Contractility of resistance arteries of spontaneously hypertensive rats related to their media

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Abstract
The purpose of the study was to test the hypothesis that the increased media thickness: lumen diameter (M:L) ratio of spontaneously hypertensive rat (SHR) resistance arteries effects enhanced arterial contractile responses compared with those of Wistar-Kyoto (WKY) rat normotensive controls under pressurized conditions in vitro. Contractile responses to the vasoconstrictor agonists noradrenaline and the thromboxane A2 analogue U46619 were assessed in femoral and mesenteric resistance arteries (internal diameters approximately 150 microm) after the development of spontaneous myogenic tone at 100 mmHg and at estimated in vivo pressures. Arterial contractile responses and structure were assessed in an arteriograph. Relaxed arterial structure was determined by light microscopy. Mean arterial pressure was determined subsequent to femoral artery cannulation. Under relaxed conditions M:L ratios were significantly greater in SHR arteries at 100 mmHg (P < 0.01) and at in vivo pressures (P<0.01). Myogenic responses were not significantly different between SHR and WKY. Under both pressure conditions the contractile responses of SHR femoral arteries were not significantly different to those of WKY in response to either agonist SHR mesenteric arteries achieved smaller diameters in response to noradrenaline (P<0.05) and U46619 (P<0.05) at 100 mmHg. At in vivo pressures, concentration-response relationships of SHR mesenteric arteries for both agonists were not significantly different compared with those of WKY; however, the maximum percentage reduction of lumen diameter in SHR mesenteric arteries in the presence of noradrenaline was greater than in WKY (P<0.05). The increased M:L ratio of SHR femoral resistance arteries does not impart an exaggerated contractile function in myogenically active resistance arteries in vitro. For mesenteric arteries the relationship is less clear because increased M:L ratio is associated with increased contractile responses under some, but not all, circumstances.