Noradrenaline-activated heparin-sensitive Ca2+ entry after depletion of intracellular Ca2+ store in portal vein smooth muscle cells.

Single portal vein smooth muscle cells were voltage-clamped using the whole cell patch-clamp technique. Intracellular free Ca2+ concentration ([Ca2+]i) was simultaneously monitored using the fluorescence from the dye indo-1. Noradrenaline (NA, 10 microM) evoked a transient increase in [Ca2+]i, due to inositol 1,4,5-trisphosphate (InP3)-induced Ca2+ release, followed by a sustained increase in [Ca2+]i, caused by extracellular Ca2+ entry. This phase was maintained as long as NA was present and was never observed when the agonist was only briefly applied (3 s). Neither ryanodine, nor caffeine produced the Ca2+ influx whereas the complete depletion of intracellular Ca2+ pool in the absence of external Ca2+ allowed, when Ca2+ was readmitted, the activation of the Ca2+ entry which was used to replenish Ca2+ store. During NA stimulation, the Ca2+ entry was activated even if the Ca2+ pool had not been totally emptied. The Ca2+ entry pathway involved was blocked by Ni2+ and Mn2+, was not permeable to these ions, and was more sensitive to heparin than the InsP3-induced Ca2+ release. Thus, the complete depletion of Ca2+ store activates a Ca2+ influx which is modulated by NA such as in its presence, a partial depletion is enough to induce Ca2+ entry.