Effects of stimulatory agents on mobilization of high and low affinity site 45Ca in rabbit aortic smooth muscle.

The relationships between contractile responses to stimulatory agents and changes in binding and distribution of La+++-resistant Ca++ at kinetically distinct high and low affinity sites were investigated. The magnitude of the norepinephrine (NE)-induced decrease in 45Ca retention (associated with release of high affinity Ca++) after exposure to La+++ and low temperature was similar when NE was present either for the entire 45Ca incubation period (40--120 min) or for as short an interval as the final minute of the incubation period. Under conditions favoring high affinity binding of Ca++, uptake of 45Ca was slow (T 1/2 approximately equal to 30 min) but the NE-induced decrease in 45Ca retention was rapid. Conversely, low affinity Ca++ sites load rapidly (T 1/2 less than or equal to 5 min) retain increased 45Ca in the presence of elevated (160 mM) K+ at all incubation intervals (5--120 min) and accumulate even more 45Ca in proportion to the increase in contraction obtained when NE is also present either for the entire incubation period or for only the final portion (1 min or longer). Similar effects on both high and low affinity binding of Ca++ are obtained with equipotent concentrations of NE and histamine. Simultaneous application of NE and histamine did not alter high and low affinity binding of Ca++ significantly more than did separate administration of either agent. These results indicate that NE and histamine effect similar high and low affinity Ca++ sites and that, even though equilibrium of Ca++ with high affinity sites is a much slower process than with low affinity sites, stimulatory agents can elicit rapid changes in the amount of Ca++ present at either site.