Calmodulin- and Ca2+-Dependent Facilitation and Inactivation of the CaV1.2 Ca2+ Channels in Guinea-Pig Ventricular Myocytes

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The L-type Ca2+ channel (CaV1.2) shows clear Ca2+-dependent facilitation and inactivation. Here we have examined the effects of calmodulin (CaM) and Ca2+ on Ca2+ channel in guinea-pig ventricular myocytes in the inside-out patch mode, where rundown of the channels was controlled. At a free [Ca2+] of 0.1 μM, CaM (0.15, 0.7, 1.4, 2.1, 3.5, and 7.0 μM) + ATP (2.4 mM) induced channel activities of 27%, 98%, 142%, 222%, 65%, and 20% relative to the control activity, respectively, showing a bell-shaped relationship. Similar results were observed at a free [Ca2+] <0.01 μM or with a Ca2+-insensitive mutant, CaM1234, suggesting that apoCaM may induce facilitation and inactivation of the channel activity. The bell-shaped curve of CaM was shifted to the lower concentration side with increasing [Ca2+]. A simple model for CaM- and Ca2+-dependent modulations of the channel activity, which involves two CaM-binding sites, was proposed. We suggest that both apoCaM and Ca2+/CaM can induce facilitation and inactivation of CaV1.2 Ca2+ channels and that the basic role of Ca2+ is to accelerate CaM-dependent facilitation and inactivation.
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