Measurement of electrically-induced Ca2+ transients in Quin2-loaded cardiac myocytes.

Quin2 is a fluorescent Ca2+-sensitive indicator which in its acetoxymethyl ester form can be loaded into the cytosolic compartment of cells without any disruption of the plasma membrane. Quin2 has been used with many cell types to measure changes of [Ca2+]i in response to hormones and other stimuli. In this report we demonstrate that Quin2 can be used to follow Ca2+ transients induced by electrical depolarization of excitable cells such as myocytes. Unlike microelectrodes and the microinjection of Ca2+ indicators, the use of Quin2 allows the collection of signals from large numbers of cells simultaneously. Ca2+ transients measured using Quin2 show a rapid rise from a basal [Ca2+]i of 200 nM to peak values of up to 500 nM within 40 ms in the presence of the beta-adrenergic agonist isoproterenol. As the intracellular Quin2 load is increased, Quin2 acts to buffer the free Ca2+ change, causing a decrease of the peak Ca2+ concentration and an increase of the time required for relaxation of the transient. Thus, Quin2 can serve both to monitor and to perturb Ca2+ transients in heart cells.