SUMMARY The net passive influx of Na + and efflux of K + (orthodirection) through the red blood cell membranes from spontaneously hypertensive rats (SHR) were observed to be significantly higher (p < 0.05) than those of three strains of normotensive rats when the measurements were made at 4°C. Similar comparative studies, carried out at 37°C, in the absence or presence of ouabain, showed no difference in these fluxes through this membrane from SHR compared to those from Wistar-Kyoto (VVKY) rats, one of the normotensive strains. A study was undertaken to determine the temperature at which the greater cation fluxes in SHR red blood cells occurred. The net fluxes of Na + and K + decreased as the temperature was reduced from 37° to 15°C, but a paradoxical increase in the fluxes was observed as the temperature was decreased from 15° to 4°C. Only with this temperature shift (15°t o 4°C) was the increase in flux significantly greater in SHR than in VVKY cells. Subsequent studies were designed to determine whether the difference in the transport systems of red blood cells of SHR and VVKY could be observed in fluxes of these cations in either direction across the membrane. For "reverse direction" flux studies, red blood cells were loaded with Na + (to 130 mEq/liter cell water) and depleted of K + (to 30 mEq/liter cell water) by incubation with the ionophore monensin. The reverse passive efflux of Na + and influx of K + at 4°C of cells from SHR were significantly greater than those of VVKY. Thus, the abnormality of the red blood cell membrane in SHR behaves as if it were just an increase in the size or number of pores through which Na + and K + diffuse freely at low temperatures. (Hypertension 6: 42-48, 1984)
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