3+ also block I MIC in a voltage-dependent manner (δ = 0.4-0.5). In addition they inhibit the inward current carried by divalent cations. I MIC is regulated by pH. Decreasing or increasing extracellular pH decreased and increased I MIC , respectively (pH 0.5 = 6.9, n H = 0.98). Qualitatively similar results were obtained on I MIC in rat basophilic leukaemia cells. These effects in cardiac myocytes were absent in the presence of high intracellular buffering by 40 mM Hepes. Our results suggest that I MIC in cardiac cells is due to TRPM channels, most probably to TRPM6 or TRPM7 channels or to their heteromultimeres.

The Mg 2ϩ -inhibited cation (MIC) current (I MIC) in cardiac myocytes biophysically resembles currents of heterologously expressed transient receptor potential (TRP) channels, particularly TRPM6 and TRPM7, known to be important in Mg 2ϩ homeostasis. To understand the regulation of MIC channels in cardiac cells, we used the whole cell voltage-clamp technique to investigate the role of intracellular ATP in pig, rat, and guinea pig isolated ventricular myocytes. I MIC, studied in the presence or absence of extracellular divalent cations, was sustained for Ն50 min after patch rupture in ATP-dialyzed cells, whereas in ATPdepleted cells I MIC exhibited complete rundown. Equimolar substitution of internal ATP by its nonhydrolyzable analog adenosine 5Ј-(␤,␥-imido)triphosphate failed to prevent rundown. In ATP-depleted cells, inhibition of lipid phosphatases by fluoride ϩ vanadate ϩ pyrophosphate prevented I MIC rundown. In contrast, under similar conditions neither the inhibition of protein phosphatases 1, 2A, 2B or of protein tyrosine phosphatase nor the activation of protein kinase A (forskolin, 20 M) or protein kinase C (phorbol myristate acetate, 100 nM) could prevent rundown. In ATP-loaded cells, depletion of phosphatidylinositol 4,5-bisphosphate (PIP 2) by prevention of its resynthesis (10 M wortmannin or 15 M phenylarsine oxide) induced rundown of I MIC. Finally, loading ATP-depleted cells with exogenous PIP 2 (10 M) prevented rundown. These results suggest that PIP 2, likely generated by ATP-utilizing lipid kinases, is necessary for maintaining cardiac MIC channel activity. cation channels; hydrolysis; phosphoinositides; rundown SEVERAL ION CHANNELS PARTICIPATE in ion fluxes that are critical for cardiac function. Apart from the well-characterized cationselective voltage-gated, ligand-gated, and background ion channels as well as the pacemaker channels, the nature of other cardiac cation-permeable channels remains unclear. In cardiac and other tissues, transient receptor potential (TRP) proteins are molecular candidates for cation-permeable channels (36). The TRP proteins are members of three main (TRPC, TRPV, and TRPM) and some more distantly related subfamilies. They form channels gated by stimuli such as voltage, chemical, or physical factors and are involved in both physiological and pathological functions (36).We previously described (10, 31, 48) a Mg 2ϩ -inhibited cation (MIC) channel in cardiac myocytes that displays pharmacological and pore properties resembling those of TRPM6 and TRPM7 channels. Native TRPM7-like currents such as cardiac MIC are also referred to as magnesium-nucleotideregulated metal ion (MagNuM) currents (32). TRPM6 and TRPM7 are closely related members of the TRPM subfamily, which form channels with very similar properties (29, 45). TRPM6 is mainly expressed in the kidney and small intestines, and its mutation causes hypomagnesemia with secondary hypocalcemia (5, 41, 46). TRPM7 is widely expressed with high levels in the kidney and heart. It is essential for cell viability and prolife...

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Objectives Diclofenac and other non-steroidal anti-inflammatory drugs (NSAIDs) are widely used in the treatment of inflammation and pain. Most effects of NSAIDs are attributed to the inhibition of cyclooxygenases (COX). However, many NSAIDs may have other effects not related to COX, including the modulation of various ion channels. The clinical implications of the effects on channels are not fully understood. This review outlines the effects of NSAIDs, with special attention to diclofenac, on ion channels and highlights the possible underlying mechanisms. Key findings NSAIDs have effects on channels such as inhibition, activation or changes in expression patterns. The channels affected include voltage-gated Na + , Ca 2+ , or K + channels, ligand-gated K + channels, transient receptor potential and other cation channels as well as chloride channels in several types of cells. The mechanisms of drug actions not related to COX inhibition may involve drug-channel interactions, interference with the generation of second messengers, changes in channel expression, or synergistic/antagonist interactions with other channel modulators. Summary The effects on ion channels may account for novel therapeutic actions of NSAIDs or for adverse effects. Among the NSAIDs, diclofenac may serve as a template for developing new channel modulators and as a tool for investigating the actions of other drugs.

Background/Aims: A magnesium-inhibited, transient receptor potential melastatin 7 (TRPM7)-like channel is expressed in cardiac cell membranes. The role and regulation of this channel by intracellular nucleotides and membrane components remain unclear. Methods: We used the whole-cell voltage-clamp technique in pig isolated ventricular myocytes to investigate the effect of non-hydrolysable guanine nucleotides. Results: The TRPM7-like current, induced by intracellular dialysis with low [Mg²⁺], remained stable when the intracellular solution contained GTP. Substituting GTP by GTP-γ-S or Gp-pNp, but not GDP-β-S, induced a run-down of the current. Under dialysis with GTP-γ-S, inhibiting phospholipase C by edelfosine or intracellularly adding exogenous phosphatidylinositol-4,5-bisphosphate (PIP₂) decreased run-down, whereas extracellularly applying carbachol and phenylephrine accelerated it. Pretreatment of cells with pertussis toxin did not prevent the run-down induced by GTP-γ-S. Conclusion: Guanine nucleotides can modulate cardiac TRPM7-like channels via a mechanism linked to G proteins and to PIP₂ metabolism.