Hypoxia Alters the Sensitivity of the L-Type Ca 2+ Channel to α-Adrenergic Receptor Stimulation in the Presence of β-Adrenergic Receptor Stimulation

—The effects of α-adrenergic receptor (α-AR) stimulation alone and the effects in the presence of β-adrenergic receptor (β-AR) stimulation were examined on L-type Ca2+ currents (I Ca-L ) in the absence and presence of hypoxia. The α-AR agonist methoxamine either had no effect or had a slight inhibitory effect on basalI Ca-L in the absence and presence of hypoxia. Hypoxia significantly decreased theK 0.5 for activation ofI Ca-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 α-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-activatedI Ca-L in a concentration-dependent manner with an EC50 of 86.9±9.9 μmol/L. However, in the presence of hypoxia, the EC50 for inhibition ofI Ca-L by methoxamine was significantly increased to 266.7±10.8 μmol/L. Methoxamine had little effect onI Ca-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 β peptide inhibitor had no effect on the methoxamine-induced antagonism ofI Ca-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 A2 inhibitors mepacrine and indomethacin. These findings represent a novel regulation of the L-type Ca2+ channel by the phospholipase A2 pathway and illustrate the complexity of regulation of the channel under hypoxic conditions.