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

1. Effects of inositol-l,4,5-trisphosphate (InsP₃) applied through a patch pipette to Xenopus laevis olfactory receptor cells (ORCs) were studied using die patch-clamp technique in conjunction with calcium imaging with fura-2. 2. InsP₃ activated, first, a novel voltage-independent Ca²⁺ current (I(Ca)) and, second, a nonselective cation current (I(cat)). 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 (I(ca)) and a nonselective cation conductance (I(cat)).