Renal phospholipase C and diglyceride lipase activity in spontaneously hypertensive rats

Vasopressin lowers renal epoxyeicosatrienoic acid levels by activating soluble epoxide hydrolase

Activation of the thick ascending limb (TAL) Na⁺-K⁺-2Cl^- cotransporter (NKCC2) by the antidiuretic hormone arginine vasopressin (AVP) is an essential mechanism of renal urine concentration and contributes to extracellular fluid and electrolyte homeostasis. AVP effects in the kidney are modulated by locally and/or by systemically produced epoxyeicosatrienoic acid derivates (EET). The relation between AVP and EET metabolism has not been determined. Here, we show that chronic treatment of AVP-deficient Brattleboro rats with the AVP V2 receptor analog desmopressin (dDAVP; 5 ng/h, 3 days) significantly lowered renal EET levels (-56 ± 3% for 5,6-EET, -50 ± 3.4% for 11,12-EET, and -60 ± 3.7% for 14,15-EET). The abundance of the principal EET-degrading enzyme soluble epoxide hydrolase (sEH) was increased at the mRNA (+160 ± 37%) and protein levels (+120 ± 26%). Immunohistochemistry revealed dDAVP-mediated induction of sEH in connecting tubules and cortical and medullary collecting ducts, suggesting a role of these segments in the regulation of local interstitial EET signals. Incubation of murine kidney cell suspensions with 1 μM 14,15-EET for 30 min reduced phosphorylation of NKCC2 at the AVP-sensitive threonine residues T96 and T101 (-66 ± 5%; P < 0.05), while 14,15-DHET had no effect. Concomitantly, isolated perfused cortical thick ascending limb pretreated with 14,15-EET showed a 30% lower transport current under high and a 70% lower transport current under low symmetric chloride concentrations. In summary, we have shown that activation of AVP signaling stimulates renal sEH biosynthesis and enzyme activity. The resulting reduction of EET tissue levels may be instrumental for increased NKCC2 transport activity during AVP-induced antidiuresis.

Phospholipase A2: its usefulness in laboratory diagnostics

Phospholipase A2 (PLA2) is an enzyme that catalyzes the hydrolysis of membrane phospholipids. This article reviews the source and structure of PLA2, the involvement of the enzyme in various biological and pathological phenomena, and the usefulness of PLA2 assays in laboratory diagnostics. Of particular importance is the role of PLA2 in the cellular production of mediators of inflammatory response to various stimuli. Assays for PLA2 activity and mass concentration are discussed, and the results of enzyme determinations in plasma from patients with different pathological conditions are presented. The determination of activity and mass concentration in plasma is particularly useful in the diagnosis and prognosis of pancreatitis, multiple organ failure, septic shock, and rheumatoid arthritis. A very important result is the demonstration that PLA2 is an acute phase protein, like CRP. Indeed, there is a close correlation between PLA2 mass concentration and CRP levels in several pathological conditions. Although the determination of C-reactive protein is much easier to perform and is routinely carried out in most clinical laboratories, the assessment of PLA2 activity or mass concentration has to be considered as a reliable approach to obtain a deeper understanding of some pathological conditions and may offer additional information concerning the prognosis of several disorders.

Effects of dietary docosahexaenoic acid on survival time and stroke-related behavior in stroke-prone spontaneously hypertensive rats

1. Dietary docosahexaenoic acid (DHA) suppressed the age-dependent increase in systolic blood pressure and prolonged the average survival time of stroke-prone spontaneously hypertensive rats (SHRSP). 2. Dietary DHA (1% and 5% in diets) altered the circadian rhythm of SHRSP, causing significant increases in ambulatory activity during the dark period. At the onset of stroke, desynchronization with light and dark phases and new biological rhythms were noted in all of the control SHRSP (DHA 0%). DHA treated SHRSP did not show such behavioral changes. 3. These effects were accompanied by the increase of DHA and the decrease of AA levels in plasma and brain cortex. 4. It was concluded that dietary DHA suppresses the development of hypertension and stroke-related behavioral changes, resulting in prolongation of the SHRSP's life span.

Endothelium-derived contracting factors in resistance arteries of young spontaneously hypertensive rats before development of overt hypertension

Vascular relaxations are impaired in adult spontaneously hypertensive rats (SHRs) because of increased production of an endothelium-derived, cyclooxygenase-dependent contractile factor or factors. To test the hypothesis that alterations in endothelial function precede and contribute to the development of overt hypertension in SHRs, we compared in myographs endothelium-mediated relaxations of mesenteric resistance arteries from 4-week-old SHRs and Wistar-Kyoto (WKY) rats. Acetylcholine (10(-9) to 10(-4) M) induced comparable relaxations in SHR and WKY arteries precontracted (ED50) with norepinephrine. In arteries obtained from SHRs but not from WKY rats, relaxations were replaced by contractile responses with higher concentrations of acetylcholine (10(-6) to 10(-5) M). The contractile responses were endothelium dependent, were augmented by nitro L-arginine (10(-4) M), and were prevented by pretreatment with indomethacin (10(-5) M) or 3-amino-1,2,4-triazole (10(-3) M), an inhibitor of superoxide anion production via the cyclooxygenase pathway. Inhibition of thromboxane synthetase (CGS-13080, 5 x 10(-5) M) and antagonism of prostaglandin H2/thromboxane A2 receptors (SQ-29,548, 5 x 10(-5) M) failed to block the contractile response to acetylcholine in SHR arteries. Acetylcholine-mediated relaxations were significantly impaired in mesenteric arteries from 16-week-old SHRs but not from WKY rats. Endothelium-independent relaxations produced by sodium nitroprusside and contractile responses to norepinephrine and endothelin were comparable in arteries from SHRs and WKY rats of all ages. In summary, endothelium-dependent relaxations of mesenteric arteries from "prehypertensive" SHR rats were impaired by the production of a contractile factor (or factors) that appears to be superoxide anions.

