Dual modulation of an inwardly rectifying potassium conductance

The modulation of a constitutively active IRK1-like inwardly rectifying potassium channel, that is endogenously expressed in the RBL-2H3 cell, was studied with the whole-cell patch-clamp technique. Activation of G-proteins by intracellular application of GTP gamma S revealed a dual modulation of the inward rectifier. An initial increase in inward current amplitude was induced by GTP gamma S, followed by a profound inhibition of the current. The stimulation of the inward rectifier by GTP gamma S was abolished by pretreatment with pertussis toxin. The inhibitory phase of the GTP gamma S-induced response was pertussis toxin-insensitive. Stimulation of the m1-muscarinic receptor expressed in the RBL cell after stable transfection, induced an inhibition of the inwardly rectifying currents. Application of protein kinase C activators such as phorbol 12-myristate 13-acetate and phorbol 12,13-dibutyrate, resulted in a strong inhibition of the currents. Application of the cAMP-dependent protein kinase activator 8-bromo cAMP also induced an inhibition of the inward rectifier. It is concluded that the inward rectifier of the RBL-2H3 cell may be inhibited both by activation of protein kinase C and by cAMP-dependent protein kinase. As this type of inward rectifier is widely expressed in the nervous system, these data imply that the channel can be inhibited by receptors that stimulate phospholipase C and/or stimulate adenylyl cyclase, and can be activated by receptors that inhibit adenylyl cyclase activity.