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.
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