CA2+ CURRENT ACTIVATION RATE CORRELATES WITH ALPHA(1) SUBUNIT DENSITY

We report here that L-type Ca2+ channels activate rapidly in myotubes expressing current at high density and slowly in myotubes expressing c urrent at low density. Partial block of the current in individual cell s does not slow activation, indicating that Ca2+ influx does not link activation rate to current density. Activation rate is positively corr elated with the density of gating charge (Q(max)) associated with the L-type Ca2+ channels. They range of values for Q(max), and the relatio nship between activation rate and Q(max), are similar for myotubes exp ressing native or recombinant L-type Ca2+ channels, whereas peak Ca2current density is similar to 3-fold higher for native channels. Taken together, these results suggest that Ca2+ channel density can govern activation kinetics. Our findings have important implications for stud ies of ion channel function because they suggest that biophysical prop erties can be significantly influenced by channel density, both in het erologous expression systems and in native tissues.