Intracellular Ca2+ and force determined simultaneously in isolated resistance arteries.

A method is described for the simultaneous measurement of intracellular Ca2+ ([Ca2+]i) and force generation in isolated resistance arteries using the fluorescent Ca2+ indicator fura-2. Branch II mesenteric resistance arteries were isolated from 12-wk-old Wistar-Kyoto rats and mounted in a wire myograph. The myograph was placed on the stage of a compound microscope interfaced with a dual excitation wavelength fluorometer, and the tissue was loaded with fura-2 by incubation over a 30-min period with the cell-permeable form of the dye. When stimulated with physiological salt solution containing 100 mM KCl and 10 microM norepinephrine, a rapid and transient increase in [Ca2+], was observed to precede active force development and plateau at approximately 60% of the maximal level after 50 s. Washout of the agonists induced relaxation of these small arteries, consisting of an 85% decline in active tension over 100 s and a fall in [Ca2+]i to 50% of prerelaxation level over the same time period. Forskolin (1 microM), which increases intracellular adenosine 3',5'-cyclic monophosphate, induced a 50% relaxation over a 150-s period that was preceded by a fall in [Ca2+]i. Nearly identical results were obtained with 100 microM sodium nitroprusside, which stimulates intracellular guanosine 3',5'-cyclic monophosphate production. These findings indicate that the initiating event of forskolin- and sodium nitroprusside-induced relaxation may be a reduction of [Ca2+]i. The method described is useful for examining basic physiological events and Ca2+-related mechanisms of action of vasoactive compounds in isolated resistance arteries.