Identification of Store-independent and Store-operated Ca(2+) Conductances in Caenorhabditis elegans Intestinal Epithelial Cells

The nematode Caenorhabditis elegans offers significant experimental advantages for defining the genetic basis of diverse biological processes. Genetic and physiological analyses have demonstrated that inositol-1,4,5-trisphosphate (IP(3))–dependent Ca(2+) oscillations in intestinal epithelial cells play a central role in regulating the nematode defecation cycle, an ultradian rhythm with a periodicity of 45–50 s. Patch clamp studies combined with behavioral assays and forward and reverse genetic screening would provide a powerful approach for defining the molecular details of oscillatory Ca(2+) signaling. However, electrophysiological characterization of the intestinal epithelium has not been possible because of its relative inaccessibility. We developed primary intestinal epithelial cell cultures that circumvent this problem. Intestinal cells express two highly Ca(2+)-selective, voltage-independent conductances. One conductance, I(ORCa), is constitutively active, exhibits strong outward rectification, is 60–70-fold more selective for Ca(2+) than Na(+), is inhibited by intracellular Mg(2+) with a K(1/2) of 692 μM, and is insensitive to Ca(2+) store depletion. Inhibition of I(ORCa) with high intracellular Mg(2+) concentrations revealed the presence of a small amplitude conductance that was activated by passive depletion of intracellular Ca(2+) stores. Active depletion of Ca(2+) stores with IP(3) or ionomycin increased the rate of current activation ∼8- and ∼22-fold compared with passive store depletion. The store-operated conductance, I(SOC), exhibits strong inward rectification, and the channel is highly selective for Ca(2+) over monovalent cations with a divalent cation selectivity sequence of Ca(2+) > Ba(2+) ≈ Sr(2+). Reversal potentials for I(SOC) could not be detected accurately between 0 and +80 mV, suggesting that P(Ca)/P(Na) of the channel may exceed 1,000:1. Lanthanum, SKF 96365, and 2-APB inhibit both I(ORCa) and I(SOC) reversibly. Our studies provide the first detailed electrophysiological characterization of voltage-independent Ca(2+) conductances in C. elegans and form the foundation for ongoing genetic and molecular studies aimed at identifying the genes that encode the intestinal cell channels, for defining mechanisms of channel regulation and for defining their roles in oscillatory Ca(2+) signaling.