Nature of norepinephrine-sensitive Ca-pool in rabbit aortic smooth muscle: Effect of pH

High affinity Ca-binding to rabbit aortic smooth muscle microsomes was reduced at low pH. To investigate the role of this Ca-binding, aortic strips were briefly exposed either to pH 5.1 or 7.3, subsequently incubated in a Ca-free medium at pH 7.3, and then challenged with 1 μM norepinephrine (NE). Tissues pretreated at pH 5.1 gave smaller contractions. Tissues loaded with ⁴⁵Ca when exposed to pH 5.1 showed much larger release of ⁴⁵Ca than those exposed to pH 7.3. Subsequently, all ⁴⁵Ca-loaded tissues were placed at pH 7.3 and the effect of 100 μM NE on ⁴⁵Ca-efflux was examined. Tissues exposed previously to pH 7.3 showed a NE-sensitive ⁴⁵Ca-efflux but those pretreated at pH 5.1 did not. The results are consistent with, but do not prove, the hypothesis that the high affinity pH-sensitive Ca-binding to plasma membranes is a large Ca-pool and that the NE-sensitive Ca-pool is a small component of it.