中脳部循環に関する実験的研究

AN EXPERIMENTAL STUDY ON REGIONAL, CIRCULATION IN THE MIDBRAIN

Ⅰ．はじめに

　脳の循環がどのような因子により支配され，どのように影響されるかは古くから研究者の興味をひき，さまざまの方法によりその解明が試みられてきた^12,29）。測定法の発達につれて脳循環測定の範囲は全脳レベルから局所循環へとすすみ，部位的にも表層のみならず脳深部の循環をも定量的もしくは定性的に測定が行われるようになつた。しかしながら非観血性，連続性，時間的応答性，加えて定量的測定などをすべてみたすような測定法はいまだ開発されず問題はなお多い。また近年，脳深部の循環を測定した報告^2,6,20,36）はあるが，数分以内におこる変化について，その循環動態を論じたものは少ない。

　教室では八木ら^40）の考案した銅エナメル線による酸素電極法を用いて，比較的短時間における脳局所循環の変化を定性的であるが連続的に観察することにより，主として視床部循環を中心に研究がなされてきた^18,3,32）。このうち木元^18）は中脳部循環にも言及し，CO₂反応性の面から中脳部循環に特異性が存在することを示唆した。そこで著者は同じく銅エナメル線による酸素電極法を用いて，中脳部循環における自己調節作用および，これに関与する神経性因子，炭酸ガスに対する反応性について検索を加え，近接する視床部とは異つた特異性が存在するとの知見を得たので報告する。

In 31 dogs, regional cerebral blood flow (rCBF) was continuously measured in the midbrain, by the use of oxygen electrodes made of copper wire, and also in the cerebral and the cerebellar cortex and thalamus. Arterial blood pressure in the subclavian artery and cerebrospinal fluid pressure in the cisterna magna were simultaneously measured and recorded. Oxygen tension of arterial blood was also monitored in the common carotid artery with a copper oxygen electrode. Carbon dioxide tension (pCO₂) was measured in blood sampled from the contralateral axillary artery. Autoregulation of circulation in each part was studied before and after intravenous injection of autonomic blocking agents, such as phenoxybenzamine, propranolol and atropine. Perfusion pressure of the brain was increased by use of a Fogarty balloon catheter in thoracic aorta, and decreased by blood exsan-guination from the femoral artery. CO₂ response of mesencephalic circulation was also studied and the effect of it was observed on the autoregulation. Each experiment was conducted were condition of the dog's mean arterial blood pressure (MABP) was at approximately 120 mmHg and arterial pCO₂ was in 30-35 mmHg. The findings were as follows :

1) Autoregulation was found in the mesencephalic circulation as well as cortical and thalamic circu-lation with the relationship curve between rCBF and MABP.

2) Disautoregulation of thalamic and mesen-cephalic circulation was induced by injections of phenoxybenzamine and atropine. In the midbrain, however additional injections of the drugs were found to be needed to induce disautoregulation in the mesencephalic circulation more than in the thalamic circulation. Propranolol did not have clear effect on the autoregulation of cortical, thalamic and mesencephalic circulation.

3) When CO₂ inhalation was performed, rCBF in the midbrain was increased more markedly than the increase of cortical blood flow. A concave curve was found on the relationship between rCBF and arterial pCO₂ in the midbrain, while that of the cortex showed linear line.

4) Autoregulation of cortical circulation was disturbed with mild hypercapnia. In the thalamus, autoregulation was not disturbed until arterial pCO₂ was increased approximately 50-65 mmHg. The autoregulation of mesencephalic circulation, however, was clearly seen when arterial pCO₂ concentration increased.

With these results, it was thought that autonomic fiber activity was a factor on autoregulation of mesencephalic circulation as well as the myogenic or metabolic factor. It was also supposed that the neurogenic factor might be antagonistic with the CO₂ responce in mesencephalic circulation, because of the concave curve relationship between rCBF and CO₂ concentration of arterial blood, and because of lack of change of autoregulation in hypercapnia.