values are consistent with values of membrane properties measured previously. Application of the analysis to precise measurements of the relationship between tether radius and axial force will provide a novel method for determining the mechanical properties of biomembrane.     

Simulation of double-stranded branch point migration.

A structural and dynamic model has been developed for the branch point formed when two DNA double helices exchange strands during genetic recombination. This model, which generalizes most previous structural models, maintains the twofold symmetry inherent in the covalent and hydrogen bonded structure, yet has three degrees of freedom about virtual bonds, constituting a simplified junction. Using this structural model, a three-step dynamic model for branch point migration has been developed: longitudinal diffusion about the virtual bonds to achieve a structure in which the helix axes are approximately parallel; opening of the base pairs; and rotary diffusion about the helix axis to effect a migratory event. The model, which includes the possible role of electrostatic interactions, solves problems inherent in previous treatments. We find that no significant electrostatic torques arise that promote branch point migration. The absence of a kinetic mechanism to circumvent thermodynamic barriers due to mispairing suggests that an energy source is used for those situations in living systems.     

Uranyl acetate induces gel phase formation in model lipid and biological membranes.

The effect of uranyl acetate on the mesomorphic phase state of lipids in model membranes as well as in isolated biological membranes has been examined. As little as 0.8 mM (0.03% [wt/vol]) uranyl acetate induces a liquid crystal-to-gel phase transformation in egg phosphatidic acid, bovine brain phosphatidylserine, and in lysed chromaffin granule membranes. These results along with others in the literature indicate that the uranyl acetate used in samples for electron microscopy could alter membrane morphology.     

The mechanism of low-frequency sound production in muscle.

Frog gastrocnemius muscles stimulated isometrically in a saline bath at 20 degrees C were found to produce a single ringing sound event beginning just before the tension record began to rise. The sound event was substantially over by the time the isometric tension began to fall. Results from studies correlating the spatial pattern of the sound, the amplitude and frequency of the sound as a function of the muscle length, and the response of both the passive and active muscle to a transverse pluck were found to be consistent with the conclusion that the sounds in these muscles are caused primarily by transverse resonant vibrations. As the muscle develops force, its lack of cylindrical symmetry gives rise to lateral motions, which are most likely the initiators of the bending vibrations detected as sound.     

Analysis of compartmentation of ATP in skeletal and cardiac muscle using 31P nuclear magnetic resonance saturation transfer.

We have developed a model for the analysis of the forward creatine kinase reaction in muscle as measured by the nuclear magnetic resonance (NMR) technique of magnetization transfer. The model, accounting for the double-exponential behavior observed in some NMR magnetization transfer data, allows for the existence of two ATP pools, one that is NMR-visible (NMR-VIS) and another that is NMR-invisible (NMR-INVIS). We have applied the model to experimental data for the forward creatine kinase reaction in skeletal and cardiac muscles to study the dependence of the creatine kinase rate constants and fluxes on workload and to account for the differences between heart and skeletal muscle. The results suggest that an NMR-distinct ATP pool exists in both heart and skeletal muscles, and that phosphate exchange with this pool catalyzed by creatine kinase increases with increased workload. The results also agree with previously published estimates of the rates of mitochondrial translocase and net ATP synthesis obtained by traditional biochemical methods.     

Aerobic and anaerobic swimming speeds of spermatozoa investigated by twin beam laser velocimetry.

The motility of bovine and ovine spermatozoa has been studied under aerobic and anaerobic conditions, using a dual beam laser velocimeter. Cells swimming under aerobic conditions were found to be characterized by a translational swimming speed and a rotation rate that were approximately double those of cells swimming in an anaerobic environment. Both types of spermatozoa have been found to exhibit a sudden coordinated transition between fast and slow swimming states when the available oxygen is exhausted. This transition from aerobic to anaerobic swimming states has also been shown to be reversible. Studies of the duration of aerobic motility using the same apparatus have shown that the cells have a constant motile efficiency over the temperature range 32 degrees-42 degrees C.     

Uniformly oriented gramicidin channels embedded in thick monodomain lecithin multilayers.

Phosphatidylcholine multilayers, containing 20% water by total sample weight and gramicidin/lipid molar ratios up to 1:40 were aligned by low temperature annealing (less than 60 degrees C) and mechanical stressing. We were able to obtain large (greater than 80 micron thick X 40 mm2 area) monodomain defect-free multilayers containing approximately 10(17) uniformly oriented gramicidin channels. The alignment of lipid multilayers was monitored by conoscopy and polarized microscopy. The smectic defects, which appeared during the alignment process, were identified and dissolved. The incorporation of gramicidin into the multilayers in the form of transmembrane channels was indicated by its circular dichroic (CD) spectrum. A well-defined CD spectrum of uniformly oriented gramicidin channels was obtained. The oriented samples will allow spectroscopic studies of the ion channel in its conducting state and diffraction studies of the channel-channel organization in the membrane.