Basal calcium entry in vascular smooth muscle

Basal calcium leak into smooth muscle was identified 30 years ago yet remains poorly understood. We characterized this leak measuring Ca uptake into cultured rat aortic smooth muscle cells. Wash solution (0°C) containing lanthanum (3 mM) removed extracellular tracer and increased cellular Ca retention more effectively than EGTA (0.2 mM). Basal Ca entry was 1.45×10 Ca ·cell ·min . This translated to ∼250 μmol ·min 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 s=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 entry is mainly due to a basal open probability of excitable Ca -channels. © 2004 Elsevier B.V. All rights reserved. 45 2+ 45 2+ 2+ 9 2+ -1 -1 -1 -1 2+ 2+ 5