Defective modulation of noradrenergic neurotransmission by exogenous prostaglandins in aging spontaneously hypertensive rats

Inhibition of cyclooxygenase enhances mesenteric vascular responses to periarterial (sympathetic) nerve stimulation (PNS) in 16-week-old spontaneously hypertensive rats (SHR), but not in 25-week-old SHR. In contrast, cyclooxygenase inhibition enhances mesenteric vascular responses to PNS similarly in 16- and 25-week-old Wistar-Kyoto normotensive rats (WKY). Thus, the modulation of noradrenergic neurotransmission by endogenous PGs becomes defective as SHR age, whereas in WKY this does not occur. The purpose of this study was to determine to what extent alterations in the concentrations of PGs and/or biological response to PGs contribute to this age/hypertension-related abnormality in SHR. All studies were conducted in the in situ autoperfused rat mesentery, and plasma levels of PGE2 and 6-keto-PGF1 alpha were determined by negative-ion, chemical-ionization, gas chromatography-mass spectrometry after derivatization and clean-up of samples by two thin-layer chromatographic steps. Base-line mesenteric venous plasma levels of PGs were similar in 16-week-old SHR vs. 16-week-old WKY; however, base-line levels of PGE2 were approximately 6-fold greater than base-line levels of 6-keto-PGF1 alpha in both strains. PNS at 7 Hz approximately doubled mesenteric venous plasma levels of PGE2 in both 16-week-old SHR and WKY, but PNS did not increase levels of 6-keto-PGF1 alpha in either strain. Inasmuch as mesenteric venous plasma levels of PGE2 were responsive to PNS, the effect of aging on PGE2 levels was studied. In both strains, the base-line mesenteric venous plasma levels of PGE2 and the PNS-induced increase in PGE2 levels were similar in 16-week vs. 25-week-old animals. In 16-week-old SHR, infusions of PGE2, arachidonic acid and PGI2 directly into the mesenteric artery inhibited vascular responses to PNS. However, in 25-week-old SHR, even high doses of PGE2 or arachidonic acid failed to inhibit vascular responses to PNS, and the inhibitory potency of PGI2 was shifted 10-fold to the right compared to 16-week-old SHR. In contrast, PGE2 and arachidonic acid had similar effects on neurotransmission in 25-week-old WKY compared to 16-week-old WKY, and aging had a lesser effect on the inhibitory potency of PGI2 (i.e., 3-fold rightward shift of the dose-response curve). Adenosine also inhibited vascular responses to PNS; however, the inhibitory potency of adenosine was only slightly and similarly affected by aging in SHR and WKY.(ABSTRACT TRUNCATED AT 400 WORDS)