Antipyrylazo III Measurements of Extracellular Ca Depletion in Heart Muscle

Abstract

It has been suggested that the frog heart is activated primarily by the influx of Ca from the extracellular space [4, 5]. Given the small extracellular space observed in ultrastructural studies, it is possible that Ca influx may result in a measurable extracellular Ca depletion. Measurements with extracellular Ca-sensitive electrodes support such a hypothesis [1, 2]. However, since the response time of the Ca electrode is slow, the rapid events accompanying the action potential may remain undetected. Further, it is unclear whether potential artifacts can be eliminated completely. To study the Ca influx with high time resolution, it appears that Ca-sensitive dyes like antipyrylazo III [6] might be superior to the Ca-sensitive electrode. Recently, such dyes have been used to measure extracellular Ca depletion [3], but the study was limited to the Ca depletion which persists following single or multiple twitches.