Calcium feedback mechanisms regulate oscillatory activity of a TRP‐like Ca 2+ conductance in C. elegans intestinal cells

Inositol‐1,4,5‐trisphosphate (IP 3 )‐dependent Ca 2+ oscillations in Caenorhabditis elegans intestinal epithelial cells regulate the nematode defecation cycle. The role of plasma membrane ion channels in intestinal cell oscillatory Ca 2+ signalling is unknown. We have shown previously that cultured intestinal cells express a Ca 2+ ‐selective conductance, I ORCa , that is biophysically similar to TRPM7 currents. I ORCa activates slowly and stabilizes when cells are patch clamped with pipette solutions containing 10 m m BAPTA and free Ca 2+ concentrations of ∼17 n m . However, when BAPTA concentration is lowered to 1 m m , I ORCa oscillates. Oscillations in channel activity induced simultaneous oscillations in cytoplasmic Ca 2+ levels. Removal of extracellular Ca 2+ inhibited I ORCa oscillations, whereas readdition of Ca 2+ to the bath caused a rapid and transient reactivation of the current. Experimental manoeuvres that elevated intracellular Ca 2+ blocked current oscillations. Elevation of intracellular Ca 2+ in the presence of 10 m m BAPTA to block I ORCa oscillations led to a dose‐dependent increase in the rate of current activation. At intracellular Ca 2+ concentrations of 250 n m , current activation was transient. Patch pipette solutions buffered with 1–4 m m of either BAPTA or EGTA gave rise to similar patterns of I ORCa oscillations. We conclude that changes in Ca 2+ concentration close to the intracellular opening of the channel pore regulate channel activity. Low concentrations of Ca 2+ activate the channel. As Ca 2+ enters and accumulates near the pore mouth, channel activity is inhibited. Oscillating plasma membrane Ca 2+ entry may play a role in generating intracellular Ca 2+ oscillations that regulate the C. elegans defecation rhythm.