Effects of arachidonic acid on ATP-sensitive K+ current in murine colonic smooth muscle cells

Muscarinic suppression of ATP-sensitive K+ channels mediated by the M3/Gq/11/phospholipase C pathway contributes to mouse ileal smooth muscle contractions

ATP-sensitive K⁺ (K_ATP) channels are expressed in gastrointestinal smooth muscles, and their activity is regulated by muscarinic receptor stimulation. However, the physiological significance and mechanisms of muscarinic regulation of K_ATP channels are not fully understood. We examined the effects of the K_ATP channel opener cromakalim and the K_ATP channel blocker glibenclamide on electrical activity of single mouse ileal myocytes and on mechanical activity in ileal segment preparations. To explore muscarinic regulation of K_ATP channel activity and its underlying mechanisms, the effect of carbachol (CCh) on cromakalim-induced K_ATP channel currents ( I_KATP) was studied in myocytes of M₂ or M₃ muscarinic receptor-knockout (KO) and wild-type (WT) mice. Cromakalim (10 µM) induced membrane hyperpolarization in single myocytes and relaxation in segment preparations from WT mice, whereas glibenclamide (10 µM) caused membrane depolarization and contraction. CCh (100 µM) induced sustained suppression of I_KATP in cells from both WT and M₂KO mice. However, CCh had a minimal effect on I_KATP in M₃KO and M₂/M₃ double-KO cells. The G_q/11 inhibitor YM-254890 (10 μM) and PLC inhibitor U73122 (1 μM), but not the PKC inhibitor calphostin C (1 μM), markedly decreased CCh-induced suppression of I_KATP in WT cells. These results indicated that K_ATP channels are constitutively active and contribute to the setting of resting membrane potential in mouse ileal smooth muscles. M₃ receptors inhibit the activity of these channels via a G_q/11/PLC-dependent but PKC-independent pathways, thereby contributing to membrane depolarization and contraction of smooth muscles. NEW & NOTEWORTHY We systematically investigated the regulation of ATP-sensitive K⁺ channels by muscarinic receptors expressed on mouse ileal smooth muscles. We found that M₃ receptors inhibit the activity of ATP-sensitive K⁺ channels via a G_q/11/PLC-dependent, but PKC-independent, pathway. This muscarinic suppression of ATP-sensitive K⁺ channels contributes to membrane depolarization and contraction of smooth muscles.

Role of arachidonic acid in hyposmotic membrane stretch-induced increase in calcium-activated potassium currents in gastric myocytes

AIM

To study effects of arachidonic acid (AA) and its metabolites on the hyposmotic membrane stretch-induced increase in calcium-activated potassium currents (I(KCa)) in gastric myocytes.

METHODS

Membrane currents were recorded by using a conventional whole cell patch-clamp technique in gastric myocytes isolated with collagenase.

RESULTS

Hyposmotic membrane stretch and AA increased both I(K(Ca))) and spontaneous transient outward currents significantly. Exogenous AA could potentiate the hyposmotic membrane stretch-induced increase in I(K(Ca)). The hyposmotic membrane stretch-induced increase in I(K(Ca)) was significantly suppressed by dimethyleicosadienoic acid (100 micromol/L in pipette solution), an inhibitor of phospholipase A2. Nordihydroguaiaretic acid, a lipoxygenase inhibitor, significantly suppressed AA and hyposmotic membrane stretch-induced increases in I(K(Ca)). External calcium-free or gadolinium chloride, a blocker of stretch-activated channels, blocked the AA-induced increase in I(K(Ca)) significantly, but it was not blocked by nicardipine, an L-type calcium channel blocker. Ryanodine, a calcium-induced calcium release agonist, completely blocked the AA-induced increase in I(K(Ca)); however, heparin, a potent inhibitor of inositol triphosphate receptor, did not block the AA-induced increase in I(K(Ca)).

CONCLUSION

Hyposmotic membrane stretch may activate phospholipase A2, which hydrolyzes membrane phospholipids to ultimately produce AA; AA as a second messenger mediates Ca(2+) influx, which triggers Ca(2+)-induced Ca(2+) release and elicits activation of I(K(Ca)) in gastric antral circular myocytes of the guinea pig.