Unitary Ca2+ current through mammalian cardiac and amphibian skeletal muscle ryanodine receptor Channels under near-physiological ionic conditions |
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Authors: | Kettlun Claudia González Adom Ríos Eduardo Fill Michael |
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Institution: | Department of Physiology, Loyola University Chicago, 2160 S. First Ave., Maywood, IL 60153, USA. |
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Abstract: | Ryanodine receptor (RyR) channels from mammalian cardiac and amphibian skeletal muscle were incorporated into planar lipid bilayers. Unitary Ca2+ currents in the SR lumen-to-cytosol direction were recorded at 0 mV in the presence of caffeine (to minimize gating fluctuations). Currents measured with 20 mM lumenal Ca2+ as exclusive charge carrier were 4.00 and 4.07 pA, respectively, and not significantly different. Currents recorded at 1-30 mM lumenal Ca2+ concentrations were attenuated by physiological K+] (150 mM) and Mg2+] (1 mM), in the same proportion (approximately 55%) in mammalian and amphibian channels. Two amplitudes, differing by approximately 35%, were found in amphibian channel studies, probably corresponding to alpha and beta RyR isoforms. In physiological Mg2+], K+], and lumenal Ca2+] (1 mM), the Ca2+ current was just less than 0.5 pA. Comparison of this value with the Ca2+ flux underlying Ca2+ sparks suggests that sparks in mammalian cardiac and amphibian skeletal muscles are generated by opening of multiple RyR channels. Further, symmetric high concentrations of Mg2+ substantially reduced the current carried by 10 mM Ca2+ (approximately 40% at 10 mM Mg2+), suggesting that high Mg2+ may make sparks smaller by both inhibiting RyR gating and reducing unitary current. |
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Keywords: | ryanodine receptor Ca2+ release sarcoplasmic reticulum |
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