Ceramide inhibits the outward potassium current in rat pinealocytes

In the present study, we investigated the effect of ceramide on the outward K+ current in rat pinealocytes using whole cell and single channel recordings. Three components of the whole cell outward K+ current were separated, an iberiotoxin (IBTX)-sensitive K+ current (I-KCa), a transient A current (I-A) and a delayed rectifier current (I-K), C6-ceramide reduced all three components of the outward K+ current. C6-ceramide (30 mum) caused a 53% inhibition of I-KCa [a component that is generated by the IBTX-sensitive K+ channel (BK channel)], a 27% inhibition of I-A and a 17% inhibition of I-K. Additional studies showed that the BK channel was not inhibited by dihydroC6-ceramide, the inactive analog of C6-ceramide, but mimicked by sphingomyelinase which increased intracellular ceramide. The ceramide inhibition of the BK channel was only partly dependent on its inhibition of the L-type Ca2+ channel. Studies using specific kinase inhibitors showed that calphostin C (a protein kinase C inhibitor) and to a lesser degree lavendustin A (a tyrosine kinase inhibitor) were effective in reducing the ceramide inhibition Of I-Kca. Taken together, our results show that, in rat pinealocytes, ceramide reduces the outward K+ current predominantly by inhibiting I-Kca. Moreover, protein kinase C appears to be the main kinase involved in the ceramide inhibition Of I-KCa.