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 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 Ca(2+) currents (I(Ca-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 I(Ca-L) in the absence and presence of hypoxia. Hypoxia significantly decreased the K(0.5) for activation of I(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 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 I(Ca-L) in a concentration-dependent manner with an EC(50) of 86.9+/-9.9 micromol/L. However, in the presence of hypoxia, the EC(50) for inhibition of I(Ca-L) by methoxamine was significantly increased to 266.7+/-10.8 micromol/L. Methoxamine had little effect on I(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 beta peptide inhibitor had no effect on the methoxamine-induced antagonism of I(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 A(2) inhibitors mepacrine and indomethacin. These findings represent a novel regulation of the L-type Ca(2+) channel by the phospholipase A(2) pathway and illustrate the complexity of regulation of the channel under hypoxic conditions.