Radioactive rubidium (86Rb+) efflux was used to measure potassium (K+) permeability in a study designed to assess both the presence and the sensitivity to ions and drugs of the K+ channels in the plasma membrane of rat lactotrophs. Rb+ efflux from Rb+-pre-loaded lactotrophs into nominally calcium-free solution containing 5 mM-K+ was linear from 1 to 60 s, with a calculated rate of about 0.1%/s. Raising K+ concentrations to depolarize the cells stimulated the Rb+ efflux (0.2%/s), which was already significant after 1 s of exposure of the cells to 100 mM-K+. This component of Rb+ efflux has been designated component V (sensitive to voltage and Ca2+-independent). Addition of Ca2+ to 5 mM-K+ solution had no effect on resting Rb+ efflux (0.1 %/s), but did further stimulate Rb+ efflux into K+-rich solutions. This component, which has been designated component C, was completely inhibited by 0.5 mM-cadmium. These data fit the view that the increase in intracellular Ca2+ concentration during depolarization opens certain (Ca2+-activated) K+ channels. K+ efflux was differently affected by K+ channel blockers. Tetraethylammonium (TEA) inhibited both V and C components while 4-aminopyridine (4-AP) inhibited the component V without modifying the C component of Rb+ efflux. Dopamine appears to affect both types of Rb+ efflux components. Dopamine increased the efflux of Rb+ in a nominally Ca2+-free medium containing 5 mM-K+ (component V). This effect was statistically significant 15 s after exposure of the cells to 10 nM-dopamine. Increasing the concentrations of K+ to gradually depolarize the cells enhanced the rate of increase of Rb+ efflux induced by dopamine, being evident in the initial 2-5 s of incubations. Dopamine also increased Rb+ efflux in a 5 mM-K+ solution containing 1 mM-Ca2+ (component C). This effect was rapid (2-5 s) and inhibited by 0.5 mM-cadmium. The combined action of dopamine on both component C and V caused the cells to be less sensitive to depolarizing concentrations of K+. The increase in Rb+ efflux and the enhancement of prolactin release induced by high concentrations of K+ were, indeed, prevented by exposure of the cells to 10 nM-dopamine. The effects of dopamine on either component V or component C were pharmacologically characterized as D2 receptor mediated, being mimicked by selective D2 receptor agonists (quinpirole and RU 24213) and stereospecifically blocked by the D2 receptor antagonist sulpiride. The increase in Rb+ efflux induced by dopamine in a calcium-free medium (component V) was unaffected by treatment of the cells with the adenylate cyclase activator forskolin, but the same treatment caused the cells to be resistant to the effects of dopamine on component C. The kinetic and pharmacological data here reported suggest that lactotrophs contain at least two classes of K+ channels that participate in the regulation of K+ fluxes in both resting and stimulated conditions. Dopamine appears to promote opening of both types of K+ channels by interacting with D2 receptors. The effects of dopamine on component V appear to be both Ca2+ and cyclic AMP independent while the effects of dopamine on component C seem to be cyclic AMP mediated and calcium sensitive.
|Number of pages||15|
|Journal||Journal of Physiology|
|Publication status||Published - 1989|
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