Coupling pharmacomechanical

Regulation of force independent of changes in is called pharmacomechanical coupling. Pharmacomechanical coupling involves either changes in [Ca +ji or changes in the cellular response to [Ca +jj independent of changes in E.  

5- IP3 receptor is highly cooperative and is also dependent on cytoplasmic [Ca +J the channel opens optimally with [Ca2+]i between 200 and 500 nM. Both higher and lower [Ca +J inhibits Ca2+ release. Although 1,4,5-IP3 appears to be the predominant poly-phosphoinositol involved in Ca + release from non-mitochondrial intracellular stores (Berridge, 1987), other less well described second messengers can also release Ca2+ from intracellular stores. Cyclic ADP ribose binds to and opens the ryanodine receptor in smooth muscle (Lee and Aarhus, 1993). 

5-IP3 hypothesis has been extensively tested in smooth muscle. In skinned smooth muscle, physiological concentrations of 1,4,5-IP3 have been shown to release sufficient Ca + rapidly enough to account for transient contractions (Somlyo et al, 1985 Suematsu et al., 1984 Walker et al, 1987 Baron et al., 1989 Abdel-Latif, 1989). This response can be blocked by intracellular heparin (Kobayashi et al., 1988b), a competitive blocker of cerebellar 1,4,5-IP3 receptors (Worley et al., 1987). Agonist stimulation increased the apparent concentration of 1,4,5-IP3 when measured as either [ HJIPg in cells (Griendling etal., 1986) or tissues (Howe etal., 1986 Long and Stone, 1987 Takuwa etal., 1986 Miller-Hance et al., 1988 Salmon and Bolton, 

1988) or as 1,4,5-IP3 mass with a receptor binding assay (Chilvers et al., 1989). 1,4,5-IP3 does not typically release the entire intracellular Ca + store (as measured by caffeine-induced intracellular release). In skinned smooth muscle, GTP can release Ca + from a non-l,4,5-IP3-sensitive store (Chuch et al., 1987 Kobayashi et al., 1988a). This has been hypothesized to represent G protein-mediated transfer of Ca + from a non-l,4,5-IP3-dependent store to a 1,4,5-IP3-responsive store (Gill et al., 1989 Burgoyne et al., 

5- IP3 hypothesis that center about the two methods employed to measure changes in 1,4,5-IP3 concen- 

There are two established mechanisms for relaxation of ASM which involve pharmacomechanical coupling stimulation of /3-adrenoceptors with the production of increases in cytosolic cAMP via activation of adenylate cyclase (Rinard et al., 1983 Suematsu et /., 1984 Fig. 9.6) and activation of guanylate cyclase with resulting increases in cGMP. 

In addition to activation by /3-adrenoreceptors, adenylate cyclase in ASM is activated by VIP, a neurotransmitter of the NANC nervous system (Rhoden and Barnes, 1990). 

An additional mechanism controlling cAMP levels is metabolism by a specific phosphodiesterase enzyme, a cAMP-dependent phosphodiesterase. Such an enzyme exists in ASM and catalyses the degradation of cAMP to 5 -adenosine monophosphate. 

The biological action of cAMP, in this case relaxation of ASM, is believed to be mediated through activation of a cAMP-dependent protein kinase, also known as protein kinase A (PKA). This enzyme, like PKC, belongs to a family of serine and threonine protein kinases, which catalyse the phosphorylation of serine and threonine amino acids in target proteins. 

Phosphorylation by PKA decreases the activity of MLCK, considered to be intimately involved in ASM tension development (Gerthoffer, 1991). Phosphorylation of MLCK, at certain sites on this enzyme, decreases its affinity for the Ca /CaM complex. Ultimately this means that irrespective of the concentration of the ICzM complex, there will be fewer activated MLCK molecules, reduced myosin activation, and thus less contractile force. 

---

Sanders Did you say that there is no pharmacomechanical coupling Don t you think there is a Ca2+ sensitization mechanism in this ... 

Molecular mechanisms of Ca2+ release operate during both electromechanical and pharmacomechanical coupling (review in Somlyo Somlyo 1994, Somlyo... 

Somlyo AP, Somlyo AV 2000 Signal transduction by G-proteins, Rho-kinase and protein phosphatase to smooth muscle and non-muscle myosin II. J Physiol 522 177—185 Somlyo AP, Devine CE, Somlyo AV, N orth SR 1971 Sarcoplasmicreticulumand the temperature-dependent contraction of smooth muscle in calcium-free solutions. J Cell Biol 51 722—741 Somlyo AP, Walker JW, Goldman YE et al 1988 Inositol trisphosphate, calcium and muscle contraction. Philos Trans R Soc Lond B Biol Sci 320 399 114 Somlyo AP, Wu X, Walker LA, Somlyo AV 1999 Pharmacomechanical coupling the role of calcium, G-proteins, kinases and phosphatases. Rev Physiol Biochem Pharmacol 134 201-234... 

ASM is rich in receptors for many of the inflammatory mediators. It relies heavily upon pharmacomechanical coupling mechanisms for the transduction of such extracellular signals. In spite of the wide range of extracellular mediators for which the ASM cell expresses receptors, it appears that the diversity of the intracellular signalling mechanisms is more restricted, involving the release of sequestered Ca, turnover of membrane phospholipids such as the phosphoinositides, changes in cytosolic cyclic nucleotide levels, and aaivation of protein kinase enzymes. 

Escin and a-toxin have attracted a lot of interest since permeabilization with either one of them appears not to interfere with the receptor effector coupling. This allows the study of the intracellular signaling cascades involved in pharmacomechanical coupling. (3-Escin is a saponin ester and a-toxin is a bacterial toxin from Staphylococcus aureus. They differ by the size of fhe holes they produce in the plasma membrane. 

The permeabilization protocol with (3-escin is similar to that applied for saponin skinning (Kobayashi et ah, 1989). The pores are quite large, allowing even antibodies to permeate (lizuka et ah, 1994). In these preparations, the pharmacomechanical coupling is functionally intact as judged from the agonist-induced release of Ca + from intracellular stores and increase in Ca + sensitivity of the myofilaments (Kobayashi et ah, 1989). One criterion for complete permeabilization again is the amplitude of maximal Ca +-activated... 

Pharmacomechanical coupling was primarily studied by measuring [Ca +J and myosin regulatory light chain phosphorylation in intact and skinned smooth muscle. Several groups found that stimuli increased... 

As will be detailed in the following, both disciplines were correct. Pharmacomechanical coupling represents increases in both [Ca +Jj and the [Ca +Jj sensitivity of phosphorylation beyond that expected given the level of depolarization. 

Modulation of myosin phosphorylation independent of changes in [Ca2+]j is another form of pharmacomechanical coupling. 

Although it is convenient for physiologists to categorize smooth muscle excitation-contraction coupling mechanisms, it is important to understand that electromechanical and pharmacomechanical coupling mechanisms are complexly intertwined in intact... 

If cADPR is a mediator of pharmacomechanical coupling in some smooth muscle cells, then agonists should modulate its intracellular levels. This has not been demonstrated yet, however, two cell-surface receptor agonists that would be good candidates are cholecystokinin (CCK) in rabbit longitudinal muscle... 

---