Biophysical properties of voltage-gated Na+ channels in frog parathyroid cells and their modulation by cannabinoids

Abstract

SUMMARY The membrane properties of isolated frog parathyroid cells were studied using perforated and conventional whole-cell patch-clamp techniques. Frog parathyroid cells displayed transient inward currents in response to depolarizing pulses from a holding potential of –84 mV. We analyzed the biophysical properties of the inward currents. The inward currents disappeared by the replacement of external Na+ with NMDG+ and were reversibly inhibited by 3 μmol l–1 TTX, indicating that the currents occur through the TTX-sensitive voltage-gated Na+channels. Current density elicited by a voltage step from –84 mV to–24 mV was –80 pA pF–1 in perforated mode and–55 pA pF–1 in conventional mode. Current density was decreased to –12 pA pF–1 by internal GTPγS (0.5 mmol l–1), but not affected by internal GDPβS (1 mmol l-1). The voltage of half-maximum (V1/2)activation was –46 mV in both perforated and conventional modes. V1/2 of inactivation was –80 mV in perforated mode and –86 mV in conventional mode. Internal GTPγS (0.5 mmol l–1) shifted the V1/2 for activation to–36 mV and for inactivation to –98 mV. A putative endocannabinoid,2-arachidonoylglycerol ether (2-AG ether, 50 μmol l–1) and a cannabinomimetic aminoalkylindole, WIN 55,212-2 (10 μmol l–1) also greatly reduced the Na+ current and shifted the V1/2 for activation and inactivation. The results suggest that the Na+ currents in frog parathyroid cells can be modulated by cannabinoids via a G protein-dependent mechanism.