Elevated phospholipase C and Na+-H+ exchange activity in spontaneously hypertensive rats

Genetic and biochemical determinants of abnormal monovalent ion transport in primary hypertension

Data obtained during the last two decades show that spontaneously hypertensive rats, an acceptable experimental model of primary human hypertension, possess increased activity of both ubiquitous and renal cell-specific isoforms of the Na+/H+ exchanger (NHE) and Na+-K+-2Cl- cotransporter. Abnormalities of these ion transporters have been found in patients suffering from essential hypertension. Recent genetic studies demonstrate that genes encoding the beta- and gamma-subunits of ENaC, a renal cell-specific isoform of the Na+-K+-2Cl- cotransporter, and alpha3-, alpha1-, and beta2-subunits of the Na+-K+ pump are localized within quantitative trait loci (QTL) for elevated blood pressure as well as for enhanced heart-to-body weight ratio, proteinuria, phosphate excretion, and stroke latency. On the basis of the homology of genome maps, several other genes encoding these transporters, as well as the Na+/H+ exchanger and Na+-K+-2Cl- cotransporter, can be predicted in QTL related to the pathogenesis of hypertension. However, despite their location within QTL, analysis of cDNA structure did not reveal any mutation in the coding region of the above-listed transporters in primary hypertension, with the exception of G276L substitution in the alpha1-Na+-K+ pump from Dahl salt-sensitive rats and a higher occurrence of T594M mutation of beta-ENaC in the black population with essential hypertension. These results suggest that, in contrast to Mendelian forms of hypertension, the altered activity of monovalent ion transporters in primary hypertension is caused by abnormalities of systems involved in the regulation of their expression and/or function. Further analysis of QTL in F2 hybrids of normotensive and hypertensive rats and in affected sibling pairs will allow mapping of genes causing abnormalities of these regulatory pathways.

Sodium-proton (Na+/H+) exchange inhibition increases blood pressure in spontaneously hypertensive rat

The current study was designed to determine the role of sodium-proton (Na+/H+) exchange in blood pressure regulation in sodium chloride (NaCl)-sensitive and NaCl-resistant spontaneously hypertensive rats and control Wistar-Kyoto rats (WKY) using 5-(N,N-hexamethylene)amiloride (HMA), a potent and selective inhibitor of Na+/H+ exchange. The response of mean arterial pressure to intravenous infusion of HMA was examined in conscious, unrestrained male rats maintained on normal (1%) or high (8%) NaCl diets for 3 weeks beginning at age 7 weeks. The HMA significantly increased mean arterial pressure in NaCl-sensitive spontaneously hypertensive rats and NaCl-resistant spontaneously hypertensive rats that were fed 1% NaCl, but not in WKY rats that were fed 1% NaCl; the 8% NaCl diet enhanced this pressor response in all 3 strains. The pressor response was accompanied by significant increases in plasma norepinephrine levels in NaCl-sensitive spontaneously hypertensive rats on both diets, but not in NaCl-resistant spontaneously hypertensive rats or WKY rats on either diet. There were no differences in steady-state levels (30-60 nM) of plasma HMA between diet groups in any strain. Therefore, administration of HMA in a dose at which it is highly selective for the Na+/H+ exchanger (Ki = 160 nM) caused a systemic pressor response in spontaneously hypertensive rats that was enhanced by dietary NaCl supplementation. With these data, it is suggested that inhibition of Na+/H+ exchange in vivo has a pressor effect greater in spontaneously hypertensive rats than in WKY rats and is further enhanced by NaCl supplementation.

Heparin lowers blood pressure and vascular calcium uptake in hypertensive rats

Increased calcium uptake by vascular tissue, leading to elevated cytosolic calcium, has been implicated in the pathophysiology of hypertension. Heparin treatment of hypertensive rats has been known to lower blood pressure but its mechanism is not known. This study examined the effect of chronic heparin treatment on systolic blood pressure, aortic calcium and 87Rubidium (86Rb) uptake of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. Starting at 12 weeks of age SHR and WKY rats were given either sodium heparin 300 units s.c. or equal amounts of saline once a day for a period of 6 weeks. At 18 weeks, systolic blood pressure, uptakes of calcium and 86Rb by aortae were significantly higher (p less than 0.01) in saline-treated SHR compared with heparin-treated SHR and WKY. Heparin treatment lowered the elevated calcium and 86Rb Uptake and blood pressure in SHR but had no effect on WKY. The parallel increase in systolic blood pressure and vascular calcium uptake suggests that increased calcium uptake mechanisms are associated with hypertension in SHR. Heparin appears to lower elevated blood pressure in SHR by lowering elevated vascular calcium uptake.