Intravenous cannulation of the freely behaving rat: frequent blood sampling and volume-dependent effect on blood leukocyte counts

A procedure for chronic cannulation of the external jugular vein of freely moving adult rats is described and compared to other techniques. This metho d permits multiple daily collections of blood and application of drugs for 22 days +/- 1.2 SEM (n = 50). Animals can be used for various experiments a nd they do not need to be restrained or anesthetized for blood sampling. A distinct improvement compared to other methodologies is the combination of fixation, suspension of the catheter and the kind of anticoagulation. The a dvantages of this method are that the number of animals needed for experime ntation can be reduced, as well as the long duration of catheter patency, l ow cost and minimal postoperative care. Furthermore, by using this techniqu e, a new phenomenon of hypervolemia-induced leucocytosis is described. Whil e a single injection of 350 mu l/min saline has no effect on leukocyte numb ers, a single injection of 750 mu l/min saline induces an early rapid leuko cytosis of 160% within 1 minute and a late second peak after 1 hour of 180% . In conclusion, this blood sampling technique makes it possible to study d ynamic alterations in the blood of completely unstressed animals. However, one limitation in its use is that blood leukocyte numbers are affected at a n injected volume of >350 mu l.