Agonist-Specific Compartmentation of cGMP Action in Myometrium

Abstract

Nitric oxide relaxes myometrium in a cGMP-independent manner. Although cGMP activates its cognate kinase, this is not required for the inhibitory effect of nitric oxide. Thus, nitric oxide-mediated cGMP elevation does not enjoy the same set of substrates as it does in other smooth muscles. To further understand the regulation of relaxation of uterine muscle by cGMP, we have studied the actions of peptide-mediated cGMP action in guinea pig myometrium. We used both functional and biochemical studies of the action of the particulate guanylyl cyclase activator uroguanylin and its receptor, particulate guanylyl cyclase type C, to address the relationship between cGMP elevation acting in the membrane signaling domain to that of the nonmembrane region of the cell. Uroguanylin relaxed oxytocin-induced contractions in a dose-dependent fashion only in pregnant myometrium. Both relaxation and cGMP accumulation after uroguanylin stimulation were blocked by the putative particulate guanylyl cyclase type C inhibitors 2-chloro-ATP and isatin (1H-indole-2,3-dione), but not by the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-A]quinoxalin-1-one (ODQ). Uroguanylin stimulated cGMP accumulation only in the pregnant myometrium. Caveolin-1 expression increased in pregnancy toward term. In the caveolin-1-containing membrane domain, uroguanylin, but not the nitric-oxide donor, led to the elevation of cGMP that was insensitive to ODQ. Particulate guanylyl cyclase C was expressed and prouroguanylin was detected in pregnant myometrium. We conclude that a uroguanylin-particulate cyclase-cGMP relaxation pathway is present and cGMP is compartmented in myometrium. The agonist-mediated selectivity of relaxation to cGMP is of fundamental pharmacological interest in understanding signal transduction in smooth muscle.

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

“…Tichenor (238) proposed that activation of particulate guanylyl cyclase in caveolae would activate PKGII, leading to relaxation through MYPT1 phosphorylation at the cell membrane. This is consistent with data showing compartmentalization of cGMP signaling in pregnant guinea pig myometrium, where activation of particulate, but not soluble, guanylyl cyclase increased cGMP levels in caveolae and caused relaxation (27). However, this MYPT1 is dissociated from myosin and is possibly threonine-phosphorylated.…”

The role of S-nitrosoglutathione reductase (GSNOR) in human disease and therapy

“…This is not the dominant •NO-mediated relaxation pathway in uterine smooth muscle however. •NO can relax the myometrium even when sGC has been inhibited (Buxton et al, 2010). The pathway through which NO relaxes the myometrium independent of cGMP is unknown, but it is likely that the S-nitrosation of contractile proteins plays a role.…”

A role of stretch-activated potassium currents in the regulation of uterine smooth muscle contraction

“…The importance of the fact that an effect of NO to relax the uterus is independent of global cGMP accumulation is twofold. First, data supporting the cGMP-independence of nitroagent mediated relaxation of uterine muscle that were first introduced by the pioneering work of Jack Diamond [30][31][32] and later by our lab as cited above, establish more dramatically than any other type of study we are aware of that the uterus, particularly the human uterus, is regulated in a disparate manner compared to other smooth muscles [19,29] . The notion that the uterine smooth muscle biochemistry of relaxation signaling is fundamentally different than vascular or gastrointestinal muscle means that there is hope for discovery of therapeutic targets in the myometrium that are absent or disparately regulated in other smooth muscles and thus, can permit a reasoned line of investigation to fi nd uterine-specifi c tocolytics.…”

“…When the uterus is relaxed by uroquanylin, cGMP elevation secondary to activation of the guanylyl cyclase activity of the uroguanylin receptor, particulate guanylyl cyclase type C does relax the uterus. The presence of a uroguanylin-particulate cyclasecGMP relaxation pathway in myometrium taken together with the failure of soluble cGMP elevation to relax the muscle leads to the inevitable conclusion that cGMP is compartmented in the myometrium [29] . The importance of the fact that an effect of NO to relax the uterus is independent of global cGMP accumulation is twofold.…”