DYNAMICS OF α‐ADRENERGIC INHIBITION OF THE ADENYL CYCLASE—CYCLIC AMP SYSTEM IN HUMAN ADIPOSE TISSUE

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Abstract
In short‐term incubations of fat cells isolated from human adipose tissue, noradrenaline (0.2–2 × 10‐5 M), a mixed α‐ and β‐adrenergic agonist, stimulated only slightly cyclic AMP accumulation. In the presence of the α‐adrenergic blocking agent phentolamine (0.05–5 μg/ml), noradrenaline markedly augmented the amount of labelled cyclic nucleotide. The stimulatory effect of noradrenaline and noradrenaline plus phentolamine was abolished by propranolol (0.1–10 μg/ml). Theophylline (10‐4‐10‐3 M) slightly increased the [3H] cyclic AMP levels per se, and magnified the stimulatory action of other agents, but did not mimic the potentiating effect of phentolamine on noradrenaline action. Stimulation of [3H] cyclic AMP was rapid in onset, maximal at 3–6 min and followed by a decline or leveling off regardless of the agent or the combination of agents causing the initial stimulation. The results are compatible with the hypothesis of α‐adrenergic activity being mediated through inhibition of adenyl cyclase, though the possibility of a stimulatory action on the phosphodiesterase enzymes has not been ruled out.

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