InsP3 causes an increase in apical [Ca2+]i by activating two distinct current components in vertebrate olfactory receptor cells.

1. Effects of inositol-1,4,5-trisphosphate (InsP3) applied through a patch pipette to Xenopus laevis olfactory receptor cells (ORCs) were studied using the patch-clamp technique in conjunction with calcium imaging with fura-2. 2. InsP3 activated, first, a novel voltage-independent Ca2+ current (ICa) and, second, a nonselective cation current (Icat). 3. The activation of these currents occurred at different intracellular calcium concentrations, and the activation of either current led to a marked increase of [Ca2+]i in the dendritic knob. 4. The results suggest that InsP3 might act as a second messenger in vertebrate olfactory receptor cells by activating, through different mechanisms, a plasma membrane Ca2+ conductance (ICa) and a nonselective cation conductance (Icat).