Surplus acylcarnitines in the plasma of starved rats derive from the liver

The method used here to assess the contribution of liver to plasma acylcarnitine is based on the idea that in rat, shortly after administration of [3H]butyrobetaine the [3H]carnitine appearing in the plasma derives from the liver and so does the acyl moiety of [acyl-3H] carnitine. In the perchloric acid extracts of plasma and liver, the ester fraction of total carnitine was determined by enzymatic analysis and that of [3H]carnitines was determined by high performance liquid chromatography. The ester fraction of total carnitine in the plasma of fed rats was 32.6% while that of [3H]carnitines was 67.9%, 1 h following injection of [3H]butyrobetaine. For 48 h starved rats the equivalent values were 54.2 and 84.0%, respectively. 24 h after the administration of [3H]butyrobetaine, the ester content became the same in the total and [3H]carnitines. That the newly synthesized carnitine was more acylated (67.9 versus 32.6%, fed) indicates that liver exports acyl groups with carnitine as carrier. The observation that the ester fraction in the newly synthesized plasma carnitine increased with fasting (84.0 versus 67.9%) indicates that the surplus plasma acylcarnitine in fasting ketosis derives from the liver. Perfused livers, however, released carnitine with the same ester content (60-61%) whether they were from fed or fasted animals. Probably, the increased plasma [acylcarnitine] in fasting develops not by an increased ester output from the liver but by an altered handling in extrahepatic tissues.     

Two-site exchange revisited: a new method for extracting exchange parameters in biological systems.

A new analysis is presented which links real volume fractions, relaxation rates, and intracompartmental exchange rates directly with apparent volume fractions and relaxation rates obtained from biexponential fits of transverse magnetization decay curves. The analysis differs from previous methods in that measurements from two paramagnetic doping levels are used to close the two-site exchange equations. Both the new method and one previously described by Herbst and Goldstein (HG) have been applied to paramagnetically doped whole-blood data sets. Significant differences in the calculated exchange parameters are found between the two methods. A small dependence of the intracellular relaxation rate on extracellular paramagnetic agent concentration, assumed nonexistent with the HG method, is inferred from the new analysis. The analysis was also applied to published data on perfused rat hearts, and we obtained a limited assessment of two-site exchange in this system.     

Butyrobetaine availability in liver is a regulatory factor for carnitine biosynthesis in rat. Flux through butyrobetaine hydroxylase in fasting state

Urinary excretion of total carnitine in 48-h fasted rats dropped to 0.30 +/- 0.01 mumol/day from 2.23 +/- 0.4 mumol/day found in fed, control animals (mean +/- SEM). Despite this marked retention, the total carnitine content of the whole body remained constant, about 83 mumol, predicting a slow-down in biosynthesis. The conversion of butyrobetaine into carnitine takes place only in the liver in rats. 48 h of starvation caused a decrease in the liver butyrobetaine level from 11.6 +/- 1.19 nmol/g to 9.30 +/- 1.19 nmol/g, which in whole livers corresponds to a decrease from 138 nmol to 61.3 nmol. The conversion rate of butyrobetaine into carnitine was studied with radiolabelled butyrobetaine. 30 min after injection of [3H]butyrobetaine the carnitine pool in the liver of fasted rats was labelled to about the same extent as that in fed rats, but from a butyrobetaine pool with higher specific radioactivity. Therefore, the conversion rate of butyrobetaine into carnitine was reduced. The newly formed carnitine found in the whole body of fasted rats was estimated to be 59% of controls. We conclude that the biosynthesis of carnitine in fasted rats slows down, for which a decreased availability of butyrobetaine in the liver is responsible. Urinary excretion of butyrobetaine in the fasted group decreased to 74.1 nmol/day from the 222-nmol/day control value while the butyrobetaine content of whole body did not significantly decrease (2.85 mumol vs. 3.04 mumol). Urinary excretion of trimethyllysine was also depressed.     

The origin and ultrastructural characteristics of corticotropin-releasing factor (CRF)-immunoreactive nerve fibers in the posterior pituitary of the rat

Summary A fine network of corticotropin-releasing factor (CRF)-immunopositive fibers was found in the posterior lobe of the pituitary of the rat. The intermediate and distal lobes were free of CRF-immunoreactivity. Varicose, terminal-like axons were frequently observed around capillary vessels. Surgical isolation of the paraventricular nuclei resulted in a complete disappearance of CRF-immunoreactive fibers from the posterior lobe. CRF-immunopositive fibers show the general characteristics of peptidergic axons. These ultrastructural observations support the idea that CRF is secreted into capillary vessels.     

Cerebrospinal fluid-contacting neurons of the central canal and terminal ventricle in various vertebrates

Cell and Tissue Research - Cerebrospinal fluid (CSF)-contacting neurons were studied by means of electron microscopy in the spinal cord and/or terminal ventricle of the ray, Raja clavata...