Enhanced nitric oxide-mediated chemoreceptor inhibition and altered cyclic GMP signaling in rat carotid body following chronic hypoxia.

Multiple studies have shown that chronic hypoxia (CH) elicits a time-dependent upregulation of carotid body chemoreceptor sensitivity in mammals. In the present study, we demonstrate that enhanced excitation is accompanied by a parallel increase of nitric oxide (NO)-dependent inhibition, which acts via a CH-induced modification of the normal mechanism in O(2)-sensitive type I cells. The NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), elicits a progressively larger increase in carotid sinus nerve (CSN) chemoreceptor activity following incremental increases in CH exposure lasting 1-16 days. The inhibitory effect of the NO donor, S-nitroso-N-acetyl-penicillamine (SNAP), on CSN activity is enhanced following CH. However, the activation of soluble guanylate cyclase (sGC) by SNAP, assessed via production of cGMP, is impaired, along with decreased expression of sGC mRNA transcript. Inhibition of hypoxia-evoked Ca(2+) responses by SNAP is mediated via a cGMP/protein kinase G (PKG)-dependent mechanism in normal type I cells that is sensitive to the PKG inhibitor KT-5823, but following CH, inhibitory responses are minimally sensitive to PKG inhibition. The data are consistent with the hypothesis that CH hampers cGMP-mediated inhibition of type I cells in favor of an alternative mechanism.