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Journal of Cell Science, Vol 90, Issue 4 691-700, Copyright © 1988 by Company of Biologists

JOURNAL ARTICLES

Effects of suboptimal levels of extracellular calcium on the regulation of the cyclic AMP phosphodiesterase-inhibitor system and membrane differentiation in Dictyostelium discoideum

MB Coukell and AM Cameron
Department of Biology, York University, Ontario, Canada.

When starved wild-type amoebae of Dictyostelium discoideum were washed and incubated in 1 mM-EGTA, their ability to induce soluble cyclic AMP phosphodiesterase (PD) activity in response to either millimolar cyclic AMP or a series of nanomolar cyclic AMP pulses was reduced by 55-75%. Supplementation of EGTA-treated cells with exogenous Ca2+ stimulated PD induction in a dose-dependent fashion (EC50 = 100-200 nM free extracellular Ca2+), and enzyme production was maximal at about 1 microM free Ca2+. Ca2+ depletion also strongly impaired production of the phosphodiesterase inhibitor (PDI). In contrast, other than delaying their appearance by about 1 h, EGTA had little effect on the induction by cyclic AMP pulses of cell surface markers such as contact sites A and membrane-bound PD activity. Similar changes in both the soluble and membrane activities were observed with strain NP368, a mutant that overproduces cyclic GMP when stimulated by cyclic AMP. Thus, Ca2+ depletion does not appear to inhibit PD and PDI production by reducing intracellular cyclic GMP. To determine whether Ca2+ depletion alters signal transduction, two mutants that produce the soluble PD activities constitutively were examined. Suboptimal concentrations of free extracellular Ca2+ were found to inhibit PD production in these cells to the same degree and with the same concentration dependence as low Ca2+ inhibited PD induction by cyclic AMP in wild-type cells. These results suggest that Ca2+ depletion by EGTA probably inhibits PD and PDI production indirectly by perturbing an intracellular Ca2+ pool(s) rather than by altering a surface cyclic AMP-receptor-mediated process.


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M. B. Coukell, A. M. Cameron, and N. R. Adames
Involvement of intracellular calcium in protein secretion in Dictyostelium discoideum
J. Cell Sci., October 1, 1992; 103(2): 371 - 380.
[Abstract] [PDF]


© The Company of Biologists Ltd 1988