Dissociation of varied actions of norepinephrine on 45Ca uptake and release at different sites in rabbit aortic smooth muscle.
The basis for different observed effects of norepinephrine (NE) on 45Ca fluxes was investigated by characterizing changes in binding of various distinct Ca++ fractions. The presence of a sufficiently high concentration of Sr++ (5.0 mM) prevents 45Ca uptake at high affinity Ca++ binding sites, whereas that at low affinity Ca++ binding sites is unaffected. Addition of Sr++ after equilibration with 45Ca also specifically displaces high affinity 45Ca. Efflux of 45Ca is increased by 10(-6) M NE when muscles are washed out with 1.5 mM Ca++ solution and decreased by NE when washed out in a O-Ca plus 0.05 mM EDTA solution. NE releases a portion (approximately 20 nmol/g of tissue) of the La+++-resistant high affinity Ca++, decreases La+++-sensitive high affinity Ca++ efflux, increases La+++-sensitive low affinity Ca++ efflux and increases La+++-inaccessible low affinity Ca++ uptake in muscles previously depolarized with a 160 mM K+-substituted solution. Of these four differing actions, Sr++ only affects the first two by preventing high affinity Ca++ binding (including that fraction of Ca++ released by NE). The interrelationships that exist between these Ca++ components are not clear, but the use of combinations of Sr++, La+++ and varied Ca++ concentrations can establish conditions under which a specific type of Ca++ binding predominates. Correlations between mobilization of specific Ca++ fractions and contractile responsiveness in vascular smooth muscle can then be examined.
D000818:Animals; D001011:Aorta / Q000378:metabolism; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D066298:In Vitro Techniques; D007811:Lanthanum / Q000494:pharmacology; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology*; D011817:Rabbits; D013324:Strontium / Q000494:pharmacology
D007811:Lanthanum; D002118:Calcium; D009638:Norepinephrine; D013324:Strontium