Reduced electrogenic sodium-potassium pump in arterioles during renovascular hypertension

Protective effect of flunarizine on blood-brain barrier permeability alterations in acutely hypertensive rats

BACKGROUND AND PURPOSE

Increased cerebrovascular permeability to protein is a well-documented finding in acute and chronic hypertension. In this study, we examined the effect of pretreatment with a calcium entry blocker, flunarizine, on the increased cerebrovascular permeability to protein that develops in norepinephrine-induced acute hypertension.

METHODS

Protein transfer was assessed qualitatively with Evans blue dye and quantitatively with iodine-125-labeled serum albumin.

RESULTS

Brains of hypertensive rats showed increased permeability to both tracers. The number and size of the areas of Evans blue extravasation were smaller in the hypertensive groups pretreated with flunarizine intravenously. This was supported by the quantitative studies, which demonstrated a significant decrease in protein transfer in total brain of hypertensive rats pretreated with intravenous flunarizine, 1 mg/kg (p less than 0.005) and 2.5 mg/kg (p less than 0.001). Data from individual brain regions showed that pretreatment with flunarizine resulted in significant reduction of protein transfer in most brain regions.

CONCLUSIONS

These data support the hypothesis that calcium plays a role in increased cerebral endothelial permeability in hypertension.

Impaired potassium-induced dilation in hypertensive rat cerebral arteries does not reflect altered Na+,K(+)-ATPase dilation

We have recently demonstrated that K(+)-induced dilation of cerebral resistance-sized vessels has two independent components, only one of which seemed sodium pump dependent. In our current investigation, potassium-induced dilation of spontaneous tone was compared in cerebral arteries from normotensive Wistar-Kyoto rats and age-matched stroke-prone spontaneously hypertensive rats. Branches of the posterior cerebral artery were cannulated and pressurized, and these vessels developed spontaneous tone. After a 5-minute period in K(+)-free physiological saline solution, K+ was increased in 1-mM increments to a final concentration of 15 mM. In the normotensive arteries, K+ concentrations between 0 and 5 mM K+ resulted in dilations that had a transient (sodium pump-dependent) component, and K+ concentrations in excess of 7 mM produced dilations that lacked a transient (sodium pump-independent) component. Similar branches from the hypertensive rat also responded with transient dilations to K+ (less than 5 mM), and these were significantly greater at 3 mM K+. However, the maintained dilations to K+ (greater than 7 mM), noted in preparations from Wistar-Kyoto rats, were absent in seven of eight preparations. Thus, the impaired dilations, in the hypertensive vessels, to K+ described here is a consequence of altered function of some sodium pump-independent component rather than altered Na+,K(+)-ATPase activity.

Ultracytochemical localisation of Na+, K(+)-ATPase in cerebral endothelium in acute hypertension

This ultracytochemical study was undertaken to determine whether increased arteriolar permeability in acute hypertension is accompanied by altered localisation of the ouabain-sensitive, K(+)-dependent p-nitrophenyl-phosphatase (K(+)-NPPase), a component of the Na+, K(+)-ATPase system. Rats were injected with horseradish peroxidase (HRP) intravenously and acute hypertension was induced by a 2-min infusion of angiotensin amide. Rats were killed at 3 and 15 min, following which brains were sliced and reacted for demonstration of K(+)-NPPase and HRP reaction product. Vessels of normotensive and hypertensive rats that were nonpermeable to HRP showed discontinuous distribution of K(+)-NPPase on the outer plasma membranes of endothelial and adventitial cells of arterioles and endothelial cells and pericytes of capillaries. Arterioles of the hypertensive rats which were permeable to HRP showed marked reduction of K(+)-NPPase localisation in their walls at 3 min while at 15 min when the blood pressure had returned to resting levels the enzyme localisation was similar to controls. This study demonstrates transient alteration of the NA+, K(+)-ATPase system during increased endothelial permeability in acute hypertension. The implication of this finding and our previous observation of reduced Ca2(+)-ATPase localisation in endothelial plasma membranes in acute hypertension has been discussed.