An inositol 1,4,5‐trisphosphate receptor‐dependent cation entry pathway in DT40 B lymphocytes

We examined the roles of inositol 1,4,5‐trisphosphate (IP 3 ) receptors (IP 3 R) in calcium signaling using DT40 B lymphocytes, and a variant lacking the three IP 3 R isoforms (IP 3 R‐KO). In wild‐type cells, B cell receptor (BCR) stimulation activates a cation entry route that exhibits significantly greater permeability to Ba 2+ than does capacitative calcium entry. This cation entry is absent in IP 3 R‐KO cells. Expression of the type‐3 IP 3 R (IP 3 R‐3) in the IP 3 R‐KO cells rescued not only agonist‐dependent release of intracellular Ca 2+ , but also Ba 2+ influx following receptor stimulation. Similar results were obtained with an IP 3 R‐3 mutant carrying a conservative point mutation in the selectivity filter region of the channel (D2477E); however, an IP 3 R‐3 mutant in which this same aspartate was replaced by alanine (D2477A) failed to restore either BCR‐induced Ca 2+ release or receptor‐dependent Ba 2+ entry. These results suggest that in DT40 B lymphocytes, BCR stimulation activates a novel cation entry across the plasma membrane that depends upon, or is mediated by, fully functional IP 3 R.