Thyroid hormone is required for dietary fish oil to induce hypersecretion of biliary cholesterol in the rat

In the rat, both fish oil diet and thyroid hormone replacement are reported to augment bile cholesterol secretion out of proportion to bile flow or secretion of other bile lipids. We sought common mechanisms for these effects and evaluated the role of phospholipid fatty acid composition in the process. Methimazole-treated hypothyroid rats were fed low-fat chow or chow supplemented with 10% corn oil or fish oil, and were studied before and after thyroid hormone treatment. Serum, hepatic, and bile lipids were measured, phospholipid fatty acid composition determined, and hepatic 3-hydroxy-3-methylglutaryl CoA reductase activity assayed. Fish oil diet stimulated cholesterol secretion into bile only after thyroid hormone was given, and this action was synergistic with that of thyroid hormone. Reduced serum cholesterol in fish oil-treated rats was associated with increased biliary cholesterol secretion and diminished hepatic cholesterol content. This suggests that augmented biliary cholesterol secretion may contribute to the fish oil-induced reduction of serum cholesterol. No definite relationship between hepatic or biliary phospholipid fatty acid composition and biliary secretion was apparent, although high bile cholesterol secretion was associated with a low percentage of hepatic and bile phospholipid linoleic acid.     

The effects of population density and malnutrition on the dynamics of whooping cough

We measured the proton leak across the inner membrane of liver mitochondria isolated from six different vertebrate species and from obese and control Zucker rats. Proton leak at 37 degrees C was similar in rat and pigeon, and in obese and control Zucker rats. Compared to rat, it was lower in cane toad, shingleback lizard, and the Madeiran lizard Lacerta dugessi. Proton leak at 20 degrees C was similar in xenopus toad and higher in rainbow trout, compared to rat. In general, proton permeability and substrate oxidation activity were greater in liver mitochondria from endotherms than those from ectotherms. Analysis of this and previous data showed that proton leak per milligram of mitochondrial protein correlated with standard metabolic rate, and proton leak per milligram of inner membrane phospholipid correlated with 11 phospholipid fatty acid compositional parameters, including unsaturation index.     

Biomechanical assessment of titanium and stainless steel posterior spinal constructs: effects of absolute/relative loading and frequency on fatigue life and determination of failure modes

Peripheral-type benzodiazepine (BZ) receptors (PBRs) have been identified in various peripheral tissues as well as in glial cells in in the brain. PBRs are located mainly on the outer mitochondrial membrane and bind with high affinity the BZ Ro 5-4864 (4'-cholorodiazepam) and the non-BZ PK 11195 (an isoquinoline carboxamide derivative), but bind with very low affinity the BZ clonazepam. PBRs have been cloned from various species. PBRs are multimeric receptors composed of the 18-kDa binding site for isoquinolines, the 32-kDa voltage-dependent anion channel, and the 30-kDa adenine nucleotide carrier (which binds BZs). The expression of PBRs is especially high in steroidogenic organs. Steroid administration affects PBR density, whereas depletion of hormones by hypophysectomy in female rats, or castration (surgical or chemical) in male rats, decreases PBR density in endocrine organs, which can be elevated to normal values after administration of the appropriate hormone. PBRs are probably involved in several functions, including cell proliferation, respiration, and steroidogenesis. It has been suggested that PBRs are involved in the translocation of cholesterol from the outer to the inner membrane of the mitochondria and have an effect on the biosynthesis of steroids.     

Relationships between serum lipids, platelet membrane fatty acid composition and platelet aggregation in type 2 diabetes mellitus

In order to gain further insight into the mechanism of platelet dysfunction frequently reported in diabetes we investigated circulating fatty acids, lipid composition of platelet membrane and platelet function in Type 2 diabetic patients. In these subjects, percentages of C16 : 1n-7 and C18 : 1n-9 in serum phospholipid fraction and of C16 : 1n-7 in serum cholesterol ester fraction were decreased. Moreover, the content of C20 : 4n-6 in serum cholesterol esters was altered in Type 2 diabetic subjects: C18 : 0 and C20 : 3n-6 were increased but C20 : 4n-6 content was similar to controls. Aggregation in vitro did not differ from controls but aggregation in vivo was increased in Type 2 diabetic subjects. No correlation was observed between metabolic parameters -i.e., HbA1, blood glucose, serum triglycerides and total cholesterol, circulating fatty acids and fatty acid content of platelet membrane. A negative linear correlation was found between aggregation in vivo and C20 : 4n-6 content of platelet membrane. Moreover, a U shaped relationship was observed between platelet aggregation in vitro and C20 : 4n-6 content of platelet membrane suggesting that C20 : 4n-6 level should be tightly controlled otherwise platelet hyperreactivity may occur. These results indicate that despite a normal mean C20 : 4n-6 content in the platelet membrane, regulation of C20 : 4n-6 metabolism is less strictly controlled in Type 2 diabetes mellitus and confirm the importance of arachidonic acid platelet content in the regulation of platelet aggregability.     

