A fast‐activated inward calcium current in twitch muscle fibres of the frog (Rana montezume).

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Abstract
Voltage-clamp experiments were performed at 18.degree. C in intact twitch muscle fibers of the frog using the three micro-electrode technique. Membrane currents were recorded in the presence of 120 mM-tetraethylammonium-methanesulfonate and 10 mM-Ca2+. The recording solution was made hypertonic by adding 350 mM-sucrose to avoid contraction. Two components of inward current in the absence of external Na+ were observed. Depolarization induced a fast-activated inward current of small amplitude in addition to the well-known slow, transient Ca2+ current (ICa,s). Both components of inward current persisted in the presence of tetrodotoxin. They practically disappeared on replacing external Ca2+ with Mg2+ and were blocked by millimolar additions of Cd2+ to the bath. Thus, the fast-activated component of inward current was also carried by Ca2+ (ICa,f). Neither ICa,f nor ICa,s were reduced by 5 .mu.M-diltiazem. During 400 ms depolarizations ICa,f was detected at approximately -60 mV, 30 mV more negative than the membrane potentials at which ICa,s appeared. At about O mV the time constant for activation was 5 ms for ICa and 150 ms for ICa,s. ICa,f did not significantly decline during depolarizations up to 2 s in duration at membrane potentials between -60 and -30 mV. ICa,f tended to disappear as a function of time on exposure to the hypertonic recording solution. Its maximum amplitude decreased from about -25 .mu.A/cm2 during the first 5 min to about -5 .mu.A/cm2 after 25 min while ICa,s remained practically unchanged (maximum peak amplitude of about -60 .mu.A/cm2). These results indicate the existence of two types of voltage-dependent Ca2+ channels in intact muscle fibers. The kinetic properties of fast-activated Ca2+ channels suggest that they significantly activate during a single twitch.