An examination of the factors affecting intestinal lymphatic transport of dietary lipids

Lipophilic drugs are carried by chylomicrons secreted by the small intestine and transported in lymph. The intent of this review is to update the reader on the digestion, uptake, and transport of dietary lipids and how these processes impact the absorption of lipophilic drugs by the gut. The digestion of lipids in the gastric and intestinal lumen is discussed as well as the role of bile salts in the solubilization of lipid digestion products for uptake by the gut. Both passive and active uptake of lipid digestion products is reviewed. Also examined is how intestinal lipid transporters located at the brush border membrane may play a role in the uptake of lipids by the enterocytes. The intracellular trafficking and the resynthesis of complex lipids from lipid digestion products are explored. Finally, the formation and secretion of chylomicrons and their potential clinical disorders are described.

Nutrient absorption

Some key advances occurred last year in understanding mechanisms involved in nutrient absorption. A novel "prechylomicron transport vesicle" was identified; its movement to the Golgi is the rate-limiting step for triacylglycerol absorption. A scavenger receptor (type BI) in the brush border membrane appears to facilitate cholesterol uptake. Several studies define mechanisms for gastrointestinal peptide hormone stimulation of glucose uptake. An oligopeptide transporter, PepT1, is transcriptionally upregulated by certain dietary amino acids and dipeptides. Surprisingly, both insulin and fasting double the maximum velocity for dipeptide uptake (via PepT1), but they act by different mechanisms. Three transporters, SMVT (sodium-dependent multivitamin transporter for biotin and pantothenate), SVCT (for vitamin C), and CaT1 (for Ca uptake from the lumen) have been cloned and are active when expressed in various cells. Additional studies provide insights on Ca absorption and vitamin D action in aging, estrogen deficiency, and adaptation to a low Ca diet. Nramp2, also called DMT1 (divalent metal ion transporter), seems to be a major regulator of transferrin-independent, nonheme iron uptake. Finally, the protein HFE associates with the transferrin receptor and is part of an iron-sensing mechanism that regulates iron absorption. It is defective in hereditary hemochromatosis. HFE and Nramp2 (DMT1) genes are reciprocally regulated.

Nutrient absorption

Interesting advances occurred recently in nutrient absorption. Kinetics of triacylglycerol appearance in endoplasmic reticulum, Golgi apparatus, and lymph support the hypothesis that endoplasmic reticulum-to-Golgi transport is rate-limiting for lipid absorption. Apolipoprotein B does not appear necessary for initial formation of chylomicron-sized lipid particles in the endoplasmic reticulum, but rather for their movement out of the endoplasmic reticulum and to the Golgi. If peptides are protected from luminal proteolysis by fatty acylation, or if a nonpeptide drug, acyclovir, is esterified with valine to enhance bioavailability, the peptides nevertheless are absorbed by peptide transporters. Experimental conditions needed to use human ileal mucosa for in vitro absorption studies are described. Intestinal mucosa contains leptin receptors, and leptin inhibits galactose absorption, suggesting a new site for leptin's modulation of body mass. The enhancer element for the apoB gene is located much farther from its structural gene in the intestine than in the liver.

Nutrient absorption

Interesting advances occurred recently in nutrient absorption. Kinetics of triacylglycerol appearance in endoplasmic reticulum, Golgi apparatus, and lymph support the hypothesis that endoplasmic reticulum-to-Golgi transport is rate-limiting for lipid absorption. Apolipoprotein B does not appear necessary for initial formation of chylomicron-sized lipid particles in the endoplasmic reticulum, but rather for their movement out of the endoplasmic reticulum and to the Golgi. If peptides are protected from luminal proteolysis by fatty acylation, or if a nonpeptide drug, acyclovir, is esterified with valine to enhance bioavailability, the peptides nevertheless are absorbed by peptide transporters. Experimental conditions needed to use human ileal mucosa for in vitro absorption studies are described. Intestinal mucosa contains leptin receptors, and leptin inhibits galactose absorption, suggesting a new site for leptin's modulation of body mass. The enhancer element for the apoB gene is located much farther from its structural gene in the intestine than in the liver.

