Differential effects of propranolol on the IgE-dependent, or calcium ionophore-stimulated, phosphoinositide hydrolysis and calcium mobilization in a mast (RBL 2H3) cell line

Our previous studies demonstrated that propranolol, an inhibitor of phosphatidic acid phosphohydrolase (PAPase) (EC 3.1.3.4) blocks the IgE-dependent mediator release from a rat mast (RBL 2H3) cell line. To continue these studies, we examined the ability of propranolol to inhibit the IgE-dependent or ionomycin-mediated phosphoinositide hydrolysis and calcium mobilization in RBL 2H3 cells. RBL 2H3 cells, sensitized with mouse monoclonal anti-trinitrophenol IgE (anti-TNP IgE), were stimulated to release both histamine and peptidoleukotrienes (LT) in response to a suboptimal concentration of trinitrophenol-ovalbumin conjugate (TNP-OVA) or ionomycin. Preincubation of the cells with d, l-propranolol (300 μM) significantly (P < 0.05) inhibited the effects of both TNP-OVA and ionomycin on histamine and LT release. There was no difference in potency for the different isomers of propranolol, indicating that these effects were not a consequence of an effect on β 2-adrenergic receptors. TNP-OVA produced a rapid hydrolysis of phosphoinositides resulting in a time-dependent increase in mono- (IP 1), di- (IP 2), tri- (IP 3), and total inositol phosphate production. Ionomycin also produced a rapid increase in total inositol phosphate production; however, this largely reflected an accumulation of IP 1 Both secretagogues produced a rapid elevation in cytosolic free calcium ([Ca 2+] i); however, the effect of ionomycin maximized within a much shorter time frame than the effect of TNP-OVA. The effects of TNP-OVA on phosphoinositide hydrolysis and increase in [Ca 2+] i were inhibited by propranolol over exactly the same concentration range as the effects of this compound on TNP-OVA-stimulated mediator release. In contrast, propranolol had no effect on the increase in [Ca 2+] i and phosphoinositide hydrolysis in response to ionomycin. Taken together, these results suggest that PAPase/ phospholipase D (PLD) (EC 3.1.4.4) activation may be a prerequisite for both IgE-dependent and ionomycin-stimulated mediator release from RBL 2H3 cells. Although other explanations are possible, the data further suggest that receptor-mediated, but not ionophore-stimulated, phosphoinositide hydrolysis and [Ca 2+] i in RBL 2H3 cells may be regulated by a propranolol-sensitive pathway involving possible activation of PAPase.