The purpose of this work was to determine whether arachidonic acid and other fatty acids might directly regulate the behavior of ion channels. Arachidonic acid is known to be liberated from plasma membrane phospholipid upon activation of cell surface receptors, and to subsequently act as a precursor to biologically active metabolites. This study was based on the rationale that the liberated arachidonic acid itself was a potential regulator of plasma membrane ion channels.

The effects of arachidonic acid and other fatty acids on the behavior of ion channels were examined in two preparations of isolated smooth muscle cells. In both cell types, K⁺-selective ion channels were activated both by arachidonic acid and by fatty acids that are not converted to metabolites through the cyclooxygenase and lipoxygenase metabolic pathways for arachidonic acid. These results indicate that metabolites of these pathways did not mediate the fatty acid response. Further, fatty acids were effective in cell-free patches of membrane in the absence of nucleotides and Ca⁺⁺, showing that signal transduction mechanisms requiring these and other cytosolic factors were not required. Such signaling mechanisms include those involving phosphorylation, cyclic nucleotides, GTP-dependent proteins, and the NADPH-dependent cytochrome P450 metabolic pathway. Thus fatty acids themselves appear to directly activate K⁺ channels, much as they directly activate several enzymes, and may constitute a new class of messenger molecules acting on ion channels.

The two preparations of cells used were gastric smooth muscle cells from the toad, Bufo Marinus, and pulmonary artery smooth muscle cells from the New Zealand White Rabbit.

In gastric smooth muscle cells, a previously undescribed K⁺ channel was activated by a variety of fatty acids. This channel exhibited a conductance of approximately 50 pS, weak voltage-dependence, and K⁺ selectivity. The fatty acid structural features required for activation of this channel were examined by testing numerous fatty acids. Further, the same K⁺ channel was found to be endogenously active in the presence of Ca⁺⁺ at the extracellular surface of the membrane.

In pulmonary artery smooth muscle cells, fatty acids activated K⁺ channels of a recognizable large-conductance type that is activated by Ca⁺⁺ at the intracellular membrane surface. This channel type has been widely studied but has not been reported in this preparation. Characteristic of the large-conductance, calcium-activated K⁺ (CAK) channel type, the channels activated by fatty acids exhibited a conductance of approximately 260 pS, strong voltage-dependence, K⁺ selectivity, and activation by low concentrations of Ca⁺⁺ (10^-7-10^-6 M) at the cytosolic surface of the membrane. Lastly, these CAK channels were found to be activated by membrane stretch.