TRPM3 (transient receptor potential melastatin 3) proteins form calcium-permeable, non-selective cation channels that can be activated by the endogenous steroid pregnenolone sulfate from the extracellular side. These ion channels are expressed in several tissues and have been implicated in insulin release from pancreatic β-cells and in sensing noxious thermal stimuli in cutaneous nociceptor neurons. However, it is still poorly understood how these channels are regulated by intracellular signaling cascades.

Using electrophysiological techniques and calcium imaging of cells expressing TRPM3 proteins heterologously or endogenously (mainly pancreatic β-cells) in combination with various pharmacological probes we determined that TRPM3 channel activity is reduced (but not abolished) after strong activation of Gα_q-coupled receptors, such as M₁-muscarinic acetylcholine receptors. This inhibition seems to be caused, at least partially, by a reduction in plasma membrane PIP₂ content. This finding was confirmed using voltage-sensitive 5'-phosphatases, which, when activated, clearly reduced the activity of TRPM3 channels.

Additionally, we observed an even stronger reduction of TRPM3 channel activity after activation of Gα_i-coupled receptors (such as α₂-adrenoceptors). In contrast to the inhibiton observed after stimulating G_q-proteins, the effect of G_i-proteins appeared to be independent of PIP₂ hydrolysis, as PIP₂ levels did not change after adrenoceptor stimulation. Rather, the inhibitory effect on TRPM3 channels was brought about by β/γ-subunits of G-proteins, as it can be mimicked by overexpression of β/γ-subunits, but not by overexpressing α_i-subunits.

Together, our data indicate that TRPM3 channels are subject to complex and varied intracellular regulatory mechanisms that allow fine-tuning the activity of these channels and the resulting calcium influx.