Electromechanical effects of caffeine in isolated human atrial fibres

Effects of caffeine on the action potential and contractile force of human atrial fibres obtained at cardiac surgery were studied with standard microelectrode technique. In 4 mmol . litre-1 [K]o, the only significant action produced by 0.3 to 3 mmol . litre-1 caffeine on the electro-mechanical activity of relatively normal atrial fibres was a slight shortening of the action potential duration at 50% repolarisation. When the fibres were depolarised in 27 mmol . litre-1 [K]o or in atrial fibres showing slow responses in 4 mmol . litre-1 [K]o, however, caffeine could increase the upstroke of slow response and the force. In 18% of atrial fibres showing slow responses in 4 mmol . litre-1 [K]o, caffeine induced spontaneous discharges and potentiated afterdepolarisations. The positive inotropic and the arrhythmogenic effects of caffeine could be diminished by pretreating the fibres with propranolol or Ca antagonists (diltiazem and verapamil). In fibres beating spontaneously in normal [K]o, caffeine accelerated spontaneous rhythms initially and then depressed them. Propranolol potentiated the later depression but did not block the initial acceleration. The results suggest that caffeine increases the transmembrane Ca influx and enhances the release of Ca from the intracellular stores in human atrial fibres. As a consequence, caffeine could induce arrhythmias in atria from certain individuals.