Octreotide enhances positive calcium balance in Duchenne muscular dystrophy

Although receptors for somatostatin are found in bone cells, the effect of somatostatin analogs on calcium metabolism is unknown. The authors studied, in a metabolic ward, the effect of octreotide (a long-acting somatostatin analog) and a placebo in two 6-day calcium balance periods in 8 children with Duchenne muscular dystrophy. As expected, octreotide (2 micrograms/kg, subcutaneously, every 8 hours) reduced serum growth hormone and somatomedin (IGF-1) to levels found in growth hormone deficiency. Octreotide enhanced calcium retention by 30% (96 mg daily [P < 0.04]) in 7 boys for whom complete data (diet, urine, and fecal calcium) were available. In 6 children with urinary calcium excretion (Uca) greater than 50 mg daily, octreotide markedly lowered Uca, from 114 +/- 23 mg daily to 61 +/- 9 mg daily (P < 0.03). Calcium retention occurred in patients with or without initial hypercalciuria, but the higher the basal Uca, the greater was the inhibition by octreotide (r = 0.79; P < 0.03). Inactive, nonambulatory patients had a more pronounced response of Uca to octreotide (P < 0.02). Octreotide caused a mild, nonsignificant reduction in fecal calcium, with no major changes in serum calcium, phosphorus, parathyroid hormone, urinary excretion of sodium and potassium, or in creatinine clearance. Based on the current observations and the presence of receptors for somatostatin in bone cells, this hormone may have, at least on a short-term basis, an anabolic effect on calcium, perhaps favoring its deposition in bone.

Anabolic steroid abuse and tooth size-arch dimensions in the rat

Anabolic steroids are misused by adolescents and adults to increase muscle mass and improve appearance and athletic performance. Since anabolics strongly enhance protein synthesis, it was speculated that alterations in tooth size and arch length could occur. This study quantified the effects of the anabolic steroid nandrolone phenpropionate on these parameters in a rat model. The steroid significantly increased mandibular arch length. No difference in mesiodistal dimensions of the molars occurred. In consequence, the increased arch dimensions combined with unaltered tooth size may result in dental spacing and/or other malocclusions.

Serum amylase activity and calcium and magnesium concentrations in young cattle grazing fescue and Bermuda grass pastures

The study reported here was part of a long-term investigation of the effects of genotype on growth, reproduction, and metabolism in cattle grazing common Bermuda grass and endophyte-infected fescue pastures. In June 1990, blood samples were collected from the tail vein of yearling heifers and steers (Angus [AA], Brahman [BB], and their reciprocal crosses [AB, BA], n = 97). Serum amylase activity was assayed enzymatically; serum Ca and Mg concentrations were determined by atomic absorption spectrophotometry. The effects of endophyte-infected fescue depended on genotype (P less than 0.001). In yearlings having at least 1 Angus parent (AA, AB, BA), grazing endophyte-infected fescue was associated with higher serum amylase activity than was grazing Bermuda grass. But serum amylase activities of BB yearlings consuming either forage were similar. Moreover, for either forage, substantial differences were related to genotype (P less than 0.007) and gender (P less than 0.05). Angus yearlings had higher serum amylase activity than did Brahman yearlings; AB and BA yearlings had intermediate values. Heifers had higher amylase activity than did steers. The relationship among serum values of amylase, Ca, and Mg depended on forage. Yearlings consuming endophyte-infected fescue and having at least 1 Angus parent had a moderate negative correlation between serum amylase activity and Ca concentration (r = -0.53; P less than 0.0005); that is, in calves of genotypes with increased amylase activity while consuming endophyte-infected fescue (AA, AB, BA), the higher the amylase activity, the lower the serum Ca concentration. However, in yearlings consuming Bermuda grass, serum amylase and Ca values were not correlated.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Pluronic L81, a hydrophobic surfactant, on intestinal mucosal cholesterol homeostasis

Addition of triglyceride and phospholipid to sodium taurocholate when chylomicron output was blocked by L81 did not increase lymphatic total cholesterol output or mucosal unesterified (UC) content more than with sodium taurocholate alone, but mucosal esterified cholesterol (CE) was increased slightly. In these animals with defective chylomicron formation, excess cholesterol accumulated in the intestinal mucosa mainly as CE. The mucosal cholesterol content of animals with normal chylomicron transport expanded during cholesterol and triglyceride absorption, and the expansion led to increased lymphatic secretion of CE. These animals accumulated significantly less CE in their mucosa than did rats treated with L81, but had about the same amount of mucosal UC. However, the overall uptake of cholesterol from the lumen, as determined by either radioactivity or mass of cholesterol in mucosa and lymph, was significantly less in the L81 rats. Also, more radioactive cholesterol remained in the lumen of the L81-treated rats. Finally, the data on specific activities of free and esterified cholesterol showed that the mucosal cholesterol derived from the lumen does not mix evenly with the free cholesterol pool in the enterocytes and is preferentially esterified for export in lymph as triglyceride-rich lipoprotein.

