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

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Abstract
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)