Blood and extracellular fluid volume in whole body and tissues of the Pacific hagfish, Eptatretus stouti

Whole-body and 20 individual-tissue Cr-51-RBC (red cell space; RCS) and Tc- 99-diethylenetriaminepentaacetic acid (extracellular space; ECS) spaces wer e measured in seven unanesthetized Pacific hagfish (Eptatretus stouti). Vol ume indicators were administered via a dorsal aortic cannula implanted the previous day. Blood samples were collected at 6, 12, 18, and 24 h after inj ection. Tissues were removed at 24 h and radioactivity was measured; tissue water content (percent of wet weight) was determined by desiccation at 95 degreesC for 48 h. Mixing rates of both indicators were identical and were essentially complete by 12 h, indicating that blood convection is the rate- limiting process. At 24 h, the whole-body RCS was 19.3 +/- 2.1 mL kg(-1) bo dy weight, and the ECS was 338.5 +/- 15.2 mL kg(-1) body weight. Blood volu me estimated from the 24-h RCS and the mean central hematocrit (14%) was 13 7.9 mL kg(-1) body weight. Liver RCS (118.6 +/- 30.5 muL g(-1) tissue weigh t) was twice that of any other tissue and was also the most variable, rangi ng from 59 to 263 muL g(-1), whereas liver ECS (406.0 +/- 34.3 muL g(-1)) w as in the range of other tissues, and water content (66.9% +/- 3.5%) was lo w. Gill RCS (55.9 +/- 5.7 muL g(-1)), ECS (415.3 +/- 37.7 muL g(-1)), and p ercent water (83.1% +/- 0.8%) were higher than most other tissues. RCS, ECS , and percent water were consistently lowest in ovum (1.1 +/- 0.02 muL g(-1 ), 111.1 +/- 4.3 muL g(-1), 51.3% +/- 3.5%, respectively). Tongue, notocord , and myotome had generally lower RCS (2.1 +/- 0.4 2.2 +/- 0.5, 7.1 +/- 0.1 muL g(-1), respectively) and ECS (121.2 +/- 7.0, 246.3 +/- 17.4 185.3 +/- 16.7 muL g(-1), respectively), although their water content was in the midr ange (74.7 +/- 0.5, 81.2 +/- 1.6, 74.4% +/- 0.6%, respectively). Skin had a low RCS (6.8 +/- 1.1) and midrange ECS (387.5 +/- 28.0) but very low water content (61.2% +/- 2.1%). These findings confirm that hagfish blood volume is at 2.1% least twice as large as other fish, whereas our estimate of ext racellular fluid volume is larger than previously reported and more in line with the predicted interstitial volume. RCS, ECS, and water content vary, often independently, between tissues, which may perhaps be indicative of sp ecific tissue needs or functions. A distinct spleen is lacking in hagfish, and the liver appears to serve this function by sequestering red cells. To our knowledge, this is the first report of tissue ECS in Myxiniformes.