INHIBITION OF THE M CURRENT IN NG 108-15 NEUROBLASTOMA X GLIOMA HYBRID-CELLS

The M current, I(M), a voltage-dependent non-inactivating K current, was recorded in NG108-15 neuroblastoma X glioma hybrid cells, using the whole-cell mode of the patch-clamp technique. We studied inhibition of the M current by bradykinin, phorbol dibutyrate (PDBu), an activator of protein kinase C (PKC), and methylxanthines. Focal application of 0.1-5-mu-M bradykinin inhibited I(M) by about 60%; 5 nM bradykinin inhibited by about 40%. Bath application of 0.1-mu-M and 1-mu-M PDBu diminished I(M) to about half of the control value. Staurosporine, a PKC inhibitor, applied for 35-43 min in a concentration of 0.3-mu-M significantly reduced the effect of 1-mu-M PDBu. M current blockage by PDBu could be partly reversed by bath application of H-7 (51-64-mu-M), another PKC inhibitor. These observations suggest that the PDBu effect is really due to activation of PKC. The findings are compatible with the view [Brown DA, Higashida H (1988) J Physiol (Lond) 397:185-207] that the bradykinin effect on I(M) is mediated by PKC. However, three further observations suggest that this is only true for part of the bradykinin effect. When the suppression of I(M) by 1-mu-M PDBu was fully developed, 0.1-mu-M bradykinin produced a further inhibition of I(M). Down-regulation of PKC by long-term treatment with PDBu reduced the effect of 0.1-mu-M bradykinin significantly but did not abolish it. Staurosporine (0.3-mu-M, applied for 31-46 min) failed to reduce the effect of 5 nM bradykinin significantly. The M current could be reversibly blocked by methylxanthines (caffeine, isobutyl-methylxanthine, theophylline) in the millimolar range, probably because of a diret action on the M channels.