Membrane microviscosity and plasma triacylglycerols in the rat

Determination and comparison of how the chain number and chain length of a lipid affects its interactions with a phospholipid at an air/water interface

We determined how the number of chains in a lipid and its chain length affects its interactions with a phospholipid model membrane, and whether the number of chains or the chain length of lipids affects their interactions with the phospholipids more. This was achieved by using a Langmuir trough and a fluorescence microscope to study the interactions of mono-, di-, and triglycerides with a phospholipid monolayer at an air/water interface. The effect of the number of chains in a lipid on its interactions with phospholipids at air/water interfaces was shown by surface pressure-area per molecule isotherms and their thermodynamic analysis to worsen as the number of alkyl chains was increased to be greater than one. An increase in the packing density decreased the mixing ability of the lipids with the phospholipids, resulting in the formation of aggregates in the mixed monolayer. The aggregation was explained by the intermolecular hydrophobic and van der Waals attractions between the lipid molecules. Fluorescence microscopy revealed partial mixing without aggregation for monoglycerides, but the presence of lipid aggregation for diglycerides and triglycerides. The effect of decreasing the chain length of triglycerides from a long chain to a medium chain caused the interactions of the lipids with the phospholipid molecules at the air/water interface to significantly improve. Decreasing the chain length of monoglycerides from a long chain to a medium chain worsened their interaction with the phospholipid molecules. The effect of decreasing the triglyceride chain length on their interactions with phospholipids was much greater than the effect of decreasing the number of alkyl chains in the lipid.

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

Abnormal Igf2 gene in Prague hereditary hypertriglyceridemic rats: its relation to blood pressure and plasma lipids

Prague hypertriglyceridemic (HTG) rats represent a suitable model of metabolic syndrome. We have established the set of F(2) hybrids derived from HTG and Lewis progenitors to investigate the relationship between respective polymorphism(s) of Igf2 gene and blood pressure (BP) or other cardiovascular phenotypes. HTG rats had elevated systolic BP and plasma triglycerides but lower plasma cholesterol compared to Lewis rats of both genders. In males, there was higher mean arterial pressure, diastolic BP and relative heart weight in HTG than in Lewis rats. The results obtained in the total population of F(2) hybrids indicated strong segregation of Igf2 genotype with plasma triglycerides. There was no segregation of Igf2 genotype with any BP component except BP changes occurring after the blockade of either renin-angiotensin system (RAS) or NO synthase. When F(2) population was analyzed according to gender, male F(2) progeny homozygous for HTG Igf2 allele had significantly higher plasma triglycerides and greater BP changes after NO synthase blockade than those homozygous for Lewis allele. On the contrary, male F(2) progeny homozygous for HTG Igf2 allele had significantly lower plasma cholesterol and smaller BP changes after RAS blockade. PCR analysis of Igf2 gene by using of microsatelite D1Mgh22 has shown polymorphism between HTG and Lewis rats. Sequence analysis of cDNA revealed insertion of 14 nucleotides in HTG gene. In conclusion, polymorphism in Igf2 gene may be responsible for differences in lipid metabolism between HTG and Lewis rats. It remains to determine how these abnormalities could be involved in BP regulation by particular vasoactive systems.

Cyclic nucleotides in platelets of genetically hypertriglyceridemic and hypertensive rats. Thrombin and nitric oxide responses are unrelated to plasma triglyceride levels

Prague hereditary hypertriglyceridemic (HTG) rats constitute a genetic model of hypertension associated with hyperlipidemia and insulin resistance. Various cell alterations, including changes in membrane dynamics, ion transport, and decreased platelet responses to thrombin have been observed in this strain. As hypertriglyceridemia appears to be associated with reduced endothelium-dependent vasodilation and platelet aggregation, we examined whether triglycerides could modulate cell responsiveness through changes in cyclic nucleotides in platelets of HTG rats. From the age of 6 weeks, these hypertensive animals were subjected for 10 weeks to interventions that modified circulating triglycerides levels (2.17+/-0.09 mmol/l), leading to their reduction (gemfibrozil treatment, 0.87+/-0.05 mmol/l) or elevation (high fructose intake, 3.23+/-0.07 mmol/l). Basal cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) contents were 15% and 48% lower in isolated platelets of HTG rats than in those of Lewis controls. cAMP level was further reduced in HTG rats subjected to high fructose intake. Irrespective of their plasma triglyceride levels, the thrombin-induced increase in platelet cGMP levels present in Lewis rats was absent in platelets of HTG rats. In contrast, no strain- or treatment-related differences were observed in the magnitude or kinetics of cGMP response to exogenous nitric oxide (NO). NO-induced cGMP and cAMP changes were associated in an opposite manner with trimethylamino-diphenylhexatriene (TMA-DPH) anisotropy, a biophysical parameter that reflects the microviscosity of the outer part of the cell membrane. Our results indicate that the attenuation of platelet responsiveness to thrombin in HTG rats represents a strain difference that cannot merely be due to a difference in plasma triglyceride levels. Platelet hyporesponsiveness to agonists such as thrombin in HTG rats cannot be explained by a change in levels of inhibitory cyclic nucleotides, since they were actually found to be low and not high.

