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

The authors examined the effects of changes in the extracellular concentrations of Na, K, and Ca ions on the steady state pressure-discharge relationships of individual baroreceptors, using an in vitro aortic arch-aortic nerve preparation. We determined the concentration-response relationships for each of the three cations, as well as 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 concentration. Lowering [Na⁺]₀ or [K⁺]₀ or increasing [Ca²⁺]₀ shifted pressure-response curves to the right. Increasing [K⁺]₀ or decreasing [Ca²⁺]₀ shifted the curves to the left. The steady state effects of small increases in [Na⁺]₀ 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. Thus, the Ca²⁺ curves for threshold pressure were shifted to the left by lowering [Na⁺]₀ or to the right by increasing [K⁺]₀. 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. Hence, the main actions are likely to be 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 have particular significance due to the special role of the baroreceptors in regulatory reflexes.