THE ACTIVATION OF CA2+-DEPENDENT K+ CONDUCTANCE BY ADRENALINE IN MOUSE PERITONEAL-MACROPHAGES

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 (4-10 s in duration, 100-500 pA in amplitude) at -40 mV associated with a marked increase in conductance. The adrenaline-induced current [I(o) (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 I(o)(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 I(o)(Adr). Furthermore, I(o)(Adr) was blocked by bath-applied quinidine and charybdotoxin, but not by tetraethylammonium or apamin. Extracellular application of an alpha-1-adrenoceptor agonist phenylephrine and of noradrenaline mimicked I(o)(Adr). On the other hand, I(o)(Adr) was antagonized by a non-selective alpha-adrenoceptor antagonist phentolamine (0.2-mu-M) and an alpha-1-adrenoceptor antagonist prazosin (0.2-mu-M), but was not affected by an alpha-2-adrenoceptor antagonist yohimbine (1-mu-M) or a beta-adrenoceptor antagonist propranolol (1-mu-M). 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 alpha-1-adrenoceptor.