Latencies of membrane currents evoked in Xenopus oocytes by receptor activation, inositol trisphosphate and calcium.

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Abstract
1. Application of serum to Xenopus oocytes elicits an oscillatory chloride membrane current, which begins after a latency of several seconds or minutes, and is mediated through a phosphoinositide-calcium signalling pathway. We studied the characteristics and origin of this latency in voltage-clamped oocytes. 2. Bath application of low doses of serum evoked responses beginning after latencies of 1 min or more. The latency decreased with increasing dose and reached a minimal value of several seconds that did not decrease with further increases in serum concentration. Experiments to study this minimal latency were done by applying brief ''puffs'' of serum and other agonists at high concentrations from a local extracellular pipette. 3. The mean latency of the response evoked by local serum application was about 7 s (at 22-24.degree. C), but individual responses showed a wide variation, from 2 s to over 20 s. Diffusion of serum from the pipette tip to the membrane did not contribute appreciably to this delay, since short (< 100 ms) delays were obtained when KCl was applied in the same way. 4. Currents evoked by acetylcholine and serotonin, in oocytes induced to acquire muscarinic and serotonergic receptors following injection of brain messenger RNA, began following latencies similar to that of the serum response. 5. The response latency was shorter when serum was applied to the vegetal rather than the animal hemisphere of the oocyte, even though smaller currents were obtained. 6. The latency showed a slight dependence upon membrane potential, becoming shorter with depolarization. 7. Cooling to temperatures below about 22.degree. C produced a striking lengthening of the delay, corresponding to a Q10 of about 5. In contrast, above 22.degree. C the temperature dependence was slight, with a Q10 of about 1.25. 8. Intracellular injections of calcium and inositol 1,4,5-trisphosphate (IP3) evoked chloride currents with short (a few tens of milliseconds) latency. Short (100 ms) latency responses were also evoked when intracellularly loaded caged IP3 was photolysed by strong illumination, but weak illumination gave responses with latencies of over 1 min. 9. Measurements of intracellular free calcium, made with Fura-2 and Indo-1, showed an increase following serum application beginning coincident with the onset of the membrane current response. 10. Generation of the chloride current responses to serum, acetylcholine and serotonin is thought to involve the following sequential stages: receptor binding, activation of a G protein, activation of a phosphoinositidase enzyme which produces IP3, liberation of calcium from intracellular stores by IP3 and, finally, the opening of calcium-dependent chloride membrane channels. Our results indicate that, at high agonist concentrations, the stages betweeen receptor binding and IP3 liberation are rate-limiting and account for almost all of the response latency. However, at low agonist concentrations the latency becomes much longer, probably because an additional delay is introduced because intracellular levels of IP3 rise slowly towards a threshold required before calcium release is triggered.