Inositol trisphosphate activates a voltage-dependent calcium influx in Xenopus oocytes

Abstract

Injection of inositol trisphosphate (IP$_{3}$) into oocytes of Xenopus laevis induces the appearance of a transient inward (T$_{\text{in}}$) current on hyperpolarization of the membrane. This current is carried largely by chloride ions, but is shown to depend on extracellular calcium, because it is abolished by removal of calcium in the bathing fluid or by addition of manganese. Recordings with aequorin as an intracellular calcium indicator show that a calcium influx is activated by hyperpolarization after intracellular injection of IP$_{3}$ as well as after activation of neurotransmitter receptors thought to mediate a rise in IP$_{3}$. Furthermore, by substituting barium for calcium in the bathing solution, inward barium currents can be recorded during hyperpolarization. We conclude that intracellular IP$_{3}$ modulates the activity of a class of calcium channels, so as to allow an influx of calcium on hyperpolarization. In normal Ringer solution this then leads to the generation of a chloride current, because of the large numbers of calcium-dependent chloride channels in the oocyte membrane.