1. The action of insulin on plasma cyclic nucleotide concentrations in normal human subjects has been studied after intravenous injection, alone and in combination with glucagon.

2. After injection of insulin alone there was an initial small, though not significant, decrease in plasma cyclic AMP at 15 min followed by an increase to more than twice the initial concentration at 30 min. The increase was absent when hypoglycaemia was lessened by infusion of glucose after insulin injection.

3. Injection of insulin caused no significant change in plasma cyclic GMP concentration, whether or not glucose was infused after the hormone.

4. Glucagon (3–300 nmol, 10–1000 μg), caused a dose-dependent increase in plasma cyclic AMP concentration. The rise in plasma cyclic AMP produced by 3 or 30 nmol of glucagon was not significantly modified by simultaneous injection of insulin (44 nmol; 6 units).

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