INDOLIZINSULPHONES - A CLASS OF BLOCKERS WITH DUAL BUT DISCRIMINATIVE EFFECTS ON L-TYPE CA2+ CHANNEL ACTIVITY AND EXCITATION-CONTRACTION COUPLING IN SKELETAL-MUSCLE

The alpha-1 subunit of the L-type Ca2+ channel plays a dual role in skeletal muscle. It is essential both for L-type Ca2+ channel activity and for the functioning of the voltage-sensor structure that is situated in the triads as a key element for excitation-contraction coupling. This paper shows, with mouse muscle cells in primary culture, that indolizinsulphone SR33557 which has its binding site on the alpha-1 subunit blocks both L-type Ca2+ channel activity and contraction as the more classical 1,4-dihydropyridine blockers. However, unlike other Ca2+ channel blockers, it can pharmacologically discriminate between the two different roles of the alpha-1 subunit. SR33557 inhibition of both contractile and L-type Ca2+ channel activities is very voltage dependent and increases at depolarized potentials. Complete blockade of contraction was observed at low SR33557 concentrations (K0.5 = 20 nM) and was associated with only minor L-type Ca2+ channel blockade (30%). The remaining and major part of the L-type Ca2+ channel activity (70%) was blocked at much higher SR33557 concentrations (K0.5 = 0.6-mu-M). The results indicate that SR33557 has a much higher affinity for the alpha-1 subunit inserted into the voltage-sensor structure. They also suggest that the voltage-sensor structure, which probably includes most of the total T-tubule alpha-1 subunit, has intrinsic (but relatively small) Ca2+ channel activity. BayK8644, a L-type Ca2+ channel activator of the 1,4-dihydropyridine family, drastically increased contraction in parallel with an increase of the L-type Ca2+ current. SR33557 at low concentrations completely abolished contraction in BayK8644-treated myotubes while leaving a L-type Ca2+ current component larger than the normal L-type Ca2+ current observed in untreated control myotubes. SR33557 is the first reported molecule which can discriminate between the two functions of the alpha-1 subunit and eliminate contraction while leaving large L-type Ca2+ currents. For that reason it should become a potent tool for future studies of excitation-contraction coupling in skeletal muscle.