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

Dictyostelium discoideum pseudoplasmodia exhibit a gradient of the cytosoli c free Ca2+-concentration ([Ca2+](i)) along their anterior-posterior axis i nvolved in cell-type specific differentiation. [Ca2+](i) is high in prestal k and low in prespore cells. We determined the content and localization of calcium and other elements in cryosectioned cells of pseudoplasmodia and fr uiting bodies by X-ray microanalysis. Granular stores rich in Ca, Mg and P were identified. Average Ca was higher in prespore than prestalk granules ( 225 vs 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 differe d between the two cell types indicating different store composition. In spo res mean granular Ca was 120 mmol/kg dry weight. Stalk cells had smaller gr anules with 360 mmol Ca/kg dry weight. Complementary to microanalysis, vesi cular Ca2+-fluxes were studied in fractionated cell homogenates. The rate o f Ca2+-uptake was higher in pellet fractions of prespore than prestalk amoe bae (4.7 vs 3.4 nmol/min x mg), Ca2+-release was greater in supernatant fra ctions from prestalk than prespore cells (16.5 vs 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 invol ved in regulation of the anterior-posterior [Ca2+](i)-gradient, (C) 2001 Ha rcourt Publishers Ltd.