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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 (V_1/2)activation was –46 mV in both perforated and …