Inhibition of Receptor-Stimulated Guanylyl Cyclase by Intracellular Calcium Ions in Dictyostelium Cells

In Dictyostelium discoideum extracellular cAMP stimulates guanylyl cyclase and phospholipase C; the latter enzyme produces Ins(1,4,5)P3 which releases Ca2+ from internal stores. The following data indicate that intracellular Ca2+- ions inhibit guanylyl cyclase activity. 1) In vitro, Ca2+ inhibits guanylyl cyclase with IC50=41 nM Ca2+ and Hill-coefficient of 2.1. 2) Extracellular Ca2+ does not affect basal cGMP levels of intact cells. In electro-permeabilized cells, however, cGMP levels are reduced by 85% within 45 s after addition of 10-6 M Ca2+ to the medium; halfmaximal reduction occurs at 200 nM extracellular Ca2+. 3) Receptor-stimulated activation of guanylyl cyclase in electro-permeabilized cells is also inhibited by extracellular Ca2+ with half-maximal effect at 200 nM Ca2+. 4) In several mutants an inverse correlation exists between receptor-stimulated Ins(1,4,5)P3 production and cGMP formation. We conclude that receptor-stimulated cytosolic Ca2+ elevation is a negative regulator of receptor-stimulated guanylyl cyclase