Role of [Ca2+]i in "Ca2+ stores depletion-Ca2+ entry coupling' in fibroblasts expressing the rat neurotensin receptor.

1. Transfected Chinese hamster ovary fibroblasts expressing the rat neurotensin receptor were used to study the 'Ca2+ stores depletion-Ca2+ entry coupling' which follows stimulation with neurotensin and liberation of InsP3. 2. This coupling could be dissociated in time. Firstly, stores depletion was produced by neurotensin or thapsigargin which caused a first [Ca2+]i transient in a Ca(2+)-free external medium. Secondly, readmission of external Ca2+ produced an influx of Ca2+ and a second [Ca2+]i transient. 3. Various concentrations of thapsigargin (20 nM to 1 microM) were used to produce complete stores depletion with small or large first peaks of [Ca2+]i. Upon return to external Ca2+, small or large second [Ca2+]i peaks were observed. The amplitudes of both peaks were positively correlated. 4. The Ca2+ entry which followed stores depletion could occur at very low basal values of [Ca2+]i, was accelerated by okadaic acid and inhibited by staurosporine and the calmodulin antagonist W-7. 5. It is concluded that the rise in [Ca2+]i during Ca2+ stores depletion is an essential parameter which determines the size of the subsequent Ca2+ entry.