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Ca²⁺-Activated Cl^– Current in Retinal Arteriolar Smooth Muscle

From the Centre for Vision and Vascular Sciences, School of Medicine and Dentistry, The Queen’s University of Belfast, Institute of Clinical Sciences, The Royal Victoria Hospital, United Kingdom.

PURPOSE. To characterize the biophysical, pharmacologic, and functional properties of the Ca²⁺-activated Cl^– current in retinal arteriolar myocytes.

METHODS. Whole-cell perforated patch-clamp recordings were made from myocytes within intact isolated arteriolar segments. Arteriolar tone was assessed using pressure myography.

RESULTS. Depolarizing of voltage steps to –40 mV and greater activated an L-type Ca²⁺ current (I_Ca(L)) that was followed by a sustained current. Large tail currents (I_tail) were observed on stepping back to –80 mV. The sustained current and I_tail reversed close to 0 mV in symmetrical Cl^– concentrations. The ion selectivity sequence for I_tail was I^–> Cl^–> glucuronate. Outward I_tail was sensitive to the Cl^– channel blockers 9-anthracene-carboxylic acid (9-AC; 1 mM), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS; 1 mM), and disodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS; 1 mM), but only DIDS produced a substantial (78%) block of inward tail currents at –100 mV. I_tail was decreased in magnitude when the normal bathing medium was substituted with Ca²⁺-free solution or if I_Ca(L) was inhibited by 1 µM nimodipine. Caffeine (10 mM) produced large transient currents that reversed close to the Cl^– equilibrium potential and were blocked by 1 mM DIDS or 100 µM tetracaine. DIDS had no effect on basal vascular tone in pressurized arterioles but dramatically reduced the level of vasoconstriction observed in the presence of 10 nM endothelin-1.

CONCLUSIONS. Retinal arteriolar myocytes have I_Cl(Ca), which may be activated by Ca²⁺ entry through L-type Ca²⁺ channels or Ca²⁺ release from intracellular stores. This current appears to contribute to agonist-induced retinal vasoconstriction.