1. Investigación

A 24-h fast induced different patterns of change in the amino acid concentrations of liver, muscle, plasma and blood cells. Starvation produced generalized increases in blood amino acids despite decreases in plasma, thus increasing the blood cells amino acid pool. Muscle increased amino acid levels with fasting, while the changes were much more buffered in liver. The fraction of essential amino acids carried by the blood was considerably greater than that of muscle and liver. The size of muscle pool in the whole rat was much greater than that of liver and more than two orders of magnitude higher than that of the whole blood. Fasting-induced changes agree with the known transport of amino acids from muscle and other peripheral tissues towards the liver and other splanchnic organs.

Rats chronically treated with two daily doses of tolbutamide, glibenclamide or glipentide were compared with animals treated with placebo. Plasma individual amino acids were determined at 0, 3, 7, 10, 12, 14, 17, 24, 27 and 29 days of treatment 16 hours after the administration of the drug. Rats were fasted for 48 h periods at days 10 to 12 and 27 to 29 of the experiment. Sulfonylurea treated animals show minor changes in the plasma aminogram, although glipentide and glibenclamide produced greater effects than tolbutamide. At the 3rd day after the onset of the treatment, plasma levels of glutamate+ glutamine, arginine and histidine appeared significantly reduced in glipentide and glibenclamide treated animals. When plasma samples were collected 3 h after the drug administration at the 24th day of treatment, the only observed change was a decrease in the levels of arginine in the glipentide treated animals. Fasting produced decreases in plasma levels of alanine, pro line, cysteine, tyrosine, methionine +ornithine and tryptophan, there were no changes in serine, aspartate + asparagine, threonine citruline, arginine and lysine; and glycine, glutamate+ glutamine and leucine + isoleucine show increases. These changes were rapidly compensated with refeeding, appearing a "rebound effect" in certain amino acids. Both fasting and refeeding affect very little the effect of sultonylureas on plasma amino acid levels, although for some individual amino acid they reduce or enhance the effect of the fasting. These small effect of sulfonylureas on plasma amino acid levels could be the result of the juxtaposition of different factors, including the effects of these drugs on circulating insulin levels, on protein biosynthesis and amino acids transamination and hepatic gluconeogenesis.

Since during pregnancy the mother switches from an anabolic to a catabolic condition, the present study was addressed to determine the effect of 48 h food deprivation on days 7, 14 and 20 of pregnancy in the rat as compared to age matched virgin controls. Body weight, free of conceptus, decreased with food deprivation more in pregnant than in virgin rats, with fetal weight (day 20) also diminishing with maternal starvation. The decline of plasma glucose with food deprivation was greatest in 20 day pregnant rats. Insulin was highest in fed 14 day pregnant rats, and declined with food deprivation in all the groups, the effect being not significant in 7-day pregnant rats. Food deprivation increased plasma glycerol only in virgin and 20 day pregnant rats. Plasma NEFA and 3-hydroxybutyrate increased with food deprivation in all groups, the effect being highest in 20 day pregnant rats. Food deprivation decreased plasma triacylglycerols in 14 day pregnant rats but increased in 20 day pregnant rats. In 20-day fetuses, plasma levels of glucose, NEFA and triacylglycerols were lower than in their mothers when fed, and food deprivation caused a further decline in plasma glucose, whereas both NEFA and 3-hydroxybutyrate increased. Liver triacylglycerols concentration did not differ among the groups when fed, whereas food deprivation caused an increase in all pregnant rats and fetuses, the effect being highest in 20-day pregnant rats. Lipoprotein lipase (LPL) activity in adipose tissue was lower in 20 day pregnant rats than in any of the other groups when fed, and it decreased in all the groups with food deprivation, whereas in liver it was very low in all groups when fed and increased with food deprivation only in 20 day pregnant rats. A significant increase in liver LPL was found with food deprivation in 20 day fetuses, reaching higher values than their mothers. Thus, the response to food deprivation varies with the time of pregnancy, being lowest at mid pregnancy and greatest at late pregnancy, and although fetuses respond in the same direction as their mothers, they show a specific response in liver LPL activity

The effect of glucose, arginine, pyruvate and palmitate administration on levels of circulating glucose and RIA-insulin was studied in fed and 48 h-fasted rats. The rise in blood glucose level after oral glucose load was greater in fasted than in fed rats, whereas plasma insulin level increase was similar in both groups. When glucose was given intravenously, plasma RIA-insulin rose only in the fed animals. Arginine administration produced minor changes in these parameters in both fed and fasted rats. Oral pyruvate produced greater enhancement in blood glucose concentration in fasted than in fed animals while plasma insulin levels rose only in the fed rats. After palmitate load, blood glucose levels increased only in the fed animals in which there was also an increase in plasma insulin levels following intravenous administration of fatty acid. These results suggest that none of the metabolites used except glucose has a physiological role in insulin secretion in the fed or fasted animals; in the latter group sensitivity to glucose stimulus was greatly reduced while the release of insulinotropic gastrointestinal factors after administration of oral glucose appeared less affected. The changes in blood glucose levels observed after addition of pyruvate or palmitate are discussed in terms of the role of pyruvate as a gluconeogenetic substrate and of the effect of palmitate on glucose metabolism.

The effects of food deprivation for up to 24 hours on plasma metabolic parameters in the rat have been studied. Liver dry weight and glycogen content dropped significantly from a hours of food deprivation onwards. Total muscle glycogen supplied about as much glycosyl residues or precursors as did the liver. Plasma glucose, urea, lactate and total and essential amino acids decreased significantly from 3 hours of fasting onwards. Glycerol, free fatty acids, beta-hydroxybutyrate and acetoacetate showed significant increases with fasting. Alanine, serine, arginine, threonine, aspartate plus asparagine and proline showed significant decreases with fasting. Several other amino acids showed almost nc change with fasting. Lysine, leucine plus isoleucine and taurine showed biphasic changes in their concentrations with a minimum at 6 hours and a transient recovery at 12 hours of fasting. Essential amino acids decreased more than the non essential ones. With fasting there is a shift in ammonia disposal with lower urea concentrations as nitrogen is better conserved. The results seem to suggest that there i~ a constant release of substrates, through liver and peripheral tissue proteolysis, that is counteracted by differential utilization of amino acids during fasting.