The β1a Subunit Regulates the Functional Properties of Adult Frog and Mouse L-Type Ca2+ Channels of Skeletal Muscle (original) (raw)

In skeletal muscle, L-type, voltage-activated Ca 2+ channels play an essential role in excitation-contraction coupling as the voltage sensors that link the depolarization of the transverse tubular system (T-system) to Ca 2+ release by the sarcoplasmic reticulum (for reviews see Rios & Pizarro, 1991; Lamb, 1992; Huang, 1993; Melzer et al. 1995). In addition to their role as voltage sensors, muscle Ca 2+ channels give rise to very slowly activated L-type Ca 2+ currents (Sanchez & Stefani, 1983; for a review, see Melzer et al. 1995). Ca 2+ channels of skeletal muscle are complex molecules composed of a 1s , a 2-d, b 1 and g subunits. The a 1s subunit (now referred to as the Ca V 1.1 channel, according to Ertel et al. 2000), is the channel-forming subunit (Perez-Reyes et al. 1989) that contains the voltage sensor of excitation-contraction coupling and the dihydropyridine binding sites (for reviews see Rios & Pizarro, 1991; Hofmann et al. 1994; Isom et al. 1994; Catterall, 2000). The b 1a subunit is one of the auxiliary subunits of Ca V 1.1 channels (Isom et al. 1994) and the main isoform among the b 1 subunits present in muscle (Ren & Hall, 1997). The b 1a subunit has important effects on the surface expression of a 1s but little information is available on its functional effects on Ca V 1.1 channels. In these experiments we describe, for the first time, the actions of b 1a on the electrophysiological properties of Ca V 1.1 channels in adult tissue. We used spherical vesicles derived from skeletal muscle plasma membranes and the whole cell voltage clamp technique. This preparation allows the control of the composition of the internal medium, to which we added the b 1a subunit. In addition, it enables proper control of the membrane potential without the complications due to the presence of the T-tubular system in muscle fibres (Camacho et al. 1999). We found that the b 1a subunit produces major changes in the amplitude of L-type currents without any effect on charge movement. Part of this study has been published in abstract form (Rebolledo et al. 2002). METHODS Preparation We used spherical vesicles derived from the plasma membrane of frog and mouse skeletal muscle. Adult frogs were anaesthetized in 15 % ethanol prior to decapitation. Mice were killed by cervical dislocation, performed according to the authorized procedures of our institution. All procedures used conformed with the principles of the UK Animals (Scientific Procedures) Act 1986. The procedure of vesicle formation by enzymatic treatment was originally described for single channel experiments by Standen et