HETEROLOGOUS EXPRESSION OF BI CA2-MUSCLE( CHANNELS IN DYSGENIC SKELETAL)

We have examined the ability of BI (class A) Ca2+ channels, cloned fro m rabbit brain, to mediate excitation-contraction (E-C) coupling in sk eletal muscle. Expression plasmids carrying cDNA encoding BI channels were microinjected into the nuclei of dysgenic mouse myotubes grown in primary culture. Ionic currents and intramembrane charge movements pr oduced by the BI channels were recorded using the whole-cell patch-cla mp technique. Injected myotubes expressed high densities of ionic BI C a2+ channel current (average 31 pA/pF) but did not display spontaneous contractions, and only very rarely displayed evoked contractions. The expressed ionic current was pharmacologically distinguished from the endogenous L-type current of dysgenic skeletal muscle (I-dys) by its i nsensitivity to the dihydropyridine antagonist (+)-PN 200-110. Peak BI Ca2+ currents activated with a time constant (tau(a)) of similar to 2 ms and inactivated with a time constant (tau(h)) of similar to 260 ms (20-23 degrees C). The time constant of inactivation (tau(h)) was not increased by substituting Ba2+ for Ca2+ as charge carrier, demonstrat ing that BI channels expressed in dysgenic myotubes do not undergo Ca2 +-dependent inactivation. The average maximal Ca2+ conductance (G(max) ) produced by the BI channels was quite large (similar to 534 S/F). In contrast, the average maximal charge movement (Q(max)) produced in th e same myotubes (similar to 2.7 nC/mu F) was quite small, being barely larger than Q(max), in control dysgenic myotubes (similar to 2.3 nC/m u F). Thus, the ratio G(max)/Q(max) for the BI channels was considerab ly higher than previously found for cardiac or skeletal muscle L-type Ca2+ channels expressed in the same system, indicating that neuronal B I Ca2+ channels exhibit a much higher open probability than these L-ty pe Ca2+ channels.