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

Abstract

Abstract: : Purpose: To characterize the electrophysiological properties of a newly identified voltage–gated sodium channel of bovine ciliary muscle cells Methods: Smooth muscle cells dispersed from the bovine ciliary body by collagenase treatment were used. The 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 stimulation evoked an inward current having a peak of –40 to– 200 pA at –10 to 0 mV, which was not observed when the descending ramp–pulse protocol was used. The current was abolished by extracellular application of tetrodotoxin (1 µM), whereas it was not affected by either carbachol (2 µM) or verapamil (1 µM). A similar current was observed in response to the ascending ramp pulse stimulation when all extracellular monovalent and divalent metal cations were isosmotically replaced with either Li⁺ or Na⁺. In contrast little or no current was evoked when Cs⁺, Mg²⁺, Sr²⁺ or Ba²⁺ was used as the substitute ion. A small but clearly monitored inward current was observed when Ca²⁺ was the substitute. Conclusions: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.

Keywords: ciliary muscle • ion channels • pharmacology