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Department of Pathophysiology, Semmelweis University, H-1089 Budapest, Hungary; and Department of Physiology, New York Medical College, Valhalla, New York 10595
To clarify the contribution of intracellular
Ca2+ concentration
([Ca2+]i)-dependent and -independent
signaling mechanisms in arteriolar smooth muscle (aSM) to modulation of
arteriolar myogenic tone by nitric oxide (NO), released in response to
increases in intraluminal flow from the endothelium, changes in aSM
[Ca2+]i and diameter of isolated rat gracilis
muscle arterioles (pretreated with indomethacin) were studied by
fluorescent videomicroscopy. At an intraluminal pressure of 80 mmHg, [Ca2+]i significantly increased and
myogenic tone developed in response to elevations of extracellular
Ca2+ concentration. The Ca2+ channel
inhibitor nimodipine substantially decreased
[Ca2+]i and completely inhibited myogenic
tone. Dilations to intraluminal flow (that were inhibited by
N
-nitro-L-arginine methyl ester)
or dilations to the NO donor
S-nitroso-N-acetyl-DL-penicillamine (that were inhibited by the guanylate cyclase inhibitor
1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) were not
accompanied by substantial decreases in aSM
[Ca2+]i. 8-Bromoguanosine cGMP and the
cGMP-specific phosphodiesterase inhibitor zaprinast significantly
dilated arterioles yet elicited only minimal decreases in
[Ca2+]i. Thus flow-induced endothelial
release of NO elicits relaxation of arteriolar smooth muscle by a
cGMP-dependent decrease of the Ca2+ sensitivity of the
contractile apparatus without substantial changes in the
pressure-induced level of [Ca2+]i.
arteriolar smooth muscle; signal transduction; dilation; shear stress; pressure; nitric oxide; calcium
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