Hypotensive effect associated with a phospholipase C-δ1 gene mutation in the spontaneously hypertensive rat

To identify the genes responsible for blood pressure in the spontaneously hypertensive rat strain, we performed a cosegregation analysis between the genotype and blood pressure in a set of male F2 rats obtained by crossmating SHR with Wistar-Kyoto rats, a parental normotensive strain. Our investigation revealed that the phospholipase C-delta 1 polymorphism, which resulted in missense mutation, cosegregates with the lower blood pressure in SHR, and that PLC-delta 1 gene is located on chromosome 8. On the other hand, we found the lack of cosegregation between blood pressure and the nerve growth factor receptor gene, which is linked to a hypertensinogenic gene locus (denoted as BP/SP-1) on chromosome 10. We propose that PLC-delta 1 gene itself of closely linked gene on chromosome 8 is a new candidate with the hypotensive effect, and that BP-SP1 locus does not directly contribute to blood pressure elevation in original SHR.

Diminished phospholipase C activation by dopamine in spontaneously hypertensive rats

It is reported that a defect in dopamine-1 (DA-1) receptor adenylate cyclase coupling in the proximal convoluted tubule in the spontaneously hypertensive rat may contribute to the diminished natriuretic response to DA-1 receptor agonists. Since the tubular DA-1 receptor is also coupled to phospholipase C, and both of these cellular signaling processes are involved in DA-1 receptor-mediated diuresis and natriuresis, it is important to know whether a similar defect is also present in DA-1 receptor-coupled phospholipase C pathway. The present study was therefore designed to determine the functional status of DA-1 receptor-phospholipase C coupling system of adult spontaneously hypertensive rats using a renal cortical slice preparation. In addition, the renal response to exogenously administered dopamine (1 microgram/kg/min i.v.) was also determined. We found that basal phospholipase C activity was significantly higher in hypertensive rats than in age-matched Wistar-Kyoto rats (7.36 +/- 0.32% versus 5.61 +/- 0.27%, p less than 0.05). However, compared with the normotensive controls, dopamine-induced increases in phospholipase C activity were significantly attenuated in the preparations of hypertensive rats in a concentration-dependent manner (13 +/- 6% versus 38 +/- 6% for 1 mM dopamine, p less than 0.05; 49 +/- 6% versus 71 +/- 9% for 3 mM dopamine, p less than 0.05; 50 +/- 16% versus 106 +/- 22%, p less than 0.05 for 10 mM dopamine).(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipid metabolism in cardiomyopathic hamster heart cells

We demonstrated that the activities of phosphatidylinositide-specific phospholipase C, inositol 1,4,5-trisphosphate (IP3) kinase, and IP3 phosphatase were enhanced in cardiomyopathic hamster hearts (BIO 14.6 and BIO 53.58) in comparison to control hamsters (F1b). Release of both arachidonic acid and prostacyclin was markedly enhanced by norepinephrine in the cardiomyopathic hamsters. Phospholipase C in heart has high substrate specificity to phosphatidylinositol. IP3 production was markedly enhanced in the cardiomyopathic hamsters. We also determined the intracellular calcium concentration, which was higher in BIO 53.58 hamsters than in BIO 14.6 hamsters at 5-20 weeks of age. There was no significant difference in the intracellular calcium level between F1b and BIO 14.6 hamsters at 5 weeks of age. These results suggest that phosphatidylinositol turnover stimulated by norepinephrine may produce high intracellular calcium levels in both BIO 14.6 and BIO 53.58 myocytes. In addition, in BIO 53.58 hamsters, some mechanism such as the sarcoplasmic reticulum, which controls the intracellular calcium level, may deteriorate in function. We concluded from these results that a prolonged high intracellular calcium level may lead to the death of BIO 53.58 myocytes.