Modification of the Ehrlich ascites tumor cell nuclear lipids

The fatty acid composition of Ehrilich ascites tumor cell nuclei was differend when the tumor-bearing mice were fed diets rich in either coconut or sunflower oil. When coconut oil was fed, the monoenoic fatty acid content of many of the nuclear lipids was increased and their polyenoic fatty acid content was reduced as compared with the sunflower oil diet. By contrast, only small changes were produced in the saturated fatty acid contents of the nuclear lipids. The nuclear membrane choline phospholipid, ethanolamine phospholipid and combined serine phospholipid plus inositol phospholipid fractions exhibited statistically significant changes in fatty acid composition, but the sphingomyelins were not altered appreciably by dietary lipid modification. The fatty acid composition of the small quantity of phospholipids associated with the chromatin was much more resistant to diet-induced mosification. Except for sphingomyelin, the fatty acid composition of the chromatin phospholipids was different from that of the corresponding nuclear membrane phospholipids, containing much larger amounts of fatty acids having less than 16 carbon atoms. The fatty acid compositons of the nuclear triaclglycerols and cholesterol esters, which were associated almost entirely with the chromatin, were modified by the dietary lipid modifications. There were no changes in the DNA, RNA or lipid content of these nuclei. Therefore, this experimental system can be used to prepare mamalian nuclei that differ appreciably only in their fatty acyl composition.     

Plasmatic and membrane lipid alterations in erythrocytes from diabetic rats fed either n-6 or n-3 fatty acids

We examined the effect of dietary n-6 and n-3 fatty acids on the lipid composition and physical properties of erythrocyte membranes together with cholesterol and triglyceride plasmatic levels in normal and experimental diabetic rats. Plasmatic total cholesterol and triglyceride did not change in normal rats under the dietary regime, but both parameters decreased significantly in the diabetic animals after the consumption of either n-6 or n-3 fatty acids. Lipid analyses of erythrocyte membranes revealed a significant decrease in the total cholesterol together with an increase in the phospholipid amount in the diabetics compared to the normal rats. As a consequence, cholesterol/phospholipid ratio decreased in these groups of animals and markedly in those fed n-3 fatty acids. These changes would be responsible for the lower fluorescent polarization of DPH observed in the latter group. We conclude that equivalent and adequate amounts of dietary either n-6 or n-3 fatty acids produce plasmatic and red cell membrane lipid changes in diabetic rats that may improve the evolution of the disease.     

Neurochemical findings in a perinatal sudanophilic leukodystrophy rich in steryl ester

Neurochemical and neuropathological studies have been made of a 10-day-old child who suffered from a sudanophilic leukodystrophy. The brain white matter contained abundant sudanophilic material. The patient's grey matter total cholesterol content was 30% higher than whole brain tissue derived from a comparable control. White matter cholesterol content was more than double the control value. Nearly 80% of the white matter cholesterol was esterified. Subcellular fractionation of the white matter resulted in a "floating fraction" rich in cholesteryl ester. The steryl ester fatty acid composition was not typical of control tissue or demyelinating tissue. Patient phospholipid fatty acid composition patterns differed from control, but white matter galactolipid fatty acid composition appeared normal. Cholesteryl ester hydrolase activity appeared normal. Myelin and myelin-like fractions, isolated from diseased and normal brain tissue, were of a primitive developing nature but appeared to be comparable. The findings indicate a neonatal sudanophilic leukodystrophy which doubtless began in prenatal life and which was rich in cholesteryl ester. The aetiology of the leukodystrophy is unknown.     

