The effects of calcium on adrenergic neuron blockade

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Abstract
The effects of increasing extracellular calcium were investigated on the responses to sympathetic nerve stimulation of three isolated organs; rabbit ileum, guinea-pig vas deferens and rabbit ear artery. A rise in the calcium concentration increased the responses of the ileum to low frequency stimulation, the maximum increase being obtained at 8.8 mM calcium. After partial blockade by guanethidine of the responses of the ileum to high frequency stimulation, raised calcium concentrations again increased the responses. The increase was similar in guanethidine-treated and untreated preparations and the maximum increase in both was obtained using 8.8 mM calcium. In the vas deferens and rabbit ear artery preparations an increase in extracellular calcium did not antagonize the blocking action of guanethidine. These experiments do not therefore support the theory that guanethidine acts by preventing the entry of calcium into the sympathetic nerve endings.