Fat feeding increases size, but not number, of chylomicrons produced by small intestine

To test the regulatory effect of dietary triglyceride (TG) on rat lymphatic apolipoprotein B (apo B) transport, lymph-fistula rats were infused intraduodenally for 8 h at 3 ml/h with a lipid emulsion containing 40 mumol TG labeled with glycerol [9,10-3H(N)]triolein, 7.8 mumol egg phosphatidylcholine, and 57 mumol sodium taurocholate in phosphate-buffered saline with or without 1 mg/h Pluronic L81 (L81). L81 is known to prevent lipid transport in the intestine by inhibiting the formation of chylomicrons (CM). This action of L81 is quickly reversible by merely replacing L81 infusion by saline infusion. In the control rats (without L81 added to the infusate), the amount of apo B secreted in either whole lymph, CM, or the very-low-density lipoprotein (VLDL) fractions did not change significantly during lipid infusion compared with fasting. Compared with the fasting, the apo B output in lymph during L81 plus lipid or saline infusion in the experimental rats did not change significantly. The lymphatic apo B output data were also supported by the incorporation studies using [3H]leucine. In summary, these data demonstrate that the absorption of a physiological load of lipid into lymph does not affect the apo B synthesis in the mucosa or the secretion of apo B in lymph. Furthermore, the action of L81 is probably not by inhibiting intestinal apo B production because apo B secretion was not affected by the presence of L81. This study also demonstrates that the number of CM particles made by the small intestine remains relatively constant during fasting or active lipid uptake and transport. During active lipid absorption, instead of increasing the number of CM, the enterocytes expand the size of the CM particles. Lastly, the number and TG content of VLDL particles synthesized and secreted by the small intestine also seems to remain relatively constant during fasting and active lipid absorption.

Transport of lipid and apolipoproteins A-I and A-IV in intestinal lymph of the rat

Intestinal lipid absorption is associated with marked increases in the synthesis and secretion of apolipoprotein A-IV (apoA-IV) by the small intestine. Whether the increased intestinal apoA-IV synthesis and secretion results from increased fat uptake, increased cellular triglyceride (TG) content, or increased secretion of TG-rich lipoproteins by the enterocytes is unknown. Previous work from this laboratory has shown that a hydrophobic surfactant, Pluronic L-81 (L-81), is a potent inhibitor of intestinal formation of chylomicrons (CM), without reducing fat uptake or re-synthesis to TG. Furthermore, this inhibition can be reversed quickly by the cessation of L-81 infusion. Thus L-81 offers a unique opportunity to study the relationship between lymphatic TG, apoA-I and A-IV secretion. In this study, we studied the lymphatic transport of TG, apoA-I, and apoA-IV during both the inhibitory phase (L-81 infused together with lipid) and the subsequent unblocking phase (saline infusion). Two groups of lymph fistula rats were used, the control and the experimental rats. In the experimental rats, a phosphate-buffered taurocholate-stabilized emulsion containing 40 mumol [3H]triolein, 7.8 mumol of phosphatidylcholine, and 1 mg L-81 per 3 ml was infused at 3 ml/h for 8 h. This was then replaced by glucose-saline infusion for an additional 12 h. The control rats received the same lipid emulsion as the experimental rats, but without L-81 added, for 8 h. Lymph lipid was determined both by radioactivity and by glyceride-glycerol determination, and the apoA-I and apoA-IV concentrations were determined by rocket electroimmunophoresis assay. L-81 inhibited the rise in lymphatic lipid and apoA-IV output in the experimental rats after the beginning of lipid + L-81 infusion. Upon cessation of L-81 infusion, the mucosal lipid accumulated as a result of L-81 treatment was rapidly cleared into lymph as CM. This was associated with a marked increase in apoA-IV output; the maximal output was about 3 times that of the fasting level. There was a time lag of 4-5 h between the peak lymph lipid output and the peak lymph apoA-IV output during the unblocking phase in the experimental rats. There was also a comparable time lag between the maximal lipid and apoA-IV outputs in the control animals. Incorporation studies using [3H]leucine showed that apoA-IV synthesis was not stimulated during lipid + L-81 infusion, perhaps explaining the lack of increase in lymphatic A-IV secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypolipidemic effect of intravenous pluronic L-81 in fasted rats treated with Triton WR-1339: possible inhibition of hepatic lipoprotein secretion

