Ca2+ release from the sarcoplasmic reticulum is required for sustained TRPM4 activity in cerebral artery smooth muscle cells

Abstract

The melastatin transient receptor potential (TRP) channel TRPM4 is a critical regulator of vascular smooth muscle cell membrane potential and contractility. Activation of the channel is Ca2+-dependent, but prolonged exposure to high (>1 μM) levels of intracellular Ca2+ causes rapid (within ∼2 min) desensitization of TRPM4 currents under conventional whole cell and inside-out patch-clamp conditions. The goal of the present study was to establish a novel method to record sustained TRPM4 currents in smooth muscle cells under near-physiological conditions. Using the amphotericin B-perforated patch-clamp technique, we recorded and characterized sustained (up to 30 min) transient inward cation currents (TICCs) in freshly isolated cerebral artery myocytes. In symmetrical cation solutions, TICCs reversed at 0 mV and had an apparent unitary conductance of 25 pS. Replacement of extracellular Na+ with the nonpermeable cation N-methyl-d-glucamine abolished the current. TICC activity was attenuated by the TRPM4 blockers fluflenamic acid and 9-phenanthrol. Selective silencing of TRPM4 expression using small interfering RNA diminished TICC activity, suggesting that the molecular identity of the responsible ion channel is TRPM4. We used the perforated patch-clamp method to test the hypothesis that TRPM4 is activated by intracellular Ca2+ signaling events. We found that TICC activity is independent of Ca2+ influx and ryanodine receptor activity but is attenuated by sarco(endo)plasmic reticulum Ca2+-ATPase inhibition and blockade of inositol 1,4,5-trisphosphate receptor-mediated Ca2+ release from the sarcoplasmic reticulum. Our findings suggest that TRPM4 channels in cerebral artery myocytes are regulated by Ca2+ release from inositol 1,4,5-trisphosphate receptor on the sarcoplasmic reticulum.