Complications associated with rapid caffeine application to cardiac myocytes that are not voltage clamped

The rapid application of caffeine to cardiac myocytes is commonly used to a ssess changes in the Ca2+ content of the sarcoplasmic reticulum (SR) and to study other parameters of intracellular Ca2+ regulation. Here we examined the effects of rapid caffeine application on membrane potential, intracellu lar Ca2+, and cell shortening in ventricular myocytes (rat, rabbit, guinea pig, dog) and atrial myocytes (rabbit) that were not voltage clamped. Condi tioning pacing was used to achieve a steady-state level of SR Ca2+ loading prior to caffeine (10 mM) application. Caffeine transiently depolarized myo cytes as expected from activation of forward Na+-Ca2+ exchange. However, we also found in each species (50% rat, 36% rabbit ventricular, 53% rabbit at rial, 56% guinea pig, 31% dog) that the caffeine-induced depolarization cou ld also trigger an action potential. Caffeine-triggered potentials were com pletely blocked by thapsigargin (1 mu M). The Ca2+ transient and contractio n that accompanied caffeine-triggered action potentials had a larger magnit ude and slower rate of decline (or relaxation) than occurred during caffein e-induced subthreshold depolarizations. Thus, the use of rapid caffeine app lication to study SR function and [Ca2+](i) regulation in myocytes that are not voltage clamped can yield erroneous results. (C) 1998 Academic Press.