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ALISON SPEIGHT
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G. FINK
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Dispersed pituitary cells obtained from female rats with regular oestrous cycles were suspended in Bio-Gel columns and perfused with pulses of luteinizing hormone releasing hormone (LH-RH). There was a close relationship between the amount of LH released and the concentration of LH-RH in the perfusate. It was not possible to elicit the priming effect of LH-RH, but the LH-response changed markedly during the oestrous cycle in a manner similar to that seen in vivo; i.e. the responses of cells prepared from rats killed at pro-oestrus were much greater than the responses of cells prepared from rats killed on other days of the cycle. A similar change in responsiveness was obtained when the columns were perfused with 60 mmol K+/1, suggesting that at least part of the increase in pituitary responsiveness that occurs at pro-oestrus is not dependent upon changes in specific receptors for LH-RH.

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ALISON SPEIGHT
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RACHEL POPKIN
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A. G. WATTS
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G. FINK
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We have investigated the mechanism by which oestradiol-17β augments pituitary responsiveness to luteinizing hormone releasing factor (LH-RF). Adult rats were ovariectomized on the morning of dioestrus and implanted with either an empty silicone elastomer capsule or a capsule containing oestradiol-17β. Twelve hours later the LH response, tested by injecting 50 ng LH-RF/100 g i.v., was significantly greater in animals implanted with an oestradiol capsule compared with that in animals implanted with an empty capsule. The effect of oestradiol was blocked by sodium pentobarbitone administered 4 h before the test, and this block was overcome by infusing LH-RF during the 4 h period at doses which by themselves were not sufficient to evoke a large release of LH. We also measured LH-RF in pituitary stalk blood collected under Althesin anaesthesia between 4–6 and 12–13 h after ovariectomy and capsule implantation. The concentration of LH-RF in stalk plasma fell between these two collection periods in animals implanted with empty but not with oestradiol-filled capsules. The concentrations of LH-RF in stalk plasma, although relatively low, were significantly higher in animals bearing an oestradiol-containing capsule than the concentrations in peripheral plasma from similarly treated animals, and, by comparison with the LH-RF concentrations in peripheral plasma from animals infused with LH-RF, were sufficiently high to increase significantly the responsiveness of the pituitary gland. These data show that as well as acting directly on the pituitary gonadotrophs, oestradiol-17β increases the responsiveness of the anterior pituitary gland by a mechanism that involves the release and the priming effect of LH-RF.

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ALISON C. SPEIGHT
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K. W. HANCOCK
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R. E. OAKEY
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The concentrations of oestrone and oestradiol-17β in peripheral plasma and the urinary excretion of unconjugated oestrone and unconjugated oestradiol-17β were measured by radioimmunoassay in 31 matched samples from seven young women. The concentrations of oestrone and oestradiol-17β not bound to protein in the plasma samples were measured following equilibrium dialysis.

The urinary excretion of unconjugated oestrone (0·70 ± 0·34 nmol/24 h, mean ±s.d., n = 28) was found to be significantly, but poorly, correlated with the concentration of non-protein-bound oestrone in plasma (10·2 ± 3·8 pmol/l) (r = 0·44, P < 0·05). Similarly, the urinary excretion of unconjugated oestradiol-17β (0·29 ± 0·16 nmol/24 h, n = 30) was found to be significantly, but still rather poorly correlated with the concentration of non-protein-bound oestradiol-17β in plasma (7·4 ± 5·3 pmol/l) (r = 0·58, P< 0·001). Since the calculated proportions of oestrone and oestradiol-17β in plasma not bound to protein (3·4 ± 0·3% and 1·7 ± 0·2% respectively) remained fairly constant (coefficient of variation 9 and 10% respectively), measurement of oestrone or oestradiol-17β in plasma provided a better guide to the biologically available (non-protein-bound) hormone than did measurement of urinary unconjugated oestrogen.

The mean renal clearance of both non-protein-bound oestrone and non-protein-bound oestradiol-17β (50 ± 21 and 36 ± 23 ml/min) was less than that of creatinine (114 ± 31 ml/min) indicating absorption and/or metabolism of each hormone by the kidney tubule.

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