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Effects of neomycin on calcium channel currents in clonal GH3 pituitary cells
1Departamento de Farmacologia, Universidade Federal do Rio de Janeiro, Brazil.
Abstract:
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.
Insights
Neomycin, an aminoglycoside, inhibits both transient and slowly-inactivating calcium currents in pituitary cells. This blockade suggests neomycin binds to sites distinct from the calcium channel
Area of Science:
- Cellular Electrophysiology
- Pharmacology
- Neuroendocrinology
Background:
- Pituitary cells (GH3) possess distinct calcium (Ca) currents crucial for cellular function.
- Aminoglycosides, like neomycin, are known to interact with cellular membranes and ion channels.
Purpose of the Study:
- To investigate the effects of neomycin on specific calcium currents in GH3 pituitary cells.
- To characterize the inhibition kinetics and voltage dependence of neomycin's action on Ca currents.
Main Methods:
- Whole-cell patch-clamp electrophysiology was used to record membrane Ca currents.
- Na and K channel currents were blocked to isolate Ca currents (ICa,t and ICa,s).
- Dose-response and voltage-dependence experiments were conducted with neomycin application.
Main Results:
- Neomycin inhibited both transient (ICa,t) and slowly-inactivating (ICa,s) Ca currents in a dose-dependent manner.
- ICa,s was more sensitive to neomycin than ICa,t; inhibition was reversible.
- Neomycin's blockade mechanism appears to involve displacement of Ca ions, with binding sites distinct from channel pore sites.
Conclusions:
- Neomycin effectively blocks both major Ca currents in GH3 pituitary cells.
- The findings suggest a general aminoglycoside property of interfering with Ca binding sites on channels.
- Neomycin binding sites are likely external to the primary Ca channel conduction pathway.