Is myosin phosphatase regulated in vivo by inhibitor-1? Evidence from inhibitor-1 knockout mice

1. The Ca2+ sensitivity of smooth muscle contractility is modulated via reg ulation of phosphatase activity. Protein phosphatase inhibitor-1 (I-1) is t he classic type-1 phosphatase inhibitor, but its presence and role in cAMP- dependent protein kinase (PKA) modulation of smooth muscle is unclear. To a ddress the relevance of I-1 in vivo, we investigated smooth muscle function in a mouse model lacking the I-1 protein (I-1((-/-))mice). 2. Significant amounts of I-1 protein were detected in the wild-type (WT) m ouse aorta and could be phosphorylated by PKA, as indicated by P-32-labelle d aortic extracts from WT mice. 3. Despite, the significant presence of I-1 in WT aorta, phenylephrine and KCl concentration-isometric force relations in the presence or absence of t he PKA pathway activator isoproterenol (isoprenaline) were unchanged compar ed to aorta. cGMP-dependent protein kinase (PKG) relaxation pathways were a lso not different. Consistent with these findings, dephosphorylation rates of the 20 kDa myosin light chains (MLC20), measured in aortic extracts, wer e nearly identical between WT and mice. 4. In the portal vein, I-1 protein ablation was associated with a significa nt (P < 0.05) rightward shift in the EC50 of isoproterenol relaxation (EC50 = 10.4 +/- 1.4nM) compared to the WT value (EC50 = 3.5 +/- 0.2 nM). Contra ction in response to acetylcholine as well as Ca2+ sensitivity were similar between WT and aorta. 5. Despite the prevalence of I-1 and its activation by PKA in the aorta, I- 1 does not appear to play a significant role in contractile or relaxant res ponses to any pharmacomechanical or electromechanical agonists used. I-1 ma y play a role as a fine-tuning mechanism involved in regulating portal vein responsiveness to beta -adrenergic agonists.