BEAT-TO-BEAT MEASUREMENTS OF [CA2-MUSCLE AFTER CHEMICAL LOADING OF AEQUORIN(](I) AND FORCE IN FERRET CARDIAC)

This communication reports the development of a modified procedure for chemical loading of aequorin in small multicellular cardiac preparati ons, with special emphasis directed toward the implementation of a new method for computer-controlled low-photon counting and digital proces sing and analysis of the data to obtain intracellular Ca2+ concentrati on ([Ca2+]i). In eight ferret right ventricular trabeculae, we measure d the mechanical performance and found that, at 1.25 mM extracellular Ca2+ concentration ([Ca2+]o), resting tension, developed tension, and time to peak tension were unchanged by the loading procedure. Estimate d resting and peak systolic [Ca2+]i were 299 +/- 65 and 766 +/- 131 nM , respectively. Thirty minutes after raising the [Ca2+]o to 5 mM, ther e was a robust increase in mechanical performance, with peak systolic [Ca2+]i averaging 1,218 +/- 222 nM. The diastolic [Ca2+]i remained unc hanged. In four other trabeculae, exposure to a low-Na+-containing sup erfusate demonstrated a remarkable beat-to-beat correspondence of incr eases in diastolic [Ca2+]i and resting tensions. The same beat-to-beat concordance was also observed between the rapidly changing amplitudes of peak [Ca2+]i and developed tension. In additional experiments, sim ultaneous recordings of [Ca2+]i and force transients were obtained dur ing rapid pace pause maneuvers. These studies showed distinct and quan tifiable fluctuations of [Ca2+]i in a 1:1 relation to the mechanical r ecord to a frequency of at approximately 300 beats/min. These results demonstrate that beat-to-beat measurements of [Ca2+]i and tension tran sients can be obtained with good resolution in multicellular cardiac p reparations.