眼鏡蛇(Naja atra)的血壓調節研究

Abstract

習性偏向樹棲的蛇類，為了避免攀爬時腦部供血不足，其安置於塑膠管內所測得的血壓較其他棲地之蛇種高，約40～70 mmHg；此外，以人為方式將蛇由頭部全身上揚90°時，偏向樹棲的蛇類頭部血壓還能維持50～90 %，而陸棲與水棲蛇類常降至0 %以下。眼鏡蛇 (Naja atra) 為陸棲性蛇類，但禦敵時具有特殊的抬頭擴頸行為，當前半身豎起時，血液循環所面臨的問題也與攀爬時的樹棲性蛇類似；因此預期眼鏡蛇雖為陸棲蛇，但可能具有偏向樹棲性蛇類的生理特徵。如果答案是否定的，特別是眼鏡蛇的血壓並未偏高時，則進一步檢測在受驚嚇刺激後，眼鏡蛇的血壓是否能快速地大幅上升，以因應禦敵時劇烈的姿勢改變?本實驗另選取與眼鏡蛇同科且陸棲的雨傘節(Bungarus multicinctus multicinctus)，以及善攀爬的錦蛇 (Elaphe taeniura) 與南蛇 (Ptyas mucosus) 作為比較物種。 實驗結果發現，眼鏡蛇並不具有類似樹棲性蛇類的生理特徵，其在塑膠管內水平未抬頭時的血壓並不高，眼鏡蛇與雨傘節分別為39與33 mmHg；而善攀爬的錦蛇與南蛇分別為48與46 mmHg。當全身上揚90°時，四種蛇中眼鏡蛇頭部血壓下降的幅度最大，調節能力也最差，頭部血壓僅剩32 %，顯著低於錦蛇與南蛇 (70與80 %)。 眼鏡蛇頭部受拍擊後血壓可快速上升二倍，平均約70 mmHg，顯著高於同科的雨傘節，而上升的速度則顯著較錦蛇與南蛇快速。眼鏡蛇主動抬頭禦敵時，血壓也是大幅上升，此時頭部血壓能維持在40 mmHg以上；然而當眼鏡蛇在塑膠管中，若前半身被動上揚且無視覺刺激的情況下，頭部血壓平均僅22 mmHg。經注射α與β交感神經抑制劑後，眼鏡蛇抬頭禦敵時的血壓上升明顯被抑制，而抬頭高度也顯著較低；顯示眼鏡蛇受刺激後的升壓反應，交感神經興奮應為主要機制，且抬頭時頭部血壓若無法維持，抬頭高度會受到影響。In order to climb without brain hypotension, arboreal snakes tend to maintain a higher (40～70) blood pressure (BP) than other snakes when measured in a plastic tube. Moreover, when passive tilted head up to 90°, the BP of head is still maintained at 50～90 % of pre tilt value. Terrestrial and aquatic snakes frequently drop bellow 0 % when treated as the same way. The Chinese cobra (Naja atra) raise it’s head up quickly when threatened. Although it is a terrestrial snake, the circulation problem of orthostatic posture is the same as climbing behavior of arboreal snakes. Consequently, I speculate that the cardiovascular and BP characteristics of cobras may be similar to that of arboreal snakes. If the speculation is not true, then, the cobra should be able to increase the BP largely and quickly when raising their heads during defending. Another terrestrial snake, banded krait (Bungarus multicinctus multicinctus) and two well climbing snakes, beauty snake (Elaphe taeniura) and rat snake (Ptyas mucosus) were selected for comparison species in this experiment. Unlike arboreal snakes, the BP of cobra measured in a plastic tube is not high, only 39 mmHg, so did banded krait (33 mmHg). Whereas The BPs were higher in the other two well climbing snakes (48 and 46 mmHg). When the head was passively tilted up at 90°, the head BP of cobra was dropped to the lowest level among four species, and the average BP of head is only 32 % of pre tilt value, significantly lower than beauty snake and rat snake (70 and 80 %). When the cobras were stressed bytapping their heads, the BP increased quickly to two times of the resting value (average, 70 mmHg), which is significantly higher than that of banded krait and faster than that of beauty snake and rat snake. A large and quick increase in BP seen in the cobra was also observed at active head raising. In this situation, the BP at head level was maintained above 40 mmHg. Nevertheless, when the cobra was settled in a plastic tube, the average BP of head is only 22 mmHg during a passive front body tilt without any visual stress. When pretreated with α and β sympathetic inhibitors, increasing in BP seen in the cobra during active head raising was abolished and the height of head raising was low. These results indicate that the mechanism of BP increase in the cobra during head raising is probably dependent on sympathetic excitation induced by mental stress and that blood supply to the head by way of BP increase may play a role in maintaining the height of head raising during defense reaction.