The activity of voltage-sensitive (VS) Ca2+ channels depends upon the redox state of the thiol containing amino acids within the channels. Peroxynitrite (ONOO) is formed endogenously by the near diffusion-limited reaction of nitric oxide with Superoxide anion. As a potent oxidant, ONOO may interfere with the function of VS Ca2+ channels. The present study utilized cultured porcine aortic smooth muscle (PASM) cells to examine: (i) the effect of repeated application of ONOO (100 pM) on intracellular calcium levels ([Ca2+]i), (ii) the effect of pretreatment with the VS Ca2+ channel antagonist nifedipine (10 μM) on the changes in [Ca2+]i produced by a single application of ONOO (100 μM); and (iii) the effect of pretreatment with ONOO on the changes in [Ca2+]i produced by the VS Ca2+ channel agonist Bay K 8644 (50 pM). The first application of ONOO (100 μM) produced a marked increase in [Ca2+]i (+76 ±14%) which subsequently decreased and stabilized at a value 29 ±3% higher than baseline. A second application of ONOO produced a significantly smaller increase in |Ca2+]j (+2J ±5%) and a third application of ONOO minimally increased [Ca2+]i (+3 ±1%). The increase in lCa2+]i produced by a single application of ONOO in vehicle-treated PASM cells was markedly attenuated by pretreatment for one minute with nifedipine (-t-104 ± 14% vs +19 ± 2%, p < 0.05). The increase in [Ca2+]i in PASM cells produced by Bay K 8644 was markedly attenuated following three repeated applications of 100 p.M ONOO (+151 ±3% vs +33 ± 6%, p < 0.05). These studies demonstrate that: (i) ONOO unexpectedly activates VS Ca2+ channels; and (ii) repeated exposure to ONOO inhibits activation of VS Ca2+ channels by a specific channel agonist. Whether the tachyphylaxis to ONOO represents a use-dependent inhibition of VS Ca2+ channel function by oxidation of critical thiols within the channel remains to be determined.
|Original language||English (US)|
|State||Published - Dec 1 1996|