Role of Cyclic AMP and Ca++ in the Metabolic and Relaxing Effects of Catecholamines in Intestinal Smooth Muscle

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Abstract
In rabbit colon muscle there was a quantitative correlation and a co‐ordination in time between relaxation and an increase in cyclic AMP on stimulation of adrenergic β‐receptors. The cyclic AMP content had increased before the muscle had started to relax. There was an increase in the phosphorylase a activity and reduction of the ATP content. The adrenergic β‐receptor blocking agent sotalol inhibited the relaxation and the increase in cyclic AMP content. Reduction of the Ca++ content of the muscle decreased the cyclic AMP content and the phosphorylase a activity. These effects were restored on addition of Ca++ ions. In a Ca‐poor muscle adrenergic β‐receptor stimulation still produced an increase in cyclic AMP content and phosphorylase a activity but there was an increase in the ATP content instead. The reduction of cyclic AMP which followed an adrenergic a‐receptor stimulation was eliminated in the Ca‐poor muscle. The existence of a Ca++‐accumulating ATP‐utilizing mechanism, stimulated by cyclic AMP, might explain the relation between relaxation, cyclic AMP and Ca++ and adrenergic β‐receptor stimulation.