Effects of DCB or bepridil on steady-state Na⁺_i/Ca²⁺_oexchange in chymotrypsin-treated platelets. (A) Washed platelets (2.0 × 10⁶/μL) suspended in modified Tyrode Hepes buffer containing 1 mmol/L CaCl₂ were treated with 60 U/mL -chymotrypsin for 10 minutes at 37°C. Fifty μL aliquots of treated platelets were mixed with 950 μL of either HBS-CaCl₂ ([Na⁺]_o = 140 mmol/L) or Na⁺-free HBS-CaCl 2 ([Na⁺]_o = 7 mmol/L), each of which contained 200 μmol/L ouabain. Afterward, 2 μCi/mL of⁴⁵CaCl ₂ was added and incubated for 15 minutes at 37°C. Net ⁴⁵Ca²⁺ influx into platelets under indicated condition ([Na⁺]_o and inhibitors) was determined by liquid scintillation counting after rapid filtration of cells on 0.45 μm filters. (B) Na⁺_i-dependent⁴⁵Ca²⁺ influx was calculated by subtracting⁴⁵Ca^{2 +} influx in HBS-CaCl₂ from that in Na⁺-free HBS-CaCl₂ in the presence of DCB (open circles) or bepridil (closed circles) and relative amount of Na⁺_i-dependent⁴⁵Ca²⁺ influx was normalized to a 100% value for that in the absence of inhibitors. Results are the mean ± SD from three separate experiments.