Attractant-induced changes and oscillations of the extracellular Ca++ concentration in suspensions of differentiating Dictyostelium cells.

Abstract

A Ca2+-sensitive electrode was used to measure changes in extracellular Ca2+ concentration in cell suspensions of D. discoideum during differentiation and attractant stimulation. The cells maintained an external level of 3-8 .mu.M Ca2+ until the beginning of aggregation, and then started to take up Ca2+. The attractants, folic acid, cAMP and cGMP, induced a transient uptake of Ca2+ by the cells. The response was detectable within 6 s and peaked at 30 s. Half-maximal uptake occurred at 5 nM cAMP or 0.2 .mu.M folic acid, respectively. The apparent rate of uptake amounted to 2 .times. 107 Ca2+/cell min-1. Following uptake, Ca2+ was released by the cells with a rate of 5 .times. 106 ions/cell min-1. Specificity studies indicated that the induced uptake of Ca2+ was mediated by cell surface receptors. The amount of accumulated Ca2+ remained constant as long as a constant stimulus was provided. No apparent adaptation occurred. The cAMP-induced uptake of Ca2+ increased during differentiation and was dependent on the external Ca2+ concentration. Saturation was above 10 .mu.M external Ca2+. The time course and magnitude of the attractant-induced uptake of external Ca2+ agree with a role of Ca2+ during contraction. During development, the extracellular Ca2+ level oscillated with a period of 6-11 min. The change of the extracellular Ca2+ concentration during 1 cycle would correspond to a 30-fold change of the cellular free Ca2+ concentration.