Effects of endogenous and exogenous nitric oxide on electrical responses of circular smooth muscle isolated from the guinea-pig stomach antrum

Inhibition of pacemaker currents by nitric oxide via activation of ATP-sensitive K+ channels in cultured interstitial cells of Cajal from the mouse small intestine

We investigated the role of nitric oxide (NO) in pacemaker activity and signal mechanisms in cultured interstitial cells of Cajal (ICC) of the mouse small intestine using whole cell patch-clamp techniques at 30 degrees C. ICC generated pacemaker potential in the current clamp mode and pacemaker currents at a holding potential of -70 mV. (+/-)-S-nitroso-N-acetylpenicillamine (SNAP; a NO donor) produced membrane hyperpolarization and inhibited the amplitude and frequency of the pacemaker currents, and increased resting currents in the outward direction. These effects were blocked by the use of glibenclamide (an ATP-sensitive K+ channel blocker), but not by the use of 5-hydroxydecanoic acid (a mitochondrial ATP-sensitive K+ channel blocker). Pretreatment with ODQ (a guanylate cyclase inhibitor) almost blocked the NO-induced effects. The use of cell-permeable 8-bromo-cyclic GMP also mimicked the action of SNAP. However, the use of KT-5823 (a protein kinase G inhibitor) did not block the NO-induced effects. Spontaneous [Ca2+]i oscillations in ICC were inhibited by the treatment of SNAP, as seen in recordings of intracellular Ca2+ ([Ca2+]i). These results suggest that NO inhibits pacemaker activity by the activation of ATP-sensitive K+ channels via a cyclic GMP dependent mechanism in ICC, and the activation of ATP-sensitive K+ channels mediates the inhibition of spontaneous [Ca2+]i oscillations.

Effects of 5-hydroxytryptamine on electrical responses of circular smooth muscle isolated from the guinea-pig gastric antrum

The effects of 5-hydroxytryptamine (5-HT) on electrical responses of the membrane were investigated in circular smooth muscle isolated from the guinea-pig stomach antrum. Small segment of circular muscle tissue produced a periodical generation of slow potentials at frequency of 0.1-2 cycles min(-1), during random generation of unitary potentials. Application of 5-HT (10(-7)-10(-5) M) hyperpolarized the membrane and either increased or decreased the frequency of slow potentials, both with associated increase in amplitude of slow potential. These effects of 5-HT were abolished by methysergide. N(omega)-nitro-L-arginine (L-NA) increased the frequency of spontaneously generated slow potentials and also increased the frequency of slow potentials generated during stimulation with 5-HT, suggesting an involvement of the increased production of nitric oxide (NO) by 5-HT. Atropine did not alter spontaneous and 5-HT-induced electrical responses. The hyperpolarization produced by 5-HT was associated with a decrease in input resistance and time constant of the membrane. The amplitude of the 5-HT-induced hyperpolarization was increased in low [K(+)](o) solution and decreased in high [K(+)](o) solution or in the presence of glybenclamide, suggesting that the hyperpolarization was produced by activation of ATP-sensitive K-channels. The increase in amplitude of slow potentials by 5-HT may be secondary due to hyperpolarization of the membrane. The inhibition by 5-HT of the frequency of slow potentials may be partly due to the increased release of NO, however the mechanism by which dual effects of 5-HT on the frequency of slow potentials remains unsolved.