Stimulation of beta‐adrenoceptors inhibits calcium‐dependent potassium‐channels in mouse macrophages

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Abstract
K+ efflux in mouse macrophages exhibited a rate constant (kK) of 0.67 ± 0.04 (h)−1 (mean ± SEM of 16 experiments). This was strongly stimulated by increasing concentrations of the Ca2+ ionophore A23187 up to a maximal value of 4.01 ± 0.25 (h)−1 with an IC50 of 7.6 ± 1.9 μM (mean ± SEM of 6 experiments). Similar results were obtained with the Ca2+ ionophore ionomycin. Binding experiments with 3H-dihydroalprenolol revealed a high density of beta-adrenergic receptors (97.5 ± 5.2 fmol/mg protein) with apparent dissociation constant of 2.03 ± 0.06 nM. Isoproterenol at a concentration of 10−6-10−5 M induced a two- to threefold stimulation of endogenous levels of cyclic AMP (cAMP). A23187-stimulated K+ efflux was partially inhibited by (i) stimulation of adenylate cyclase with isoproterenol, forskolin or, PG1; (ii) exogenous cAMP; and (iii) inhibition of phosphodiesterase with MIX (1-methyl-3-isobutylxanthine). Maximal inhibition of K+ efflux was obtained by simultaneous addition of isoproterenol and MIX. In dose-response curves, the isoproterenol-sensitive K+ efflux was half-maximally inhibited (IC50) with 2–5 × 10−10 M of isoproterenol concentration. Propranolol was able to completely block the effect of isoproterenol, with an IC50 of about 1–2 × 10−7 M. Isoproterenol and MIX were also able to partially inhibit ionomycin-stimulated K+ efflux. Isoproterenol and MIX did not inhibit A23187-stimulated K+ efflux in an incubation medium where NaCI was replaced by sucrose (or choline), suggesting the involvement of an Na + : Ca2+ exchange mechanism. Our results show that stimulation of beta-adrenoceptors in mouse macrophages counterbalances the opening of K+ channels induced by the calcium ionophore A23187. This likely reflects a decrease in cytosolic free calcium content via a cAMP-mediated stimulation of Na + :Ca2+ exchange.