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1 John B. Pierce Laboratory & Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, CT, USA
2 Department of Pharmacology and Toxicology, Michigan State University, East Lansing, MI, none
* To whom correspondence should be addressed. E-mail: sssegal{at}jbpierce.org.
Intracellular calcium concentration ([Ca2+]i) governs the contractile status of arteriolar smooth muscle cells (SMC). While studied in vitro, little is known of SMC [Ca2+]i dynamics during the local control of blood flow. We tested the hypothesis that the rise and fall of SMC [Ca2+]i underlies arteriolar constriction and dilation in vivo. Aparenchymal segments of second-order arterioles (diameter 35±2µm) were prepared in the superfused cheek pouch of anesthetized hamsters (n=18) and perifused with the ratiometric dye Fura-PE3 (AM) to load SMC (1 µM, 20 min). Resting SMC [Ca2+]i was 406±37 nM. Elevating superfusate O2 from 0 to 21% produced constriction (11±2 µm) that was unaffected by dye loading; [Ca2+]i increased by 108±53 nM (n=6, P<0.05). Cycling of [Ca2+]i during vasomotion (amplitude, 150±53 nM; n=4) preceded corresponding diameter changes (7±1 µm) by ~2 s. Microiontophoresis (1 µm pipette tip; 1 µA, 1 s) of phenylephrine (PE) transiently increased [Ca2+]i by 479±64 nM (n=8, P<0.05) with constriction (26±3 µm). Flushing blood from the lumen with saline increased fluorescence at 510 nm by ~45% during excitation at both 340 and 380 nm with no difference in resting [Ca2+]i, diameter, or respective responses to PE (n=7). Acetylcholine microiontophoresis (1 µA, 1 s) transiently reduced resting SMC [Ca2+]i by 131±21 nM (n=6, P<0.05) with vasodilation (17±1 µm). Superfusion of sodium nitroprusside (10 µM) transiently reduced SMC [Ca2+]i by 124±18 nM (n=6, P<0.05) while dilation (23±5 µm) was sustained. Resolution of arteriolar SMC [Ca2+]i in vivo discriminates key signaling events that govern the local control of tissue blood flow.
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