Increased Na + - H + exchange activity in cultured vascular smooth muscle cells from stroke-prone spontaneously hypertensive rats

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Abstract
Na+ -H+ exchange activities were studied in vascular smooth muscle cells (VSMC) obtained from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY). The cytoplasmic pH of VSMC was detected by 2''-7''-bis (carboxyethyl)-carboxyfluorescein (BCECF), which was used as a fluorescent pH probe with a fluorescent microplate reader; this enabled us to measure th cytoplasmic pH of cells attached to multiwell culture plates. We measured cytoplasmic pH recovery in quiescent VSMC after acid loading with 20 mmol/l NH4Cl in HCO3- -free buffer. This recovery was inhibited either by removal of extracellular sodium or by addition of 5-N-ethyl-N-isopropyl amiloride (EIIPA). The initial recovery rate was dependent on extracellular sodium concentration. Therefore, this change in cytoplasmic pH was presumed to be due to amiloride-sensitive Na+ -H+ exchange activity represented by the cytoplasmic pH recovery rate was significantly higher in VSMC from SHRSP than in WKY cells at extracellular sodium concentrations .gtoreq. 20 mmol/l. In contrast, the steady-state cytoplasmic pH of quiescent VSMC from SHRSP was lower than that of WKY cells in HCO3 -free buffer. These findings seemed to contradict each other, and suggested that cytoplasmic pH was regulated not only by Na+ -H+ exchange but also by other complicated mechanisms.

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