Maturation attenuates the effects of cGMP on contraction, [Ca2+]i and Ca2+ sensitivity in ovine basilar arteries

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

The fetal cerebral circulation: three decades of exploration by the LLU Center for Perinatal Biology

For more than three decades, research programs in the Center of Perinatal Biology have focused on the vascular biology of the fetal cerebral circulation. In the 1980s, research in the Center demonstrated that cerebral autoregulation operated over a narrower pressure range, and was more vulnerable to insults, in fetuses than in adults. Other studies were among the first to establish that compared to adult cerebral arteries, fetal cerebral arteries were more hydrated, contained smaller smooth muscle cells and less connective tissue, and had endothelium less capable of producing NO. Work in the 1990s revealed that pregnancy depressed reactivity to NO in extra-cerebral arteries, but elevated it in cerebral arteries through effects involving changes in cGMP metabolism. Comparative studies verified that fetal lamb cerebral arteries were an excellent model for cerebral arteries from human infants. Biochemical studies demonstrated that cGMP metabolism was dramatically upregulated, but that contraction was far more dependent on calcium influx, in fetal compared to adult cerebral arteries. Further studies established that chronic hypoxia accelerates functional maturation of fetal cerebral arteries, as indicated by increased contractile responses to adrenergic agonists and perivascular adrenergic nerves. In the 2000s, studies of signal transduction established age-dependent roles for PKG, PKC, PKA, ERK, ODC, IP3, myofilament calcium sensitivity, and many other mechanisms. These diverse studies clearly demonstrated that fetal cerebral arteries were functionally quite distinct compared to adult cerebral arteries. In the current decade, research in the Center has expanded to a more molecular focus on epigenetic mechanisms and their role in fetal vascular adaptation to chronic hypoxia, maternal drug abuse, and nutrient deprivation. Overall, the past three decades have transformed thinking about, and understanding of, the fetal cerebral circulation due in no small part to the sustained research efforts by faculty and staff in the Center for Perinatal Biology.

Involvement of guanylate cyclase and K+ channels in relaxation evoked by ferulate nitrate in rat aorta artery

Vasorelaxant properties of N-2-(ferulamidoethyl)-nitrate (ferulate nitrate, FLNT), a newly synthesized nitrate, were compared with those of isosorbide dinitrate, nicorandil, nitroglycerin, and 8-bromoguanosine 3,5-cyclic monophosphate (8-Br-cGMP) in rat aorta pre-contracted by phenylephrine. FLNT produced vasorelaxation in a concentration-dependent manner (0.1 - 100 µM). The degree of relaxation induced by FLNT was similar to that induced by isosorbide dinitrate. In addition, removal of endothelium did not affect the relaxant effect of FLNT. FLNT caused a rightward shift of the cumulative concentration-response curves of phenylephrine and reduced the maximal efficacy of contraction. 1H-[1,2,4]Oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ, 10 µM) and K(+)-channel blockers charybdotoxin (CHT, 0.1 µM) and BaCl(2) (1 µM) reduced the relaxant effect of FLNT in the endothelium-denuded arteries, whereas glibenclamide (1 µM) and 4-aminopyridine (1 mM) failed to influence FLNT-induced vasorelaxation. Furthermore, in the presence of ODQ, both CHT (0.1 µM) and BaCl(2) (1 µM) still significantly reduced the relaxation evoked by FLNT. Pretreatment of vessels with hydroxocobalamin, a nitric oxide scavenger, abolished the FLNT effect. These findings demonstrate that FLNT induces relaxation of the rat aorta rings endothelium-independently. Furthermore, we demonstrated that FLNT-induced vasorelaxation is related to its stimulation of soluble guanylate cyclase and activation of K(+) channels.

Cilostazol prevents endothelin-induced smooth muscle constriction and proliferation

Cilostazol is a phosphodiesterase inhibitor that has been shown to inhibit platelet activation. Endothelin is known to be the most potent endogenous growth promoting and vasoactive peptide. In patients and animal models with stroke, the level of circulating endothelin increases and complicates the recovery progress contributed by vascular constriction (an immediate pathology) and vascular proliferation (a long-term pathology). However, the effects of cilostazol on endothelin have not been explored. To demonstrate the dual-antagonizing effects of cilostazol on vasoconstriction and cell proliferation induced by endothelin, we used primary culture of mouse vascular smooth muscle cells in vitro, mouse femoral artery ex vivo, and intracranial basilar artery ex vivo. We show that the dual-inhibition effects of cilostazol are mediated by blocking endothelin-induced extracellular calcium influx. Although cilostazol does not inhibit endothelin-induced intraorganellar calcium release, blockade of extracellular calcium influx is sufficient to blunt endothelin-induced vasoconstriction. We also show that cilostazol inhibits endothelin-induced cellular proliferation by blocking extracellular calcium influx. Inhibition of cAMP-dependent protein kinase (PKA) can block anti-proliferation activity of cilostazol, confirming the downstream role of PKA in cellular proliferation. To further demonstrate the selectivity of the dual-antagonizing effects of cilostazol, we used a different phosphodiesterase inhibitor. Interestingly, sildenafil inhibits endothelin-induced vasoconstriction but not cellular proliferation in smooth muscle cells. For the first time, we show selective dual-antagonizing effects of cilostazol on endothelin. We propose that cilostazol is an excellent candidate to treat endothelin-associated diseases, such as stroke.

