Sláma Karel, Lukáš Jan
Laboratory of Insect Physiology, Intereco, Evropská, Praha, Czech Republic.
Crop Research Institute, Drnovská, Praha, Czech Republic.
Int J Insect Sci. 2016 Oct 19;8:81-93. doi: 10.4137/IJIS.S40566. eCollection 2016.
The physiological and biochemical mechanisms that enable insects to feed on dry food to secure enough water for larval growth were investigated. The study was carried out with a plethora of physiological methods, ranging from the simple volumetric determination of O consumption and water intake to more advanced methods such as scanning microrespirography and thermovision imaging of insect's body temperature. The experiments were done on the European firebug, , which feeds exclusively on dry linden seeds. In order to survive, it needs to drink water or suck a sap from plants occasionally. It was found that the young larval instars compensate the occasional water deficiency by the increased production of metabolic water. The juvenile hormone (JH)-dependent production of metabolic water, which was previously found in other species consuming dry food, was achieved in by total metabolic combustion of the dietary lipid (neutral seed oil). The water-producing, hypermetabolic larvae were heated from inside by endothermic energy released from the uncoupling of oxidation from oxidative phosphorylation. The "warm", hypermetabolic larvae burning the dietary oil into CO and water showed the increased rates of respiratory metabolism. Microrespirographic recording of these larvae revealed the ratio of the respiratory quotient (RQ, CO/O) of 0.7, which indicated the breakdown of a pure triglyceride. The warm hypermetabolic larvae could be easily spotted and distinguished from the "cold" larvae on the screen of a thermovision camera. The last instar larvae lacking the JH were always only cold. They metabolized a carbohydrate substrate exclusively (RQ = 1.0), while the dietary lipid was stored in the fat body. In comparison with the hypermetabolic larvae of some other species fed on dry food, which exhibited the highest rates of O consumption ever recorded in a living organism (10-20 mL O/g per hour), the metabolic difference between the warm and cold larvae of was only some 30% (not a reported 10-fold difference), which was presumably due to their ability to drink. We conclude that a very important, though still largely neglected, epigenetic biochemical role of insect JH depends on switchover between the utilization of dietary lipid (+JH; production of metabolic water) and carbohydrate (-JH; lipid storage in the fat body). The hypermetabolic water supply in insects fed on dry food, which is associated with enormous rates of O consumption, liberates endothermic energy that heats the body and potentially influences the insect thermoregulation. A possibility that the JH-dependent lipolytic hormone stimulates the total metabolic breakdown of nutritional lipids may be absolutely different from the currently known adipokinetic peptides that have been emphasized.
研究了昆虫以干燥食物为食以获取足够水分用于幼虫生长的生理和生化机制。该研究采用了大量生理方法，从简单的氧气消耗和水分摄入的容量测定到更先进的方法，如扫描微呼吸描记法和昆虫体温的热成像。实验以欧洲火蝽为对象，它仅以干燥的椴树种子为食。为了生存，它需要饮水或偶尔吸食植物汁液。研究发现，幼虫早期通过增加代谢水的产生来弥补偶尔出现的水分不足。在其他以干燥食物为食的物种中先前发现的依赖保幼激素（JH）产生代谢水的现象，在欧洲火蝽中是通过膳食脂质（中性种子油）的完全代谢燃烧实现的。产生水分的高代谢幼虫通过氧化磷酸化解偶联释放的吸热能量从内部加热。将膳食油燃烧成二氧化碳和水的“温暖”、高代谢幼虫显示出呼吸代谢率增加。对这些幼虫的微呼吸描记记录显示呼吸商（RQ，二氧化碳/氧气）为0.7，这表明是纯甘油三酯的分解。在热成像相机的屏幕上，温暖的高代谢幼虫很容易被发现并与“寒冷”的幼虫区分开来。缺乏JH的末龄幼虫总是只处于寒冷状态。它们仅代谢碳水化合物底物（RQ = 1.0），而膳食脂质则储存在脂肪体中。与其他以干燥食物为食的高代谢幼虫相比，后者表现出生物体中记录到的最高氧气消耗率（每小时10 - 20毫升氧气/克），欧洲火蝽温暖和寒冷幼虫之间代谢差异仅约为30%（而非报道中的10倍差异），这可能是由于它们有饮水能力。我们得出结论，昆虫JH一个非常重要但仍在很大程度上被忽视的表观遗传生化作用取决于膳食脂质利用（+JH；产生代谢水）和碳水化合物利用（-JH；脂质储存在脂肪体中）之间的转换。以干燥食物为食的昆虫中的高代谢水分供应与极高的氧气消耗率相关，释放出加热身体并可能影响昆虫体温调节的吸热能量。依赖JH的脂解激素刺激营养脂质的完全代谢分解的可能性可能与目前强调的已知脂肪动肽完全不同。
