Tetrodotoxin‐Sensitive Sodium Current in Sheep Lymphatic Smooth Muscle

Abstract

1. Fast inward currents were elicited in freshly isolated sheep lymphatic smooth muscle cells by depolarization from a holding potential of -80 mV using the whole-cell patch-clamp technique. The currents activated at voltages positive to -40 mV and peaked at 0 mV. 2. When sodium chloride in the bathing solution was replaced isosmotically with choline chloride inward currents were abolished at all potentials. 3. These currents were very sensitive to tetrodotoxin (TTX). Peak current was almost abolished at 1 /,M with half-maximal inhibition at 17 nM.4. Examination of the voltage dependence of steady state inactivation showed that more than 90 % of the current was available at the normal resting potential of these cells (-60 mV). 5. The time course of recovery from inactivation was studied using a double-pulse protocol and showed that recovery was complete within 100 ms with a time constant of recovery of 20 ms.6. Under current clamp, action potentials were elicited by depolarizing current pulses. These had a rapid upstroke and a short duration and could be blocked with 1 /LM TTX. 7. Spontaneous contractions of isolated rings of sheep mesenteric lymphatic vessels were abolished or significantly depressed by 1 /uM TTX.

Outward Currents in Smooth Muscle Cells Isolated from Sheep Mesenteric Lymphatics

“…When all of the BK current was blocked, there was a remaining voltage-dependent outward current. This was absent in cells which were dialysed with Cs¤, instead of K¤ (Hollywood et al 1997), and was reduced by TEA and 4-AP. It is highly likely, therefore, that this current was carried by K¤ ions.…”

“…Although our cell yields are low, as might be expected from preparations in which the muscle layer is some 3-4 cells thick (unpublished observations) it has proved possible to obtain enough cells to allow a characterization of their membrane currents. We have demonstrated that these cells contain several inward currents (Hollywood et al 1997) and a variety of outward currents, including a Ca¥-sensitive current, and a voltage-dependent current which itself probably consists of more than one component. The Ca¥-sensitive current observed under the conditions of our experiments appears to be mediated mainly by large conductance Ca¥-activated K¤ channels (BK channels) as (1) it was sensitive to both iberiotoxin and penitrem A, which are thought to be highly selective blockers of these channels (Garcia, Galvez, GarciaCalvo, King, Vazquez & Kaczorowski, 1991;Knaus et al 1994), and (2) large channels with the characteristics of BK channels were present in inside-out patches derived from lymphatic cell membranes and, indeed, were by far the most prominent channel observed.…”

“…1A, bottom panel). Evidence presented in the following paper (Hollywood, Cotton, Thornbury & McHale, 1997) suggests that this remaining current is carried by Na¤ ions. A mean current-voltage relationship is shown for the peak outward current in nine cells in Fig.…”

The cardiac sodium current Nav1.5 is functionally expressed in rabbit bronchial smooth muscle cells

“…Currents were recorded as previously described using the ruptured-patch configuration of the whole cell patch-clamp technique (14). Pipettes were pulled from borosilicate glass capillary tubing (1.5 mm OD, 1.17 mm ID; Clark Medical Instruments) to a tip of diameter ϳ1-1.5 m and resistance of 2-4 M⍀.…”

“…In freshly dispersed smooth muscle cells, VGSC were previously regarded as a rarity, but evidence for their existence in this cell type has been gradually accumulating. In phasic tissues, such as uterine, lymphatic, vas deferens, portal vein, and gastrointestinal smooth muscle, they may play a part in the generation and/or propagation of spontaneous electrical activity underlying myogenic contractions (14,15,29,31,38). However, they have also been found in a variety of tonic arterial smooth muscles (7,9,21,27), although mostly only in cultured cells, suggesting that they are indicative of an altered phenotype (9,27).…”