45Ca fluxes in rat mesenteric resistance vessels

In order to evaluate previously utilized 45Ca influx measurements in Wistar-Kyoto (WKY) rat mesenteric resistance vessels, 45Ca compartments obtained under steady-state conditions have been determined. Curve fitting of 45Ca efflux curves resulting from a 50 min washout of 45Ca from vessels incubated in 45Ca-labeled physiological saline solution (PSS) for 1.5 h yields a biexponential as the best fit. The rapidly exchanging compartment has a t1/2 of 58 +/- 5 s, while the slowly exchanging compartment has a t1/2 of 10.5 +/- 1.5 min. A two-compartment model is proposed, with the rapidly exchanging compartment hypothesized to be extracellularly bound Ca2+, and the more slowly exchanging compartment to be intracellular Ca2+. This model is supported by the observation that high K+ increases the 45Ca content of the slowly exchanging fraction, while decreasing that of the rapidly exchanging fraction. Two components of 45Ca efflux can be further distinguished by washout in ice-cold Ca2+-free PSS containing 2 mM EGTA. Under these conditions, the rapidly lost fraction has a t1/2 of 70 +/- 7 s, while the more slowly lost fraction has a t1/2 of 28.9 +/- 5 min. High K+ also increases the 45Ca content of this fraction. The curve relating the steady-state uptake of 45Ca into this fraction to time can best be fitted to a single exponential, for which the t1/2 and 45Ca content are not different from those determined from steady-state 45Ca efflux data.(ABSTRACT TRUNCATED AT 250 WORDS)