STRUCTURAL AND FUNCTIONAL-ANALYSIS OF SMALL ARTERIES FROM YOUNG SPONTANEOUSLY HYPERTENSIVE RATS

We studied structural and functional changes of small muscular arterie s from the mesenteric Vascular bed of young spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) using a new morph ometric protocol involving the use of confocal microscopy and a pressu rized artery system. At 3 and 4 weeks of age, systolic pressure of SHR and WKY was similar; however, significant structural changes in the m esenteric vasculature were already present in SHR. Arteries fixed unde r pressure in vitro from SHR had a larger medial volume and increased number of smooth muscle cell layers but similar lumen size compared wi th arteries from WKY in maximally relaxed conditions. Functional studi es showed that SHR arteries contracted more in response to stimulation by KCI and norepinephrine, resulting in a significantly smaller lumen size in these vessels than in those from WKY. SHR arteries precontrac ted with KCl were also able to maintain a smaller lumen diameter than WKY arteries when challenged with increasing pressure levels. No diffe rence in the sensitivity of response of these arteries to norepinephri ne stimulation was found. At 3 and 4 weeks of age, mesenteric arteries from some SHR and WKY were not responsive to periarterial nerve stimu lation, and the number of responders was higher in the WKY than SHR. H owever, a greater degree of contraction was found in SHR arteries resp onding to field stimulation at 4 weeks than in WKY arteries. We conclu de that there is a temporal difference in the rate of functional matur ation of the innervation in SHR arteries compared with WKY arteries. S tructural changes of the small muscular arteries, caused by an increas e in the medial volume, and increased number of smooth muscle cell lay ers are primary changes that contribute to the development of hyperten sion in the SHR because these changes are present at the age when bloo d pressure is similar in SHR and WKY.