The activation of Ca2+-dependent K+ conductance by adrenaline in mouse peritoneal macrophages

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Abstract
Responses to adrenaline in mouse peritoneal macrophages were investigated with perforated and cell-attached patch-clamp recording, and with a combination of the perforated-patch recording and fura-2 fluorescence measurements. Extracellularly applied adrenaline induced a transient outward current (4–10s in duration, 100–500 pA in amplitude) at −40 mV associated with a marked increase in conductance. The adrenaline-induced current [Io (Adr)] reversed polarity near −80 mV. The reversal potential depended distinctly on the external K+ concentration but not on external Cl− concentration. Removal of external Ca2+ did not affect Io(Adr) within 2–4 min but subsequent responses to adrenaline were progressively depressed. In contrast, treatment with an intracellular Ca2+ chelator, the acetoxymethyl ester of 1,2-bis-(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid completely abolished Io(Adr). Furthermore, Io(Adr) was blocked by bath-applied quinidine and charybdotoxin, but not by tetraethylammonium or apamin. Extracellular application of an α1-adrenoceptor agonist phenylephrine and of noradrenaline mimicked Io(Adr). On the other hand, Io(Adr) was antagonized by a non-selective α-adrenoceptor antagonist phentolamine (0.2 μM) and an α1-adrenoceptor antagonist prazosin (0.2 μM), but was not affected by an α2-adrenoceptor antagonist yohimbine (1 μM) or a β-adrenoceptor antagonist propranolol (1 μM). Cell-attached single-channel recordings with the pipette solution containing 145 mM KCl revealed the activation of single-channel currents with a conductance of 40 pS during application of adrenaline outside the patch. Parallel measurements of membrane current and fura-2 fluorescence in the same cell demonstrated a correlation between the rise in [Ca2+]i and an increase in K+ conductance. Therefore, it is concluded that adrenaline activates a Ca2+-dependent K+ conductance by release of Ca2+ from internal stores through an activation of an α1-adrenoceptor.