Effect of sodium chloride on opiate receptor binding in spontaneously hypertensive rats: dependence on age.

The effect of 100 mM sodium chloride on the binding of [3H]-naltrexone to rat brain opiate receptors in spontaneously hypertensive (SHR) and normotensive (WKY) rats was studied. The percentage increase in binding in the presence of sodium chloride did not vary with age in WKY rats. Brain homogenates from 4 week old SHR rats incubated with 100 mM sodium chloride exhibited a similar increase in binding compared to age matched WKY rats. In contrast, brain preparations from 6, 14 and 20 week old SHR rats were more sensitive to sodium chloride, and the increase in binding of [3H]-naltrexone was significantly greater in these animals than in corresponding normotensive ones. Since blood pressure is increased in SHR rats compared to WKY rats at these ages, these results suggest that elevated blood pressure may be correlated with an increase in opiate receptor sensitivity to sodium chloride. The effect of in vivo sodium chloride was examined by feeding the animals a diet containing 4% salt. This concentration of salt did not significantly alter the binding of [3H]-naltrexone to rat brain homogenates prepared from 8 week old SHR rats. These results suggest that higher levels of sodium chloride and longer exposure to the diet may be required to observe the salt sensitivity produced by 100 mM salt in the in vitro radioreceptor assay.