ENHANCED RESISTANCE ARTERY SENSITIVITY TO AGONISTS UNDER ISOBARIC COMPARED WITH ISOMETRIC CONDITIONS

We have compared the responsiveness of rabbit mesenteric resistance ar teries with agonists under isometric and isobaric conditions. When pre ssurized (60 mmHg), arteries spontaneously reduced their diameter by 1 8.1%. An equivalent isometric stress did not generate force in a ''wir e'' myograph. Subsequently, much higher concentrations of norepinephri ne (NE) and histamine were required to cause isometric contractions th an were needed to reduce vascular diameter of pressurized vessels, whe reas angiotensin II produced a maintained response only in pressurized arteries. Reducing transmural pressure to 20 mmHg abolished pressure- induced myogenic tone and decreased arterial sensitivity to NE. Under isometric conditions, partial depolarization with KCl increased sensit ivity to NE and histamine to within the concentration range effective in pressurized vessels and also ''revealed'' responses to angiotensin II, The membrane potential of the vascular smooth muscle cells under p artially depolarized conditions was similar to that found in vivo and in vessels studied isobarically. These observations demonstrate a fund amental interaction between pressure-induced myogenic tone and the sen sitivity of resistance arteries to vasoactive stimuli. This influence was mimicked in isometrically mounted vessels by partial depolarizatio n, indicating a possible pivotal role for membrane potential in determ ining the reactivity of the resistance vasculature.