Reduction of Norepinephrine-Induced Tonic Contraction and Phosphoinositide Turnover in Arteries of Spontaneously Hypertensive Rats

Abstract

Experiments were conducted to determine whether a difference in receptor-induced phosphatidylinositol hydrolysis occurred in aorta from spontaneously hypertensive rats (SHR) v Wistar-Kyoto (WKY) rats, and whether such a difference was correlated with contractile response. Basal incorporation of 32P into phosphatidylinositol (PI), phosphatidylinositol phosphate (PIP), phosphatidylinositol diphosphate (PIP2) and phosphatidic-acid (PA) was not different between SHR and WKY groups. However, after five minutes of norepinephrine (NE; 10 mumol) exposure, increases in 32P labeling were markedly lower in SHR arteries. The percentage decrease amounted to 45% for PI, 68% for PIP, 100% for PIP2 and 58% for PA. Basal incorporation of 3H-myo-inositol into inositol monophosphate (IP) was similar for SHR and WKY groups. However, after 30 minutes of NE (10 mumol), SHR arteries failed to show an increase in 3H-IP levels, whereas labeling was increased 219% in WKY arteries. The contractile response of SHR arteries to 10 mumol NE showed a marked reduction in the rate of development of the tonic phase that has previously been shown to be supported by activity of protein kinase C. Higher Ca2+ levels failed to augment the SHR response, whereas WKY responses were significantly increased. Contractions in the presence of the phorbol ester tetradecanoylphorbolacetate exhibited a similar reduction in NE-induced tonic phase tension. These results indicate an impairment in SHR arteries at the level of receptor-induced formation of inositol cycle second messengers, possibly due to elevated basal levels of protein kinase C. These differences may be important in explaining altered vascular responses in primary hypertension.