Nonlinear Binding of Valproic Acid (VPA) and E-Δ2-Valproic Acid to Rat Plasma Proteins

The binding characteristics of valproic acid (VPA) and its pharmacologically active monounsaturated metabolite, E-²-VPA, to rat plasma proteins were compared. The plasma free fraction was determined by a rapid equilibrium procedure, which minimizes the interfering effects of nonesterified fatty acids liberated by in vitro lipolysis. Nonlinear binding behavior was observed with both compounds over their respective pharmacologic concentration range. Multiple binding-site models were invoked to explain the binding isotherm. The 2-unsaturated compound has a much higher affinity for the rat plasma proteins (mainly albumin) than its saturated precursor. The equilibrium association constants for the high- and intermediate-affinity sites were more than an order of magnitude higher with E-²-VPA than with VPA (10⁴–10⁶ versus 10³ M ^–1). This difference in binding affinity was also reflected by a lower plasma free fraction for E-²-VPA compared with VPA (<<10 versus >20% at total concentrations of less than 100 µg/ml). A more pronounced dose- and concentration-dependent variation in the distribution and clearance kinetics is predicted for the 2-unsaturated analogue compared to VPA. Also, the structural dependency in plasma protein binding observed with these branched-chain fatty acids may provide insights into the mechanism of interaction between fatty acyl molecules and albumin.     

Hypofluorescent spots in indocyanine green angiography of peau d‘orange and fundus

Purpose. Hypofluorescent spots were seen inindocyanine green (ICG) angiography of peau dorangefundus in eyes with angioid streaks. Origin of the hypofluorescentspots were examined with attention to their correlationwith a peau dorange appearance of the central fundususing a computer-assisted image comparison system. Methods. ICG angiography was performed in 5 patientshaving peau dorange appearance of fundus using ascanning laser ophthalmoscope (SLO) and a digitalvideo-fundus camera. The same central fundus areas corresponding to hypofluorescent spots in an ICGangiogram were then digitally identified in afluorescein angiogram and in a red-free picture in all10 eyes of the 5 patients. Monochromatic lightobservation was also performed with a dark fieldobservation using a SLO to see subretinal orintrachoroidal pigment clumping. Results. In no patient, the areas identified withhypofluorescent spots did show relevant changes ina fluorescein angiogram or a red-free picture. SLOexamination revealed not perfusion defect at the sameareas. The dark field observation showed no pigmentclumping at the peripapillary and papillomacularbundle regions where hypofluorescent spots were seen.Conclusions: Hypofluorescent spots seen in ICGangiograms did not show exact consistency with peau dorange changes in their location and shape. Perfusion defects or blocking by pigments were not acause of hypofluorescent spots. The scatteredhypofluorescent spots were considered to be relevantwith irregular affinity of the fundus to ICG dye.     

Selective attenuation of neuropeptide-Y-mediated contractile responses in blood vessels from patients with diabetes mellitus

Vascular smooth muscle contractile responses to neuropeptide Y, ,ß-methyleneATP and noradrenaline were studied in circular segments of isolated vessels with intact endotheliumin vitro from 12 patients with diabetes mellitus type 2 (NIDDM) and 12 control subjects. The dilatory effect of acetylcholine was used to test the function of the endothelium. Subcutaneous arteries and veins (diameter 0.1–1.1 mm) were obtained during surgery. There was no difference in contractile responses to noradrenaline or ,ß-methyleneATP between diabetic and control vessels. The contractile response to neuropeptide Y, however, was markedly reduced in the diabetic group. The maximal contractile effect (46.0 ± 14.0%,p < 0.05) but not the sensitivity to neuropeptide Y was significantly less in diabetic veins compared to control (107.5 ± 19.6%). Thus, the attenuation of neuropeptide Y responses was present in humans as previously observed in alloxan-induced diabetes mellitus in rabbits. There was no difference in the dilator effect of acetylcholine between the diabetic and the control group in any of the vessel types, indicating that the difference in vascular reactivity to neuropeptide Y was not endothelium-dependent. In conclusion, the present study has shown that the postjunctional effects of neuropeptide Y, a co-transmitter of the peripheral sympathetic nervous system, is selectively attenuated in diabetes mellitus.     

Up-regulation of β1-adrenergic receptors in rat brain after chronic citalopram and fluoxetine treatments

Quantitative receptor autoradiography was used to study the effects of the selective serotonin reuptake inhibitors citalopram and fluoxetine and the tricyclic antidepressant imipramine on the regulation of ₁-adrenergic receptors in the rat brain. Rats were treated with saline, citalopram (10 mg kg^–1), fluoxetine (10 mg kg^–1), or imipramine (15 mg kg^–1) SC once daily for 14 days. [¹²⁵I]Iodocyanopindolol binding to ₁-adrenergic receptors was found to increase significantly in the caudate-putamen and the somatosensory areas of the frontal cortex after both citalopram and fluoxetine treatments. Imipramine treatment elicited a marked decrease in ₁ binding in the outer laminae of the cingulate cortex, as well as in the motor and somatosensory areas of the frontal cortex. In a separate experiment, rats were treated with saline, citalopram (2.5, 10 and 20 mg kg^–1) or fluoxetine (2.5, 10 and 20 mg kg^–1) SC once daily for 14 days. The effects of citalopram and fluoxetine on ₁ receptors in the somatosensory cortex and caudate-putamen were replicated. These results demonstrate that chronic administration of selective serotonin reuptake inhibitors, in contrast to imipramine, can cause a regional up-regulation of ₁-adrenergic receptors in the rat brain.     

