The capacity of the median eminence to transport LRF from CSF to hypophysial portal blood and the efficacy of intraventricularly and intravenously administered LRF in releasing LH were studied. Ten to 15 min after 125 ng LRF was injected intraventricularly, LRF was detected in portal blood. The maximal rate of transport ranged from 35 to more than 200 pg’min. The rates of transport were much less following the administration of 25 or 5 ng LRF. When these same quantities of LRF were injected intraventricularly into rats with intact pituitary glands, plasma LH levels increased in proportion to the dose of LRF. The releasing action of LRF was long, and 120 min after the administration of 125 ng LRF, the concentration of plasma LH was 40- to 50-fold greater than the control level. Intravenous injection of 5, 25, or 125 ng LRF caused corresponding increases in LH release. Intravenously administered LRF stimulated LH release slightly faster than did LRF given intraventricularly, but the effect of intraventricular LRF injections lasted much longer. The disappearance curve of LRF from systemic plasma after an intravenous injection consisted of a fast and a slow component with estimated half-times of 7 and 48 min, respectively. The half-time for the disappearance curve of LRF from plasma after intraventricular injection was 107 min, indicating slow, prolonged entry of LRF into the systemic vasculature from CSF. It is concluded that LRF can be transported from CSF to hypophysial portal blood in significant quantity. In addition, LRF given intraventricularly is more effective on a prolonged basis in stimulating LH release than when given intravenously.
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