Lipid content and fatty acid composition of heart and muscle of the BIO 82.62 cardiomyopathic hamster

Microscopic and analytical studies of the lipids in the heart and muscle of the BIO 82.62 myopathic hamsters and age-matched normal animals at the average ages of 33, 67, and 108 days were performed. Microscopic examinations did not show increased lipid depositions in the hearts of the diseased animals as was found in the BIO 14.6 strain. No consistent differences in the lipid content of the cardiomyopathic hamsters (BIO 82.62) and age-matched controls were observed in the three age groups except in the cholesterol content of muscle. Cholesterol increased significantly (P less than 0.01) in the 67 and 108 day old animals. This increase elevated the cholesterol/phospholipid ratio. Analysis of the fatty acid composition of triglycerides showed that the cardiomyopathic hamsters store more saturated fatty acids in both heart and muscle than do their normal counterparts. The abundance of more saturated fatty acids may imply that either the desaturation mechanism is altered in the diseased animals or that unsaturated fatty acids are preferentially utilized in other processes.     

Synthesis and removal of different cholesterol esters by aortic smooth muscle cells in culture

The incorporation of 3H-labelled oleic acid and of 14C-labelled linoleic acid into phospholipid, triglyceride and cholesterol ester in smooth muscle cells grown in incubation medium supplemented with either 5% normal or 5% hyperlipemic serum has been studied. Both fatty acids were incorporated into cholesterol esters to a greater extent when cells grown in incubation medium containing hyperlipemic serum. Oleic acid was incorporated into cholesterol esters in preference to linoleic acid. The addition of hyperlipemic serum to the incubation medium did not increase the incorporation fo either 3H-labelled oleic acid or of 14C-labelled linoleic acid into phospholipid or triglyceride. The removal of labelled lipid fractions has also been followed for four days in cells pulse labelled for 24 hours with 3H-labelled oleic acid and 14C-labelled linoleic acid. Both 3H- and 14C-labelled cholesterol esters were removed more rapidly when the smooth muscle cells were grown in medium containing normal serum than in medium containing hyperlipemic serum. The removal of both phospholipid and triglyceride was similar in normal and hyperlipemic serum. Comparison of the 3H/14C ratio indicated that the cholesterol oleate and cholesterol linoleate were removed at similar rates.     

Feed-back control of cholesterol synthesis in partially hepatectomized rats

Partial hepatectomy caused a marked stimulation of cholesterol and fatty acid syntheses without affecting serum total cholesterol, total phospholipid and triacylglycerol concentrations of rats so far examined 48 h after the operation. Serum free cholesterol level, however, was increased by the treatment and the ratio of lysophosphatidylcholine to phosphatidylcholine was concomitantly decreased, suggesting the impairment of serum lecithin: cholesterol acyltransferase activity. The lipid content in the liver, especially triacylglycerol and ester cholesterol, was increased markedly by the operation. Feeding of a high cholesterol diet which elevated serum cholesterol and phospholipid levels to the partially hepatectomized rats, accelerated the accumulation of hepatic triacylglycerol and ester cholesterol by the partial hepatectomy. The weight of the regenerating liver was not influenced by cholesterol feeding, which suggested that cholesterol feeding did not inhibit the regeneration mechanism of the liver. The increase of cholesterol synthesis after partial hepatectomy was inhibited by cholesterol feeding. Therefore, it is conceivable that the negative feed-back control of cholesterol synthesis is induced by cholesterol feeding under the stimulated cell divisions of the liver after partial hepatectomy. It is suggested from the present data that a large amount of the cholesterol which is necessary for cell growth can be taken up from serum, when serum cholesterol concentration is high.     

