Glucocorticoids inhibit acetylcholine-induced current in chromaffin cells

The effects of dexamethasone, a synthetic glucocorticoid, on membrane excitability and on the acetylcholine (ACh)-induced inward current (IACh) were studied in dispersed guinea pig adrenal medullary cells by using the whole cell version of the patch-clamp technique. Bath application of 10 microM dexamethasone had no effect on the resting membrane conductance or voltage-gated Na+, Ca2+, or K+ channels, whereas it reversibly inhibited the IACh at a holding potential of -70 mV. Intracellular application of dexamethasone through the patch electrode did not modify the IACh. Application of 10 microM dexamethasone neither shifted the dose-response curve of the peak IACh to the right [dissociation constant (Kd) = 50 microM] nor affected its Hill coefficient (1.2) but inhibited the current amplitudes by approximately 41% in the cells sufficiently pretreated with dexamethasone. Furthermore, fractional inhibition of the IACh at the end of approximately 50-s application was the same for any concentration of ACh (3-100 microM). The dose-response curve of the inhibition by dexamethasone showed a good fit to the theoretical line assuming an inhibition constant (Ki) of 10 microM and a Hill coefficient of 1. Hydrocortisone 21-hemisuccinate sodium salt (25 microM) and prednisolone 21-hemisuccinate sodium salt (25 microM) inhibited the IACh to a lesser extent than 25 microM dexamethasone. These results suggest that dexamethasone binds to the specific site on the outer cell membrane, probably on the ACh receptor-coupled channel, and inhibits the IACh in a noncompetitive manner, thus controlling the immediate catecholamine release from the adrenal medulla.