TETRODOTOXIN-SENSITIVE SODIUM CURRENT IN SHEEP LYMPHATIC SMOOTH-MUSCLE

1. Fast inward currents mere elicited in freshly isolated sheep lympha tic smooth muscle cells by depolarization from a holding potential of -80 mV using the whole-cell patch-clamp technique. The currents activa ted at voltages positive to -40 mV and peaked at 0 mV. 2. When sodium chloride in the bathing solution was replaced isosmotically with choli ne chloride inward currents were abolished at all potentials. 3. These currents were very sensitive to tetrodotoxin (TTX). Peak current was almost abolished at 1 mu M with half-maximal inhibition at 17 nM. 4. E xamination of the voltage dependence of steady state inactivation show ed ed that more than 90 % of the current was available at the normal r esting potential of these cells (-60 mV). 5. The time course of recove ry from inactivation was studied using a double-pulse protocol and sho wed that recovery was complete within 100 ms with a time constant of r ecovery of 20 ms. 6. Under current damp, action potentials were elicit ed by depolarizing current pulses. These had a rapid upstroke and a sh ort duration and could be blocked with 1 mu M TTX. 7. Spontaneous cont ractions of isolated rings of sheep mesenteric lymphatic vessels were abolished or significantly depressed by 1 mu M TTX.