Inhibitory mechanisms for cross‐bridge cycling: the nitric oxide‐cGMP signal transduction pathway in smooth muscle relaxation

Abstract

Relaxation follows sequestration of Ca2+ mobilized by an excitatory stimulus in striated muscle. Removal of excitatory stimuli also relaxes smooth muscle in vitro after reductions in the myoplasmic [Ca2+] and dephosphorylation of the myosin regulatory light chains. However, there are several experimental procedures that produce relaxation in the presence of excitatory stimuli and elevated Ca(2+)-dependent cross-bridge phosphorylation. Of potential widespread physiological importance are treatments that increase myoplasmic [cGMP] owing to the ubiquity of nitric oxide (NO) as a signalling molecule for endothelial-mediated vasodilation and inhibitory nerves in most types of smooth muscle. Several mechanisms are implicated in the NO-cGMP mediated relaxation. Most studies support reductions in myoplasmic Ca2+. However, there is evidence that increases in cGMP also lower the Ca(2+)-sensitivity of cross-bridge phosphorylation. This would contribute to a decline in force through actions on the myosin light chain kinase/phosphatase system. In addition, changes in the dependence of force on phosphorylation are observed in tissues partially relaxed by treatments that elevate cGMP. This demonstrates that either the attachment and cycling of phosphorylated cross-bridges is impaired or blocked, or that the formation of dephosphorylated, force-generating cross-bridges ('latch-bridges') is reduced. Protein kinase G-catalysed phosphorylation of either a thin filament protein that blocks attachment of cross-bridges or a protein that inhibits myosin light chain phosphatase may explain the NO-induced relaxation with elevated cross-bridge phosphorylation.

Vascular Biology in Genetically Altered Mice

“…Thus, these data provide direct evidence that cGKI is the primary mediator of relaxation of vascular muscle in response to NO and cGMP ( Figure 2). This concept is consistent with earlier work, 71 including reports suggesting that the soluble guanylate cyclase/cGMP pathway mediates relaxation of large arteries and microvessels in response to NO. 31,32,38,[73][74][75] In addition to regulation of vascular tone, effects of NO on vascular gene expression and cell migration, proliferation, and permeability may also be mediated by soluble guanylate cyclase, cGMP, and cGKI.…”

“…70 The latter finding is important because some previous work has suggested that high concentrations of cAMP could potentially activate cGK. 71,72 At this time, it is not known whether normal vascular responses to cAMP in cGKIdeficient mice result from expression of other compensatory mechanisms. Thus, these data provide direct evidence that cGKI is the primary mediator of relaxation of vascular muscle in response to NO and cGMP ( Figure 2).…”

cGMP signals mainly through cAMP kinase in permeabilized murine aorta

“…Intracellular [Ca 2ϩ ] is reduced by 1) inhibiting Ca 2ϩ release from intracellular stores via phosphorylation of the inositol receptor-associated G-kinase substrate (IRAG) (16); 2) attenuating inositol (1,4,5)-trisphosphate [Ins(1,4,5)P 3 ]-mediated Ca 2ϩ release by calming the synthesis of Ins(1,4,5)P 3 via modulation of the regulator of G protein signaling 2 (RGS-2), which regulates GTPase activity of G q (44); and 3) hyperpolarization of the membrane potential and closure of L-type calcium channels via cGKIinduced increase in the open probability of Ca 2ϩ -activated K ϩ channels (BK Ca ) (1,13,38,50). An alternative mechanism to induce relaxation at constant [Ca 2ϩ ] is to increase MLCP activity, a process that has been described as Ca 2ϩ desensitization (35,42). cGMP/cGKI has been reported to increase the activity of myosin phosphatase leading to dephosphorylation of regulatory light chain (RLC) (9, 28, 49), probably via phosphorylation of MYPT1 (43, 49), albeit at another phosphorylation site than ROCK (49).…”

Calcium-dependent and calcium-independent inhibition of contraction by cGMP/cGKI in intestinal smooth muscle

“…Further studies have shown that the latter mechanism involves a protein complex that consists of at least three proteins, i.e., the inositol-1,4,5 trisphosphate receptor (IP 3 R), the cGKI, and, as a scaffold protein between both, the IP 3 R-associated G kinase substrate (IRAG) (21). Ca 2ϩ -independent mechanisms include cGMP/cGKI-dependent phosphorylation of myosin phosphatase targeting subunit 1 (MYPT-1) that increases or resumes the activity of myosin light chain phosphatase (MLCP) and leads to dephosphorylation of myosin regulatory light chain (14,22). A further regulatory mechanism is the cGKI-induced increase in the open probability of Ca 2ϩ -activated K ϩ channels (BK Ca ) (1,5,20,31), resulting in hyperpolarization of the membrane potential and closure of L-type Ca 2ϩ channels.…”