Always single and single again women: a qualitative study

This study reports the effects of a novel polyunsaturated 3-thia fatty acid, methyl 3-thiaoctadeca-6,9,12,15-tetraenoate on serum lipids and key enzymes in hepatic fatty acid metabolism compared to a saturated 3-thia fatty acid, tetradecylthioacetic acid. Palmitic acid treated rats served as controls. Fatty acids were administered by gavage in daily doses of 150 mg/kg body weight for 10 days. The aim of the present study was: (a) To investigate the effect of a polyunsaturated 3-thia fatty acid ester, methyl 3-thiaoctadeca-6,9,12,15-tetraenoate on plasma lipids in normolipidemic rats: (b) to verify whether the lipid-lowering effect could be consistent with enhanced fatty acid oxidation: and (c) to study whether decreased activity of esterifying enzymes and diversion to phospholipid synthesis is a concerted mechanism in limiting the availability of free fatty acid as a substrate for hepatic triglyceride formation. Repeated administration of the polyunsaturated 3-thia fatty acid ester for 10 days resulted in a reduction of plasma triglycerides (40%), cholesterol (33%) and phospholipids (20%) compared to controls. Administration of polyunsaturated and saturated 3-thia fatty acids (daily doses of 150 mg/kg body weight) reduced levels of lipids to a similar extent and followed about the same time-course. Both mitochondrial and peroxisomal fatty acid oxidation increased (1.4-fold- and 4.2-fold, respectively) and significantly increased activities of carnitine palmitoyltransferase (CPT) (1.6-fold), 2,4-dienoyl-CoA reductase (1.2-fold) and fatty acyl-CoA oxidase (3.0-fold) were observed in polyunsaturated 3-thia fatty acid treated animals. This was accompanied by increased CPT-II mRNA (1.7-fold). 2,4-dienoyl-CoA reductase mRNA (2.9-fold) and fatty acyl-CoA oxidase mRNA (1.7-fold). Compared to controls, the hepatic triglyceride biosynthesis was retarded as indicated by a decrease in liver triglyceride content (40%). The activities of glycerophosphate acyltransferase, acyl-CoA: 1,2-diacylglycerol acyltransferase and CTP:phosphocholine cytidylyltransferase were increased. The cholesterol lowering effect was accompanied by a reduction in HMG-CoA reductase activity (80%) and acyl-CoA:cholesterol acyltransferase activity (33%). In hepatocytes treated with methyl 3-thiaoctadeca-6,9,12,15-tetraenoate, fatty acid oxidation was increased 1.8-fold compared to controls. The results suggest that treatment with methyl 3-thiaoctadeca-6,9,12,15-tetraenoate reduces plasma triglycerides by a decrease in the availability of fatty acid substrate for triglyceride biosynthesis via enhanced fatty acid oxidation, most likely attributed to the mitochondrial fatty acid oxidation. It is hypothesized that decreased phosphatidate phosphohydrolase activity may be an additive mechanism which contribute whereby 3-thia fatty acids reduce triglyceride formation in the liver. The cholesterol-lowering effect of the polyunsaturated 3-thia fatty acid ester may be due to changes in cholesterol/cholesterol ester synthesis as 60% of this acid was observed in the hepatic cholesterol ester fraction.     

Thermal sensitivity of growth and feeding in Manduca sexta caterpillars

We explore how the thermal sensitivity of organismic performance emerges from the thermal sensitivity of the underlying component processes involved, using growth and feeding of Manduca sexta caterpillars as a model system. We measured thermal performance curves for the short-term rates of growth, consumption, protein (casein) digestion, amino acid (methionine) uptake, and respiration in fifth-instar caterpillars over a biologically realistic temperature range from 14 degrees to 42 degrees C. Growth and consumption rates increased between 14 degrees and 26 degrees C, reached a maximum value near 34 degrees C, and declined rapidly above 38 degrees C. In contrast, protein digestion rate and respiration rate increased monotonically over the entire temperature range, and amino acid uptake rate increased with temperatures up to 38 degrees C and then leveled off between 38 degrees and 42 degrees C. These results suggest that the shape and position of the thermal performance curve for growth rate--in particular the maximum at 34 degrees C and rapid decline above 38 degrees C--was most closely correlated with the thermal sensitivity of consumption rate; the declining growth performance above 38 degrees C was not associated with declines in digestion or uptake rates or with accelerated respiration rates at these temperatures.     

Effect of tyrosine kinase and tyrosine phosphatase inhibitors on ATP- and thapsigargin-induced CA2+ entry in rat peritoneal macrophages

