. Fifteen putative calmodulin kinase II phosphorylation sites were identified mostly in the carboxyl-terminal tail of Ca v 1.2. Neither truncation at amino acid 1728 nor changing the II-III loop serines 808 and 888 to alanines affected facilitation of the calcium current. In contrast, facilitation was decreased by the single mutations S1512A and S1570A and abolished by the double mutation S1512A/S1570A. These serines flank the carboxyl-terminal EF-hand motif. Immunoprecipitation of calmodulin kinase II with the Ca v 1.2 channel was not affected by the mutation S1512A/S1570A. The phosphorylation of the Ca v 1.2 protein was strongly decreased in the S1512A/S1570A double mutant. These results suggest that voltagedependent facilitation of the Ca v 1.2 channel depends on the phosphorylation of Ser 1512

is carried by L-type calcium channels. In B-cells, the predominant calcium channel is Ca V 1.2, whereas Ca V 1.2 and Ca V 1.3 were identified in A-cells. These results were confirmed by using mice carrying A-or B-cell-specific inactivation of the Ca V 1.2 gene. In B-cells, the remaining I Ca flows in equal amounts through Ca V 2.1, Ca V 2.2 and Ca V 2.3. In A-cells, 30 and 15% of I Ca is due to Ca V 2.3 and Ca V 2.1 activity, respectively, whereas Ca V 2.2 current was not found in these cells. Low-voltage-activated T-type calcium channels could not be identified in A-and B-cells. Instead, two TTX-sensitive sodium currents were found: an early inactivating and a residual current. The residual current was only recovered in a subpopulation of B-cells. A putative genetic background for these currents is Na V 1.7.