We have tracked the cell surface area of CHO cells by measuring the membrane capacitance, Cm. An increase in cytosolic [Ca2+], [Ca2+]i, increased the cell surface area by 20–30%. At micromolar [Ca2+]i the increase occurred in minutes, while at 20 microM or higher [Ca2+]i it occurred in seconds and was transient. GTPgammaS caused a 3% increase even at 0.1 microM [Ca2+]i. We conclude that CHO cells, previously thought capable only of constitutive exocytosis, can perform Ca2+‐triggered exocytosis that is both massive and rapid. Ca2+‐triggered exocytosis was also observed in 3T3 fibroblasts. Our findings add evidence to the view that Ca induces exocytosis in cells other than known secretory cells.

Low-voltage-activated (l-v-a) and high-voltage-activated (h-v-a) Ca2+ currents (ICa) were recorded in whole-cell voltage clamped NG108-15 neuroblastoma x glioma hybrid cells. We studied the effects of arachidonic acid (AA), oleic acid, myristic acid and of the positively charged compounds tetradecyltrimethylammonium (C14TMA) and sphingosine. At pulse potentials > -20 mV, AA (25-100 microM) decreased l-v-a and h-v-a ICa equally. The decrease developed slowly and became continually stronger with increasing time of application. It was accompanied by a small negative shift and a slight flattening of the activation and inactivation curves of the l-v-a ICa. The shift of the activation curve manifested itself in a small increase of l-v-a ICa at pulse potentials < -30 mV. The effects were only partly reversible. The AA effect was not prevented by 50 microM 5, 8, 11, 14-eicosatetraynoic acid, an inhibitor of the AA metabolism, and not mimicked by 0.1-1 microM phorbol 12, 13-dibutyrate, an activator of protein kinase C. Probably, AA directly affects the channel protein or its lipid environment. Oleic and myristic acid acted similarly to AA but were much less effective. The positively charged compounds C14TMA and sphingosine had a different effect: They shifted the activation curve of l-v-a ICa in the positive direction and suppressed l-v-a more than h-v-a ICa; their effect reached a steady-state within 5-10 min and was readily reversible. C14TMA blocked l-v-a ICa with an IC50 of 4.2 microM while sphingosine was less potent.

The M current, IM, of NG108-15 neuroblastoma x glioma hybrid cells, a non-inactivating K+ current, is decreased by arachidonic acid (5-25 microM), often after an initial transitory increase. To test the possibility that the decrease is caused by activation of protein kinase C (PKC) we used the PKC 19-31 peptide, which is an effective inhibitor of PKC. With 1 microM peptide in the pipette solution the normally observed strong reduction of IM by 1 microM phorbol 12,13-dibutyrate (PDB) was almost totally prevented, indicating that PKC is completely inhibited; also the voltage dependence of the M conductance, gM(V), was shifted to more negative membrane potentials. In the presence of 1 microM peptide the effect of 25 microM arachidonic acid on IM was significantly reduced, suggesting that the effect, or at least a large part of it, is mediated by PKC.

No abstract

No abstract