CALCIUM CURRENT ACTIVATED BP MUSCARINIC RECEPTORS AND THAPSIGARGIN INNEURONAL CELLS

The activation of muscarinic receptors in N1E-115 neuroblastoma cells elicits a voltage-independent calcium current. The current turns on sl owly, reaches its maximum value similar to 45 s after applying the ago nist, is sustained as long as agonist is present, and recovers by one half in similar to 10 s after washing the agonist away. The current de nsity is 0.11 +/- 0.08 pA/pF (mean +/- SD; n = 12). It is absent in ze ro-Ca++ saline and reduced by Mn++ and Ba++. The I(V) curve characteri zing the current has an extrapolated reversal potential > +40 mV. The calcium current is observed in cells heavily loaded with BAPTA indicat ing that the calcium entry pathway is not directly gated by calcium. I n fura-2 experiments, we find that muscarinic activation causes an ele vation of intracellular Ca++ that is due to both intracellular calcium release and calcium influx. The component of the signal that requires external Ca++ has the same time course as the receptor operated calci um current. Calcium influx measured in this way elevates (Ca++)(i) by 89 +/- 41 nM (n = 7). Thapsigargin, an inhibitor of Ca++/ATPase associ ated with the endoplasmic reticulum (ER), activates a calcium current with similar properties. The current density is 0.22 +/- 0.20 pA/pF (n = 6). Thapsigargin activated current is reduced by Mn++ and Ba++ and increased by elevated external Ca++. Calcium influx activated by thaps igargin elevates (Ca++)(i) by 82 +/- 35 nM. The Ca++ currents due to a gonist and due to thapsigargin do not sum, indicating that these proce dures activate the same process. Carbachol and thapsigargin both cause calcium release from internal steres and the calcium current bears st rong similarity to calcium-release-activated calcium currents in nonex citable cells (Hoth, M., and R. Penner. 1993. Journal of Physiology. 4 65:359-386; Zweifach, A., and R. S. Lewis, 1993. Proceedings of the Na tional Academy of Sciences, USA. 90:6295-6299).