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A blood sampling/blood transfusion technique is described for chronically cannulated freely behaving rats. The procedure permits high frequency sampling for several hours at a maximum rate of one blood sample per min. Prolactin and corticosterone were used as indicators of stress. In male rats the prolactin concentration in blood obtained by rapid decapitation was not significantly different from that obtained through a cannula. A blood volume reduction of 1 or 2 ml did not affect prolactin or corticosterone secretion; however, a reduction of 3 ml or more increased corticosterone secretion but did not consistently increase prolactin secretion. When blood volume reduction was compensated for by blood transfusion, frequent blood sampling did not affect prolactin or corticosterone secretion in dioestrous rats. The surges of prolactin secretion during the afternoon of pro-oestrus and pseudopregnancy were also unaffected by high frequency blood sampling. It is therefore concluded that the blood sampling/transfusion procedure described does not stress the animals. Its advantages include not only the possibility of following individual hormone profiles, but also the economic and ethical aspects of reducing the number of animals needed for experimentation.

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In order to characterize plasma prolactin (Prl) patterns during the estrous cycle and pseudopregnancy (PSP), I collected sequential blood samples between 8.00 and 19.00 h at 1-hour, 1.5-min or 1-min intervals from conscious, unrestrained female rats. During the afternoon of proestrus and estrus a single Prl surge was observed, while during PSP two daily Prl surges occurred, one nocturnal and one diurnal. During PSP the secretion of Prl occurred occasionally in substantial bursts from baseline levels. This contrasted with the proestrus afternoon surge when plasma Prl levels were constantly elevated due to a more or less continuous release of fluctuating amounts of Prl. The differences in timing and secretion patterns of the three surges suggest a separate neural regulation. The diurnal surge on day 0 of PSP may be composed of two different surges which also may have a separate neural regulation: one peak occurs at 14.00–16.00 h, and is comparable in timing to the Prl surge during the afternoon of estrus; a second surge starts at 17.00 h, and is comparable in time of onset to the diurnal surge on the other days of PSP. Plasma Prl always appeared to increase in an unpredictable manner, discontinuously by means of several bursts, with maximum increments of about 600 ng/ml/min. The shortest half-time values, as calculated from the disappearance of Prl from the circulation, were about 2.2 min. The individual patterns show that Prl release must be the consequence of a very dynamic neural regulatory process.

Haematological, immunological and endocrinological aspects of blood transfusions with either freshly collected or preserved donor blood were investigated in chronically cannulated unrestrained rats. Three anticoagulant preservatives were tested: citrate, citrate-dextrose and citrate-phosphate-dextrose-adenine (CPDA-1). Prolactin was used as an indicator of stress in endocrine studies. The repeated collection of 4 ml blood at 2-week intervals did not affect normal blood composition. Whole blood of rats could be stored in citrate, citrate-dextrose or CPDA-1 for 8, 22 or 35 days, respectively. Blood transfusions with fresh or preserved donor blood of F1 (R X U) rats did not affect normal blood composition nor did it induce immunological responses in F1 rats. Frequent blood sampling for several hours at highest rates of 1 sample/min did not affect prolactin secretion when blood volume reduction was replaced by blood transfusions with fresh donor blood. However, compensation with preserved blood affected prolactin secretion significantly. Blood transfusions did not affect health, behaviour, cyclicity or pseudopregnancy. The application of blood transfusion in chronically cannulated rats appeared to be quite simple. Its advantages are the possibility of following individual secretion patterns of blood-bound substances, the repeated use of animals and the reduction of the number of animals.