Phenylalkylamine-sensitive calcium channels in osteoblast-like osteosarcoma cells. Characterization by ligand binding and single channel recordings

(-)-[³H]Desmethoxyverapamil ((-)-DMV) binds saturably to homogenates of the osteoblast-like cell lines UMR 106 and ROS 17/2.8 with K(D) values of 45 and 61 nM and B(max) values of 6.0 and 5 pmol/mg protein, respectively. Binding is stereoselective with (-)-DMV 8-10 times more potent than (+)-DMV. None of the dihydropyridine or benzothiazepine Ca²⁺ antagonists examined affect (-)-[³H]DMV binding. Monovalent cations such as Li⁺, Na⁺, and K⁺ inhibit (-)[³H]DMV binding in the 100-400 mM range. Divalent cations such as Ba²⁺, Sr²⁺, Ca²⁺, and Mg²⁺ are effective binding inhibitors in the 2-5 mM range. ROS 17/2.8 cells express a channel on the apical plasma membrane which conducts Ba²⁺ and Ca²⁺. With 110 mM BaCl₂ or CaCl₂ as charge carriers the single channel conductance is 3-5 picosiemens. In cell-excised patches the channel selects for Ba²⁺ over Na⁺ 3.3:1. In the absence of divalent ions the channel conducts Na⁺ ions with a single channel conductance of 13 picosiemens. This Na⁺ conductance decreases with psychological levels of Ca²⁺. The channel appears related to the (-)-[³H]DMV binding site, since its conductance is blocked by verapamil in a dose-dependent manner. Moreover, DMV blocks the channel stereoselectively with relative potencies of the isomers corresponding to their affinities for the binding site. The dihydropyridine drugs BAY K 8644 or (+)-202-791 do not affect channel opening. These binding and biophysical data indicate that osteoblast cells have a phenylalkylamine receptor associated with a Ca²⁺ channel.