Effects of open- and closed-system temperature changes on blood O_2-binding characteristics of Atlantic bluefin tuna (Thunnus thynnus)

摘要

We investigated the effects of open-and closed-system temperature changes on the O_2 affinity of Atlantic bluefin tuna (Thunnus thynnus) blood using in vitro methods essentially identical to those previously employed on tropical tuna species. Bluefintuna blood has a general O_2 affinity (P_(50) = 2.6-3.1 kPa or 19-23 mm Hg at 0.5% CO_2) similar to that of skipjack tuna, yellowfin tuna, and kawakawa blood (P_(50) = 2.8-3.1 kPa at 0.5% CO_2) but significantly above that of bigeye tuna blood (P_(50) =1.6-2.0 kPa at 0.5% CO_2). We therefore hypothesize that bluefin tuna are less tolerant of hypoxia than bigeye tuna. Further, we found the P_(50) of bluefin tuna blood to be slightly reduced by a 10°C open-system temperature increase (e.g., from 4.83 kPa at 15°C to 3.95 kPa at 25°C) and to be completely unaffected by a 10°C closed-system temperature change. Bluefin tuna blood, therefore, had a significantly reduced Bohr effect when subjected to the inevitable changes in PCO_2 and plasma pH that accompany closed-system temperature shifts (0.04-0.09 log P_(50) pH~(-1)) compared with the effects of changes in plasma pH accomplished by changing PCO_2 alone (0.81-0.94 log P_(50) pH~(-1)). This response is similar to that of skipjack tuna blood, but different from yellowfin or bigeye tuna blood. During closed-system temperature changes at oxygen levels above P_(50), however, bluefin tuna blood showed a reversed temperature effect (i.e., PO_2 decreased in response to an increase in temperature). Unlike in other tuna species, temperature effects on O_2 affinity of bluefin tuna whole blood were similar to those previously reported for hemoglobin solutions, suggesting that red cell-mediated ligand changes are not involved.