Alterations of phosphoinositide-specific phospholipase C and protein kinase C in the myocardium of spontaneously hypertensive rats

In order to determine whether phosphoinositide metabolism is altered in hypertensive cardiac hypertrophy, phospholipase C (PLC) and protein kinase C activities were measured in hearts from 4- and 20-week-old spontaneously hypertensive rats (SHR) and age-matched, normotensive Wistar-Kyoto rats (WKY). PLC activities were assayed using phosphatidylinositol (PI) and phosphatidylinositol-4,5-bisphosphate (PIP2) as substrates to assess the substrate specificity. PI-hydrolyzing PLC activity (PI-PLC) was predominantly located in the cytosol, and its activity was similar in both strains. Membrane-bound PIP2-hydrolyzing PLC activity (PIP2-PLC) was significantly lower in 20-week-old SHR than in WKY, but there was no significant difference in soluble PIP2-PLC. Protein kinase C activity was significantly elevated in 20-week-old SHR and Ca2(+)-phospholipid-dependent phosphorylation was observed in the proteins of molecular weight 26, 32, 43, and 95 KDa. In 4-week-old prehypertensive SHR, there were no significant differences in PI-PLC, PIP2-PLC, or protein kinase C activities as compared with age-matched WKY. These data demonstrated that protein kinase C and membrane-bound PIP2-PLC are altered during the period of hypertension development. These alterations may have important roles in the development or maintenance of hypertensive cardiac hypertrophy in SHR.

Role of the kidney in the pathogenesis of primary hypertension

Primary hypertension in animals and humans probably represents several different pathophysiological states rather than being a uniform nosological entity. Among other factors, renal mechanisms may be primarily and secondarily involved. The availability of genetically homologous animal models for hypertension has greatly promoted studies on the etiology and pathogenesis of high blood pressure disease. In particular, renal transplantation studies between genetically hypertensive and normotensive rats from three different models have provided strong evidence for a primary role of the kidney in genetic hypertension. Other factors, such as vascular, neural, and humoral mechanisms have also been shown to be involved and may be particularly effective in increasing blood pressure, when they act through the kidney. Several functional and biochemical differences have been identified between kidneys from genetically hypertensive and normotensive animals. However, the relative contribution of each of these factors to the development of primary hypertension remains to be determined. Evidence from studies on human renal graft recipients also indicates that, among other factors, the kidney plays an important role in the development of primary hypertension in humans.

Lipid alterations in renal membrane of stoke-prone spontaneously hypertensive rats

Phospholipase A2 activity, phospholipids, and phospholipid fatty acids were investigated in renal membrane of male stroke-prone spontaneously hypertensive rats (SHRSP) and age-matched Wistar-Kyoto rats. Renal phospholipase A2 activity increased and membranous phospholipids especially phosphatidylcholine and phosphatidylethanolamine, decreased with age in SHRSP. Arachidonate in phospholipid also decreased with age in SHRSP. To determine the effect of pressure load on the lipid alterations in renal membrane, SHRSP that received antihypertensive treatment with hydralazine, enalapril, or nicardipine for 5 weeks were compared with those without treatment. Antihypertensive treatments prevented phospholipid degradation and increased arachidonate in phospholipid relative to the control group. Phospholipase A2 activity in each group treated with antihypertensive drugs did not differ from that in the control group. These results suggest that the course of hypertension causes renal membranous phospholipid degradation and increases phospholipase A2 activity. Antihypertensive treatments may prevent these lipid alterations in SHRSP. These renal membranous structural changes may provide an explanation not only for functional abnormalities such as decreased membrane fluidity but also for the progress of hypertension.

Renal alpha 1-adrenergic receptor response coupling in spontaneously hypertensive rats

Renal sympathetic antidiuretic, antinatriuretic, and vasoconstrictor responses are mediated by alpha 1-adrenergic receptors in the normal rat. Since the renal nerve has been implicated in the pathogenesis of rat genetic hypertension, we investigated renal alpha 1-adrenergic receptor coupling to phosphoinositide turnover in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). In cortical slices from adult (13-week-old) SHR and WKY, stimulation with norepinephrine (10(-7)-10(-3) M) caused a concentration-dependent increase in accumulation of [3H]inositol phosphates. However, dose-response curves for SHR characteristically displayed a depression of the maximum response as compared with those for WKY. Baseline accumulation of [3H]inositol phosphates was not different between strains (39.4 +/- 2.2 cpm/mg tissue/hr for WKY and 34.4 +/- 2.1 cpm/mg tissue/hr for SHR slices; n = 5 rats/group, determined in triplicate). Antagonist competition studies revealed that norepinephrine-stimulated (10(-4) M) [3H]inositol phosphate accumulation was mediated by alpha 1-adrenergic receptors (IC50) for prazosin: 65 +/- 11 nM for SHR and 64 +/- 5 nM for WKY). The reduction in norepinephrine-stimulated [3H]inositol phosphate accumulation in SHR cortex was not the result of the hypertension, since it was also present in cortical slices from young (4-week-old) SHR in which the blood pressure was not yet significantly different from that in WKY and since [3H]inositol phosphate accumulation was unchanged from control values in rats made hypertensive by treatment with deoxycorticosterone acetate. Scatchard analysis of [3H]prazosin binding in renal cortical membranes of young and adult SHR and WKY revealed no significant differences in alpha 1-adrenergic receptor density or affinity between strains at either age. Our results suggest that renal alpha 1-adrenergic receptor coupling to phospholipase C is less efficient in SHR than in WKY. This impaired response is not the result of hypertension or changes in receptor density; this defect may play a role in increased renal sympathetic nerve activity and in the development or maintenance of hypertension in SHR.