Pluronic L-81 (L-81), a non-ionic hydrophobic surfactant, is a powerful inhibitor of the secretion of lipid-transporting chylomicrons from intestinal epithelial cells to lymph. Since the other major organ that secretes lipoproteins into the circulation is the liver, whose principal lipid secretory product is very low density lipoprotein (VLDL), we tested the hypothesis that L-81 will also inhibit hepatic lipid secretion. Rats were fasted so that they had little lipid input from the intestine. We then administered Triton WR-1339 (tyloxapol) intravenously to block peripheral utilization of VLDL, causing plasma lipids to rise rapidly. Some animals were also given L-81 intravenously to test whether the L-81 would retard the tyloxapol-induced rise in plasma lipids. Administration of tyloxapol alone (250 mg/kg) increased plasma triglyceride, phospholipid and cholesterol concentrations considerably. Simultaneous administration of a small dose of L-81 (6 mg/kg) markedly reduced the rise in plasma triglyceride, particularly in the first hour (by 45%). L-81 also diminished the rise in plasma phospholipid and cholesterol, but to a lesser extent (30%). In the fasting rat, most of the plasma triglyceride is in VLDL; therefore, L-81 probably acts by decreasing the secretion of hepatic VLDL. Thus, Pluronic L-81 may be a useful tool for examining the secretion and metabolism of hepatic lipoproteins, in particular, VLDL.

Vasopressin in rats with genetic and streptozocin-induced diabetes

Rats were administered streptozocin (STZ; 50 or 75 mg/kg i.v., tail vein) or vehicle. Approximately 2 wk later, venous and arterial catheters was implanted for subsequent (24 h later) vasopressin, electrolyte, and hemodynamic measurements. STZ-induced diabetic (STZ-D) rats demonstrated a dose-dependent increase in the plasma glucose concentration, plasma osmolality, and plasma vasopressin concentration. Mean arterial blood pressure (MABP) was unchanged, but heart rate was reduced. Diabetes-prone BB rats, maintained on or withdrawn from insulin treatment for 24-48 h, and diabetes-resistant rats were instrumented and studied as above. Spontaneous-diabetes-prone rats demonstrated increase in plasma glucose concentration and plasma osmolality similar to STZ-D rats but had significantly greater plasma vasopressin concentrations. The significant decrease in MABP observed in these animals probably contributed to the enhanced vasopressin response. We conclude that both osmotic and cardiovascular parameters play important roles in vasopressin secretion in diabetic rats.

Effect of chronic treatment with the calcium antagonist diltiazem in Duchenne muscular dystrophy

We conducted a double-blind trial with the calcium antagonist, diltiazem (8 mg/kg/d), for 24 to 32 months in 22 boys with Duchenne muscular dystrophy, who were paired by functional activity and age. No adverse clinical or ECG effects of diltiazem were detected. In eight matched pairs, completing 28 months, manual muscle testing scores fell somewhat less in the diltiazem group (from 5.5 to 4.6) than in the placebo group (from 5.3 to 4.2), although the difference between groups was not significant (p = 0.06). The 95% confidence interval for the difference in slopes of regression lines obtained from trimonthly manual muscle tests on all subjects was markedly asymmetric in favor of the diltiazem group, but this difference was also not significant. There was less deterioration of functional activity of lower extremities in the diltiazem-treated group, when beginning and end values were analyzed (p = 0.03). However, the difference in slopes of regression lines obtained from trimonthly determinations was nonsignificant. Similarly, the beginning versus end comparisons of systolic and diastolic blood pressure showed a significantly (p less than 0.05) smaller elevation of blood pressure in the diltiazem-treated group, but no difference was observed when the slopes of all values were analyzed. All other clinical and laboratory variables were unaffected by diltiazem treatment. The findings in manual muscle tests and functional activity suggest a beneficial trend with chronic diltiazem treatment in DMD.

