Automated repetitive microsampling of blood: growth hormone profiles in conscious male rats

A system is described for the automatic collection of small samples of blood from conscious rats. Rats bearing chronic indwelling i.v. catheters were connected via swivels to a multichannel peristaltic pump, solenoid valves and a fraction collector. A microcomputer controlled the operations involved in the removal of blood and its deposition into a fraction collector for subsequent direct radioimmunoassay for GH. Blood samples of 10-20 microliter could be collected, into a total volume of 100 microliter heparinized saline, from up to eight rats simultaneously every few minutes for many hours. This collection method avoided major blood loss and did not require transfusions of donor blood to maintain blood volume. Using a double-lumen cannula it was possible to inject or infuse into the animals while sampling blood. The system was used to investigate in detail the secretion of GH in conscious male rats. The 3-hourly endogenous secretory rhythm of GH was maintained for up to 44 h with episodes of GH secretion being multicomponent. Endogenous secretion was suppressed by constant i.v. infusions of somatostatin, with repetitive sampling showing in detail a rapid rebound secretion of GH after terminating the somatostatin infusions. Four injections of a fragment of GH-releasing factor, given at 3-hourly intervals, produced entrained GH responses, but the subsequent recovery of endogenous GH pulsing was delayed for up to 12 h. This method for the automatic microsampling of blood in small animals gives a very detailed description of the blood levels of hormones secreted in a highly episodic fashion, and could be widely applicable to other endocrine studies.