Molecules undergo non-Brownian diffusion in the plasma membrane, but the mechanism behind this anomalous diffusion is controversial. To characterize the anomalous diffusion in the complex system of the plasma membrane and to understand its underlying mechanism, single-molecule/particle methods that allow researchers to avoid ensemble averaging have turned out to be highly effective. However, the intrinsic problems of time-averaging (resolution) and the frequency of the observations have not been explored. These would not matter for the observations of simple Brownian particles, but they do strongly affect the observation of molecules undergoing anomalous diffusion. We examined these effects on the apparent motion of molecules undergoing simple, totally confined, or hop diffusion, using Monte Carlo simulations of particles undergoing short-term confined diffusion within a compartment and long-term hop diffusion between these compartments, explicitly including the effects of time-averaging during a single frame of the camera (exposure time) and the frequency of observations (frame rate). The intricate relationships of these time-related experimental parameters with the intrinsic diffusion parameters have been clarified, which indicated that by systematically varying the frame time and rate, the anomalous diffusion can be clearly detected and characterized. Based on these results, single-particle tracking of transferrin receptor in the plasma membrane of live PtK2 cells were carried out, varying the frame time between 0.025 and 33 ms (0.03-40 kHz), which revealed the hop diffusion of the receptor between 47-nm (average) compartments with an average residency time of 1.7 ms, with the aid of single fluorescent-molecule video imaging.

Ultraspeed single-molecule tracking with <25-μs resolution and electron tomography show that transmembrane proteins and phospholipids in the plasma membrane hop among submicrometer compartments of the same size, probably delimited by the anchored-transmembrane-protein pickets lining the actin-based membrane-skeleton fence, once every 1–58 ms.

Voltage-gated Na þ channel (I Na ) is expressed under culture conditions in human smooth muscle cells (hSMCs) such as coronary myocytes. The aim of this study is to clarify the physiological, pharmacological and molecular characteristics of I Na expressed in cultured hSMCs obtained from bronchus, main pulmonary and coronary artery. I Na , was recorded in these hSMCs and inhibited by tetrodotoxin (TTX) with an IC 50 value of approximately 10 nM. Reverse transcriptase/polymerase chain reaction (RT-PCR) analysis of mRNA showed the prominent expression of transcripts for SCN9A, which was consistent with the results of real-time quantitative RT-PCR. These results provide novel evidence that TTX-sensitive Na þ channel expressed in cultured hSMCs is mainly composed of Na v 1.7.

1 The eects of oestrogens on action potential and membrane currents were examined in single guinea-pig atrial myocytes. 2 17b-estradiol (3 ± 10 mM) shortened the action potential duration without signi®cant changes in the resting membrane potential. E-4031 (1 mM) markedly prolonged the action potential duration and induced early afterdepolarization, and 17b-estradiol (10 mM) abolished it. 3 When cells were perfused in isoproterenol-containing solution, action potentials due to abnormal automaticity caused by membrane depolarization developed, and were also inhibited by 17b-estradiol. 4 Under voltage clamp conditions, the voltage-dependent Ca 2+ currents consisted of both T-(I Ca.T ) and L-type (I Ca.L ). 17b-estradiol reduced I Ca.L concentration-dependently, while it (10 mM) suppressed I Ca.T only by approximately 10%. 17b-estradiol did not aect time courses of I Ca.L inactivation, but it shifted the steady-state inactivation curve to more negative potentials. 5 17b-estradiol (10 mM) did not aect the time-dependent K + current (I K ), referred to as I Kr and I Ks , and inwardly rectifying K + current. However, 17b-estradiol (30 mM) or diethylstilbestrol (10 mM) inhibited K + currents. 6 DES and ethinylestradiol (EES) also suppressed I Ca.L , but testosterone and progesterone failed to inhibit I Ca.L . The potency of the inhibitory eect on I Ca.L was DES4EES417b-estradiol. 7 17b-estradiol and DES also inhibited the cyclic AMP-enhanced I Ca.L , but cyclic GMP in the pipette or pretreatment of L-NAME could not block the eects of oestrogen on I Ca.L . 8 These results suggest that oestrogen speci®cally has antiarrhythmic eects, possibly by acting the L-type Ca 2+ channels. The antiarrhythmic eects of oestrogens may contribute to the cardioprotective actions of oestrogens.