Abstract
Objectives: We tested the hypothesis that adrenomedullin reduces calcium influx independent of potassium channels in depolarized endothelium-denuded mesenteric artery from pregnant rats. Results: Adrenomedullin reduced the CaCl<sub>2</sub>-induced contraction, while the receptor antagonist calcitonin gene-related peptide (CGRP)<sub>8–37</sub>, but not adrenomedullin<sub>22–52</sub>, reversed these effects. Adenylate cyclase inhibition by SQ22536 did not prevent adrenomedullin effects on CaCl<sub>2</sub>-induced contraction. Adrenomedullin did not inhibit depolarization-induced calcium entry to isolated vascular smooth muscle. Inhibition of myosin light-chain (MLC) phosphatase by calyculin A reversed the effects of adrenomedullin on contraction caused by submillimolar concentrations of CaCl<sub>2</sub>, while adrenomedullin still inhibited contraction caused by higher concentrations of CaCl<sub>2</sub>. However, the ratio of phosphorylated to total myosin phosphatase target 1, the regulatory subunit of MLC phosphatase, did not change with adrenomedullin, indicating a lack of MLC phosphatase activation. Interestingly, sodium fluoride, a nonspecific protein phosphatase inhibitor, completely blocked the effect of adrenomedullin on CaCl<sub>2</sub>-induced contraction. Adrenomedullin inhibited calcium mobilization from intracellular stores induced by thapsigargin. Conclusion: Adrenomedullin inhibits CaCl<sub>2</sub>-induced contraction, without affecting calcium influx, through a CGRP<sub>8–37</sub>-sensitive receptor, but not using the cyclic adenosine monophosphate pathway, probably through activation of protein phosphatases. Inhibition of intracellular calcium release is an additional role played by adrenomedullin in calcium homeostasis in vascular smooth muscle.
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