Hypoxia increases the sensitivity of the L-type Ca(2+) current to beta-adrenergic receptor stimulation via a C2 region-containing protein kinase C isoform.

The effects of hypoxia on the L-type Ca(2+) current (I:(Ca-L)) in the absence and presence of the ss-adrenergic receptor agonist isoproterenol (Iso) were examined. Exposing guinea pig ventricular myocytes to hypoxia alone resulted in a reversible inhibition of basal I:(Ca-L). When cells were exposed to Iso in the presence of hypoxia, the K:(0.5) for activation of I:(Ca-L) by Iso was significantly decreased from 5.3+/-0.7 to 1.6+/-0.1 nmol/L. The membrane-impermeant thiol-specific oxidizing compound 5, 5'-dithio-bis(2-nitrobenzoic acid) (DTNB) attenuated the inhibition of basal I:(Ca-L) by hypoxia 81.3+/-9.4% but had no effect on the increase in sensitivity of I:(Ca-L) to Iso. In addition, DTT mimicked the effects of hypoxia on basal I:(Ca-L) and the increase in sensitivity to Iso. Neither the inhibitors of guanylate cyclase LY-83583 or methylene blue nor the NO synthase inhibitor N:(G)-monomethyl-L-arginine monoacetate had any effect on the basal inhibition of I:(Ca-L) or the decrease in K:(0.5) for activation of I:(Ca-L) by Iso during hypoxia. However, the protein kinase C (PKC) inhibitors bisindolylmaleimide I and Gö 7874 significantly attenuated the increase in sensitivity of I:(Ca-L) to Iso. More specifically, the response was attenuated when cells were dialyzed with a peptide inhibitor of the C2 region-containing classical PKC isoforms. The same effect was not observed with the PKCepsilon peptide inhibitor. These results suggest that hypoxia regulates I:(Ca-L) through the following 2 distinct mechanisms: direct inhibition of basal I:(Ca-L) and an indirect effect on the sensitivity of the channel to ss-adrenergic receptor stimulation that is mediated through a classical PKC isoform.