Pituitary regulation of preovulatory estrogen secretion in the rat

The factors stimulating estrogen secretion in the preovulatory phase and an attempt to explain the mechanism of termination of estrogen secretion are discussed. Female Wistar rats, hypophysectomized at 1 p.m. in proestrus, were injected with rat pituitary extracts. Ovarian venous blood was collected and the estrogen activity of the plasma was measured. The estrogen secretion was minimized within 3 hours after hypophysectomy. The rat pituitary extract caused an 11-fold increase of estrogen concentration in the ovarian venous blood within 1 hour. Either LH or FSH alone was able to restore the estrogen secretion: LH took 1 hour to reach maximal response, FSH 2 hours. In the 1-hour test, the minimal effective dose for LH appeared to be less than .25 mcg per rat, for FSH, 2.5 mcg per rat. The total ability of the two preparations to produce estrogen appeared to be the same. 10 I.U. of prolactin slightly stimulated estrogen secretion, but 20 mU of ACTH was quite negative. These results demonstrate the pituitary gonadotropin dependency of estrogen secretion from the ovary having ripened follicles. It also showed that the ovary, after completion of ovulatory surge of LH, abolished its reactivity to the pituitary extract containing sufficient amount of substances in promoting estrogen secretion. Either LH or FSH was able to terminate estrogen secretion even at minute doses as small as 10 mcg. This shows that both FSH and LH provide a dual effect on ovarian estrogen secretion at the preovulatory stage, promotion and suppression. Promotion is an acute and direct action of hormones on steroidogenesis and suppression probably a delayed and indirect action of ovulation-inducing hormone, the release of which initiates the differentiation of estrogen-forming cells towards ovulation unfavorable to estrogen synthesis.     

Lectin histochemistry in rat thyroid tumours

The thyroid tumours and background goiterous and adenomatous lesions induced in rats with diisopropanolnitrosamine (DIPN) plus methylthiouracil (MTU), and regenerative thyroid tissues after wounding were studied by lectin histochemistry. Ten weeks after cessation of the carcinogen treatments, carcinomas invading the surrounding tissues and blood vessels (13/20) and papillary micronodules (11/20) were formed in the thyroid tissues. In general, the carcinoma lesion was solitary, and the papillary micronodules were multiple in a single thyroid gland. Among the lectins tested, Maclura pomifera (MPA) and Solanum tuberosum (STA) showed specific binding with both carcinoma and papillary micronodule lesions, but not with the background goiterous and adenomatous lesions and regenerative thyroid tissues. The former both lesions showed higher labelling indices with BUdR or 3H-thymidine and poorer thyroglobulin accumulation than the latters, thereby indicating their enhanced proliferative capability and depressed potency of cyto-differentiation. The common cytological and histochemical properties of carcinoma lesions and papillary micronodules allow us to regard the latter as pre-invading carcinoma lesions. The lectins MPA and STA may be, therefore, used as the specific markers of malignancy in rat thyroid carcinogenesis.     

Regulation of intracellular Mg2+ by superoxide in amnion cells.

Changes of intracellular free Mg2+ concentration ([Mg2+]i) in human amnion cells induced by superoxide anion were determined using a highly Mg(2+)-sensitive fluorescent dye Mg(2+)-fura2 or Mg(2+)-indol. Superoxide anion, produced by addition of xanthine oxidase to hypoxanthine, induced decrease of [Mg2+]i. The decrease was significantly inhibited by an anion channel blocker, 4,4'diisothiocyano-2,2' disulfonic acid stilbene (DIDS). Superoxide dismutase (SOD), injected into cells by cell fusion, also inhibited the change of [Mg2+]i, but catalase did not. Superoxide anion induced prompt increase of intracellular pH (pHi) as well as decrease of [Mg2+]i and subsequently activated the increase of intracellular free Ca2+ ([Ca2+]i) and the release of arachidonate. In contrast to superoxide anion, NH4Cl which induces increase of pHi in amnion cells increased [Mg2+]i. The elevation of basal level of [Mg2+]i by Mg(2+)-ionophore inhibited the change of [Ca2+]i and the release of arachidonate induced by superoxide anion. These results suggest that superoxide anion, transported through anion channels into cells, decreases [Mg2+]i directly, not due to a pH-effect and that the decrease of [Mg2+]i may regulate biological functions of the cells via increase of [Ca2+]i.     

A site-directed mutagenesis study on the role of isoleucine-23 of human epidermal growth factor in the receptor binding.

The isoleucine-23 residue of human epidermal growth factor (hEGF) was substituted by a variety of amino acid residues and the receptor-binding activities of variant hEGFs were determined by the use of human KB cell. Tight receptor binding was found of variants with hydrophobic amino acid residues in position 23. The size of the isoleucine residue was nearly optimum for the receptor binding as compared with other hydrophobic residues. The structure analysis by two-dimensional nuclear magnetic resonance spectroscopy showed that the substitution at position 23 only slightly affected the tertiary structure of hEGF. These indicate that the side chain of isoleucine residue in position 23, which is exposed on the protein surface, directly binds to a hydrophobic pocket of the receptor.