Intracellular Calcium Responses to External Calcium Stimuli in <i>Dictyostelium</i>

Intracellular calcium (Ca²⁺) plays a vital role in numerous cellular processes, including signal transduction, cell motility, and development. Despite extensive research on intracellular calcium dynamics, the specific mechanisms by which extracellular calcium influences intracellular calcium levels remain unclear. In this study, we generated a <i>Dictyostelium</i>-specific plasmid expressing the calcium biosensor GCaMP3 and visualized real-time intracellular calcium fluctuations in live cells. In response to external calcium, vegetative cells displayed a rapid and transient increase in intracellular calcium levels, peaking within 20 s, whereas aggregation-competent cells showed no such response. <i>Gβ</i>-null cells showed a fast and slight induction at approximately 5 s, with a slower and more efficient, extended high induction at 30 s. Meanwhile, <i>gca/sgc</i>-null cells showed a slightly delayed response, and a significantly lower proportion of <i>iplA</i>-null cells responded to external calcium stimuli compared to wild-type cells. In the presence of W7, a calmodulin antagonist that inhibits calcium release from acidic stores, cells exhibited a marked reduction in the major calcium induction peak at 20 s post-stimulation. Our results suggest that there are at least two pathways to increase the intracellular calcium level in response to external calcium stimuli, and that intracellular acidic calcium stores partially contribute to the second major peak of calcium induction following external stimuli. It appears that G proteins, cGMP, and IplA are involved in calcium homeostasis upon external calcium stimulation, and play an important role in modulating the timing and amplitude of calcium responses.