Hypoxia alters the sensitivity of the L-type Ca2+ channel to α-adrenergic receptor stimulation in the presence of β-adrenergic receptor stimulation

The effects of alpha -adrenergic receptor (alpha -AR) stimulation alone and the effects in the presence of beta -adrenergic receptor (beta -AR) stimulation were examined on L-type Ca2+ currents (ICa-L) in the absence and presence of hypoxia. The alpha -AR agonist methoxamine either had no effect or had a slight inhibitory effect on basal ICa-L in the absence and presence of hypoxia. Hypoxia significantly decreased the K-0.5 for activation of ICa-L by norepinephrine from 79.8 +/-6.6 to 13.3 +/-0.7 nmol/L. To determine whether hypoxia specifically altered the sensitivity of the channel to alpha -AR stimulation, cells were exposed to increasing concentrations of methoxamine in the presence of 100 nmol/L isoproterenol (Iso). In the absence of hypoxia, methoxamine inhibited the Iso-activated ICa-L in a concentration-dependent manner with an EC50 of 86.9 +/-9.9 mu mol/L. However, in the presence of hypoxia, the EC50 for inhibition of ICa-L by methoxamine was significantly increased to 266.7 +/- 10.8 mu mol/L. Methoxamine had little effect on ICa-L activated by forskolin or histamine in the absence or presence of hypoxia. In addition, inhibition of protein kinase C by bisindolylmaleimide 1 or protein kinase C beta peptide inhibitor had no effect on the methoxamine-induced antagonism of ICa-L in the absence or presence of hypoxia. The tyrosine kinase inhibitor genistein attenuated the methoxamine response in nonhypoxic cells only. However, during hypoxia it was attenuated with the phospholipase A(2) inhibitors mepacrine and indomethacin. These findings represent a novel regulation of the L-type Ca2+ channel by the phospholipase A(2) pathway and illustrate the complexity of regulation of the channel under hypoxic conditions.