Specific Alterations in Binding and Uptake of High and Low Affinity Ca⁺⁺ in Canine Aortic Microsomes

Abstract

The effects of Sr⁺⁺, aminoglycoside antibiotics and other agents on Ca⁺⁺ incorporation by canine aortic microsomes have been examined in 0.03 vaM Ca⁺⁺ (high affinity Ca⁺⁺ components predominate) and in 3.0 mMCa⁺⁺ (low affinity Ca⁺⁺ components predominate) in the presence (⁴⁵Ca uptake) and absence (⁴⁵Ca binding) of ATP. Sr⁺⁺ inhibited high affinity Ca⁺⁺ uptake more than binding of Ca⁺⁺ and reduced low affinity Ca⁺⁺ uptake and binding only at higher concentrations (3.0–5.0 mM). Similar results were obtained with Sr⁺⁺ in canine aortic media intimal strips. Neomycin (0.4–1.4 mM) or gentamicin (0.6–2.0 mM) had no significant effect on high affinity Ca⁺⁺ uptake, but reduced high affinity Ca⁺⁺ binding and inhibited low affinity Ca⁺⁺ uptake and binding. Nitroprusside inhibited only high affinity Ca⁺⁺ uptake and D-600 was without effect. Thus, measurement of Ca⁺⁺ uptake and binding in smooth muscle microsomal membrane preparations demonstrates that agents affecting smooth muscle contractility can alter binding or uptake of Ca⁺⁺ in differing ways or, in some cases (e.g. D-600), exert their action by blocking other Ca⁺⁺-related effects.