Calcium stores in differentiated Dictyostelium discoideum: prespore cells sequester calcium more efficiently than prestalk cells.

Dictyostelium discoideum pseudoplasmodia exhibit a gradient of the cytosolic free Ca2+-concentration ([Ca2+]i) along their anterior-posterior axis involved in cell-type specific differentiation. [Ca2+]i is high in prestalk and low in prespore cells. We determined the content and localization of calcium and other elements in cryosectioned cells of pseudoplasmodia and fruiting bodies by X-ray microanalysis. Granular stores rich in Ca, Mg and P were identified. Average Ca was higher in prespore than prestalk granules (225vs 111 mmol/kg dry weight). Total Ca stored in granules was also higher in prespore than prestalk cells. The amount of P and S in granules differed between the two cell types indicating different store composition. In spores mean granular Ca was 120 mmol/kg dry weight. Stalk cells had smaller granules with 360 mmol Ca/kg dry weight. Complementary to microanalysis, vesicular Ca2+-fluxes were studied in fractionated cell homogenates. The rate of Ca2+-uptake was higher in pellet fractions of prespore than prestalk amoebae (4.7 vs 3.4 nmol/min x mg). Ca2+-release was greater in supernatant fractions from prestalk than prespore cells (16.5vs 7.7 nmol/10(8)cells). In summary, prestalk and prespore cells possess qualitatively different, high-capacity stores containing distinct amounts of Ca and probably being involved in regulation of the anterior-posterior [Ca2+]i-gradient.