A central question concerning vision is the identity of the biochemical pathway that underlies phototransduction. The large size of the ventral photoreceptors of Limulus polyphemus renders them a favourite preparation for investigating this problem. The fact that a single photon opens approximately 1,000 ionic channels in these photoreceptors suggests the need for an internal transmitter. We have investigated whether inositol 1,4,5-trisphosphate (InsP3) functions as such an internal transmitter, given that InsP3 may act as an intracellular messenger in other cellular processes. Here we report that in Limulus, intracellular pressure injection of InsP3 both excites and adapts ventral photoreceptors in a manner similar to light.
InsP3 has two important functions in generating Ca2+ oscillations. It releases Ca2+ from the internal store and it can contribute to Ca2+ entry. A hypothesis has been developed to describe a mechanism for Ca2+ oscillations with particular emphasis on the way agonist concentration regulates oscillator frequency. The main idea is that the InsP3 receptors are sensitized to release Ca2+ periodically by cyclical fluctuations of Ca2+ within the lumen of the endoplasmic reticulum. Each time a pulse of Ca2+ is released, the luminal level of Ca2+ declines and has to be replenished before the InsP3 receptors are resensitized to deliver the next pulse of Ca2+. It is this loading of the internal store that explains why frequency is sensitive to external Ca2+ and may also account for how variations in agonist concentration are translated into changes in oscillation frequency. Variations in agonist-induced entry of external Ca2+, which can occur through different mechanisms, determine the variable rates of store loading responsible for adjusting the sensitivity of the InsP3 receptors to produce the periodic pulses of Ca2+. The Ca2+ oscillator is an effective analogue-to-digital converter in that variations in the concentration of the external stimulus are translated into a change in oscillator frequency.
1. The preparation of isolated Malpighian tubules is described. The rate of urine flow increases with increasing serum dilution, and vice versa. Urine is almost isotonic with haemolymph over a wide range of osmotic pressure. 2. Serum collected from 3-day-old insects promotes urine formation, whereas that from 6-day-old insects does not. 3. A factor which was extracted from the m.n.c. accelerates the rate of urine flow, from a normal value of 0·87 to 3·1 mm.3 x 10- 3/min. The osmotic pressure of the urine, however, remains unchanged. 4. The hormone concentration of different parts of the nervous system was assayed with these isolated tubules. Most activity occurs in the m.n.c., but some activity is present in extracts from the c.c. and the fused ganglionic mass in the mesothorax. 5. Malpighian tubules isolated from 6-day-old insects remain inactive, but after the addition of hormone they immediately begin to produce urine. 6. These observations have been incorporated into a tentative hypothesis on the control of excretion in Dysdercus.
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