O2-sensitive K+ current in undifferentiated and NGF-treated PC12 cell variants

Characterization of the K+ channel opening effect of the anticonvulsant retigabine in PC12 cells

Retigabine (D-23129) is a new anticonvulsant compound which acts as a K+ channel opener in neuronal cells. The aim of the present study was to further characterize the retigabine induced K+ current. In nerve growth factor treated PC12 cells and in rat cortical neurones the application of retigabine activated a K+ current. In contrast, however, no K+ current activation was observed in untreated PC12 and in glial cells which were cultivated together with the neuronal cells. To characterise the retigabine activated K+ current, K+ channel blockers were used. The retigabine induced current was not affected by 1 and 10 mM 4-aminopyridine (4AP). Ba2+ 1 mM resulted in a reduction of 88.6+/-3.0% (n = 5); 10 mM abolished the current. Tetraetylamonium (TEA), 1 and 10 mM, reduced the current by 23.6+/-3.1 and 61.6+/-3.7%, respectively. To investigate the current/voltage (I/V) relation of the current initiated by retigabine (10 microM), cells were clamped to a holding potential of -80 mV and a ramp stimulation protocol (-120 to +60 mV in 5 s) was applied prior to and during application of retigabine. Subtraction of the two traces yielded the current induced by retigabine. A nearly linear relationship was determined between - 120 and -40 mV. At potentials positive to - 40 mV, the response was variable. This was due to the additionally observed weak blocking effect of retigabine on delayed rectifier (Kdr) currents. If the ramp was applied in the presence of 10 mM 4AP to block Kdr, a nearly linear I/V-relationship was present from -120 to +60 mV. The comparison of the I/V relation and pharmacology with published K+ channel subtypes gives evidence that an unknown neuronal K+ channel subtype may be involved.

Hypoxia evokes catecholamine secretion from rat pheochromocytoma PC-12 cells

We have monitored exocytosis of catecholamines from individual PC-12 cells by amperometry using carbon fiber microelectrodes in order to investigate possible secretory responses to acute hypoxia. In normoxia, no secretion was detected from cells perfused with a solution containing 5 mM K+. However, when [K+] was raised (10-100 mM), exocytotic events were observed. Hypoxia (PO2 11 mmHg) stimulated secretion from PC-12 cells, and in hypoxic conditions exocytosis was greater at each [K+] studied as compared with normoxia. Hypoxia-evoked secretion was abolished in Ca2+ free solutions containing 1 mM EGTA and by the non-specific Ca2+ channel blocker, Cd2+ (200 microM). Secretion was also largely inhibited by omega-conotoxin GVIA (1 microM). Exocytosis was also observed in normoxia when cells were exposed to tetraethylammonium (1-10 mM), but not 4-aminopyridine (3 mM). Our findings indicate that hypoxia evokes exocytosis via depolarization arising from inhibition of a TEA-sensitive K+ conductance, leading to Ca2+ influx primarily via N-type Ca2+ channels.

Release of dopamine and norepinephrine by hypoxia from PC-12 cells

We examined the effects of hypoxia on the release of dopamine (DA) and norepinephrine (NE) from rat pheochromocytoma 12 (PC-12) cells and assessed the involvement of Ca2+ and protein kinases in stimulus-secretion coupling. Catecholamine release was monitored by microvoltammetry using a carbon fiber electrode as well as by HPLC coupled with electrochemical detection (ECD). Microvoltammetric analysis showed that hypoxia-induced catecholamine secretion (PO2 of medium approximately 40 mmHg) occurred within 1 min after the onset of the stimulus and reached a plateau between 10 and 15 min. HPLC-ECD analysis revealed that, at any level of PO2, the release of NE was greater than the release of DA. In contrast, in response to K+ (80 mM), DA release was approximately 11-fold greater than NE release. The magnitude of hypoxia-induced NE and DA releases depended on the passage, source, and culture conditions of the PC-12 cells. Omission of extracellular Ca2+ or addition of voltage-gated Ca2+ channel blockers attenuated hypoxia-induced release of both DA and NE to a similar extent. Protein kinase inhibitors, staurosporine (200 nM) and bisindolylmaleimide I (2 microM), on the other hand, attenuated hypoxia-induced NE release more than DA release. However, protein kinase inhibitors had no significant effect on K+-induced NE and DA releases. These results demonstrate that hypoxia releases catecholamines from PC-12 cells and that, for a given change in PO2, NE release is greater than DA release. It is suggested that protein kinases are involved in the enhanced release of NE during hypoxia.