Overexpression of human CD38/ADP-ribosyl cyclase enhances acetylcholine-induced Ca2+ signalling in rodent NG108-15 neuroblastoma cells.

The role of cyclic ADP-ribose (cADPR) and its synthetic enzyme, CD38, as a downstream signal of muscarinic acetylcholine receptors (mAChRs) was examined in neuroblastoma cells expressing M1 mAChRs (NGM1). NGM1 cells were further transformed with both wild-type and mutant (C119K/C201E) human CD38. The dual transformed cells exhibited higher cADPR formation than ADPR production and elevated intracellular free Ca(2+) concentrations ([Ca(2+)](i)) in response to ACh. These phenotypes were analyzed in detail in a representative CD38 clone. The intracellular cADPR concentration by ACh application was significantly increased by CD38 overexpression. Digital image analysis by a confocal microscopy revealed that topographical distribution of the sites of Ca(2+) release was unchanged between control and overexpressed cells. These results indicate that cADPR is an intracellular messenger of Ca(2+) signalling, suggesting that CD38 can contribute to mAChR-cADPR signalling.

Modulation of single channels underlying hippocampal L-type current enhancement by agonists depends on the permeant ion.

The influx of calcium (Ca(2+)) ions through L-type channels underlies many cellular processes, ranging from initiation of gene transcription to activation of Ca(2+)-activated potassium channels. L-type channels possess a diagnostic pharmacology, being enhanced by the dihydropyridine BAY K 8644 and benzoylpyrrole FPL 64176. It is assumed that the action of these compounds is independent of the ion conducted through the channel. In contrast to this assumption, modulation of L-type channel activity in acutely dissociated rat CA1 hippocampal neurons depended on the divalent ion identity. BAY K 8644 and FPL 64176 substantially increased single-channel open time only when barium (Ba(2+)) was the permeant ion. BAY K 8644 increased single-channel conductance when either Ba(2+) or Ca(2+) ions were the charge carrier, an effect not observed with FPL 64176. BAY K 8644 enhanced the whole cell L-type channel Ca(2+)- or Ba(2+)-carried current without a change in deactivation tail kinetics. In contrast, enhancement by FPL 64176 was associated with a dramatic slowing of deactivation kinetics only when Ba(2+) and not Ca(2+) was the charge carrier. Current activation was slowed by FPL 64176 with either charge carrier, an effect arising from a clustering of agonist-modified long-duration openings toward the end of the voltage step. These data indicate that agonists enhanced L-type current by distinct mechanisms dependent on the permeant ion, indicating that care must be considered when used as diagnostic tools.