Category: receptor_pharmacology
Explains the fluttery 'skipped-beat' sensation many coffee-drinkers notice — premature ventricular complexes (PVCs) and atrial ectopy that the conscious heart-tap registers as a pause-then-thud. Caffeine is a competitive antagonist at adenosine A1 and A2A receptors at ordinary intake concentrations (Fredholm 1999). Endogenous adenosine, acting on A1 receptors at the sinoatrial and atrioventricular nodes, slows automaticity and conduction — this is the mechanism behind IV adenosine as the drug of choice for terminating SVT (DiMarco 1990). When caffeine blocks A1, the tonic adenosinergic brake on nodal pacemaking is lifted, which combined with secondary catecholamine release (β1 → cAMP → PKA → If/IK shifts) raises ectopy in susceptible substrates. The 2023 CRAVE trial (Marcus 2023) is the cleanest prospective data: a randomized day-on / day-off design in 100 ambulatory adults showed coffee days had measurably more PVCs (though fewer atrial ectopic beats — the atrial story is more nuanced). CYP1A2 polymorphism gates exposure: slow metabolizers (CYP1A2*1F) accumulate caffeine longer and show stronger cardiovascular responses (Cornelis 2006 linked slow metabolizer + heavy coffee to MI risk). Effect is dose-dependent and tachyphylactic — habituated users have upregulated A1 receptors and largely lose the ectopy response. Importantly, the prospective evidence does NOT support clinical atrial-fibrillation risk from moderate coffee intake (Klatsky 2011 review); the user-felt 'skipped beats' are mostly benign PVCs amplified by caffeine-driven sympathetic tone.