Convergence of Ca²⁺-desensitizing mechanisms activated by forskolin and phenylephrine pretreatment, but not 8-bromo-cGMP

Abstract

Contractile stimuli can sensitize myosin to Ca2+ by activating RhoA kinase (ROK) and PKC that inhibit myosin light chain phosphatase (MLCP) activity. Relaxant stimuli, acting through PKA and PKG (cyclic nucleotide-dependent protein kinases), and pretreatment with contractile agents such as phenylephrine (PE), can desensitize myosin to Ca2+. It is unknown precisely how these stimuli cause Ca2+ desensitization. To test the hypothesis that PKA, PKG, and PE pretreatment signaling systems converge to cause relaxation by inhibition of ROK in intact, isolated tissues, we examined the effects of forskolin (FSK; PKA activation), 8-bromo-cGMP (8br-cGMP; PKG activation), and PE pretreatment on KCl-induced force maintenance in rabbit arteries, a response nearly completely dependent on ROK activation. PE pretreatment and agents activating PKA and PKG caused Ca2+ desensitization by inhibiting KCl-induced tonic force and MLC phosphorylation without inhibiting intracellular [Ca2+]. At pCa 5 in beta-escin-permeabilized muscle, FSK and 8b-cGMP accelerated the relaxation rate when tissues were returned to pCa 9, suggesting that both agents can elevate MLCP activity. However, a component of the Ca2+ desensitization attributed to PKG activation in intact tissues appeared to involve a MLC phosphorylation-independent component. Inhibition of KCl-induced tonic force by the ROK inhibitor, Y-27632, and by PE pretreatment, were synergistically potentiated by 8b-cGMP, but not FSK. FSK and PE pretreatment, but not 8b-cGMP, inhibited the KCl-induced increase in site-specific myosin phosphatase target protein-1 phosphorylation at Thr853. These data support the hypothesis that PKA and PE pretreatment converge on a common Ca2+-desensitization pathway, but that PKG can act by a mechanism different from that activated by PKA and PE pretreatment.

G protein-coupled estrogen receptor 1 mediates relaxation of coronary arteries via cAMP/PKA-dependent activation of MLCP

“…These results indicate that cAMP/PKA is involved in the response to G-1, and the result that G-1 increased PKA activity in porcine coronary artery SMCs further confirmed this notion. Consistent with our results, it was reported previously that forskolin and db-cAMP, a PKA activator, inhibited a KClinduced increase in MYPT-1 phosphorylation at Thr 853 and the phosphorylation of MLC, thus relaxing femoral arteries from adult New Zealand white rabbits (56). Taken together, these findings suggest that through cAMP/PKA signaling, G-1 induced activation of MLCP, which resulted in dephosphorylation of MLC in coronary artery SMCs.…”

Hypotension in the chronically hypoxic chicken embryo is related to the β-adrenergic response of chorioallantoic and femoral arteries and not to bradycardia

“…In addition, and depending on vascular beds, the endothelial NO pathway has been found to be involved in the relaxation elicited by activation of ␤ARs (8, 28). The final transduction mechanisms of these pathways involve several processes, including membrane hyperpolarization due to opening of K ϩ channels, (34), enhanced Ca 2ϩ sequestration by the endoplasmic reticulum (33), or reduction in Ca 2ϩ sensitization (35). When the femoral arteries were stimulated by high K ϩ (62.5 mM), opening of K ϩ channels could not cause further hyperpolarization and thus could not exert any further relaxation effect (40).…”

Forskolin Changes the Relationship between Cytosolic Ca2+ and Contraction in Guinea Pig Ileum

“…For example, high K ＋ induces less contractile tension for a given increase in [Ca 2＋ ]i than agonists, such as norepinephrine in ferret portal veins, norepinephrine, prostaglsndins, and endothelin-1 in rat aortas, thromboxane analogues and phenylephrine in rabbit pulmonary arteries, carbachol in canine tracheas, and pilocarpine in ileum (Karaki, 1989). Furthermore, several reports have suggested that receptor-agonists induce greater myosin light chain phosphorylation than high K ＋ at a given [Ca 2＋ ]i (Ratz, 1999;Porter et al, 2006). A decrease in Ca 2＋ sensitivity due to cAMP has also been reported (Rembold and Chen, 1998;Rembold et al, 2001).…”

“…It is generally accepted that adenosine 3',5'-cyclic monophosphate (cAMP) is the intracellular messenger by which many smooth muscle relaxants inhibit the generation or maintenance of muscle tension (Murray, 1990;Lincoln and Cornwell, 1991;Ozaki et al, 1992;Murthy, 2006;Porter et al, 2006). Smooth muscle relaxation induced by cAMP occurs after activation of cAMP-dependent protein kinase (Eckly-Michel et al, 1997;Porter et al, 2006).…”

“…Smooth muscle relaxation induced by cAMP occurs after activation of cAMP-dependent protein kinase (Eckly-Michel et al, 1997;Porter et al, 2006). In smooth muscle, cAMP decreases resting tone and inhibits contraction induced by various agonists, although the mechanism of the inhibitory effect is not fully understood.…”