Ca 2+ -Dependent Rapid Ca 2+ Sensitization of Contraction in Arterial Smooth Muscle

• Published in: Circulation research Vol. 100; no. 1; pp. 121 - 129
• Main Authors: Dimopoulos, George J., Semba, Shingo, Kitazawa, Kazuyo, Eto, Masumi, Kitazawa, Toshio
• Format: Journal Article
• Language: English
• Published: 05-01-2007
• ISSN: 0009-7330; 1524-4571
• DOI: 10.1161/01.RES.0000253902.90489.df

Ca
2+
ion is a universal intracellular messenger that regulates numerous biological functions. In smooth muscle, Ca
2+
with calmodulin activates myosin light chain (MLC) kinase to initiate a rapid MLC phosphorylation and contraction. To test the hypothesis that regulation of MLC phosphatase is involved in the rapid development of MLC phosphorylation and contraction during Ca
2+
transient, we compared Ca
2+
signal, MLC phosphorylation, and 2 modes of inhibition of MLC phosphatase, phosphorylation of CPI-17 Thr38 and MYPT1 Thr853, during α
1
agonist-induced contraction with/without various inhibitors in intact rabbit femoral artery. Phenylephrine rapidly induced CPI-17 phosphorylation from a negligible amount to a peak value of 0.38±0.04 mol of Pi/mol within 7 seconds following stimulation, similar to the rapid time course of Ca
2+
rise and MLC phosphorylation. This rapid CPI-17 phosphorylation was dramatically inhibited by either blocking Ca
2+
release from the sarcoplasmic reticulum or by pretreatment with protein kinase C inhibitors, suggesting an involvement of Ca
2+
-dependent protein kinase C. This was followed by a slow Ca
2+
-independent and Rho-kinase/protein kinase C-dependent phosphorylation of CPI-17. In contrast, MYPT1 phosphorylation had only a slow component that increased from 0.29±0.09 at rest to the peak of 0.68±0.14 mol of Pi/mol at 1 minute, similar to the time course of contraction. Thus, there are 2 components of the Ca
2+
sensitization through inhibition of MLC phosphatase. Our results support the hypothesis that the initial rapid Ca
2+
rise induces a rapid inhibition of MLC phosphatase coincident with the Ca
2+
-induced MLC kinase activation to synergistically initiate a rapid MLC phosphorylation and contraction in arteries with abundant CPI-17 content.