The production of eicosapentaenoic acid [20:5omega3; EPA] from Shewanella gelidimarina (ACAM 456T) was investigated with respect to growth temperature and growth on sole carbon sources. The percentage and quantitative yield of EPA remained relatively constant at all growth temperatures within or below the optimal growth temperature region. At higher growth temperatures, these values decreased greatly. Growth on differing sole carbon sources also influenced the percentage and amount of EPA produced, with the fatty acid composition influenced by provision of potential acyl chain primers as sole carbon sources. The highest amounts of EPA occurred from growth on propionic acid and L-leucine respectively, while the highest percentage of EPA occurred from growth on L-proline. Monounsaturated fatty acid components and EPA were concentrated in phosphatidylglycerol (PG), while the proportion of branched-chain fatty acids was elevated in phosphatidylethanolamine (PE); the two major phospholipid classes. Specific associations of EPA with other acyl chains were identified within cellular phospholipid classes. The association of EPA with 17:1 and 18:0 acyl chains in phospholipid species was specific to PG, whereas the association of EPA with i13:0/13:0 and 14:0/i14:0 was specific to PE. Such acyl chain 'tailoring' is indicative of the important role of EPA in bacterial membrane adaptive responses. EPA was also a large component (22%) of a non-esterified fatty acid (NEFA) fraction within the total lipid extract of the bacterium. This may point toward a particular role of NEFA in polyunsaturated fatty acid (PUFA) metabolism. The formation of EPA was investigated by labelling with L-[U-14C]serine and sodium [1-14C]acetate. The accumulation of radiolabel within unsaturated intermediates (di-, tri- and tetraunsaturated fractions) was low, indicating a rapid formation and derivatisation of these components. Similar results were found for the unsaturated fatty acid fractions of both PE and PG using sodium [1-14C]acetate radiolabel. The regulation of triunsaturated fatty acid components may be a potential control site in PUFA biosynthesis.     

Fractal analysis of photolysis of nitrosyl haemoglobin at low temperatures

In Mycobacterium phlei, fatty acid unsaturation increased with decreasing temperature. The 10-hexadecenoic acid content increased as the temperature was reduced from 35 degrees C to 26-20 degrees C. At lower temperatures tuberculostearic acid decreased while oleic and linoleic acids increased, the latter being found in M. phlei for the first time. Concomitantly palmitic acid content decreased, and the 6- and 9-hexadecenoic acids increased slightly on reducing the temperature from 35 to 10 degrees C. Thus, down to 26-20 degrees C palmitic acid was mainly replaced by 10-hexadecenoic acid. From this range down to 10 degrees C, palmitic and tuberculostearic acids were replaced by oleic and linoleic acids. Consequently, fatty acid branching decreased and mean chain length increased, as the temperature was reduced. These observations support the view that regulation of membrane fatty acid composition is part of microbial temperature adaptation, and that the mechanism behind the responses might be more complex than generally believed.     

Changes in the cholesterol and phospholipid content of mouse spleen after Raucscher leukemia virus infection

The effect of Rauscher Leukemia Virus (MuLV-R) infection on the lipid composition of mouse spleen from BALB/c mice was investigated. Drastic changes in the lipid composition of the spleen as a result of tumor growth induced by the virus could be demonstrated at 21 days after infection. The molar ratio of cholesterol to phospholipids was found to be low, while a shift within the choline containing phospholipid classes resulted into a lower sphingomyelin and a higher phosphatidyl choline content of the MuLV-R infected spleen. The cholesterol ester content increased more than two-fold during tumor growth, and shifts in the fatty acid patterns of the lipids were demonstrated.     

Effect of dietary fats and sesamin on the lipid metabolism and immune function of Sprague-Dawley rats

We examined the effect of three dietary fats, safflower oil (SAF) rich in linoleic acid, borage oil (BOR) rich in gamma-linolenic acid, and perilla oil (PER) rich in alpha-linolenic acid, on the lipid metabolism, and chemical mediator and immunoglobulin levels in Sprague-Dawley rats, as well as the dietary effect of sesame-derived antioxidative sesamin. The serum cholesterol, phospholipid, triglyceride, prostaglandin E2 level and splenic leukotriene B4 level were lower in the rats fed on BOR or PER than in those fed on SAF. SES feeding suppressed the expression of the lipid-decreasing effect of BOR, but not in the rats fed on PER. In respect of the fatty acid composition of the liver and spleen, PER feeding gave a lower arachidonic acid level, and higher eicosapentaenoic and docosahexaenoic acid levels than SAF feeding did, while the effect of BOR feeding was marginal. The effect of SES feeding on fatty acid composition was much smaller than that of dietary fats. In respect of immunoglobulin production, PER + SES feeding gave the lowest IgE productivity in the mesenteric lymph node lymphocytes. These results suggest that PER feeding regulated lipid metabolism and exerted an anti-allergic effect by a different mechanism from that with BOR feeding.     

