Chimeric L-type Ca2+ channels expressed in Xenopus laevis oocytes reveal role of repeats III and IV in activation gating.

1. Chimeric alpha 1 subunits consisting of repeat I and II from the rabbit cardiac (alpha 1C-a) and repeat III and IV from the carp skeletal muscle Ca2+ channel (alpha 1S) were constructed and expressed in Xenopus laevis oocytes without co-expressing other channel subunits. Ba2+-current kinetics of five chimeric channel constructs were studied in Xenopus oocytes using the two-microelectrode technique. 2. Exchange of repeats III and IV of alpha 1C-a with sequences of alpha 1S results in a significantly slower and biexponential activation (apparent activation time constants tau 1act = 19.8 +/- 1.8 ms and tau 2act = 214 +/- 28.7 ms, n = 7) of expressed Ca2+ channel currents; no current inactivation was observable during an 800 ms test pulse to 0 mV. 3. Activation of a chimera consisting of repeats I, II and IV from the alpha 1C-a subunit and repeat III from alpha 1S was fast and monoexponential (tau 1act = 6.33 +/- 1.7 ms, n = 5) and the current inactivated during a 350 ms test pulse to 0 mV (tau inact = 175 +/- 22 ms, n = 5). The current kinetics of this construct did not significantly differ from kinetics of a construct consisting of repeats I to IV from alpha 1C-a (tau 1act = 6.6 +/- 2.1 ms; tau inact = 198 +/- 14 ms; n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)