Abstract
The role of calmodulin (CaM) in modulating calcium (Ca) uptake by sarcoplasmic reticulum (SR) of vascular smooth muscle was studied in saponin skinned strips of rat caudal artery. Exogenous CaM concentrations ranging 0.3–1.8 μM did not statistically change the steady state MgATP‐dependent Ca content, the MgATP‐independent Ca content, or the oxalate‐stimulated Ca influx. Calmidazolium (CDZ), W‐7, and trifluoperazine (TFP) were used to examine the potential effect of an endogenous CaM pool on inward Ca transport. The IC~50~ of these antagonists for inhibiton of Ca‐CaM‐stimulated phosphodiesterase activity and Ca‐activated superprecipitiation of canine aortic actomyosin was measured and found to be be in the low micromolar range with a rank order of potency of inhibition of CDZ > TFP > W‐7. In skinned tissues, micromolar concentrations of antagonists that inhibited CaM‐mediated reactions in isolated enzyme systems did not reduce Ca content or oxalate‐stimulated Ca influx. At higher concentration of 100–200 μM, the MgATP‐dependent Ca content was significantly reduced by TFP and W‐7 but not by CDZ. The order of potency for inhibition of Ca uptake was TFP > W‐7 > CDZ. The MgATP‐independent Ca content was significantly decreased only by 200 μM TFP. Although none of these inhibitors significantly altered Ca efflux at concentrations up to 100 μM, Ca release was significantly stimulated by all three at 200 μM. The TFP‐stimulated Ca release was partially inhibited by ruthenium red. The results indicate that neither exogenous CaM nor an endogenous CaM pool directly modulates inward Ca transport by the SR of saponin skinned caudal artery. The inhibition of Ca uptake produced by hundred micromolar concentrations of CaM antagonists fails to correlate with the order of and with the potency of inhibition measured in isolated enzyme systems. This suggests that the inhibition of Ca uptake produced by high concentrations of these antagonists may be independent of a specific interaction with CaM. The activation of Ca release by high concentrations of CaM antagonists may involve a nonspecific increase in membrane permeability as well as modulation of a membrane Ca Chanel. © 1992 Wiley‐Liss, Inc.