CA2+-Activated K+ Channels in Erythrocytes and Excitable Cells

Abstract

The proper execution of many physiological functions, such as the conduc­ tion of impulses in nerve and muscle, the activities of cardiac and neuronal _.pacemakers, light reception, secretion, and fertilization, usually requires accurately timed and selective changes of K+ permeability. These changes are brought about by the activation of distinct channels that can be divided into two classes: those activated by variations in the membrane potential, and others activated by Ca2+ (5,9,22,29,37,47,70, 84a) and modulated by the membrane potential. In nerve axons, only the potential-activated K+ channels have been found (66). However, in the soma of neurons and in most other types of cells that have been studied so far, Ca2+-activated channels were discovered, either alone or together with the potential­ activated channels. The other cells include even nonexcitable cells such as macrophages, L-cells, and erythrocytes. Only the Ca2+-activated K+-selec­ tive channels form the subject of the present review (Table I). Ca2+-activated, K+-selective channels were first discovered in red cells, whose functions do not seem to depend to any extent on variations of K+ permeability or the associated variations of membrane potential. Ob­ servations made by Wilbrandt (99) in the late 1930s showed that the glucolytic inhibitors fluoride and iodoacetate could induce a selective increase of K+ emux that exceeded the increment expected to occur after an interruption of the energy supply to the K-Na pump. These observa­ tions were confirmed and extended by Gw-dos (26), who showed that the inhibition of metabolism must be accompanied by the presence of Ca2+ to obtain the response. Later work has shown that the presence of cer­ tain drugs [e.g. propranolol (19, 83)] renders the cells sensitive to