Effects of neomycin on calcium channel currents in clonal GH3 pituitary cells

Membrane Ca currents were recorded from voltage-clamped clonal (GH3) pituitary cells under conditions where currents through Na and K channels were abolished. Two Ca currents, with distinct kinetics and voltage dependence for activation and inactivation, were identified. Neomycin, an aminoglycoside polycation, inhibited both the "transient" (ICa,t) and the "slowly-inactivating" (ICa,s) Ca currents in a dose-dependent manner (100-1,000 microM). The blockade was reversible and ICa,s was more sensitive to neomycin than ICa,t. The inhibition of ICa,s was frequency and time-independent, and was not affected by changes in the holding membrane potential (-35 to -100 mV). Neomycin did not affect the voltage dependence for inactivation of ICa,t. The blockade of both Ca currents by neomycin is ascribed to the general property of aminoglycosides to compete with, and displace Ca ions from membrane binding sites that determine the currents and selectivity of Ca channels. Because comparable concentrations of neomycin were required for blocking the currents conveyed by Ca or, in the absence of external divalent cations, by Na ions through the slowly-inactivating Ca channels, we suggest that the neomycin binding sites are distinct from the high-affinity transition sites within the Ca channel path.