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.
Kuwayama
H
H
Department of Biochemistry, University of Groningen, The Netherlands.
van Haastert
P J
PJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002132:Calcium Radioisotopes; D002633:Chemotaxis; D000242:Cyclic AMP / Q000378:metabolism / Q000494:pharmacology*; D006152:Cyclic GMP / Q000378:metabolism* / Q000494:pharmacology*; D004023:Dictyostelium / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D006162:Guanylate Cyclase / Q000172:deficiency / Q000235:genetics; D007700:Kinetics; D008954:Models, Biological; D011953:Receptors, Cyclic AMP / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction
D015220:Calcium Channels; D002132:Calcium Radioisotopes; D011953:Receptors, Cyclic AMP; D000242:Cyclic AMP; D006162:Guanylate Cyclase; D006152:Cyclic GMP; D002118:Calcium