Characteristics of calcium currents in rabbit portal vein myocytes

The properties of voltage-dependent Ca²⁺ channels were studied in isolated portal vein myocytes using the whole cell voltage-clamp method. Ca²⁺ currents (I(Ca)) were identified based on their activation and inactivation potential, their dependence on external Ca²⁺ ([Ca²⁺](o)), their suppression by organic or inorganic Ca²⁺ channel blockers, their augmentation by BAY K 8644, and their insensitivity to tetrodotoxin or alterations in external Na⁺ ([Na⁺](o)). Changing the holding potential from -90 to -40 mV decreased I(Ca) from 4.6 ± 0.6 to 2.0 ± 0.3 pA/pF at 0 mV but did not shift its voltage dependence significantly. The voltage dependence of steady-state inactivation and activation was represented by Boltzmann distributions with the following parameters: inactivation, half-maximal voltage (V_0.5) = -32 ± 7 mV and slope factor (k) = 6.1 ± 0.2 mV; activation, V_0.5 = -15 ± 4 mV and k = 5.6 ± 0.6 mV. Doubling the [Ca²⁺](o) increased I(Ca) and shifted the voltage dependence of its activation and inactivation by ~10 mV toward more positive potentials without altering the window currents. Substituting Na⁺, Ba²⁺, or Sr²⁺ for Ca²⁺ as the charge carrier through the Ca²⁺ channel slowed the rate of its inactivation and shifted its voltage dependence toward more negative potentials. Divalent selectivity of the Ca²⁺ channel showed an apparent concentration dependence: at 2 mM I(Sr) < I(Ba) = I(Ca), while at 10 mM I(Ca) < I(Sr) = I(Ba). Because 50-100 μM ethylene glycol-bis(β-aminoethyl ether)- N,N,N',N'-tetraacetic acid abolished the apparent concentration dependence of the divalent ion selectivity, this phenomenon was attributed to a high Ca²⁺ selectivity of the channel. Our data support the presence of only one type of Ca²⁺ channel in rabbit portal vein myocytes with characteristics similar to the L-type Ca²⁺ channel described in other cells, but with somewhat different divalent selectivity, holding potential, and [Na⁺](o) dependence.