Inositol trisphosphate analogues induce different oscillatory patterns in Xenopus oocytes.

Agonists that utilize the calcium-mobilizing second messenger inositol(1,4,5)trisphosphate Ins(1,4,5)P3 usually generate oscillations in intracellular calcium. Such oscillations, based on the periodic release of calcium from the endoplasmic reticulum, can also be induced by injecting cells with Ins(1,4,5)P3. The mechanism responsible for oscillatory activity was studied in Xenopus oocytes by injecting them with different inositol trisphosphates. The plasma membrane of Xenopus oocytes has calcium-dependent chloride channels that open in response to calcium, leading to membrane depolarization. Oscillations in calcium were thus monitored by recording membrane potential. The naturally occurring Ins(1,4,5)P3 produced a large initial transient followed by a single transient or a burst of oscillations. By contrast, two analogues (Ins(2,4,5)P3 and Ins(1,4,5)P(S)3) produced a different oscillatory pattern made up of a short burst of sharp transients. Ins(1,3,4,5)P4 had no effect when injected by itself, and it also failed to modify the oscillatory responses to either Ins(2,4,5)P3 or Ins(1,4,5)P(S)3. Both analogues failed to induce a response when injected immediately after the initial Ins(1,4,5)P3-induced response, indicating that they act on the same intracellular pool of calcium. The existence of different oscillatory patterns suggests that there may be different mechanisms for setting up calcium oscillations. The Ins(2,4,5)P3 and Ins(1,4,5)P(S)3 analogues may initiate oscillations through a negative feedback mechanism whereby calcium inhibits its own release. The two-pool model is the most likely mechanism to describe the Ins(1,4,5)P3-induced oscillations.