Regulation of pacemaker currents in interstitial cells of Cajal from murine small intestine by cyclic nucleotides

1. Electrical rhythmicity (slow waves) in gastrointestinal muscles (GI) is generated by interstitial cells of Cajal (ICC). Cultured ICC from the murin e small intestine were studied with the patch-clamp technique to characteri ze regulation of pacemaker currents by cyclic nucleotides. Cyclic nucleotid e agonists were also tested on intact strips of murine small intestine. 2. Nitric oxide donors slowed the frequency of pacemaker currents in a conc entration-dependent manner. These effects depended on cGMP formation and we re reduced by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The effect s of nitric oxide donors were mimicked by meabrane-permeable analogues of c GMP. The specific cGMP phosphodiesterase inhibitor zaprinast reduced the fr equency of spontaneous pacemaker currents. 3. The cGMP-dependent effects on pacemaker currents were not affected by ok adaic acid or KT-5823, an inhibitor of protein kinase G. 4. Forskolin, but not dideoxy forskolin, reduced the frequency of spontaneo us pacemaker activity and activated a sustained outward current. The latter was likely to be due to ATP-dependent K+ channels because it was blocked b y glibenclamide. 5. The effects of forskolin were not mimicked by membrane-permeable cAMP an alogues. A membrane-permeable inhibitor of protein kinase A, myristoylated PKA inhibitor, and the adenylyl cyclase inhibitor SQ-22536, had no effect o n responses to forskolin. 6. Responses of intact muscles to cGMP and cAMP agonists were similar to th e responses of pacemaker cells. Changes in resting membrane potential and s low wave amplitude, however, were noted in intact jejunal muscles that were not observed in ICC. Differences in responses may have been due to the eff ects of cyclic nucleotide agonists on smooth muscle cells that would sum wi th responses of ICC in intact jejunal muscle strips. 7. A cGMP-dependent mechanism regulates slow wave frequency, but this occur s through direct action of cGMP not via protein phosphorylation. Regulation of pacemaker currents by cAMP-dependent mechanisms was not observed.