Direct measurements of free Ca2+ in heart cells are needed for an understanding of the regulation of contractility. We developed and used Ca2+-sensitive microelectrodes with fine tips, stable properties and ample sensitivity to free Ca2+ in the sub-micromolar range. In quiescent ventricular muscle, measurements which passed tests for electrode sealing and cell viability gave a mean free Ca2+ concentration of 0.26 μM. During contractures, we recorded Ca2+ transients rising as high as 10 μM. In studying the effects of catecholamines on free Ca2+ and force, we found evidence that adrenaline can reduce myofibrillar Ca2+ sensitivity in intact heart muscle.
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