TRPM3, a calcium permeable channel of the transient receptor potential melastatin channel family, is expressed in pancreatic b cells and can be activated by the endogenous neurosteroid pregnenolone sulphate (PS). Activation of TRPM3 leads to an increased intracellular calcium concentration followed by a reinforced glucose-stimulated insulin release. So far, the exact function and regulation of TRPM3 in pancreatic b cells is not known. For a better understanding of the role of TRPM3 in pancreatic b cells we examined whether TRPM3 is subject to regulation by intracellular signalling cascades in b cells. In rat insulinoma cells (INS-1) and mouse primary pancreatic b cells noradrenaline and adrenaline strongly inhibit the TRPM3 activity via activation of a2-adrenoreceptors. Experiments with pertussis toxin (PTX), an inhibitor of Gi/o-proteins, revealed that the mechanism is G-protein coupled. Further experiments with IBMX, a nonselective phosphodiesterase inhibitor, and forskolin, an activator of the adenylyl cyclase, showed that the inhibitory effect is independent of the intracellular cAMP concentration and still detectable when the intracellular cAMP concentration is increased. In contrast, overexpression of the b/g-scavenging peptides myr-b-ARKct or myr-phosducin (that bind b/g-subunits of G-proteins particularly at the plasma membrane) led to a reduction of the inhibitory effect, indicating that b/g-subunits could be at least partly involved. Furthermore we examined whether the Gai subunit is directly participating in the inhibition mechanism.