Coupling Optical and Electrical Measurements in Artificial Membranes: Lateral Diffusion of Lipids and Channel Forming Peptides in Planar Bilayers

Neuronal sodium channels in ventricular heart cells are localized near T-tubules openings

Cardiac voltage-dependent sodium channels (VDSC) are known to be tetrodotoxin (TTX)-resistant. However, recent immunochemical studies suggest the presence of TTX-sensitive neuronal-type VDSC in the heart. Scanning ion conductance microscopy (SICM) coupled to electrophysiology was used to obtain more direct functional evidence. TTX sensitivities of whole-cell sodium currents (I(Na)) in control and detubulated cells were compared. Addition of 200 nM TTX decreased I(Na) of control cells by 20%, whereas detubulated cells were hardly effected. The remaining current peaked slightly earlier and inactivation decay was faster (as in neuronal VDSC) than in detubulated cells. Single-channel activity was first assayed at random on the plasmalemma, and after topography had been revealed by SICM, at patched T-tubules openings. In the latter case, a single-channel conductance of 11-12pS was observed with a higher rate of success. This study provides independent evidence for neuronal VDSC in cardiomyocytes where they could rapidly and synchronously couple T-tubule and cell surface depolarizations.

Investigations of Polypeptide Rotational Diffusion in Aligned Membranes by 2H and 15N Solid-State NMR Spectroscopy

Transmembrane and in-plane oriented peptides have been prepared by solid-phase peptide synthesis, labeled with 3,3,3-2H3-alanine and 15N-leucine at two selected sites, and reconstituted into oriented phophatidylcholine membranes. Thereafter, proton-decoupled 15N and 2H solid-state NMR spectroscopy at sample orientations of the membrane normal parallel to the magnetic field direction have been used to characterize the tilt and rotational pitch angle of these peptides in some detail. In a second step the samples have been tilted by 90 degrees . In this setup the spectral line shapes are sensitive indicators of the rate of rotational diffusion. Whereas monomeric transmembrane peptides exhibit spectral averaging and well-defined resonances, larger complexes are characterized by broad spectral line shapes. In particular the deuterium line shape is sensitive to association of a few transmembrane helices. In contrast, the formation of much larger complexes affects the 15N chemical shift spectrum. The spectra indicate that in liquid crystalline membranes an amphipathic peptide of 14 amino acids exhibits fast rotational diffusion on both the 2H and 15N time scales (>10(-5) s). Extending the sequences to 26 amino acids results in pronounced changes of the 2H solid-state NMR spectrum, whereas the signal intensities of 15N solid-state NMR spectra degrade. Below the phase transition temperature of the phospholipid bilayers, motional averaging on the time scale of the 2H solid-state NMR spectrum ceases for transmembrane and in-plane oriented peptides. Furthermore at temperatures close to the phase transition the total signal intensities of the deuterium solid-state NMR spectra strongly decrease.