AGE-RELATED DECREASE OF CALCITONIN GENE-RELATED PEPTIDE CONTAINING VASODILATOR INNERVATION IN THE MESENTERIC RESISTANCE VESSEL OF THE SPONTANEOUSLY HYPERTENSIVE RAT

We previously demonstrated that the mesenteric resistance blood vessels have nonadrenergic, noncholinergic vasodilator innervation in which calcitonin gene-related peptide (CGRP) is a possible neurotransmitter. The role of CGRP-containing vasodilator nerves in hypertension was investigated in perfused mesenteric vascular beds isolated from spontaneously hypertensive rats (SHR). The adrenergic vasoconstrictor responses to perivascular nerve stimulation in both SHR (8-, 15-, and 30-week-old) and age-matched Wistar-Kyoto (WKY) rat preparations increased with aging, but the response was greater in SHR than in WKY rats at all ages. The preparation isolated from SHR and WKY rats precontracted by continuous perfusion of Krebs' solution containing 7 x 10-6 M methoxamine plus 5 x 10-6 M guanethidine. In both SHR and WKY rats, perivascular nerve stimulation (1-8 Hz) produced frequency-dependent vasodilation, which was blocked by 1 x 10-7 M tetrodotoxin, pretreatment with 5 x 10-7 M capsaicin, and denervation by cold storage (4°C for 72 hours). The vasodilation induced by perivascular nerve stimulation in SHR greatly decreased with age, whereas a slight decrease in the response with age was found in WKY rats. The neurogenic vasodilation in the young SHR preparation was similar in magnitude to the vasodilation in age-matched WKY rats, whereas the vasodilation in 15- and 30-week-old SHR was significantly smaller than that in age-matched WKY rats. In both SHR and WKY rats, perfusion of rat CGRP (1 x 10-10 to 3 x 10-8 M) produced marked vasodilation in a concentration-dependent manner. The CGRP-induced vasodilation in SHR increased with age, whereas an age-related decrease in vasodilation was found in WKY rats. Perivascular nerve stimulation (4 and 8 Hz) of the perfused mesenteric vascular bed evoked an increased release of CGRP-like immunoreactive substance in the perfusate, which was significantly less in 15-week-old SHR than in age-matched WKY rats. Immunohistochemical studies showed an age-related decrease in CGRP-like immunoreactive fibers in SHR but not in WKY rats. These results suggest that CGRP-containing vasodilator innervation is greatly decreased when SHR develop and maintain hypertension. It is also suggested that the decreased vasodilator mechanism by CGRP-containing nerves contributes to the development and maintenance of hypertension.