Competing Effects Among Protein Kinase G, Chronic Hypoxia, and Postnatal Maturation on 5HT Agonist Affinity in Ovine Carotid Cerebral Arteries

Abstract

In ovine arteries, contractile responses to 5HT are differentially influenced by artery type, chronic hypoxia (CH), postnatal maturation (PM), and maximal Protein Kinase G activation (PKG). Here we explore the hypothesis that all of these effects involve competing changes in agonist binding affinity at the 5HT receptor (pKa). pKa was measured using the Furchgott method of partial irreversible blockade with phenoxybenzamine. In cerebral arteries, pKa was similar in fetal (6.3±0.1) and adult (6.5±0.2) arteries. CH increased pKa in both fetal (6.5±0.1) and adult (6.6±0.1) arteries. PKG activation decreased pKa in both fetal (5.8±0.2) and adult (6.3±0.1) arteries. However, PKG activation together with CH had little net effect on pKa in both fetal (6.3±0.3) and adult (6.3±0.2) arteries. Results in carotid arteries were qualitatively different: PM decreased pKa, CH increased pKa in fetal arteries but decreased it in adult arteries, and combined PKG activation and CH had little effect in fetal arteries but decreased pKa in adult arteries. In all cases, changes in pKa were closely correlated with changes in both the Emax and pD2 for 5HT. Together, these results demonstrate that PKG and CH often produce opposing effects on pKa values for 5HT. More importantly, changes in agonist binding affinity play a key role in the effects of artery type, CH, PM, and PKG on contractile responses to 5HT. Supported by NIH HD31226 and HL64867