Abstract
Basal calcium leak into smooth muscle was identified 30 years ago yet remains poorly understood. We characterized this leak measuring 45Ca 2+ uptake into cultured rat aortic smooth muscle cells. Wash solution (0 °C) containing lanthanum (3 mM) removed extracellular tracer and increased cellular 45Ca 2+ retention more effectively than EGTA (0.2 mM). Basal Ca 2+ entry was 1.45×10 9 Ca 2+·cell −1·min −1. This translated to ∼250 μmol −1·min −1 given cell volumes of 4–15 pl as determined by 3-D image reconstruction. Gadolinium (100 μM) blocked 80% of the leak and exhibited a biphasic concentration–response relation (IC 50s=1 μM and 2 mM). Organic ion channel blockers also inhibited ∼80% of the leak; 45% by nifedipine (10 μM), 7% was exclusively blocked by SKF 96365 (1-[ b-[3-(4-Methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole) (50 μM) and 23% was exclusively sensitive to 2-aminoethoxy-diphenylborate (2-APB, 75 μM). Reverse transcriptase polymerase chain reaction revealed TrpC1, 4 and 6 mRNA, and we propose that 2-APB may selectively block TrpC4-containing channels. We conclude that basal Ca 2+ entry is mainly due to a basal open probability of excitable Ca 2+-channels.
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