The role of intracellular Ca2+ in the regulation of the plasma membrane Ca2+ permeability of unstimulated rat lymphocytes.

The mechanism responsible for the increase in cytosolic free Ca2+ concentration ([Ca2+]i) during mitogenic stimulation of lymphocytes has been widely investigated. By contrast, little is known about the processes underlying Ca2+i homeostasis in resting (unstimulated) cells. It has been suggested that [Ca2+]i is an important determinant of the rate of Ca2+ influx following mitogenic activation. Using rat thymic lymphocytes, we investigated whether the resting influx pathway is similarly controlled by [Ca2+]i. Otherwise untreated cells were Ca(2+)-depleted by loading with Ca2+ chelators while suspended in Ca(2+)-free solution. Ca2+ depletion induced an 8-fold increase in the rate of unidirectional Ca2+ uptake. The depletion-activated flux was voltage-sensitive and was blocked by La3+ and by compound SK&F 96365, a receptor-operated Ca2+ channel blocker. Upon reintroduction to Ca(2+)-containing solution, the increased influx brought about a rapid recovery of [Ca2+]i. Detailed analysis of the magnitude of the 45Ca2+ flux during this recovery indicated that [Ca2+]i is not the primary determinant of the plasmalemmal Ca2+ permeability. Instead, depletion of an internal thapsigargin-sensitive store correlates with and appears to be responsible for the increased permeability of the plasma membrane. Accordingly, the Ca2+ fluxes induced by intracellular Ca2+ depletion and by thapsigargin were pharmacologically indistinguishable. Mitogenic lectins also released Ca2+ from a thapsigargin-sensitive store and activated a plasmalemmal Ca2+ permeability displaying identical pharmacology. The data support the existence of a coupling process whereby the degree of filling of an internal Ca2+ store dictates the Ca2+ permeability of the plasma membrane. This coupling mechanism is important not only in mediating the effects of mitogens and other agonists, as suggested before, but seemingly also in the control of resting Ca2+i homeostasis in unstimulated cells.