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Calcium channel inactivation in frog (Rana pipiens and Rana moctezuma) skeletal muscle fibres.
Author(s) -
Cota G,
Nicola Siri L,
Stefani E
Publication year - 1984
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1984.sp015365
Subject(s) - rana , depolarization , chemistry , membrane potential , voltage clamp , time constant , biophysics , analytical chemistry (journal) , pulse (music) , tonicity , anatomy , voltage , chromatography , biology , biochemistry , physics , engineering , quantum mechanics , electrical engineering
The decay of the Ca2+ current (ICa) during a maintained depolarization was studied in intact twitch skeletal muscle fibres of Rana pipiens and Rana moctezuma with the three‐micro‐electrode voltage‐clamp technique. ICa was recorded at 23 degrees C, after blocking K+ currents, in TEA methanesulphonate saline with 10 mM‐Ca2+ made hypertonic by adding 350 mM‐sucrose. In two‐pulse experiments, ICa during the test pulse was reduced to about 80% (R. pipiens) or 50% (R. moctezuma) of the control value, without any detectable inward ICa during 7 s conditioning pre‐pulses. The experimental points of the steady‐state inactivation curve (h infinity) were fitted to h infinity = (1 + exp [Em ‐ Vh)/kh]‐1, where Em is the membrane potential and with Vh = ‐33 +/‐ 3 mV and kh = 6 +/‐ 1 mV for R. pipiens, and Vh = ‐44 +/‐ 3 mV and kh = 9.5 +/‐ 1.0 mV for R. moctezuma. The rate constant of decay for inactivated currents (range ‐8 to ‐47 mA cm‐3) and for control currents (range ‐23 to ‐62 mA cm‐3), was independent of ICa amplitude. The average rate constant of decay at 0 mV was 1.18 +/‐ 0.02 s‐1 (66). These results indicate that in intact fibres under hypertonic solution ICa decay can be explained by a voltage‐dependent inactivation process and not by depletion of tubular Ca2+. The absence of depletion could be due to a large fractional tubular volume or to the presence of a Ca2+ pump in the tubular system.