Decreased cytosolic calcium and prostaglandin synthesis in prehypertensive rats

Cholesterol affects flow-stimulated cyclooxygenase-2 expression and prostanoid secretion in the cortical collecting duct

Essential hypertension (eHTN) is associated with hypercholesterolemia, but how cholesterol contributes to eHTN is unknown. Recent evidence demonstrates that short-term dietary cholesterol ingestion induces epithelial Na channel (ENaC)-dependent Na absorption with a subsequent rise in blood pressure (BP), implicating cholesterol in salt-sensitive HTN. Prostaglandin E2 (PGE2), an autocrine/paracrine molecule, is induced by flow in endothelia to vasodilate the vasculature and inhibit ENaC-dependent Na absorption in the renal collecting duct (CD), which reduce BP. We hypothesize that cholesterol suppresses flow-mediated cyclooxygenase-2 (COX-2) expression and PGE2 release in the CD, which, in turn, affects Na absorption. Cortical CDs (CCDs) were microperfused at 0, 1, and 5 nl·min(-1)·mm(-1), and PGE2 release was measured. Secreted PGE2 was similar between no- and low-flow (151 ± 28 vs. 121 ± 48 pg·ml(-1)·mm(-1)) CCDs, but PGE2 was greatest from high-flow (578 ± 146 pg·ml(-1)·mm(-1); P < 0.05) CCDs. Next, mice were fed either a 0 or 1% cholesterol diet, injected with saline to generate high urine flow rates, and CCDs were microdissected for PGE2 secretion. CCDs isolated from cholesterol-fed mice secreted less PGE2 and had a lower PGE2-generating capacity than CCDs isolated from control mice, implying cholesterol repressed flow-induced PGE2 synthesis. Next, cholesterol extraction in a CD cell line induced COX-2 expression and PGE2 release while cholesterol incorporation, conversely, suppressed their expression. Moreover, fluid shear stress (FSS) and cholesterol extraction induced COX-2 protein abundance via p38-dependent activation. Thus cellular cholesterol composition affects biomechanical signaling, which, in turn, affects FSS-mediated COX-2 expression and PGE2 release via a p38-dependent mechanism.

ETA receptor-mediated Ca2+ signaling in thin descending limbs of Henle's loop: impairment in genetic hypertension

BACKGROUND

Endothelins (ET) have diuretic and natriuretic actions via ETB receptors that are found in most renal tubular segments, although the thin limbs have not been studied. Data also suggest that dysfunction of the renal ET system may be important in the pathogenesis of hypertension. The present study was aimed at determining the presence and nature of ET receptors in the thin limbs of Henle's loop and their ability to activate a Ca2+-dependent signaling pathway, as well as whether ET-induced Ca2+ signals are altered in hypertension.

METHODS

Reverse transcription-polymerase chain reaction (RT-PCR) and Fura 2 fluoreselected strains of Lyon rats with low-normal (LL), normal (LN), and high (LH) blood pressure.

RESULTS

In SD rats, ET induced Ca2+ signals in DTL of long-looped nephrons, but not in DTL of short loops, or in ascending thin limbs. Ca2+ increases were abolished by BQ123, an antagonist of the ETA receptor, but not by BQ788, an antagonist of the ETB subtype. Endothelin-3 and sarafotoxin 6c, two ETB receptor agonists, were both inactive. RT-PCR showed the presence of both ETA and ETB receptor mRNA. Ca2+ signals measured scence measurements of [Ca2+]i were made to characterize ET receptors in descending thin limbs (DTL) of Sprague-Dawley rats, spontaneously hypertensive (SH) rats, and control Wistar-Kyoto (WKY) rats, and the three in DTL of WKY LL and LN rats were similar to those in Sprague-Dawley rats, but were significantly diminished (LH) or abolished (SH) in hypertensive rats.

CONCLUSION

A functional ETA receptor activating a Ca2+-dependent pathway is expressed in DTL. This ETA-induced calcium signaling is impaired in two strains of genetically hypertensive rats.

