A fast-activated inward calcium current in twitch muscle fibres of the frog (Rana montezume).
AUTOR(ES)
Cota, G
RESUMO
Voltage-clamp experiments were performed at 18 degrees C in intact twitch muscle fibres of the frog using the three micro-electrode technique. Membrane currents were recorded in the presence of 120 mM-tetraethylammonium-methanesulphonate 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 microM-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 0 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 2s 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 microA/cm2 during the first 5 min to about -5 microA/cm2 after 25 min while ICa.s remained practically unchanged (maximum peak amplitude of about -60 microA/cm2). These results indicate the existence of two types of voltage-dependent CA2+ channels in intact muscle fibres. The kinetic properties of fast-activated Ca2+ channels suggest that they significantly activate during a single twitch.
ACESSO AO ARTIGO
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1192673Documentos Relacionados
- Calcium channel inactivation in frog (Rana pipiens and Rana moctezuma) skeletal muscle fibres.
- Calcium action potentials and calcium currents in tonic muscle fibres of the frog (Rana pipiens).
- Inactivation of the slow calcium current in twitch skeletal muscle fibres of the frog.
- Effect of Ca2+ channel blockers on K+ contractures in twitch fibres of the frog (Rana pipiens).
- Aequorin-calcium transients in frog twitch muscle fibres.