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A. V. NALBANDOV

Even though I had met Professor Hammond and talked with him at length both at Cambridge and elsewhere, the meeting I cherish most occurred totally unexpectedly at Midway Airport which was then the international airport of Chicago. This was probably in 1958 or 1959. He had been a Visiting Professor at Washington State University and, when I met him at Midway, he was returning home, as was I. Both of us were `weathered in' and we sat for some hours in the rather unattractive lounge at Midway and talked. Professor Hammond, who at that time had not yet been knighted, discussed at length the subject matter which had preoccupied him for years and which concerned ways and means of applying newly discovered scientific facts and methods to the improvement of domestic animals, their growth, productivity, and reproduction. Finally, he asked me about my own work. I was then in the

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BRIAN COOK and A. V. NALBANDOV

Summary.

Luteinized ovaries of the opossum incubated in Krebs-Ringer bicarbonate buffer synthesized progesterone. Such synthesis could be enhanced by lh but not by fsh or prolactin. The ovaries did not incorporate acetate-1-14C into progesterone but did convert cholesterol-1,2-3H and pregnenolone-7α-3H into progesterone. lh tended to stimulate cholesterol conversion into progesterone but the effect was not significant. The thesis is advanced that, regardless of the luteotrophic or luteolytic mechanisms operating in different species, lh is steroidogenic. It is suggested that the control mechanisms for the oestrous cycle are simpler in marsupials than in other mammalian species and an understanding of the processes involved in marsupials would give a useful baseline for comparisons with other species.

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P. V. TIGCHELAAR and A. V. NALBANDOV

Summary.

Daily injections of various commercial preparations of melatonin into immature female rats yielded different results. Melatonin obtained from Regis and Sigma resulted in decreased ovarian and uterine weights when compared to controls, while melatonin obtained from Calbiochem and Nutritional Biochemicals did not alter ovarian or uterine weights. No change was noted in anterior pituitary weights in any of the groups. At present, the explanation for these differences is unknown.

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P. L. KEYES and A. V. NALBANDOV

Summary.

An explanation is proposed for the fact that luteinizing hormone (lh) injected into intact pseudopregnant rabbits acts in a luteolytic manner while in hypophysectomized rabbits it acts like a luteotrophin. Recent evidence shows that oestrogen is both luteotrophic and steroidogenic in rabbits. It is pointed out that injection of lh into intact rabbits causes ovulation of follicles which presumably serve as the endogenous source of oestrogen. In the absence of endogenous oestrogen, corpora lutea degenerate. In hypophysectomized rabbits, follicles do not ovulate in response to lh, and the latter is assumed to cause the unovulated follicles to secrete oestrogen which in turn acts in a luteotrophic manner on the corpora lutea.

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MARILYN J. HASLER and A. V. NALBANDOV

Dryden (1969) reported that the testes in the musk shrew resided in the posterior position of the abdominal cavity in shallow coelomic evaginations called cremaster sacs. Dryden & McAllister (1970) found the testes in this species were resistant to cadmium damage and suggested this was due to the similarity between testicular temperature and body temperature and the apparent lack of a pampiniform plexus. They reported a mean rectal temperature of 33·7±0·3°C and a mean cremaster sac temperature of 32·4±0·3°C under ether anaesthesia. Since it is known that the process of spermatogenesis is very heat-sensitive, the control of deep body temperature may be as important to testicond species as the control of testis temperature is to scrotal mammals. For this reason, we performed two experiments: (1) measurement of the deep body temperatures

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J. P. ALDRED, P. H. SAMMELWITZ and A. V. NALBANDOV

Summary.

High doses of progesterone can apparently inhibit secretion or release of hypophyseal hormones, including the luteotrophic hormone (lth). Injection of progesterone into non-pregnant and pregnant guinea-pigs led to partial destruction of formed corpora lutea but did not prevent their initial formation.

The data presented suggest that in the guinea-pig lth is released about the time of ovulation, but for a period of less than 3 days. This temporary release of lth is sufficient to initiate formation and function of corpora lutea. No further lth support seems to occur in non-pregnant females.

In pregnant animals, a neurohumoral signalling mechanism initiated by intrauterine events of implantation is assumed to call forth a second release of hypophyseal lth, which is responsible for the maintenance of corpora lutea during the remainder of their functional life span during gestation. Prolactin is not luteotrophic in guinea-pigs.

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P. H. SAMMELWITZ, J. P. ALDRED and A. V. NALBANDOV

Summary.

Progesterone injected into pregnant pigs from the time of ovulation until Days 10 to 13 of gestation does not prevent formation of corpora lutea. Injections of progesterone beyond Days 12 to 16 of pregnancy, i.e. after corpus luteum formation has been completed, results in complete and rapid destruction of the formed corpora lutea.

In contrast, progesterone injected into rats throughout the entire pregnancy or pseudopregnancy has no visible or measurable effect on corpora lutea.

Since in both rats and pigs a luteotrophic hormone (lth) is presumed to be involved in the formation and maintenance of corpora lutea, it appears that progesterone is able to inhibit lth release and hence maintenance of corpora lutea during pregnancy, but not during the luteal phase of the cycle. The possible theoretical implications of these findings concerning the role of lth in maintenance of corpora lutea and the mechanisms of release of lth in rats and in pigs are discussed.

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B. Cook, F. J. Karsch, Jean W. Graber and A. V. Nalbandov

In many mammalian species, pregnancy triggers a luteotrophic mechanism that causes the corpora lutea (CL) to persist for a period longer than the lifespan of the cyclic CL. In marsupials, however, the life span of CL is not changed by pregnancy. In Didelphis marsupialis, for example, the CL reach maximum size on Day 3 after ovulation, begin to regress on Day 7 and are almost fully regressed by Day 13 when parturition occurs (Hartman, 1923). The next ovulation takes place about Day 28 regardless of whether pregnancy has intervened, provided no young are suckling (Hartman, 1923). Unlike the effects in sheep and some other species (Anderson, Bowerman & Melampy, 1963), hysterectomy also fails to prolong the life of CL in the opossum (Hartman, 1925) and there appears to be no known way in which the life of the didelphine CL can be extended. Because luteotrophic mechanisms induced by pregnancy and luteolytic mechanisms depending on the uterus seem to be absent in the opossum, the direct action of oestrogen on the lifespan of the opossum CL was investigated. Hoffmann (1960) suggested, as a consequence of his studies in women, that oestrogen acted directly on CL to induce regression, and a direct effect of oestrogen has been demonstrated for the CL of sheep (Cook, Karsch, Foster & Nalbandov, 1974) and rhesus monkeys (Karsch & Sutton, 1976).