Macrophage heterogeneity, antigen presentation, and membrane fluidity: implications in visceral Leishmaniasis

Morphological and functional heterogeneity of the splenic macrophage (M phi) population was studied in Leishmania donovani (LD) infected BALB/c mice. On a discontinuous percoll gradient two distinct M phi populations were separated. They differed significantly in size as evident from Scanning Electron Microscopy (SEM). Morphologically, the bigger M phi (LM) showed surface projections, whereas the smaller M phi (SM) was round. As regards the antigen-presenting abilities, the LM of infected animals showed defective antigen-presenting abilities at a later stage of the disease, i.e. 6 months post infection ((6)I-LM) but not earlier, whereas the SM population remained functionally intact throughout the course of the infection. Further, the (6)I-LM showed a much enhanced Ad status as compared to their controls. Interestingly, both the (6)I-LM and the control set showed a comparable level of binding of a known A(d) restricted peptide. Despite the presence of sufficient A(d) molecules and the ability to bind the appropriate peptide, (6)I-LM were unable to stimulate peptide specific T-cell hybridoma. Further, the (6)I-LM showed an increase in membrane fluidity and distorted morphology with membrane fissure and blebs as evident from SEM. It is possible that an increase in the membrane fluidity may lead to the defective antigen-presenting ability of (6)I-LM. Thus, the LD infection functionally keep the (6)I-LM out of antigen presentation and this may contribute to the defective cell mediated immune response in leishmaniasis.

Membrane microviscosity, blood pressure and cytosolic pH in Dahl rats: the influence of plasma lipids

OBJECTIVE

To determine the relationships between blood pressure, membrane microviscosity, plasma lipids and cytosolic pH in Dahl rats susceptible or resistant to salt hypertension.

DESIGN AND METHODS

Blood pressure, plasma triglycerides and total cholesterol, platelet cytosolic pH (pHi) and the microviscosity of both outer membrane leaflet (TMA-DPH fluorescence anisotropy) and membrane lipid core (DPH fluorescence anisotropy) were studied in platelets and erythrocyte ghosts of Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats fed either a low-salt diet (0.3% NaCl) until the age of 9, 15 or 24 weeks or a high-salt diet (4% NaCl) for 5 or 10 weeks after weaning.

RESULTS

At low salt intake, DPH but not TMA-DPH anisotropy increased with age in platelets of SS/Jr rats. Chronic high salt intake was accompanied by an increase of DPH anisotropy in platelets but not in erythrocyte ghosts of SS/Jr rats. Platelet DPH anisotropy correlated positively with blood pressure of salt-loaded SS/Jr rats. Chronic high salt intake also reduced pHi in platelets, the regulation of which seemed to be related to the changes in TMA-DPH anisotropy. This especially concerns the thrombin-induced pHi rise which was inversely related to basal pHi, plasma lipids and TMA-DPH anisotropy. Altered membrane lipid composition might be the underlying mechanism because both membrane microviscosity and platelet pHi regulation were reported to correlate significantly with plasma triglycerides and/or cholesterol.

CONCLUSIONS

Platelets of salt hypertensive Dahl rats are characterized by an increased microviscosity of membrane lipid core which correlated positively with blood pressure. The major influence of plasma triglycerides on DPH anisotropy should be taken into consideration when investigating the links between membrane microviscosity and blood pressure. On the other hand, the changes in microviscosity of the outer membrane leaflet might be involved in pHi regulation (probably through control of the Na+/H+ exchanger).