. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 0 25 50 75 100 125 150 0 25 50 75 100 125 150 PO2 (mm Hg) PO2 (mm Hg) Figure 3. Fractional oxygen uptake of two B. califoriiU' single animals (A and B) plotted as a function of the partial pressure of oxygen. The dashed lines represent the best tit of the data to the second-degree polynomial model proposed hy Mangum and Van Winkle (19). causes the intracellular partial pressure of oxygen to drop rapidly as the result of the high energetic demands of contraction (10). Myoglobin in heart muscle cells has been shown to effe
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 0 25 50 75 100 125 150 0 25 50 75 100 125 150 PO2 (mm Hg) PO2 (mm Hg) Figure 3. Fractional oxygen uptake of two B. califoriiU' single animals (A and B) plotted as a function of the partial pressure of oxygen. The dashed lines represent the best tit of the data to the second-degree polynomial model proposed hy Mangum and Van Winkle (19). causes the intracellular partial pressure of oxygen to drop rapidly as the result of the high energetic demands of contraction (10). Myoglobin in heart muscle cells has been shown to effectively increase O2 transport to 30 times the amount that can be delivered by diffusion alone (10). Amphioxus notochord Hb, with its high oxygen- binding affinity in situ and hyperbolic oxygen saturation curve, may facilitate diffusion of oxygen into the noto- chord cells (10). Although a lower Hb concentration in the thicker region of the notochord might reduce the amount of facilitated O: diffusion, this decrease might also be ascribed to a greater passive stiffness of the larger diameter thick region compared to the thinner regions. When acting as the skeletal support during fast swim- ming, amphioxus notochord cells maintain high levels of activity in tonic contracture at peak tension (7,9). This high level of activity requires an adequate supply of ATP energy that could be supplied by aerobic metabolism. By providing a constant supply of oxygen to mitochondria in the notochord cells even under active conditions, the notochord Hb may be crucial for the production of ATP by aerobic pathways, thereby supporting the maintenance of a semi-rigid skeleton. Acknowledgments This work is dedicated to the memory of Charlotte Preston Mangum. This investigation was supported by The Ralph M. Parsons Foundation, a grant from the Uni- versity Research Council of Pepperdine University, Na- tional Science Foundation Grants IBN9343430 and 1BN9219658 to JED and DWK. and NSF Research Expe- rie
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