No abstract

1 Phosphoinositide-speci®c phospholipase C (PLC) is involved in the regulation of many cellular functions. In the myocardium, PLC-generated second messengers play a role in the regulation of contractile function and in the pathophysiology of myocardial hypertrophy. 2 In the present study, the eect of mastoparan, a tetradecapeptide which is capable of activating heterotrimeric G proteins by mimicking the action of an activated receptor, on membrane-bound human myocardial PLC, was investigated in a cell-free assay with exogenous phospholipids as a substrate. 3 Mastoparan stimulated human myocardial PLC approximately two fold with a half-maximal eect at approximately 2 mM and a maximal eect at 10 mM. The peptide did not alter the dependence of PLC on free calcium ions. In order to exclude non-speci®c eects of mastoparan due to its amphiphilic properties, dierent mastoparan derivatives were used as positive and negative controls. Mas17, an inactive mastoparan analogue with phsyical properties very similar to mastoparan, did not induce substantial PLC stimulation in human myocardial membranes. In contrast, Mas7, the most active mastoparan derivative known, caused a more pronounced PLC activation compared with the mother compound indicating that the eect was sequence-speci®c. Human myocardial PLC stimulation was pertussis toxin-insensitive and could not be abolished by addition of excess a-subunits from puri®ed retinal transducin or by excess GDP or GDPbS. In order to investigate whether mastoparan stimulated PLC via pertussis toxin-insensitive a q , a deletion mutant of PLCb 2 de®cient of the site of interaction with a q -subunits was expressed in COS-1 cells. Both wild-type and mutant PLCb 2 were similarly sensitive to stimulation by mastoparan. 4 It is concluded that mastoparan stimulates human myocardial PLC by a mechanism distinct from heterotrimeric G proteins.