RESPONSES OF THE RED-BLOOD-CELLS FROM 2 HIGH-ENERGY-DEMAND TELEOSTS, YELLOWFIN TUNA (THUNNUS ALBACARES) AND SKIPJACK TUNA (KATSUWONUS-PELAMIS), TO CATECHOLAMINES

In fishes, catecholamines increase red blood cell intracellular pH thr ough stimulation of a sodium/proton (Na+/H+) antiporter. This response can counteract potential reductions in blood O-2 carrying capacity (d ue to Bohr and Root effects) when plasma pH and intracellular pH decre ase during hypoxia, hypercapnia, or following exhaustive exercise. Tun a physiology and behavior dictate exceptionally high rates of O-2 deli very to the tissues often under adverse conditions, but especially dur ing recovery from exhaustive exercise when plasma pH may be reduced by as much as 0.4 pH units. We hypothesize that blood O-2 transport duri ng periods of metabolic acidosis could be especially critical in tunas and the response of rbc to catecholamines elevated to an extreme. We therefore investigated the in vitro response of red blood cells from y ellowfin tuna (Thunnus albacares es) and skipjack tuna (Katsuwonus pel amis) to catecholamines. Tuna red blood cells had a typical response t o catecholamines, indicated by a rapid decrease in plasma pH. Amilorid e reduced the response, whereas 4,4'diisothiocyanatostilbene-2,2'-disu lphonic acid enhanced both the decrease in plasma pH and the increase in intracellular pH. Changes in plasma [Na+], [Cl-], and [K+] were con sistent with the hypothesis that tuna red blood cells have a Na+/H+ an tiporter similar to that described for other teleost red blood cells. Red blood cells from both tuna species were more responsive to noradre naline than adrenaline. At identical catecholamine concentrations, the decrease in plasma pH was greater in skipjack tuna blood, the more ac tive of the two tuna species. Based on changes in plasma pH, the respo nse of red blood cells to catecholamines from both tuna species was le ss than that of rainbow trout (Oncorhynchus mykiss) red blood cells, b ut greater than that of cod (Gadus morhua) red blood cells. Noradrenal ine had no measurable influence on the O-2 affinity of skipjack tuna b lood and only slightly increased the O-2 affinity of yellowfin tuna bl ood. Our results, therefore, do not support our original hypothesis. T he catecholamine response of red blood cells from high-energy-demand t eleosts (i.e., tunas) is not enhanced compared to other teleosts. Ther e are data on changes in cardio-respiratory function in tunas caused b y acute hypoxia and modest increases in activity, but there are no dat a on the changes in cardio-respiratory function in tunas accompanying the large increases in metabolic rate seen during recovery from exhaus tive exercise. However, we conclude that during those instances where high rates of O-2 delivery to the tissues are needed, tunas' ability t o increase cardiac output, ventilation volume, blood O-2 carrying capa city, and effective. respiratory (i.e., gill) surface area are probabl y more important than are the responses of red blood cells to catechol amines. We also use our data to investigate the extent of the Haldane effect and its relationship to blood O-2 and CO2 transport in yellowfi n tuna. Yellowfin tuna blood shows a large Haldane effect; intracellul ar pH increases 0.20 units during oxygenation. The largest change in i ntracellular pH occurs between 40-100% O-2 saturation, indicating that yellowfin tuna, like other teleosts, fully exploit the Haldane effect over the normal physiological range of blood O-2 saturation.