Calcium feedback mechanisms regulate oscillatory activity of a TRP-like Ca²⁺ conductance in C. elegans intestinal cells

Abstract

Inositol-1,4,5-trisphosphate (IP₃)-dependent Ca²⁺ oscillations in Caenorhabditis elegans intestinal epithelial cells regulate the nematode defecation cycle. The role of plasma membrane ion channels in intestinal cell oscillatory Ca²⁺ signalling is unknown. We have shown previously that cultured intestinal cells express a Ca²⁺-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 mm BAPTA and free Ca²⁺ concentrations of ∼17 nm. However, when BAPTA concentration is lowered to 1 mm, I_ORCa oscillates. Oscillations in channel activity induced simultaneous oscillations in cytoplasmic Ca²⁺ levels. Removal of extracellular Ca²⁺ inhibited I_ORCa oscillations, whereas readdition of Ca²⁺ to the bath caused a rapid and transient reactivation of the current. Experimental manoeuvres that elevated intracellular Ca²⁺ blocked current oscillations. Elevation of intracellular Ca²⁺ in the presence of 10 mm BAPTA to block I_ORCa oscillations led to a dose-dependent increase in the rate of current activation. At intracellular Ca²⁺ concentrations of 250 nm, current activation was transient. Patch pipette solutions buffered with 1–4 mm of either BAPTA or EGTA gave rise to similar patterns of I_ORCa oscillations. We conclude that changes in Ca²⁺ concentration close to the intracellular opening of the channel pore regulate channel activity. Low concentrations of Ca²⁺ activate the channel. As Ca²⁺ enters and accumulates near the pore mouth, channel activity is inhibited. Oscillating plasma membrane Ca²⁺ entry may play a role in generating intracellular Ca²⁺ oscillations that regulate the C. elegans defecation rhythm.