Cation channels of the trp-superfamily provide a ubiquitously used pathway for Ca²⁺ entry. This family can be subdivided into the receptor-operated short (canonical) TRPC channels, the long TRPM (melastatin) channels, and a subfamily, TRPV, which is related to the vanniloid receptor VR-1. Since cloning of the capsaicin receptor VR-1 (TRPV1), five related proteins have been identified which show a surprising diversity in their mechanisms of activation. Focus of this communication is on TRPV4 (OTRPC4, VR-OAC, TRP12) and the highly Ca²⁺-selective channels TRPV5 and -6 (ECaC1, ECaC2 or CaT1).

TRPV4, heterologously expressed in HEK 293 cells, is a Ca²⁺– and Mg²⁺-permeable cation channel, which only weakly discriminates between monovalent and divalent cations (P_Ca/P_Na~6.9, P_Mg/P_Na~2.5). TRPV4 is activated by cell swelling but much more efficient by a ligand, 4α-PDD (phorbol 12,13-didecanoate). Candidates for endogenous ligands refer to anandamide related compounds. Increasing the temperature above 30 °C activates TRPV4. Both types of activation require an intact N-terminus which comprises at least three ankyrin binding repeats. Deletion of one single domain already reduced the ligand activation but not activation by cell swelling of TRPV4. Activation by the ligands and by heat shows desensitisation during repetitive activation. Interestingly, activation is modulated by exchanging single amino acids in the pore region. Activation of TRPV4 is also modulated by a potential-dependent open pore block by extracellular Ca²⁺, which is bound within the pore at an identified low affinity binding site. As a probe for dissecting TRPV4 activation from co-activation of other ion channels, distinct properties of a submicromolar voltage-dependent block by ruthenium red will by described. TRPV4 is inactivated by an increase of the intracellular Ca²⁺ concentration, [Ca²⁺]_i. IC₅₀ for this inactivation is approximately 450 nM at -80 mV and is dramatically changed by mutations in the sixth transmembrane loop (TM6). This Ca²⁺-dependent inactivation is much less efficient than for the highly Ca²⁺-selective channels TRPV5 and -6.

All features of activation can be measured in freshly isolated mouse aorta endothelial cells, which endogenously express TRPV4. Activation of TRPV4 in those cells is accompanied by an increase in [Ca²⁺]_i, indicating that these channels are involved in endothelial Ca²⁺ signalling triggered by diverse physical and chemical stimuli. Possible functional consequences will be discussed.

Gating of TRPV5 and -6, which are both highly Ca²⁺ selective channels, involve as a critical step the removal of blocking Mg²⁺ or Ca²⁺ ions from a high-affinity binding site within the pore. This step requires hyperpolarisation of the membrane. Both TRPV channels sensitively inactivate by an increase in [Ca²⁺]_i. This Ca²⁺-dependent inactivation occurs with an IC₅₀ of approximately 120 nM. Three regions in the C terminus have been identified which are necessary for Ca²⁺ dependent inactivation of TRPV5 and -6. The very slow recovery from inactivation hints to the possible interaction with a modulating protein. In addition, it will be shown that the inactivation includes as a critical determinant the intracellular linker between TM2 and TM3. This linker seems also to determine the differences in the inactivation behaviour between TRPV5 and -6 and is responsible for distinct phenotypes of the both channels types in relation to the speed of the fast initial inactivation at hyperpolarizing steps, the degree of inactivation and the Ca²⁺ over Ba²⁺ discrimination. So far, no ligand activation for TRPV5 and -6 could be identified. A model for gating of TRPV5/6 will be discussed.