Impaired myogenic responsiveness of renal microvessels in Dahl salt-sensitive rats

The mechanisms mediating abnormal renal autoregulation in Dahl salt-sensitive (DS) rats have not been fully defined. In the present study, we assessed myogenic responsiveness of interlobular arteries (ILAs), afferent arterioles (AAs), and efferent arterioles in isolated perfused hydronephrotic Dahl rat kidneys. Dahl rats were divided into four groups according to strain (Dahl salt-resistant [DR] or DS rats) and dietary sodium manipulation (rats fed low or high salt diets). Systolic blood pressure was elevated only in DS rats fed the high salt diet (202 +/- 4 mm Hg, p less than 0.05). Myogenic responses were obtained by stepwise elevation of renal arterial pressure. Vessel diameters were determined by computer-assisted videomicroscopy. Preglomerular microvessels of DS and DR rats responded differently to changes in renal arterial pressure. AAs and ILAs manifested diminished myogenic responsiveness to increasing renal arterial pressure in DS rats compared with DR rats (p less than 0.05). Both AAs and ILAs in DS rats manifested a higher threshold pressure for eliciting myogenic responses and a decrease in maximal pressure-induced vasoconstriction. The sensitivity of the AA myogenic response to nifedipine was enhanced in DS rats compared with DR rats (p less than 0.05). For rats fed the high salt diet, preglomerular vessels exhibited reduced myogenic responsiveness in both strains. In contrast to preglomerular microvessels, efferent arterioles from all four groups of rats failed to exhibit pressure-induced vasoconstriction. Our data suggest that diminished myogenic responsiveness of AAs and ILAs in DS rats contributes to impaired renal autoregulation in this strain.

Pathogenesis of the essential hypertensions

The pathogenesis of essential hypertension (EH) is reviewed with a special focus on the development phase or the pre-hypertensive period. Three animal models are presented: the spontaneously hypertensive rat, the Dahl's salt-sensitive rat, and the Milan hypertensive rat. Some of the findings in animal models have inspired new fields and technical approaches for studying EH in man. From the original idea of Page, a new mosaic of various etiological parameters serves as a basis for reviewing the multiple facets of EH in man. One must conclude that EH is heterogeneous disease and most likely every single hypertensive patient belongs to a subgroup of the whole population of hypertensives.

Calcium and prostaglandin E2 in renomedullary interstitial cells

Renomedullary interstitial cells cultured from the Dahl salt-resistant rat have higher levels of basal cytosolic calcium and prostaglandin E2 and are more responsive to vasopressin than interstitial cells from the Dahl salt-sensitive rat. We examined the potential role of inositol phospholipid hydrolysis in mediating these differences. Vasopressin-induced increases in labeled inositol phosphates were enhanced in renomedullary interstitial cells from Dahl salt-resistant compared with those from salt-sensitive rats. Addition of neomycin reduced basal production of labeled inositol phosphates and abolished the increase in inositol phosphates induced by vasopressin. Neomycin also prevented the peak decline pattern in cytosolic Ca2+ seen with vasopressin but did not influence basal cytosolic Ca2+. In the presence of neomycin, vasopressin induced a modest but prolonged increase in cytosolic calcium. In contrast to its marked effects on inositol phosphate production, neomycin was without effect on basal or vasopressin-responsive prostaglandin E2 production. Moreover, basal and vasopressin-induced increases in cytosolic Ca2+ remained higher in renomedullary interstitial cells from Dahl salt-resistant versus those from salt-sensitive rats exposed to neomycin. The results do not support a requirement for phospholipase C-induced inositol phospholipid hydrolysis in the mediation of vasopressin actions on prostaglandin E2 production by renomedullary interstitial cells and imply that the differences in cytosolic Ca2+ and prostaglandin E2 seen in these two cell lines are not related to differences in inositol phospholipid metabolism.