The effect of blood sampling on plasma levels of LH and FSH in male rats.

Plasma levels of LH and FSH in male rats were compared in blood samples collected by decapitation or through permanently implanted aortic cannulae. The hormone levels were higher in the samples taken by decapitation. After cannulation, the gonadotropin levels remained unchanged when sampling occurred only once a day and each rat was bled 1-3 times over a period of 7 days (sample volume 2.3 ml). The hematocrits of these rats decreased from 42% to 34% after two samplings. In another experiment the LH and FSH levels increased when seven samples had been taken during 24 h. In this experiment the blood cells were reinfused into the circulation, and as a result after 5 samples the hematocrit decreased only from 45% to 40%. A drop in food consumption was observed after cannulation. It is suggested that the increase in gonadotropin levels may be due to the acute stress caused by handling during and before decapitation or decrease in gonadotropin levels may be due to the chronic stress caused by the implanted cannula. The frequent bleedings and cell infusions into the cannulated rats may stimulate the secretion of gonadotropins, opposing the effect of chronic stress.