Free calcium in sheep cardiac tissue and frog skeletal muscle measured with Ca²⁺-selective microelectrodes

Abstract

Microelectrodes filled with neutral carrier selective to Ca²⁺ were used to measure the free intracellular Ca²⁺ concentration ([Ca²⁺]_i) in sheep cardiac tissue and frog skeletal muscle. Calibration of the electrodes was performed in the presence of a solution resembling the cationic composition of the cytoplasm. [Ca²⁺]_i at rest in normal physiological saline (20–22° C) was 240 nM in Purkinje fibres, 270 nM in ventricular muscle, and 52 nM in skeletal muscle. In Purkinje fibres, elevation of [Ca²⁺]_o from 1.8 mM to 5.4 mM produced a 1.7-fold increase in [Ca²⁺]_i. Elevation of [Ca²⁺]_o from 1.8 mM to 18 mM induced a 2.6-fold increase in [Ca²⁺]_i. Exposure to Na⁺-free solution (Li⁺-substituted) gave rise to elevation of [Ca²⁺]_i by factors of 5.8 and 14 in ventricular muscle and Purkinje fibres, respectively. These latter changes in [Ca²⁺]_i were associated with the development of contractures which reached 34% and 172% of the corresponding twitch tension.