The effects of diltiazem in dystrophic hamsters

Calcium accumulates in muscles of dystrophic hamsters (DH) and patients with Duchenne muscular dystrophy. Various Ca antagonists were beneficial to the cardiomyopathy of DH, but had only minor effects on skeletal muscle. We administered a new Ca antagonist, diltiazem, 25 mg/kg/day orally to normal and dystrophic hamsters from ages 37 to 92 days. We observed a marked reduction in muscle Ca in DH treated with diltiazem: 73% in the heart, 61% in the diaphragm, and 48% in the rectus femoris. Plasma CK was significantly lower (by 37%) in treated DH, while the elevated rate of noncollagen protein synthesis in the diaphragm was not diminished. Histologically, the most important change was a reduction in Ca deposits in the heart. Diltiazem was well-tolerated by all animals and did not modify Ca content in normal hamsters. This study suggests that diltiazem may have therapeutic value in those conditions that are accompanied by excessive accumulation of Ca in tissues, such as muscular dystrophy.

Parathyroid ablation in dystrophic hamsters. Effects on Ca content and histology of heart, diaphragm, and rectus femoris

Cumulative evidence indicates that there is an increased accumulation of calcium in dystrophic muscle and that this may have a pathophysiological role in the progression of the dystrophic process. The accumulation may be related to a defect of the plasma membrane. Because parathyroid hormone (PTH) stimulates calcium influx into the cytosol, the chronic effects of surgical ablation of the parathyroid glands on muscle Ca, Mg, protein synthesis, and histology, as well as plasma creatine phosphokinase (CPK), Ca, and Mg, were studied in normal and dystrophic (BIO 14.6) hamsters. Thyroparathyroidectomized (TPTX) hamsters receiving replacement doses of l-thyroxine were killed at age 90 d, 55 d after TPTX. In intact dystrophic hamsters, the Ca content in the heart was 20 times higher than in normal animals and was reduced by half in TPTX dystrophic hamsters. Similar results were observed in diaphragm and rectus femoris. No abnormalities in Mg content were observed in intact or TPTX dystrophic hamsters. Ether-extractable fat of the heart and diaphragm was reduced in dystrophic hamsters and was not modified by TPTX. Protein synthesis was enhanced in the diaphragm of dystrophic hamsters but was not changed by TPTX. The concentration of CPK in plasma was elevated in dystrophic hamsters and fell significantly after TPTX. In the latter animals, microscopic examination of the heart showed lesser signs of dystrophy, particularly in the degree of fibrosis. To determine the degree of dystrophy at the age when TPTX was performed, identical analyses were made in 35-d-old hamsters. Definitive histological signs of dystrophy were observed, and although the Ca content in heart, diaphragm, and rectus femoris was elevated, the values were lower than in 90-d-old intact and TPTX dystrophic hamsters. This indicates that chronic TPTX in dystrophic hamsters reduces, but does not arrest, the dystrophic process. In normal hamsters, a 50% reduction in plasma Ca concentration was observed 6 h after TPTX; 55 d after TPTX, however, plasma Ca was within normal limits in both normal and dystrophic hamsters. No parathyroid tissue was observed in serial sections of the trachea and adjacent tissues in TPTX animals. This suggests that in chronically TPTX hamsters fed a standard laboratory diet, plasma Ca can be maintained by mechanisms independent of parathyroid function. THE DATA INDICATE THAT IN DYSTROPHIC HAMSTERS TPTX CAUSES A MARKED REDUCTION IN: (a) muscle Ca accumulation, (b) levels of plasma CPK and, (c) intensity of histological changes in the heart. These changes were independent of the levels of plasma Ca and were not observed in normal hamsters. We conclude that PTH accentuates the dystrophic process, probably by enhancing the already increased Ca flux into muscle (apparently caused by defective sarcolemma). We postulate that normal secretion of PTH may have a deleterious effect in congenital or acquired conditions associated with altered plasma membranes.

