THE EFFECT OF THE HEAT-SHOCK RESPONSE ON ULTRASTRUCTURE OF THE CENTROSOME OF DROSOPHILA CULTURED-CELLS IN INTERPHASE - POSSIBLE RELATION WITH CHANGES IN THE CHEMICAL-STATE OF CALCIUM

Previous observations have shown that the heat shock response affects the centrosome function. We compared the ultrastructural organization of the centrosome in control (23 degrees C) and heat-shocked (37 degre es C, 50 min) interphase Drosophila cells to detect the nature of the lesions that could alter this organelle. The centrosome apparatus show ed only minor modifications after the stress and the architecture of t he centrioles appeared unaffected. The main difference concerned the o rganization of pericentriolar material which appeared more condensed a nd clotted. In extreme cases this material seemed to collapse on the c entrioles. Recent reports proposed that Ca2+ concentrations could modi fy the distribution of pericentriolar material. In this study, we meas ured the changes in total and bound calcium in control or heat-shocked cell samples. The hyperthermia stress induced an increase of about 80 % in global calcium. However, there was a decrease of about 50% in bou nd calcium. A heat shock stress seemed therefore to promote a change f rom the bound to the free state for a noticeable proportion of the ele ment. As a preliminary hypothesis, these changes in the chemical state of calcium could be related to alterations in the pericentriolar mate rial and thus with the functional inactivation of the centrosome. This view is also supported by calcium analysis on early Drosophila embryo s. Contrary to cultured cells, Drosophila embryos did not present a st ress inactivation of centrosomes. Equally, a heat shock did not distur b the bound calcium level in embryos.