Postnatal maturation modulates relationships among cytosolic Ca2+, myosin light chain phosphorylation, and contractile tone in ovine cerebral arteries

The present study tests the hypothesis that age-related changes in patterns of agonist-induced myofilament Ca(2+) sensitization involve corresponding differences in the relative contributions of thick- and thin-filament regulation to overall myofilament Ca(2+) sensitivity. Posterior communicating cerebral arteries from term fetal and nonpregnant adult sheep were used in measurements of cytosolic Ca(2+), myosin light chain (MLC) phosphorylation, and contractile tensions induced by varying concentrations of K(+) or serotonin [5-hydroxytryptamine (5-HT)]. The results were used to assess the relative contributions of the relationships between cytosolic Ca(2+) and MLC phosphorylation (thick-filament reactivity), along with the relationships between MLC phosphorylation and contractile tension (thin-filament reactivity), to overall myofilament Ca(2+) sensitivity. For K(+)-induced contractions, both fetal and adult arteries exhibited similar basal myofilament Ca(2+) sensitivity. Despite this similarity, thick-filament reactivity was greater in fetal arteries, whereas thin-filament reactivity was greater in adult arteries. In contrast, 5-HT-induced contractions exhibited increased myofilament Ca(2+) sensitivity compared with K(+)-induced contractions for both fetal and adult cerebral arteries, and the magnitude of this effect was greater in fetal compared with adult arteries. When interpreted together with our previous studies of 5-HT-induced myofilament Ca(2+) sensitization, we attributed the present effects to agonist enhancement of thick-filament reactivity in fetal arteries mediated by G protein receptor activation of a PKC-independent but RhoA-dependent pathway. In adult arteries, agonist stimulation enhanced thin-filament reactivity was also probably mediated through G protein-coupled activation of RhoA-dependent and PKC-independent mechanisms. Overall, the present data demonstrate that agonist-enhanced myofilament Ca(2+) sensitivity can be partitioned into separate thick- and thin-filament effects, the magnitudes of which are different between fetal and adult cerebral arteries.

Chronic hypoxia modulates relations among calcium, myosin light chain phosphorylation, and force differently in fetal and adult ovine basilar arteries

The present study tests the hypothesis that age-related differences in contractility of cerebral arteries from hypoxic animals involve changes in myofilament Ca2+ sensitivity. Basilar arteries from term fetal and nonpregnant adult sheep maintained 110 days at 3,820 m were used in measurements of cytosolic calcium concentration ([Ca2+]i), myosin light chain phosphorylation, and contractile tensions induced by graded concentrations of K+ or serotonin (5-HT). Slopes relating [Ca2+]i to tension were similar in fetal (0.83 +/- 0.07) and adult (1.02 +/- 0.08) arteries for K+ contractions but were significantly greater for fetal (3.77 +/- 0.64) than adult (2.00 +/- 0.13) arteries for 5-HT contractions. For both K+ and 5-HT contractions, these relations were left shifted in fetal compared with adult arteries, indicating greater Ca2+ sensitivity in fetal arteries. In contrast, slopes relating [Ca2+]i and %myosin phosphorylation for K+ contractions were less in fetal (0.37 +/- 0.08) than adult (0.81 +/- 0.07) arteries, and the fetal curves were right shifted. For 5-HT contractions, the slope of the Ca2+-phosphorylation relation was similar in fetal (0.33 +/- 0.09) and adult (0.33 +/- 0.23) arteries, indicating that 5-HT depressed Ca2+-induced myosin phosphorylation in adult arteries. For slopes relating %myosin phosphorylation and tension, fetal values (K+: 1.52 +/- 0.22, 5-HT: 7.66 +/- 1.70) were less than adult values (K+: 2.13 +/- 0.30, 5-HT: 8.29 +/- 2.40) for both K+- and 5-HT-induced contractions, although again fetal curves were left shifted relative to the adult. Thus, in hypoxia-acclimatized basilar arteries, a downregulated ability of Ca2+ to promote myosin phosphorylation is offset by an upregulated ability of phosphorylated myosin to produce force yielding an increased fetal myofilament Ca2+ sensitivity. Postnatal maturation reprioritizes the mechanisms regulating hypoxic contractility through changes in the source of activator Ca2+, the pathways governing myosin light chain phosphorylation, and its interaction with actin.