Activation of L‐type calcium channel in twitch skeletal muscle fibres of the frog.

Abstract

1. The activation of the L-type calcium current (ICa) was studied in normally polarized (-100 mV) cut skeletal muscle fibres of the frog with the double Vaseline-gap voltage-clamp technique. Both external and internal solutions were Ca2P buffered. Solutions were made in order to minimize all but the Ca2P current.2. The voltage-dependent components of the time course of activation were determined by two procedures: fast and slow components were evaluated by multiexponential fitting to current traces elicited by long voltage pulses (5 s) after removing inactivation; fast components were also determined by short voltage pulses having different duration (0 5-70 ms). 3. The components of deactivation were evaluated after removing the charge-movement current from the total tail current by the difference between two short (50 and 70 ms) voltage pulses to 10 mV, moving the same intramembrane charge. Two exponential components, fast and slow (time constants, 6 + 0 3 and 90 + 7 ms at -100 mV; n = 26), were found. 4. The time onset of ICa was evaluated either by multiexponential fitting to the ICa activation or by pulses of different duration to test the beginning of the 'on' and 'off' inequality. This was at about 2 ms, denoting that it was very early. 5. The time constants vs. voltage plots indicated the presence of four voltage-dependent components in the activation pathway. Various kinetic models are discussed. Models with independent transitions, like a Hodgkin-Huxley scheme, were excluded. Suitable models were a five-state sequential and a four-state cyclic with a branch scheme. The latter gave the best simulation of the data. 6. The steady-state activation curve saturated at high potentials. It had a half-voltage value of 1 + 0-02 mV and the opening probability was only 0-82 + 0-2 at 20 mV (n = 32). This result implies a larger number of functional calcium channels than was previously supposed and is in agreement with the number of dihydropyridine (DHP) receptors calculated for the tubular system.

Sphingosine 1-phosphate induces Ca2+ transients and cytoskeletal rearrangement in C2C12 myoblastic cells

“…The role for Ins(1,4,5)P 3 R in the generation of Ca 2ϩ signals has been well established in several cell types, including immature and developing skeletal muscle cells (12,20 (14). Their expression is developmentally regulated in embryos and in muscle cell lines, because their plasma membrane density sharply increases on muscle differentiation (23,44).…”

L‐type Ca2+ channel and ryanodine receptor cross‐talk in frog skeletal muscle

“…Solutions were the same as those used by Francini et al (1996). The composition of the external solution for ICM and I Ca recording (control external solution) was (m m ): free Ca 2+ , 2; Ca 2+ , 13.5; l‐ malate 2− , 74.5; Mops − , 18; TEA + , 131.4; TTX, 0.001; 3,4‐DAP, 1; Rb + , 2.5; 9‐anthracene carboxylic acid, 1; SO 4 2− , 1.25.…”

“…In the present study on single muscle fibres the investigation of ICM and I Ca was extended to positive potentials in order to examine all three kinds of charges, q β, q γ and q h , and a number of pharmacological interventions were carried out, mainly aimed at interfering with (a) the DHPR/ L ‐type Ca 2+ channel, by using channel blockers such as Cd 2+ (Hui, 1991; Francini et al 1996), nifedipine (Huang, 1990; Chen & Hui, 1991) and 1‐alkanols (Oz et al 2001), and (b) RyR/CRC, by using specific blockers such as ryanodine (García et al 1991; Huang, 1996) and ruthenium red (RR) (Csernoch et al 1991), known to suppress Ca 2+ release from RyR/CRC.…”

Kinetics of inactivation and restoration from inactivation of the L‐type calcium current in human myotubes

“…In addition they permit an extremely slow calcium inward current (L_type current) whose physiological role is still unclear (for a review see Melzer et al 1995). L_type current characteristics have been extensively studied in adult amphibian and mammalian skeletal muscle (for references see Delbono, 1992;Feldmeyer et al 1990Feldmeyer et al , 1992Feldmeyer et al , 1995Garcia et al 1992;Francini et al 1996). On the other hand, important data on structure-function relationships of the channel have been obtained by heterologous expression of recombinant DHP receptors in cultured immature muscle cells (myotubes) (Tanabe et al 1988;Garcia et al 1994).…”