Changes in fibrinolytic activity in diving grey seals

In order to test the hypothesis that enhanced fibrinolytic activity is a factor which prevents the blood of diving seals from clotting, we instrumented two female grey seals (Halichoerus grypus) with subcutaneous electrodes for measurements of heart rate (HR) and an extradural intravertebral venous catheter for collection of blood samples before, during and after simulated dives of 10 min duration. Blood samples were used for in vitro determination of clot lysis time (CLT), which is a measure of the level of fibrinolytic activity, and for analyses of plasma levels of cortisol, noradrenaline and adrenaline (A). The seals displayed profound diving bradycardia indicative of a substantial reduction in blood flow rates (pre-dive HR: 78 (63-98) bpm; dive HR: 8 (7-10) bpm; (median (range); n = 2)) and elevated catecholamine levels (pre-dive A: 121 (98-184) pg.ml-1; peak dive/post-dive A: 3510 (447-6181) pg.ml-1), both of which are factors which promote blood coagulation. Nevertheless, we found that CLT always increased in connection with diving (pre-dive CLT: 436 (356-568) min; peak CLT during diving: 1380 (640-1800) min), which implies a reduced, rather than enhanced, fibrinolytic activity in this situation. These results show that enhanced fibrinolytic activity is not part of the defence system which prevents fatal clotting from occurring in diving grey seals.