A technique was developed for superfusion of rat hemipituitaries with hypophysial portal blood. One hemipituitary was placed in a special chamber and superfused with portal blood from a donor rat at a rate of 7 μl/min for 80 min, while the contralateral hemipituitary was superfused with arterial blood from the same animal. The effluent blood from each superfusion chamber was divided into 40 fractions (each equivalent to 2 min of superfusion) and assayed for LH by RIA. No difference in the LH output by a pair of hemipituitaries was observed when one was superfused with blood from the left femoral artery and the other with blood from the right femoral artery. However, hemipituitaries superfused with portal blood from castrated female rats secreted LH at the rate of 132 ± 4.1 ng/min (mean and SE) during the 80 min of the experiment, whereas the contralateral hemipituitaries superfused with arterial blood secreted LH at 63 ± 2.3 ng/min. The rate of LH output by hemipituitaries superfused with portal or arterial blood from diestrous rats was 42 ± 1.1 or 39 ± 0.7 ng/min, respectively. Endogenous LH in the blood entering the chamber constituted no more than 4% of the LH in the blood leaving the chamber. The data indicate that in the castrated rat there is an increased concentration of LRF in hypophysial portal blood. During the diestrous stage of the ovulatory cycle, there is little LRF activity in portal blood. Some LRF activity may be present in arterial blood of castrated rats.
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