In vitro effects of fatty acids on the actions of serum on rat and pig cartilage

It has been reported that fatty acids preferentially inhibit serum-stimulated incorporation of sulfate by embryonic chick cartilage, suggesting that they may interfere with the effects of a proposed mediator (serum somatomedin) of the actions of growth hormone (GH). This was studied further in mammalian cartilage. Butyrate and octanoate at concentrations of 0.5 to 5 mM produced a concentration-dependent inhibition of both basal and serum-stimulated sulfate and thymidine incorporation by costal cartilage from hypophysectomized rats. Butyrate also inhibited basal and serum-stimulated sulfate incorporation in cartilage from normal pigs and normal sucklings rats. In all 3 test systems, oleate (0.2--5 mM) bound to serum albumin (4 g/dl) was ineffective. There was no evidence that fatty acids preferentially inhibited the stimulation of sulfate incorporation produced by serum.

Effects of growth hormone, thyroxine, and age on diaphragm muscle from dwarf mice

The ability of GH in vitro to stimulate leucine incorporation into protein and the uptake of 2-aminoisobutyric acid (AIB) and 3-O-methyl-glucose (3-OMG) was studied in diaphragm muscle from dwarf (dw/dw) mice. Ovine GH (25 mug/ml) significantly enhanced the rate of protein synthesis in muscle from untreated dwarfs. In contrast, GH usually failed to stimulate transport, although occasionally a small increase in AIB uptake was seen. Insulin (50 muU/ml) readily stimulated both AIB transport and protein synthesis in muscle from untreated dwarfs. Dwarfs were treated for 4 days with thyroxine (T4) (0.3-5 mug/day) or 3,5,3'-triiodo thyronine (T3) (0.1 mug/day). This enhanced the responsiveness of the transport systems to GH and seemed to increase the basal transport rates for AIB and 3-OMG. After the T4-treatment 0.75 mug/ml GH stimulated AIB transport in mice aged 15 weeks; 25 mug/ml GH stimulated AIB transport in dwarfs aged 5 weeks-1 yr. and leucine incorporation into protein in dwarfs aged 5 weeks-2 yr. This last result suggests that response to GH does not cease merely due to increasing age. Synergism by T3/T4 was not permanent, and the maximum response to GH occurred 1 week after the last injection of T4. The possible mechanisms whereby thyroid hormones enhance responsiveness of the transport systems to GH are discussed.

Ontogeny of sensitivity to growth hormone in rat diaphragm muscle

In order to delineate the ages of onset and decline in sensitivity to growth hormone (GH) in rats, the ability of ovine GH in vitro to stimulate amino acid uptake and protein synthesis was studied in diaphragm muscle. GH (25 mug/ml) produced a significant, but small (12-17%), stimulation of alpha-aminoisobutyric acid (AIB, 1 mM) uptake in diaphragms from fed intact rats 7-24 days old, but not from 21-day fetuses or from fed rats 4, 30, 50, 100, or 130 days old. When intact rats were fasted for 24 h, both the magnitude and consistency of the responses to GH increased considerably. In fasted rats, GH significantly stimulated AIB transport by 10,29, 77,111, and 58% at ages 4, 7, 11, 15, and 23 days, respectively; GH was not effective at age 3 or 30 days. At age 15 days, 0.5 mug/ml GH stimulated AIB transport by 96%. GH (25 or 0.5 mug/ml) significantly increased leucine incorporation into protein at age 15 days in fasted intact rats. When rats were hypophysectomized at age 28, 47, or 98 days and tested 2-3 days later, the basal rate of protein synthesis fell, but the basal rate of AIB transport did not; however, GH now stimulated both AIB uptake and amino acid incorporation into protein. These results suggest that rat diaphragm muscle first becomes sensitive to GH at about age 4-7 days (AIB transport), but that further development may be necessary for the anabolic action of the hormone (protein synthesis) to be seen. Stimulation of AIB uptake is maximal about 15 days after birth. Thereafter, diaphragm muscles from normal rats becomes progressively more refractory to the acute stimulatory effects of GH on transport processes. Hypphysectomy of rats 30 days old or more leads to the re-establishment of GH-responsiveness by the AIB transport system. This cannot be attributed merely to a lowering of the basal transport rate; rather hypophysectomy appears to result in the loss of an inhibitor of growth hormone's acute stimulatory actions on transport processes. Previous work suggests that this refractory state is produced in normal rats by occasional surges of endogenous growth hormone. The dominance of the refractory state appears about the time of weaning. This may protect the weaned rat, whose diet is more variable than it is during the suckling period, from periodic depletion of amino acids and glucose from the plasma in response to intermittent surges of GH, thereby allowing GH to produce a sustained stimulation of protein synthesis and growth.