Ca2+ sensitization due to myosin light chain phosphatase inhibition and cytoskeletal reorganization in the myogenic response of skeletal muscle resistance arteries
Authors: Moreno‐Domínguez Alejandro , Colinas Olaia , El‐Yazbi Ahmed , Walsh Emma J. , Hill Michael A. , Walsh Michael P. , Cole William C. 
Published: December 2012

Key points
Blood flow to our organs is maintained within a defined range to provide an adequate supply of nutrients and remove waste products by contraction and relaxation of smooth muscle cells of resistance arteries and arterioles.
The ability of these cells to contract in response to an increase in intravascular pressure, and to relax following a reduction in pressure (the ‘myogenic response’), is critical for appropriate control of blood flow, but our understanding of its mechanistic basis is incomplete.
Small arteries of skeletal muscles were used to test the hypothesis that myogenic constriction involves two enzymes, Rho‐associated kinase and protein kinase C, which evoke vasoconstriction by activating the contractile protein, myosin, and by reorganizing the cytoskeleton.
Knowledge of the mechanisms involved in the myogenic response contributes to understanding of how blood flow is regulated and will help to identify the molecular basis of dysfunctional control of arterial diameter in disease.
Abstract  The myogenic response of resistance arteries to intravascular pressure elevation is a fundamental physiological mechanism of crucial importance for blood pressure regulation and organ‐specific control of blood flow. The importance of Ca2+ entry via voltage‐gated Ca2+ channels leading to phosphorylation of the 20 kDa myosin regulatory light chains (LC20) in the myogenic response is well established. Recent studies, however, have suggested a role for Ca2+ sensitization via activation of the RhoA/Rho‐associated kinase (ROK) pathway in the myogenic response. The possibility that enhanced actin polymerization is also involved in myogenic vasoconstriction has been suggested. Here, we have used pressurized resistance arteries from rat gracilis and cremaster skeletal muscles to assess the contribution to myogenic constriction of Ca2+ sensitization due to: (1) phosphorylation of the myosin targeting subunit of myosin light chain phosphatase (MYPT1) by ROK; (2) phosphorylation of the 17 kDa protein kinase C (PKC)‐potentiated protein phosphatase 1 inhibitor protein (CPI‐17) by PKC; and (3) dynamic reorganization of the actin cytoskeleton evoked by ROK and PKC. Arterial diameter, MYPT1, CPI‐17 and LC20 phosphorylation, and G‐actin content were determined at varied intraluminal pressures ± H1152, GF109203X or latrunculin B to suppress ROK, PKC and actin polymerization, respectively. The myogenic response was associated with an increase in MYPT1 and LC20 phosphorylation that was blocked by H1152. No change in phospho‐CPI‐17 content was detected although the PKC inhibitor, GF109203X, suppressed myogenic constriction. Basal LC20 phosphorylation at 10 mmHg was high at ∼40%, increased to a maximal level of ∼55% at 80 mmHg, and exhibited no additional change on further pressurization to 120 and 140 mmHg. Myogenic constriction at 80 mmHg was associated with a decline in G‐actin content by ∼65% that was blocked by inhibition of ROK or PKC. Taken together, our findings indicate that two mechanisms of Ca2+ sensitization (ROK‐mediated phosphorylation of MYPT1‐T855 with augmentation of LC20 phosphorylation, and a ROK‐ and PKC‐evoked increase in actin polymerization) contribute to force generation in the myogenic response of skeletal muscle arterioles.