Specific alterations in binding and uptake of high and low affinity Ca++ in canine aortic microsomes

The effects of Sr++, aminoglycoside antibiotics and other agents on Ca++ incorporation by canine aortic microsomes have been examined in 0.03 mM Ca++ (high affinity Ca++ components predominate) and in 3.0 mM Ca++ (low affinity Ca++ components predominate) in the presence of 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.