A sodium channel activated by depolarization in smooth muscle cells freshly isolated from the bovine ciliary body

Abstract

PURPOSE To examine the properties of a newly identified voltage-gated sodium channel of bovine ciliary muscle cells.
METHODS Bovine ciliary myocytes freshly dispersed by collagenase treatment were used. Whole-cell membrane potential was held constant at -50 mV and ramp pulses of 200 ms duration either ascending or descending between -120 mV and 80 mV at a rate of 1 V/s were applied. The bath was perfused with a HEPES-Krebs solution (pH 7.4, 30°C), and borosilicate-glass pipette electrodes were filled with 100 mM-Cs⁺ solution containing 70 nM-Ca²⁺ and 200 μM-GTP (pH 7.0).
RESULTS Ascending ramp-pulse evoked an inward current having a peak of 40 to 200 pA at -10 to 0 mV, which was not observed when descending ramp pulse protocol was used. The current was abolished by tetrodotoxin (1 μM), whereas it was not affected by carbachol (2 μM) or verapamil (1 μM). A similar current was observed in response to the ascending ramp pulse when all extracellular metal cations were isosmotically replaced with either Li⁺ or Na⁺. No currents were evoked when Cs⁺, Mg²⁺, Sr²⁺ or Ba²⁺ was used as the substitute ion. A small but clearly discernible inward current was observed when Ca²⁺ was the substitute.
DISCUSSION The bovine ciliary muscle possesses a voltage-dependent sodium channel similar to the ones widely distributed in neurons or skeletal muscle. This channel, which is measurably permeable to Ca²⁺, may serve as a pathway for Ca²⁺ entry from the extracellular space in the early phase of the contraction. [Jpn J Physiol 54 Suppl:S135 (2004)]