Store-operated Ca2+ channels in human glomerular mesangial cells

Experiments were performed to identify the biophysical properties of store-operated Ca2+ channels (SOC) in cultured human glomerular mesangial cells (MC). A fluorometric technique (fura 2) was utilized to monitor the change in intracellular calcium concentration ([Ca2+]i) evoked by elevating external [Ca2+] from 10 nM to 1 mM (Δ[Ca2+]). Under control conditions, Δ[Ca2+] averaged 6 nM and was unaffected by elevating bath [K+]. After treatment with 1 μM thapsigargin to deplete the intracellular Ca2+ store, the change in [Ca2+]i(Δ[Ca2+]th) averaged 147 ± 16 nM. In thapsigargin-treated MC studied under depolarizing conditions (75 mM bath K+), Δ[Ca2+]th was 45 ± 7 nM. The Δ[Ca2+]th response of thapsigargin-treated cells was inhibited by La3+(IC50 = 335 nM) but was unaffected by 5 μM Cd2+. In patch clamp studies, inward currents were observed in cell-attached patches with either 90 mM Ba2+ or Ca2+ in the pipette and 140 mM KCl in the bathing solution. The single-channel conductance was 2.1 pS with Ba2+ and 0.7 pS with Ca2+. The estimated selectivities were Ca2+ > Ba2+ >> K+. These channels were sensitive to 2 μM La3+, insensitive to 5 μM Cd2+, and voltage independent, with an average channel activity ( NP o) of 1.02 at command potential (− V p) ranging from 0 to −80 mV. In summary, MC exhibited an electrogenic Ca2+ influx pathway that is suggestive of Ca2+entry through SOC, as well as a small-conductance divalent-selective channel displaying biophysical properties consistent with SOC. Based on estimates of whole cell Ca2+ influx derived from our data, we conclude that SOC with low single-channel conductance must be highly abundant in MC to allow significant capacitative Ca2+ entry in response to depletion of the intracellular store.