Receptor-Operated Cation Entry—More Than Esoteric Terminology?

Many hormones and neurotransmitters elicit an increase in the intracellular calcium concentration by binding to phospholipase C–linked G protein–coupled receptors. Activated receptors signal to calcium-permeable cation channels in the plasma membrane, which are distinct from those engaged by emptying of intracellular stores of calcium. The TRPC family of the mammalian homologs of the Drosophila transient receptor potential (TRP) cation channel represents likely molecular correlates underlying receptor-operated cation entry. While all TRPC family members are gated in a phospholipase C–dependent manner, the exact activation mechanism still remains elusive, although lipids such as diacylglycerol and polyunsaturated fatty acids are potential diffusible messengers. Functional TRPC channel complexes in the plasma membrane are thought to be composed of four distinct subunits whose stoichiometry and composition under physiological conditions are still largely unknown. However, recent progress in defining the combinatorial rules of TRPC channel assembly may lead to the identification of TRPC-dependent ion fluxes in living cells. Because of the large number of TRP proteins and their frequently overlapping functional characteristics, the central question is whether TRP proteins are functionally interchangeable or whether unique physiological roles can be ascribed to them. Receptor-operated cation entry is critically involved in the control of airway and vascular smooth muscle tone; hence, TRPC proteins are promising new drug targets.