Age-dependent hyperresponsiveness of spontaneously hypertensive rats to the pressor effects of intravenous neuropeptide Y (NPY): role of mode of peptide administration and plasma NPY-like immunoreactivity.

The effects of various doses of intravenously (i.v.) infused (5-min duration, 0.1-3.2 nmol/kg/min) or bolus-injected (0.1-3.2 nmol/kg) porcine and/or rat/human neuropeptide Y (NPY) on mean arterial pressure (MAP), heart rate (HR), and plasma concentrations of porcine NPY-like immunoreactivity (pNPYir) were examined in conscious, unrestrained spontaneously hypertensive (SHR), Wistar-Kyoto (WKY), and Sprague-Dawley (SD) rats of various ages. When administered as an infusion to 12- to 17-week-old SHR, WKY, and SD rats, porcine NPY (pNPY) was more potent in increasing MAP in SHR than in either WKY or SD rats. Infusions of rat/human NPY instead of pNPY resulted in similar increases in potency in 12- to 17-week-old SHR as compared with WKY. This potency-associated hyperresponsiveness to infused pNPY was also observed when 36- to 41-week-old and 6-week-old SHR and WKY were examined, but infused NPY induced similar HR reductions in age-matched rats regardless of rat strain. Furthermore, doses of infused pNPY that elicited significantly greater pressor responses in SHR and WKY (0.32 nmol/kg/min in 12- to 17-week-old rats and 1.0 nmol/kg/min in 6-week-old rats) resulted in essentially identical plasma pNPYir concentrations in the two rat strains. In contrast, hyperresponsiveness to the MAP effects of bolus injections of pNPY in 12- to 17-week-old SHR was manifested as an increase in efficacy rather than potency, was associated with significantly smaller reductions in HR in SHR than in WKY and occurred at plasma pNPYir concentrations that were significantly larger than those required for infusion-associated hyperresponsiveness. These results are consistent with the hypothesis that NPY is an important contributor to the development and maintenance of essential hypertension.