Effects of salinity manipulations on blood pressures in an osmoconforming chordate, the hagfish, Eptatretus cirrhatus.

Arterial and venous pressures were measured in hagfishes subjected to acute changes in salinity. The osmotic pressure of the seawater (SW) was increased or decreased by approximately 10%. Sixty minutes after the change in medium osmolarity the osmotic pressure of the blood corresponded with that of the medium. Following transfer to 90% SW all measured parameters changed as predicted for a passive increase in blood volume, apart from the pressure in the posterior cardinal vein (PCV) which fell. By 2 h dorsal aortic (DA) pressure and pressure in the PCV and supraintestinal vein had returned to pre-change values. In contrast, following exposure to 110% SW, pressures fell and apart from the supraintestinal vein they remained low at 120 min. At 24 h, DA pressure was lower than pre-change values for both groups. The data are consistent with the concept of central venous tone being regulated in hagfishes, which cope better with volume expansion than volume depletion.