Functional properties of Drosophila inositol trisphosphate receptors

The functional properties of the only inositol trisphosphate (IP3) receptor subtype expressed in Drosophila were examined in permeabilized S2 cells. The IP3 receptors of S2 cells bound (1,4,5)IP3 with high affinity (Kd = 8.5±1.1nM), mediated positively co-operative Ca2+ release from a thapsigargin-sensitive Ca2+ store (EC50 = 75±4nM, Hill coefficient = 2.1±0.2), and they were recognized by an antiserum to a peptide conserved in all IP3 receptor subtypes in the same way as mammalian IP3 receptors. As with mammalian IP3 receptors, (2,4,5)IP3 (EC50 = 2.3±0.3μM) and (4,5)IP2 (EC50 approx. 10μM) were approx. 20- and 100-fold less potent than (1,4,5)IP3. Adenophostin A, which is typically approx. 10-fold more potent than IP3 at mammalian IP3 receptors, was 46-fold more potent than IP3 in S2 cells (EC50 = 1.67±0.07nM). Responses to submaximal concentrations of IP3 were quantal and IP3-evoked Ca2+ release was biphasically regulated by cytosolic Ca2+. Using rapid superfusion to examine the kinetics of IP3-evoked Ca2+ release from S2 cells, we established that IP3 (10μM) maximally activated Drosophila IP3 receptors within 400ms. The activity of the receptors then slowly decayed (t1/2 = 2.03±0.07s) to a stable state which had 47±1% of the activity of the maximally active state. We conclude that the single subtype of IP3 receptor expressed in Drosophila has similar functional properties to mammalian IP3 receptors and that analyses of IP3 receptor function in this genetically tractable organism are therefore likely to contribute to understanding the roles of mammalian IP3 receptors.