Two different store-operated Ca2+ entry pathways in MDCK cells

Whole cell patch clamp experiments in conjunction with Fura-2 fluorescence microscopy were performed to study the mechanisms of 'store-operated' (capacitative) Ca2+ entry. In MDCK cells, depletion of inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ stores activates a store-operated cation current (SOCC) predominantly selective for Ca2+ than for Na+ or Mn2+ [Delles C., Haller T., Dietl P. A highly calcium-selective cation current activated by intracellular calcium release in MDCK cells. J Physiol 1995; 486: 557-569]. In the presence of extracellular Ca2+, thapsigargin (TG) stimulated both SOCC and a Ca2+-dependent K+ current (I-K(Ca), reflecting stimulation of store-operated Ca2+ entry. The Ca2+ entry blocker 1-[3-(4-methoxyphenyl) propoxyl]-1-(4-methoxyphenyl)-ethyl-1H-imidazole HCl (SK & F96365; 30 mu M) did not inhibit SOCC. At the same concentration, it exerted a transient partial inhibition on I-K(Ca) activated by TG-induced Ca2+ entry. It did, however, not directly inhibit I-K(Ca). This was demonstrated by an unchanged relationship between the cytosolic Ca2+ concentration ([Ca2+](i)) and I-K(Ca) in experiments where [Ca2+](i) was measured under whole cell patch clamp conditions and by a lacking effect of SK&F96365 on I-K(Ca) prestimulated by a high 'clamped' [Ca2+](i). La3+ partially, but not directly, inhibited the TG-induced I-K(Ca) at a concentration (10 mu M) sufficient to entirely block SOCC. La3+ and SK&F96365 in combination exerted an additive reduction on the TG-induced whole cell conductance (G) and completely blocked I-K(Ca) stimulated by TG. We conclude that two Ca2+ entry pathways with different pharmacological and biophysical properties are involved in 'store-operated' Ca2+ entry in MDCK cells.