Phosphodiesterase inhibition and rising cAMP
Beyond blocking adenosine receptors, caffeine can inhibit phosphodiesterase, the enzyme that breaks down the intracellular messenger cyclic AMP (cAMP). With less breakdown, cAMP accumulates inside cells. Because cAMP is the second messenger that relays signals from adrenaline and noradrenaline, higher cAMP amplifies catecholamine-driven responses such as increased heart rate, fat mobilization, and heightened alertness.
An important caveat: meaningful phosphodiesterase inhibition tends to require caffeine concentrations higher than those reached with ordinary dietary intake, so at normal doses adenosine-receptor blockade is generally considered the dominant mechanism.
Why the two mechanisms reinforce each other
Adenosine-receptor blockade and cAMP accumulation point in the same direction, toward greater sympathetic, stimulant-like activity, which is why the diagram presents them as a cascade. The net effect depends strongly on dose, individual metabolism, and habitual use.
This is general educational information, not medical advice. High caffeine intake can raise heart rate and blood pressure, and people with cardiovascular concerns should consult a qualified professional.