Individual and combined actions of calcium, sodium, and potassium ions on baroreceptors in the rat.

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Abstract
The effects of changes in the extracellular concentrations of Na, K and Ca ions on the steady state pressure-discharge relationships of individual baroreceptors were examined using an in vitro aortic arch-aortic nerve preparation. The concentration-response relationships were determined for each of the 3 cations, and for the combined effects of changes in Ca2+-K+ and Ca2+-Na+. The concentrations tested ranged from normal values to values well above and below normal. For each ion and each combination, effects on the pressure-discharge curves appeared as continuous functions of concentrations. Lowering [Na+]o or [K+]o or increasing [Ca2+]o shifted pressure-response curves to the right. Increasing [K+]o or decreasing [Ca2+]o shifted the curves to the left. The steady state effects of small increases in [Na+]o were inconsistent. Combinations of changes in ion concentration enhanced the effects in a way that was consistent with the results of individual changes, although quantitative comparisons were not made. The Ca2+ curves for threshold pressure were shifted to the left by lowering [Na+]o or to the right by increasing [K+]o. Measurements of static volume, diameter and distensibility showed no significant changes in the vessel wall during these changes so that, for equivalent strains, nervous discharge was clearly altered. The main actions are probably on the receptor and its axon rather than on the smooth muscle of the vessel wall, although local mechanical effects cannot be excluded completely. The results were interpreted by applying the electrophysiological theory of nerve membranes. These effects of extracellular ions on baroreceptors may be significant due to the special role of the baroreceptors in regulatory reflexes.