脱水症ラットにおける下垂体循環動態の変化

EFFECTS OF DEHYDRATION OF THE PITUITARY MICROCIRCULATION IN RATS

　抄録　バゾプレッシンの分泌促進状態が下垂体の循環動態に与える影響を明らかにするために，脱水症モデルを用いて，オートラジオグラフィー法により，下垂体の毛細血管透過度（K），血漿量（Vp），赤血球量（Ve）を測定した。雄成ラットを用いて5日間の脱水状態とした後，Kは¹⁴C-alpha-aminoisobutyric acid（AIB），Vpは¹²⁵I-albumin,Veは⁵¹Cr-erythrocytesをトレーサーとして，各々のオートラジオグラムを作成し測定した。正常ラットのKは，後葉》前葉〉中間葉の順に高値を示した。一方，VpとVeは共に前葉〉後葉》中間葉の順であった。K値は脱水により後葉でのみ著しい増加が認められたが，VpとVeはいずれの部位でも有意の変化を示さなかった。正常下垂体では，各葉間の循環動態の差は，各葉内の毛細血管床と血液供給の相違によると考えられた。しかし，脱水症によりバゾプレッシンの分泌促進が起こると，分泌に関与する後葉内の毛細血管透過性が亢進し，相互に密接な関係が存在することが示唆された。

The contiguous three lobe of the pituitary gland have different endocrine functions and vascular bed. This study was designed to investigate the effects of dehydration, which stimulates the secre-tion of vasopressin, on the pituitary microcircula-tion.

Male Sprague-Dawley rats, weighing 300-380 g, were divided into two groups. Twenty three rats were allowed free access to food and water, and 24 rats had free access to food but were deprived of water for 5 days. Capillary solute transfer (K), plasma volume (Vp) and erythrocyte volume (Ve) in each pituitary lobe were determined for both control and dehydrated rats. Quantitative auto-radiographic techniques were used to measure the K with ¹⁴C-alpha-aminoisobutyric acid (AIB, a small neutral amino acid), the Vp with ¹²⁵I-albumin and the Ve with ⁵¹Cr-erythrocytes.

Body weight was lower (-23%) and the arte-rial hematocrit was higher (+22%) in the de-hydrated rats. In the lobes of the control pituitary gland, the order of K from the highest to the lowest was posterior>anterior >intermediate lobe. K was increased by several folds only in the posterior lobe in dehydrated rats (p<0. 05). Even under normal hydrated conditions, K for AIB in the posterior lobe was several hundreds times greater than in cerebral gray matter structures.

The rank of Vp and Ve in each lobe of normal animals was about the same order and anterior> posterior>>intermediate. Normal microvascular Vp in the anterior and posterior lobes was 5-10 times larger than in cerebral gray matter. Ve of anterior and posterior lobes also were as large asVp. Both Vp and Ve increased by 120% in the posterior lobes in dehydration, however, which did not attain statistical significance.

Common morphological alterations in the pos-terior lobes of dehydrated animals include prolife-ration of the capillary endothelium and hyperplasia of the pituicytes. It is possible that these changes increase the tissue uptake of aminoacids such as AIB simply by enlarging the amount of membrane surface area. Such functional and structual chan-ges may be associated with an increase in protein turnover and neurovascular contacts in the pos-terior lobe during dehydration.