Ontogenesis of lipids in chick embryo retina

PURPOSE: The effects of embryonic development on lipid composition in the retina were studied in 7, 11, 15, and 18-day-old chick embryos and newly hatched chicks. METHODS: The proportions of phospholipids, free and esterified cholesterol, diacylglycerides, and free fatty acids were determined using the Iatroscan TLC/FID procedure. Gas chromatography and mass spectrometry were used to determine the fatty acid composition. RESULTS: The major phospholipid species were phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, lysophosphatidylcholine, and sphingomyelin. Concentrations of the analyzed components have been related to the chronology of concrete stages of retinal development. The fatty acid composition of the total lipids, (n-6):(n-3) and saturated: unsaturated fatty acid ratios, and other parameters are reported. The proportions of total saturated and total monounsaturated fatty acids decreased very little from day 7 to hatching, whereas total polyunsaturated fatty acids nearly doubled over the same period. The increase in C18:2(n-6) from day 11 onwards was not followed by a similar increase in C20:4(n-6), hence the C20:4 to C18:2 ratio decreased with age. CONCLUSIONS: The cholesterol:phospholipid ratio decreased from day 7 to day 15 and increased from day 15 to hatching. High proportions of esterified cholesterol, very probably originating in the retinal pigment epithelium, were also recorded. Total saturated and monounsaturated fatty acids decreased, while polyunsaturated fatty acids increased during the period of initial retinal growth.     

Effects of hypophysectomy and prolactin replacement therapy on prostatic response to androgen in orchiectomized rats

The effects of hypophysectomy and prolactin replacement therapy on prostatic response to androgen in orchiectomized rats were studied. Castration 23 days prior to treatment with testosterone propionate (TP), followed by hypophysectomy 13 days before TP treatment, and then treatment with 1 mg TP every other day for 16 days caused a greater decrease in body weight, prostatic weight, and the level of citric acid in the prostate than did TP treatment, castration, and sham hypophysectomy. This suggests the existence of a pituitary factor in the maintenance of prostatic integrity. Prolactin replacement therapy in hypophysectomized, castrated,TP-treated rats significantly (p less than .005) increased the prostatic weight and both the content and concentration of citric acid. The results confirm previously reported observations that the prostatic response to androgens is markedly reduced by hypophysectomy in castrated rats, and that prolactin acts synergistically wtih testosterone in promoting prostatic growth and the concentration of citric acid in the prostate. The direct effects of both hypophysectomy and prolactin replacement therapy on the ventral and dorsolateral lobes of the prostate were also demonstrated.     

The utilisation of creatine and its analogues by cytosolic and mitochondrial creatine kinase

We have investigated the utilisation of four analogues of creatine by cytosolic Creatine Kinase (CK), using 31P-NMR in the porcine carotid artery, and by mitochondrial CK (Mt-CK), using oxygen consumption studies in isolated heart mitochondria and skinned fibers. Porcine carotid arteries were superfused for 12 h with Krebs-Henseleit buffer at 22 degrees C, containing 11 mM glucose as substrate, and supplemented with either 20 mM beta-guanidinopropionic acid (beta-GPA), methyl-guanidinopropionic acid (m-GPA), guanidinoacetic acid (GA) or cyclocreatine (cCr). All four analogues entered the tissue and became phosphorylated by CK as seen by 31 P-NMR, Inhibition of oxidative metabolism by 1 mM cyanide after accumulation of the phosphorylated analogue resulted in the utilisation of PCr, beta-GPA-P, GA-P and GA-P over a similar time course (approximately 2 h), despite very different kinetic properties of these analogues in vitro. cCr-P was utilised at a significantly slower rate, but was rapidly dephosphorylated in the presence of both 1 mM iodoacetate and cyanide (to inhibit both glycolysis and oxidative metabolism respectively). The technique of creatine stimulated respiration was used to investigate the phosphorylation of the analogues by Mt-CK, Isolated mitochondria were subjected to increasing [ATP], whereas skinned fibres received a similar protocol with increasing [ADP]. There was a significant stimulation of respiration by creatine and cCr in isolated mitochondria (decreased K(m) and increased Vmax vs control), but none by GA, mGPA or beta-GPA (also in skinned fibres), indicating that these latter analogues were not utilised by Mt-CK. These results demonstrate differences in the phosphorylation and dephosphorylation of creatine and its analogues by cytosolic CK and Mt-CK in vivo and in vitro.