Actions of sodium and potassium ions on baroreceptors of normotensive and spontaneously hypertensive rats.

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Abstract
Although mechanoreceptors are known to be sensitive to extracellular Na+ and K+ concentrations [(Na+]O and [K+]O), the ionic sensitivity of baroreceptors as it relates to their function was not reported. An in vitro aortic arch-aortic nerve preparation was used to study this problem in normotensive (NTR) and spontaneously hypertensive (SHR) rats. The threshold of the response to pressure is increased and the sensitivity reduced by lowering [Na+]O; an increase in [K+]O produces changes in the opposite direction. These changes in ionic environment have no effect on the pressure-volume relationship of the isolated aortic arch segment, so the effects are on the baroreceptors themselves. Differences in ionic sensitivity between SHR and NTR baroreceptors were not observed, but the number of receptors compared in this way was small. A proposed model based on conventional membrane electrophysiological theory describes the ionic effects observed in SHR and NTR baroreceptors. The increased threshold and decreased sensitivity of SHR baroreceptors can be accounted for by assuming that their membrane Na to K permeability ratio is reduced and the distensibility of their aortas is not significantly reduced. A mechanism such as this might be involved in acute resetting of baroreceptors. Na-sensitive baroreceptor reflexes are predicted, and reductions of as little as 5% in [Na+]O bathing baroreceptors reflexly increase arterial blood pressure and urine output.