比較探討不同頭足類動物鰓部對於氨的調節策略

Abstract

蛋白質是頭足類動物重要的營養來源，而氨是胺基酸代謝的主要產物，因此氨的調節對於頭足類動物生理恆定十分重要。本研究運用真蛸(Octopus vulgaris)、虎斑烏賊(Sepia pharaonis)以及萊氏擬烏賊(Sepioteuthis lessoniana)的鰓進行離體灌流實驗，比較並探討採取不同運動模式的頭足類動物對於氨之調節策略。實驗結果顯示：頭足類動物鰓部對氨的運輸皆具有雙面向的調節功能：真蛸、虎斑烏賊與萊氏擬烏賊鰓部血液中的含氨濃度分別高於300 μM與100 μM會進行排氨；而血液含氨濃度在分別低於300 μM (真蛸與虎斑烏賊)與100 μM (萊氏擬烏賊)時，則不排氨以維持鰓部氨濃度。檢測鰓部的排氨速率則發現：萊氏擬烏賊之排氨速率高於其他兩物種。另外，真蛸與虎斑烏賊正常血液含氨濃度皆高於萊氏擬烏賊。進一步經由血酸處理(pH 7.2)與對照組(pH 7.6)的比較結果發現：氫離子伴隨氨排除濃度增加的現象只出現在真蛸鰓部。這些結果暗指頭足類動物體內和鰓部組織的含氨濃度與牠們採取不同運動模式有著密切關聯。除此之外，本研究利用離體灌流優勢進行藥理實驗探討鰓部氨運輸之機制，成功地在軟體動物導入嶄新的離體生理研究模式並釐清頭足類對於氨的調節機制。

Regulation of ammonia homeostasis is essential for cephalopods since protein is the major constituent of their diet and large amounts of ammonia will be produced by extensive amino acid catabolism. Cephalopods have successfully evolved different lifestyles to accommodate their own specific ecological niches in benthic and pelagic habitats. The cephalopod gill has been suggested to represent the major excretory organ thus we hypothesize that cephalopods evolved diverse mechanisms for ammonia regulation depending on their different lifestyles. We developed an in vitro system to study branchial NH4+ transport by using perfusion technic in isolated gills of octopus (Octopus vulgaris), cuttlefish (Sepia pharanois) and squid (Sepioteuthis lessoniana). The gills in all three species possess a bi-phasic NH4+ regulation. In octopus and cuttlefish gills, NH4+ is excreted at blood NH4+ levels higher than 300 μM and increased via ammoniagenesis at NH4+ levels lower than 300 μM. In contrast, squid gills excreted NH4+ at blood NH4+ level higher than 100 μM and accumulated NH4+ at blood NH4+ levels loer than 100 μM. Further experiments simulating extracellular acidosis (pH 7.2) demonstrated that the machinery of H+ secretion coupled with NH4+ excretion can be only observed in octopus gills. Moreover, the rates of NH4+ excretion were higher in squid gills compared to the other two species. The in vivo NH4+ levels were higher (~ 300 μM) in octopus and cuttlefish blood compared to those of squid (~ 25 μM). These observations inferred that the variations in ammonia homeostasis in these three octopus species are probably linked to their respective locomotory capacities. Moreover, pharmacological studies were also used to investigate the ammonia excretion mechanisms and the NH4+ regulation involved in cAMP-dependent pathways were also found in octopus and cuttlefish gills. The present study applied a new method to better understand the mechanisms of ammonia regulation in highly ammonotelic species.