Composition of experimental diets 3^ Chemical composition of experimental diets 3? Growth of male rats for period from if-9 weeks in age from parents selected for postweaning gain ifO Gross protein and energy deposition in rats from if-9 weeks in age from parents selected for postweaning gain ifl Growth of male rats during a three week metabolism study ifif Average daily nitrogen balance in rats selected for postweaning gain if5 Daily protein deposition, protein intake and protein requirement in rats from 6-9 weeks of age fed three levels of protein h7 Average daily energy balance in rats selected for postweaning gain if8 Cellular development of skeletal muscle and plasma hormone levels in male rats from parents selected for postweaning gain i+9 The growth of male and female rats from 3-12 weeks of age fed three levels of protein ^1 Biological value and digestibility of proteins 52 Cellular development of skeletal muscle in rats at 12 weeks in age 53 Effect of level of protein intake on plasma growth hormone and insulin levels in rats selected for postweaning gain 55 Growth of male rats from parents selected for postweaning gain 56 V 15" Growth of female rats from parents selected for postweaning gain Chemical composition of male rats (ingestafree) from parents selected for postweaning gain The relationship of water and lipid in the ingesta-free body male rats from parents selected for postweaning gain Relationship between ingesta-free body weight and chemical components in male rats Gross protein and energy efficiency in rats from parents selected for postweaning gain Growth and cellular development of the gastrocnemius muscle in male rats from parents selected for postweaning gain Growth and cellular development of the gastrocnemius muscle in female rats from parents selected for postweaning gain Accumulation of DNA and RNA in skeletal muscle of male rats with different growth potentials Accumulation of DNA and RNA in skeletal muscle of female rats with different growth potentials 24-Growth of the rectus femoris muscle and total sarcoplasmic and myofibrillar protein in male rats with different growth potentials Growth of the rectus femoris muscle and total sarcoplasmic and myofibrillar protein in female rats with different growth potentials Accumulation of sarcoplasmic and myofibrillar protein in male rats with different growth potentials Accumulation of sarcoplasmic and myofibrillar protein in female rats with different growth potentials 57 59 59

The inhibition of receptor binding of [3H]phorbol-12,13-dibutyrate (PDBu) by a factor from human serum was characterized. The serum factor inhibited [3H]PDBu binding in intact monolayer cultures of the rat embryo cell line CREF N and in a subcellular system containing membranes from these cells. Inhibition occurred at both 37 and 4 degrees C and was rapid and reversible. An analysis of [3H]PDBu binding in the presence of the serum factor indicated that inhibition of [3H]PDBu binding by the serum factor was noncompetitive. Using gel filtration to separate the serum factor from free [3H]PDBu, we obtained evidence that the serum factor does not act by binding or trapping the [3H]PDBu. Unlike the phorbol ester tumor promoters, the serum factor alone did not stimulate the release of choline or arachidonic acid from cellular phospholipids, nor did it inhibit the binding of 125I-labeled epidermal growth factor to cellular receptors. The factor did, however, antagonize the inhibition of epidermal growth factor binding induced by PDBu. Sera from pregnant women were, in general, more inhibitory of [3H]PDBu binding than were those from nonpregnant women, which were more inhibitory than those from men. During these studies we found that CREF N cells responded to being grown in the presence of PDBu by partial down regulation of the phorboid receptor. The 50% effective dose for down regulation was 8 nM PDBu, and the maximum effect occurred after 6 h. Taken together, our results indicate that the serum factor inhibits [3H]PDBu binding by a direct physical effect at the level of the phorboid receptors or their associated membranes. It would appear that if this factor acts in vivo, then it might antagonize certain effects of this class of tumor promoters.

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