Regenerative and non-regenerative calcium transients in hamster eggs triggered by inositol 1,4,5-trisphosphate.
1. Inositol 1,4,5-trisphosphate (InsP3) 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 InsP3 gave a single HR above a threshold value of 0.3 nM. At 5 nM and above, InsP3 induced HRs with no detectable latency. At concentrations between these two values a latency was observed. The amplitude of the HR was independent of InsP3 concentration. 3. A second HR could be elicited by injection of InsP3, but five times more InsP3 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 InsP3 could be resolved into two distinct components: a slow, early rise immediately after InsP3 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 InsP3 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 InsP3 concentrations gave slow, larger Ca2+ transients (and HRs) with no detectable latency. 6. If large amounts of InsP3 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 InsP3. 7. InsP3 can induce regenerative, all-or-none [Ca2+]i increase (CICR) in hamster eggs, often with a long latency, as well as non-regenerative increases. InsP3 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.
Galione
A
A
Department of Physiology, University College London, UK.
Swann
K
K
Georgiou
P
P
Whitaker
M
M
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D006224:Cricetinae / Q000378:metabolism* / Q000502:physiology; D004594:Electrophysiology; D005260:Female; D016257:Fura-2; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D009994:Osmolar Concentration; D010063:Ovum / Q000378:metabolism* / Q000502:physiology; D011930:Reaction Time
D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium; D016257:Fura-2