Thus, siRNA-mediated knockdown of these GRKs inhibit agonist-induced ERK1/2 phosphorylation. A surprising observation of the present study was that silencing the expression of either GRK5 or GRK6 resulted in enhanced ERK1/2 phosphorylation in response to C3a. The possibility that this enhanced response reflects attenuated C3aR desensitization is unlikely because reduced expression of these GRKs had no effect on C3aR desensitization or internalization. These findings suggest that GRK2/GRK3 and GRK5/GRK6 inhibit C3a-induced ERK1/2 phosphorylation via distinct pathways; one involving receptor desensitization and the other independent of receptor desensitization. The mechanism by which GRK5/GRK6 inhibit C3a-induced ERK1/2 phosphorylation is not known. Similar to our finding, Barthet et al., recently showed that GRK5 inhibits 5-HT4 receptor-mediated ERK1/2 phosphorylation. They also demonstrated that GRK5, but not GRK2, phosphorylates AbMole BioScience kinase inhibitors b-arrestin-1 and that this phosphorylation is required for the inhibition of ERK1/2 activity. We have recently shown that silencing the expression of b-arrestin-1 enhances C3a-induced ERK1/2 phosphorylation via a receptor desensitization-independent pathway. This raises the interesting possibility that, as for 5-HT4 receptor, agonist-induced C3aR phosphorylation by GRK5/6 recruits b-arrestin-1 to inhibit C3a-induced ERK1/2 phosphorylation. Tipping et al., recently showed that the single barrestin present in Drosophila, Kurtz directly binds to and sequesters an inactive form of ERK, thus preventing its activation by the upstream kinase, MEK. It is therefore possible that C3a induces b-arrestin-1 phosphorylation via GRK5/GRK6 to promote a complex formation between ERK and b-arrestin-1, leading to the inhibition of ERK1/2 activity. Thus, silencing the expression of GRK5/GRK6 or b-arrestin-1 removes this inhibitory constraint to enhance ERK1/2 phosphorylation. Whether this or other mechanisms participate in the regulation of ERK1/2 activity by GRK5/GRK6 in C3a-activated mast cells remains to be determined. In summary, the present study demonstrates that GRK2 and GRK3 participate in C3aR desensitization in human mast cells. It also provides the novel finding that GRK5 and GRK6 promote C3a-induced mast cell degranulation but inhibit ERK1/2 phosphorylation via mechanisms that are independent of receptor desensitization. Gompertz first described ageing as an increase in the likelihood of mortality with increasing chronological age. In man, this equates to a corresponding, progressive loss of metabolic and physiological functions.