Evidence for Na+/Ca2+ exchange in isolated smooth muscle cells: a fura-2 study

Abstract

Isolated smooth muscle cells (SMC) from guinea pig taenia coli were employed. Suspension of cells were externally loaded in saline with the fluorescent calcium indicators quin-2/AM or fura-2/AM at 20–40 μM or 4 μM respectively, resulting in an estimated intracellular concentration of 100–200 μM for quin-2 or 10–20 μM fura-2 (free acid). On addition of 100 μM carbachol or high K _o ⁺ (80 mM) depolarization, fura-2 loaded cells contracted (104±47 μm,n=121 rest: 39±13 μm,n=59 contracted) identically to control (103±35 μm,n=232 rest: 39±16 μm,n=89 contracted) cells, whereas quin-2 loaded cells were unresponsive to these protocols and there was no significant length change. The Ca _i ²⁺ of fura-2 loaded cells was 100±18 nM (mean±SD,n=15) and was not significantly different from quin-2 loaded cells 107±26 nM (n=13). Treatment of fura-2 loaded cells with 100 μM ouabain saline for 10–60 min progressively elevated the Ca _i ²⁺ to a mean of 266±83 nM (n=15). Reduction of Na _p ⁺ (96% Li⁺ replaced) significantly increased Ca _i ²⁺ to 317±77 nM (n=8). After pretreatment with ouabain (100 μM), Na _o ⁺ replacement (Li⁺) increased Ca _i ²⁺ at a significantly faster rate [3.6 nM min⁻¹ (control) cf. 19.8 nM min⁻¹ (ouabain)].