cGMP potentiates receptor-stimulated Ca2+ influx in Dictyostelium discoideum

Binding of extracellular cAMP to surface receptors induces at least two responses in Dictyostelium discoideum, the G-protein-dependent activation of guanylyl cyclase, and the opening of a plasma membrane Ca2+ channel. Some experiments suggest that intracellular cGMP opens the Ca2+ channel, while others demonstrate that the channel can open in the absence of functional G-proteins (and thus in the absence of cGMP formation). We have analysed 45Ca2+ uptake in three mutants with altered cGMP formation. Mutant stmF shows a prolonged cGMP response due to deletion of an intracellular phosphodiesterase. Uptake of receptor-stimulated 45Ca2+ is enhanced about two-fold in this mutant if compared to wild-type cells, suggesting that cGMP regulates the opening of the channel. Mutant KI-7 has very low levels of surface cAMP receptors, but nevertheless an enhanced receptor-stimulated cGMP response due to a defect in the turn-off of guanylyl cyclase. This mutant shows poor receptor-stimulated 45Ca2+ uptake, suggesting that cGMP alone is not sufficient to open the Ca2+ channel. Finally, mutant KI-8 has no cGMP due to the absence of nearly all guanylyl cyclase activity. The mutant shows significant but reduced 45Ca2+ uptake (19% of wild-type; 60% if corrected for the reduced level of surface cAMP receptors), suggesting that the channel can open in the absence of cGMP. Taken together, the results demonstrate that receptor-stimulated Ca2+ influx is not directly induced by cGMP formation; it can occur in the absence of cGMP, but is potentiated two- to four-fold by cGMP.