Role of ion channels and subcellular Ca²⁺ signaling in arachidonic acid-induced dilation of pressurized retinal arterioles

Purpose. To investigate the mechanisms responsible for the dilatation of rat retinal arterioles in response to arachidonic acid (AA). Methods. Changes in the diameter of isolated, pressurized rat retinal arterioles were measured in the presence of AA alone and following pre-incubation with pharmacologic agents inhibiting Ca²⁺ sparks and oscillations and K⁺ channels. Subcellular Ca²⁺ signals were recorded in arteriolar myocytes using Fluo-4-based confocal imaging. The effects of AA on membrane currents of retinal arteriolar myocytes were studied using whole-cell perforated patch clamp recording. Results. Arachidonic acid dilated pressurized retinal arterioles under conditions of myogenic tone. Eicosatetraynoic acid (ETYA) exerted a similar effect, but unlike AA, its effects were rapidly reversible. Arachidonic acid-induced dilation was associated with an inhibition of subcellular Ca²⁺ signals. Interventions known to block Ca²⁺ sparks and oscillations in retinal arterioles caused dilatation and inhibited AA-induced vasodilator responses. Arachidonic acid accelerated the rate of inactivation of the A-type Kv current and the voltage dependence of inactivation was shifted to more negative membrane potentials. It also enhanced voltage-activated and spontaneous large-conductance calcium-activated K⁺ (BK) currents, but only at positive membrane potentials. Pharmacologic inhibition of A-type Kv and BK currents failed to block AA-induced vasodilator responses. Arachidonic acid suppressed L-type Ca²⁺ currents. Conclusions. These results suggest that AA induces retinal arteriolar vasodilation by inhibiting subcellular Ca²⁺-signaling activity in retinal arteriolar myocytes, most likely through a mechanism involving the inhibition of L-type Ca²⁺-channel activity. Arachidonic acid actions on K⁺ currents are inconsistent with a model in which K⁺ channels contribute to the vasodilator effects of AA.