Ion concentration-dependence of rat cardiac unitary L-type calcium channelconductance

Little is known about the native properties of unitary cardiac L-type calci um currents (i(Ca)) measured with physiological calcium (Ca) ion concentrat ion, and their role in excitation-contraction (E-C) coupling. Our goal was to chart the concentration-dependence of unitary conductance (gamma) to phy siological Ca concentration and compare it to barium ion (Ba) conductance i n the absence of agonists. In isolated, K-depolarized rat myocytes, i(Ca) a mplitudes were measured using cell-attached patches with 2 to 70 mM Ca or 2 to 105 mM Ba in the pipette. At 0 mV, 2 mM of Ca produced 0.12 pA, and 2 m M of Ba produced 0.19 pA unitary currents. Unitary conductance was describe d by a Langmuir isotherm relationship with a maximum gamma (Ca) of 5.3 +/- 0.2 pS (n = 15), and gamma (Ba) of 15 +/- 1 pS (n = 27). The concentration producing half-maximal gamma, Kd((gamma)), was not different between Ca (1. 7 +/- 0.3 mM) and Ba (1.9 +/- 0.4 mM), We found that quasi-physiological co ncentrations of Ca produced currents that were as easily resolvable as thos e obtained with the traditionally used higher concentrations, This study le ads to future work on the molecular basis of E-C coupling with a physiologi cal concentration of Ca ions permeating the Ca channel.