THE ROLE OF HORMONAL AND DIETARY FACTORS IN THE FORMATION OF EXCESS RIBONUCLEIC ACID IN THE LIVERS OF PREGNANT RATS

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Abstract
1. Excess ribonucleic acid ('RNA') is defined as the difference between the RNA contents of livers of pregnant and non-pregnant rats. Large amounts of excess RNA are formed in the liver of the rat during the last week of pregnancy. Excess RNA is formed in the liver after removal, on the 14th or 15th day of pregnancy, of the foetuses, or foetuses and ovaries, or foetuses and adrenals, or foetuses, ovaries and adrenals, or pituitary, or pituitary and foetuses, or pituitary, foetuses and ovaries. Viable placentae must be present. 2. Two fractions of RNA appear to be present in the liver cells of pregnant rats. One fraction varies linearly with the protein content of the liver cells, as does the RNA of non-pregnant rats' livers. The second fraction (excess RNA) is quite independent of the protein content of the liver cells but varies linearly with the weight of the placentae and the energy, but not the protein, content of the diet. 3. Hypophysectomy lowers the amount of excess RNA by 20–25 %. After removal of the foetuses on the 14th day, the placentae do not attain the normal weight, and the amount of excess RNA is smaller than in normal pregnancy. After removal of foetuses and ovaries the placentae are larger and heavier than after removal of the foetuses alone. 4. Both adrenalectomy and ovariectomy in non-pregnant female rats cause a small rise of liver deoxyribonucleic acid ('DNA') and RNA. After hypophysectomy, there is a loss in liver RNA greater than that expected from the simultaneous loss of liver protein. The loss of RNA occurs even when the loss of liver protein is prevented by feeding the rats by stomach tube. DNA is not lost from the liver a fortnight after hypophysectomy, as long as the energy intake is normal. 5. In non-pregnant female rats oestradiol, but not progesterone, causes an increase of liver DNA and RNA. This is not found in hypophysectomized rats. Injection of an alkaline placental extract causes a significant rise of liver RNA which, however, is very much smaller than that found in pregnancy. 6. Since hypophysectomy lowers, but does not abolish excess liver RNA in pregnant rats, it is concluded that at least two factors play a role: first and foremost, an unknown factor secreted by the placenta, acting independently of the pituitary, and second, increased amounts of oestrogen apparently requiring the presence of the pituitary.