VOLUME CAPACITY AND CONTRACTION CONTROL OF THE SEAL SPLEEN

Volume changes in the spleens of hooded seals (Cystophora cristata) an d harp seals (Phoca groenlandica) were measured plethysmographically i n vitro in response to epinephrine, norepinephrine, isoprenaline, phen tolamine, and acetylcholine. Dilated spleens contracted forcefully wit hin 1-3 min of alpha-adrenoceptor activation with 1.0-5.0 mu g epineph rine/kg body mass, whereas stimulation of beta-adrenoceptors and choli nergic receptors had little effect. The mass of dilated hooded seal sp leens corresponded to 2-4% (n = 7) of body mass, with volume (V; ml) r elating to body mass (M; kg) as follows: V = 12.0M + 910 (r(2) = 0.96, n = 4). Thus the spleen of a 250-kg hooded seal maximally expels 3.9 liters, or 13%, of its estimated total blood volume. Average hematocri t in splenic venous outflow from dilated spleens was 90 +/- 3% (n = 3) in hooded seals and 85% (n = 2) in harp seals. From these data we hav e estimated that the aerobic diving limit of a 250-kg hooded seal incr eases only 105 s, at the most, if complete emptying of the spleen occu rs during diving, while the corresponding estimate for a 112-kg harp s eal is 80 s.