Plasma Factor Triggering Alternative Complement Pathway Activation by Liposomes

Several plasma components, such as complement (C) components, play a role in the clearance of liposomes from the circulation. The interactions between liposomes and the C system were investigated in this study. Multilamellar vesicle (MLV) liposomes, which were damaged by activation of the complement, became susceptible depending on the density of cetylmannoside (Man) on the liposome membrane, and activation proceeded through the alternative C pathway as observed for liposomes without Man (PC-MLV) (K. Funato et al, Biochim. Biophys. Acta 1103:198–204, 1992). In addition, the capacity of Man-modified liposomes (Man-MLV) to activate the alternative C pathway was abolished by preadsorption of plasma with Man-MLV but not with PC-MLV. The results suggest that a specific plasma factor adsorbed with Man-MLV was responsible for the augmentation of the C activation and, further, that the rapid clearance of Man-MLV from the circulation is caused by both enhanced C-mediated liposome permeability and enhanced C-mediated phagocytosis of liposomes.     

The selectivity of different external binding sites for quaternary ammonium ions in cloned potassium channels

Tetraethylammonium (TEA) is thought to be the most effective quaternary ammonium (QA) ion blocker at the external site of K⁺ channels, and small changes to the TEA ion reduce its potency. To examine the properties of the external QA receptor, we applied a variety of QA ions to excised patches from human embryonic kidney cells or Xenopus oocytes transfected with the delayed rectifying K⁺ channels Kv 2.1 and Kv 3.1. In outside-out patches of Kv 3.1, the relative potencies were TEA > tetrapropylammonium (TPA) > tetrabutylammonium (TBA). In contrast to Kv 3.1, the relative potencies in Kv 2.1 were TBA > TEA > TPA. In Kv 3.1 and Kv 2.1, external tetrapentylammonium (TPeA) blocked K⁺ currents in a fast, reversible and, in contrast to TEA, time-dependent manner. The external binding of TPeA appeared to be voltage independent, unlike the effects of TPeA applied to inside-out patches. External n-alkyl-triethylammonium compounds (C₈, C₁₀ chain length) had a lower affinity than TEA in Kv 3.1, but a higher affinity than TEA in Kv 2.1. In Kv 3.1, the decrease in QA affinity was large when one or two methyl groups were substituted for ethyl groups in TEA, but minor when propyl groups replaced ethyl groups. Changes in the free energy of binding could be correlated to changes in the free energy of hydration of TEA derivatives calculated by continuum methodology. These results reveal a substantial hydrophobic component of external QA ion binding to Kv 2.1, and to a lesser degree to Kv 3.1, in addition to the generally accepted electrostatic interactions. The chain length of hydrophobic TEA derivatives affects the affinity for the hydrophobic binding site, whereas the hydropathy of QA ions determines the electrostatic interaction energy.     

Postzygotic instability of the myotonic dystrophy p[AGC]n repeat supported by larger expansions in muscle and reduced amplifications in sperm

We have analysed the [AGC] expansion in leucocytes, muscle and sperm from 17 individuals affected by myotonic dystrophy (DM). Skeletal muscle showed a larger repeat number than leucocytes in the same patient. A similar degree of expansion was detected in differently affected muscles of a single patient. The germline mutation ( 350 repeats) was expanded in somatic cells of the progeny in all patients examined. Our results provide evidence of an early postzygotic instability of the [AGC] repeat in DM.     

The selective P2X purinoceptor agonist, β,γ-methylene-L-adenosine 5′-triphosphate,discriminates between smooth muscle and neuronal P2X purinoceptors

The effects of the putative selective P_2X purinoceptor agonist, ,-methylene-l-adenosine 5-triphosphate (me-l-ATP), were determined at rat neuronal and smooth muscle P_2X purinoceptors.Me-l-ATP had no effect on the extracellularly recorded membrane potential of the rat isolated vagus nerve preparation at concentrations up to 300 M. In contrast, the archetypal P_2X purinoceptor agonist, , methylene ATP (meATP;1–100 M), produced concentration-related depolarisation responses with a mean EC₅₀ value of 10.8 M. The depolarising effects of meATP were not attenuated by me-l-ATP (100 M). In voltage clamp experiments on single nodose ganglion neurones, ATP (100 M), but not me-l.-ATP (1–300 M), evoked rapid ( < 20 ms onset) inward currents when applied using a concentration-clamp method. In receptor binding studies to rat brain membranes, me-d-ATP and meATP competed with high affinity for [³H]Lx meATP binding sites, with mean pIC₅₀ values of 7.7 and 8.3, respectively. However, me-l-ATP possessed low affinity for these sites and competed only at concentrations in excess of 10 M (mean pIC₅₀ value 4.1).In prostatic segments of the rat vas deferens, me-l-ATP (1–100 M) and meATP (0.3–100 M) each produced concentration-related contractile responses with mean EC₅₀ values of 17.1 and 3.6 M, respectively. Me-l-ATP (1–10 M) evoked fast inward currents in freshly dispersed vas deferens smooth muscle cells, indicative of an action at ligand-gated ion channels. Binding sites in vas deferens membranes labelled using 1 nM [³H]meATP exhibited high affinity for me-l-ATP, meATP and me-d-ATP with mean PIC₅₀ values of 7.7, 8.4 and 7.3, respectively.These results indicate that me-l-ATP exhibits neither agonist nor antagonist properties at P_2X purinoceptors on rat vagal neurones and possesses only very low affinity for [³H]meATP binding sites in rat brain. In contrast, me-l-ATP is a potent, high affinity agonist at smooth muscle P_2X purinoceptors of the rat vas deferens. This selective agonist action of me-l-ATP suggests that P_2X purinoceptors in smooth muscle and neurones are different and represent distinct P_2X purinoceptor subtypes.