REGENERATIVE AND NON-REGENERATIVE CALCIUM TRANSIENTS IN HAMSTER EGGS TRIGGERED BY INOSITOL 1,4,5-TRISPHOSPHATE

1. Inositol 1,4,5-trisphosphate (InsP(3)) injected into unfertilized golden hamster eggs elicits a hyperpolarizing response (HR) that is due to stimulation of calcium-activated potassium channels in the egg plasma membrane. 2. A single injection of InsP(3) gave a single HR above a threshold value of 0.3 nM. At 5 nM and above, InsP(3) induced HRs with no detectable latency. At concentrations between these two values a latency was observed. The amplitude of the HR was independent of InsP(3) concentration. 3. A second HR could be elicited by injection of InsP(3), but five times more InsP(3) was required to trigger a second HR, and 10-100 times more to give an HR of similar magnitude to the first, and there was no latency. 4. The increase in [Ca2+](i) in response to an initial injection of 1 nM InsP(3) could be resolved into two distinct components: a slow, early rise immediately after InsP(3) injection (phase I) followed by a larger and more rapid increase (phase II). The initiation of an HR coincided with the second component of the [Ca2+](i) increase. 5. Further injection of InsP(3) resulted only in slow, smaller increases in [Ca2+](i) that resembled phase I and often did not cause an HR. Phase II appeared to be absent. However, 100-fold greater InsP(3) concentrations gave slow, larger Ca2+ transients (and HRs) with no detectable latency. 6. If large amounts of InsP(3) were allowed to leak into the eggs constantly from a pipette, repetitive calcium transients were seen. Unlike the sustained repetitive responses seen at fertilization, they were often smaller than the initial transient and less well sustained. However, a subsequent transient could still be elicited on injection of very large concentrations of InsP(3). 7. InsP(3) can induce regenerative, all-or-none [Ca2+](i) increase (CICR) in hamster eggs, often with a long latency, as well as non-regenerative increases. InsP(3) injections desensitize CICR and cannot mimic all the features of Ca2+ signalling at fertilization in the hamster egg, in particular, the sensitization of CICR caused by the sperm.